1. 2015
Bachelor thesis
presented to the
Roskilde university
partial fulfilment of
the requirements for
the degree of Bachelor
of Science
Head of Department : Garry Banta
Department of Environmental, Social and Spatial Change
Roskilde University
Internal supervisor : Benny Hansen
Department of Environmental, Social and Spatial Change
Roskilde University
External supervisor : Mads Christoffersen
DTU Aqua National Institute of Aquatic Resources
Source : “Glass Eel” Chris Bowser NYSDEC
Habitat behaviour and substratum
selection by elver (Anguilla Anguilla)
By Anastasija Martjanova
Student Number: 50713
2. Anastasija Martjanova
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ABSTRACT
For more than a century the life history of the European eel (Anguilla Anguilla) has been
studied. European eel was earlier thought as a catadromous species; however studies
conducted during the last couple of years proved that some part of the population never
reaches the freshwater, and stays in the coastal brackish water during all their growth phase.
This leads to a conclusion that eels are not truly catadromous species. The purpose for this
study is to investigate species sediment preference, burial performance and general behaviour
characteristics.
The present experiments are set up to investigate one aspect of this bury activity: elver
preference for specific coarseness of sediment. Laboratory experiments with 127 juvenile
European eels (70-130mm) were conducted at Den Blå Planet, National Aquarium of Denmark,
within 10 working days. The largest amount of eels was about 7 cm long and weighted 1.5-2.0
g. Highest rate of eels that were presented in the water column was observed after one hour of
the experiment start, whereas burial activity were observed continuously during all experimental
time. Conclusively, just 15% of juvenile eel preferred sand with vegetation and 12% preferred
small gravel. Whereas the major part of the eels (72%) preferred larger subtract such as
medium gravel and large gravel.
Such information will be of importance for field estimation of elver abundances in the
coastal areas, and a general evaluation of habitat applicability for juvenile eel. Sediment for
juvenile eels is not only a first line of defence and protection from currents, but also a place
where they spend much of their time. Decrease and damage of natural habitat of juvenile eels
in brackish water by anthropogenic factors likely is one of the main reasons of eels’ population
decline. The results of this study have implication for coastal zone management, demonstrating
the importance of habitat and subtract in the early stages of eel life ecology.
3. Anastasija Martjanova
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PREFACE
This paper is a Bachelor project report written by Anastasija Martjanova from Roskilde
University (RUC).The project was conducted in cooperation with DTU Aqua National Institute of
Aquatic Resources, particularly with Peter Munk and Mads Christoffersen. It has been set up to
acquire more information on habitat preference and use by the elvers of Anguilla anguilla. This
project addresses the readers such as students and persons with a background in fish biology
and environment biology at least at Bachelor level. The paper concerns a research project on
the information on habitat preference and use by the elvers. All laboratory work has been
conducted at the Den Blå Planet, the National Aquarium of Denmark, which has been recently
opened in Kastrup. Laboratory work was conducted under the supervision of Mads
Christoffersen. The project is a result of ten weeks of experimental work, which includes
preparation for laboratory experiment, designing and establishment of the project materials and
equipment.
It includes analysis of suggested literature and books, meetings, ten days of laboratory
experiment, processing and handling the data analysis. The project is mainly focused on
answering on the question what type of sediment preference of Anguilla anguilla juveniles, and
followed by descriptive analyze of observation results on eel general behavior during the
experiment, such as: burial activity and presence in a water column. Whole project includes an
abstract of our work, introduction about eel ecology and life cycle, methods and material part
which describe the laboratory work. It is followed by results of our experiments which include
observation and notes about eel behavior and subtract preference; and our descriptive
discussion and analyze of our results and about overall about our project. The project ends with
conclusion and perspectives. Final part includes references and an appendix, where all raw data
and sheets are illustrated for better understanding.
4. Anastasija Martjanova
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ACKNOWLEDGMENT
I would like to thank the Head of Studies of the Department of Natural Science of
Roskilde University, Morten Blomhøj for permission to write my Bachelor project alone and
outside of my home institution. It gave me opportunity to study habitat behaviour and subtract
preference of one of most mysterious and interesting species, the European eel. The laboratory
experiment was conducted at Den Blå Planet, National Aquarium of Denmark, which has been
recently opened in Kastrup. It was a great help from employees for taking care of the eels in
the aquarium, helping me feed the eels, and supplying me with all required equipment, as well
as designing the boxes and holding tanks, and taking care of regular technical maintaining staff.
A special thanks to DTU Aqua, National Institute of Aquatic Resources, and Peter Munk
in particular who made me wonder about eel life style and behaviour. Then, fortunately for me
I got introduced to my future supervisor - Mads Christoffersen. Through all the work, starting
with writing the plan for the laboratory work and building the boxes, till the last correction of
my reference list, Mads was much more than just a supervisor: mentor, tutor and most
important, person on who I could to rely as on myself. He supported me in most stressed
moments, always having a sensible advice or right explanation.
Also, I would like to mentioned participation of Peter Munk, who has joined us on our
finish line before submit this paper. Thanks to Peter’s review, helpful discussions and
comments, project found it logical structure, and will have a positive contribution to the
understanding of elver subtract preference.
5. Anastasija Martjanova
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Table of Contents
Summary ...............................................................................................................................5
1. Introduction ...............................................................................................................10
1.1. Migration and life cycle............................................................................................10
1.2. Conservation status.................................................................................................12
1.3. Habitat and behaviour.............................................................................................13
1.4. Problem description ................................................................................................14
2. Method ......................................................................................................................15
2.1. Specimen collection and husbandry..........................................................................15
2.2. Sediment preference experiments............................................................................16
2.3. Experiment design ..................................................................................................18
2.4. Observation strategy...............................................................................................19
2.5. Technique of summarizing sediment preference result ..............................................20
3. Results.......................................................................................................................21
3.1. Morphological measurements...................................................................................21
3.2. Behaviour observation of presence in water column and bury activity ........................22
3.3. Subtract preference of juvenile eels .........................................................................23
4. Discussion..................................................................................................................25
4.1. Behaviour observation of presence in water column and bury activity ........................25
4.2. Subtract preference of eel juveniles .........................................................................26
4.3. Sampling techniques ...............................................................................................27
4.4. Implication on management ....................................................................................28
4.5. Habitat destruction..................................................................................................28
4.6. Further studies .......................................................................................................29
5. Evaluation of the Project .............................................................................................30
6. Conclusion .................................................................................................................32
7. Glossary.....................................................................................................................33
8. Reference list .............................................................................................................34
9. Appendices ................................................................................................................39
6. Anastasija Martjanova
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Summary
Even at our highly progressive times, and even on our so thoroughly studied Planet
there still is quite big part of living species which lifestyle and biological cycle remain
undiscovered. Due to that fact, as well as due to the rapid population decline, Anguila anguila
species received very high priority in the list of main conservation objectives. Eels as known for
their fascinating life cycle and value: economic and ecological prospective, is a good choice of
conservation study. The studies have a long research history on their distribution and general
ecology. However, even now, this species bring a lot of controversial questions, which bring us
to presented research of this particular species.
Introduction
Eels undergo impressive migrations, and larvae of European eel (Anguilla anguilla) drift
thousands of kilometers from the oceanic spawning area in the Sargasso Sea back to the
coastal areas. At their arrival from the sea, A. Anguillia glass eels (unpigmented juveniles)
typically remain temporally in estuarine areas while they undergo some physical and behavioral
transitions (8) (6). The common belief is that all juveniles subsequently migrate to freshwater
habitats; however recent studies using otoliths microchemistry have provided evidence that the
eel catadromy is facultative, hence a large proportion of eels complete their growth without
ever reaching fresh water (10) (13).
The estuarine habitats might be of much larger importance to eel populations than it is
assumed, and there is a need for improved understanding of the life conditions and habitat use
of the juvenile eels growing in the coastal, estuarine areas.
Little is however known about the life of elvers (the pigmented juvenile eel, following
stage of the glass eel growth) in the coastal areas, thus at DTU Aqua a project has been set up
to avail more information on habitat preference and use of it by the elvers. There are some
questions that would be of interest.
First of all, our main focus were on eel sediment preference and by this project we
would like to answer what type of subtract juvenile eels prefer the most. Next, during
experiment we would observe general eel behavior like swimming or staying still in a water
column without any bury activity. Moreover, bury activity will be noted with specific time frame
during all experiment length.
7. Anastasija Martjanova
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Method
During the experiment period all eels were kept in a flow-through holding tank with
water temperature at 17°C and salinity level 14 PSU. All experiments were conducted indoors
with a constant light and air temperature in The Blue Planet. Whole experiment was conducted
in three stages. First, two weeks were used for designing the experiment and finding the
required equipment. For handling substrate and for taking it out of the experimental tank for
eel counting were made eight handmade boxes. This stage was followed by 10 working days of
the actual experiments with six trials when 20 randomly chosen elvers were released into the
two aquariums (10 in each). These experiments took part in duration of two hours; during each
trial of eel behaviour (such as burial behaviour, presence /swimming in water column) in
substrate was observed and noted. Final weeks were used for calculation of the results,
planning and discussion of experiment method, cleaning and maintaining. Each of the six trials
was done by similar method: group of 20 elvers was collected randomly from the large holding
tank, and then the first 10 individuals were released into the Tank 1. Then, after 30 minutes of
acclimatization, first observation was made, as well as the counting of elvers that present in
water column or bury into the sediment. The next 10 elvers were again randomly released, but
now into the Tank 2.
At the end of the experiment each box with juveniles and sediment was taken outside
the experiment tank and placed into a plastic bucket in order to count eels which preferred this
habitat. At the end, after all calculations and observations, all eels were fed. To avoid any
behaviour acquired eels were used only once in the experiment.
Results
In total, 127 eels were measured to estimate their weight and length. Eels’ sizes were
between 7 cm to 14 cm with an average of 10.9 cm. Most abundant group of individuals - 69
eels – was eels from 11cm to 13cm. Weight of observed individuals varied from 0.3 g to 3.13 g,
with an average of 1.6 g. Most populated group - with 55 individuals – was between 1.5 and 2
g.
Results at the end of all experiments showed that the majority of the juvenile eels
preferred to stay totally or almost covered in the habitat; only 4 juvenile eels remained in the
water column. More than 10%, which is 14 individuals, performed bury activity. We noted that
majority of those individuals, at the end of experiment days preferred to burying in medium
gravel (GM) and large gravel (GL), with 5 eels in each, respectively. We observed that there are
two groups (gravel large + gravel medium) and (gravel small + sand and vegetation) which
8. Anastasija Martjanova
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were significantly different from each other with 72% supremacy of the first one. However,
future studies needed to include higher amount of eel juveniles in order to prove and show
higher significance of substrate preference.
Discussion
A range of experiments has been carried out on behavioral aspects of the glass eels. For
example, Anguilla japonica clearly showed a nocturnal activity under laboratory conditions.
Light–dark cycle was a determinant factor affecting their photonegative behavior; they show
nocturnal locomotive activity, and feeding behavior (19). Elvers are known to hide under stones
or bury in sediment, but little precise information is available on this important aspects of the
juvenile ecology.
The present experiments are set up to investigate one aspect of this burying activity:
elver preference for specific coarseness of sediment. Our expectations were based on different
research papers and projects which noticed that eels preferred habitat were they could hide
from predators and bury while they rest. Our results prove our expectation what eels would
preferred medium and large coarse subtract due to fact that this type of substrate ideally suited
for elvers living behavior and life condition.
It is known that eel fishing is a big part of fishing industry. Unfortunately, ecologists and
fishermen's have to face the fact that overall number of eel is continuing to decline all over the
world. It is caused by various human harsh influences, from overfishing to habitat loss. Our
project implications are concentrated on the latest factor such as habitat loss.
Habitat destruction as a factor has various causes. Most common in the Baltic Sea are
marine aggregate extractions with negative influence on sand eel, when another report proved
that suctions hopper dredging and trailer dredging have also unfavourable conditions for flora
and fauna in the areas.
There is information which concludes that seemingly habitat loss and destruction is the
fundamental reason of eel population decline. Mortality level of eels caused by habitat loss
needs to be taken into account and compared to a number of damaged eels due to fishing and
hydropower turbines.
Report from FISRWG also showed that eels prefer pools with deep waters and large
substrate, making them highly vulnerable to human activities which lead to habitat degradation.
Human activities such as logging and land development make eel juveniles highly vulnerable to
land degradation, which lead to continuous habitat loss and eel disappearance. It can be
9. Anastasija Martjanova
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assumed that excavation and habitat destruction observed in recent years has negative effect
on ecology of eel juvenile and further economic profit from fishing on silver eel.
Perspective discussion
For a better understanding of the life cycle and general eel ecology would be interesting
to investigate the eel sediment preference not only in laboratory but also conduct a field
experiment. From the literature, it seems very likely that the fine coarse substrate is a perfect
habitat which helps young eels be less vulnerable and increase their chances for surviving. At
the moment we could state that, as far as we know, it is only experimental study that
contribute to the understanding the habitat choice of eel juveniles in brackish salinity water.
However, there is different type of disturbance and environment effects on eel juvenile subtract
choice and overall eel behaviour , which has not all been included and checked yet.
Conclusion
Our report shows the laboratory experiment on 127 eel juveniles which were used in our
practical analysis. Eels showed strong preference of medium and large gravel subtract, whereas
small gravel and vegetation were ignored by most of the individuals. However studies cannot
determine whether it is caused by effect of disturbance during laboratory experiment or
because of short experiment length. It is clear that further research on the field is needed to
elucidate the mechanisms of how the juvenile eels make their subtract preference, but also
investigate what factors and how effect their general behaviour in order to improve their life
conditions and habitat use of the juvenile eels growing in the coastal, estuarine areas.
Outline on the Bachelor Project Structure and Study
This project topic and problem give excellent opportunity to study both natural science
disciplines: environment ecology and fish biology. In addition it provides indispensable practical
experience based on learnt theory material. Practice and handling of individuals during
laboratory experiments, showed how substantial the knowledge of basics of species biology is,
and how strongly it affects not only on their behaviour, but also distribution and habitat
preference.
It was very advantageous to make a project with people who have been investigated
biology and population dynamics of this particular species for a long time that allows learning
how to understand general eel biology. It also is able to help uncovering the mystery of eel life
style, which occurred due to the lack of the knowledge about the species life cycle. Comments
10. Anastasija Martjanova
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and review the project work and laboratory experiment also give opportunity to understand
how method of handling trials and experiment procedure could be improved in future in order
to get more statistically significant numbers.
There was no conflict of interests in this paper, however overall assigned project provide
many difficulties, starting with design of the equipment, high number of samples (129) to the
limited time frame and contradictive theory background. It was troublesome to contract an
appropriate tools and equipment which convince us that there are appropriate to carry grave
substrate with different coarseness, and also allowed water to pass through. Additionally, it is
very important to carry experimental design which will be appropriate for such tiny and fragile
species, who are vulnerable to many different environmental aspects such as salinity, water
quality, food availability and other. During the experiment, significant part of handling and
releasing species were concentrated on accurateness and carefulness.
After finishing our experiment trials we started our measurements on species weight
and length. However, we were surprised to find only 127 individuals; it means that we were
short by two individuals. However in the discussion part we provide a reasonable explanation
for such an unpleasant situation, which might reveal the cause that lead to that. Another
important note is that due to restricted time frame we received assistance with conducting the
statistical analysis the aid was provided by one of the DTU Aqua Research Centre member.
Statistical analysis supports our main hypothesis of the project, however it also showed that
further studies with higher number of samples are required.
11. Anastasija Martjanova
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1. Introduction
European eel life history is complex and atypical among aquatic species.
However in the early 20th century, due to early marine expeditions of the Danish biologist
Johannes Schmidt was discovered major breeding location of the European eel (Anguilla
Anguilla). (1) This means that the eel has to make a spawning migrating close to 5000 km, in
the Sargasso Sea. (2)
1.1. Migration and life cycle
Despite that the life cycle of the eek is still highly unknown, there is no doubt that the
reproduction is taking place somewhere in the Atlantic Ocean, where the smallest larvae have
been found. Figure 1 shows the end product of long term research conducted by Johannes
Schmidt, which proved that eels spawned in the sea and were originally marine fishes. Schmidt
found that both the European and the American eel (Anguilla rostrata) spawned in the Sargasso
Sea. On the map the length in millimetres at each stage of their journey is shown. The edge of
the dotted zone marks the locations, at which leptocephalae metamorphose into elvers. (2)
Munk et al 2010 found that European eels follow eastward route back to Europe, helped by the
Subtropical Counter Current, and that this is important in the life-cycle completion of European
eels. (3)
Despite continuous research of the eels’ distribution, there have been
misunderstandings and lack of knowledge about different parts of the European eel life cycle.
Harden Jones (1968) has been expressing his outline about this issue in his book: ‘‘No adult
eels have ever been caught in the open Atlantic nor eggs definitely identified in the wild.
Figure 1 - Distribution patterns of eel larvae with the size of the larvae in mm
(source: Schmidt 1923)
12. Anastasija Martjanova
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Migration routes and spawning conditions for adults are unknown or conjectural, as are many
details of the development, feeding and growth, migration or drift are uncertain’’. (5)
European Eels are famous for their impressive migration during their life cycle, which is
the longest and most oceanographically complex among of all Anguilla species. (5) Figure 2
illustrates all life cycles stages with specific regional biological terminology. There are a number
of phases in an eel’s life that have terminology with specific morphology (Figure 2).
As amphihaline fish species, leaf like leptocephalus (marine larvae) start their migration
to the continental shelf with drifting thousands of kilometers from the oceanic spawning area by
ocean currents. (6)
During migration they believed to feed on ‘marine snow’ – organic matter. (7) By
arriving in coastal waters, the leaf-like larvae undergo metamorphosis, becoming glass eels. By
using passive tidal transport and active migration, they enter estuarine (8). The most significant
factors which influence and defines the transition area is the combination of salinity and water
temperature. (8) Eel-shaped, transparent, colourless juveniles in freshwater or saline waters
growth and become pigmented, reaching the elver stage and then yellow eels (9). Increased
size, yellowish bicolour dorsum, and lighter ventral are main morphological difference between
yellow eel and elver. Yellow eel stage lasts for 5-20 years, when they grow into silver eels
Figure 2 - Life Cycle of European Eel (source Rob Slaupkaukas proposal FOR THE INCLUSION OF
THE European EEL (Anguilla anguilla) On CMS Appendix II)
13. Anastasija Martjanova
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before embarking on their spawning migration. During this period they undergo significant
enlarging of their eyes, eels lose their ability to feed, the dorsal side turns black, as do the
pectoral fins; the ventral side, in contrast, turns white with silvery reflections. At full sexual
maturation, their snake-like elongated body would have dorsal, pectoral, anal and caudal with
absent ventral fin. (4) At the end they migrate, through the ocean to the Sargasso Sea, where
they spawn and die. (10)
1.2. Conservation status
The European eel is an important resource in conservation, ecological and socio-
economic terms. There is a strong positive affect between habitat health and fisheries because
most of eel catching areas are based at the vegetated coastal habitats. According to “Facts
about Swedish fisheries", catchment of eel approximately 50% of the fishermen profit in area of
Skagerrak and Kattegat, making this habitat and species highly significant not only as an
important biological species, but also economically, providing places for employment and source
of income. (11) (12)
As mentioned in ICES (16) report, there are various factors which have influence on the
eel life cycle ecology and distribution of the eel. Studies conclude that not only European eel,
but also Japanese (Anguilla japonica) and North American eel the most important
environmental aspects that influence glass eel migration is temperature of water, time of the
day, lunar phase, water discharge, tidal cycle, water conductivity, salinity and water
(6)(16)(18)(16). In addition, it is also important to consider that all mentioned factors differ
according to location, estuary characteristics and physiological status of the eels. (15) (16)
Figure 3 shows the time trends for European eel among others. At first glance there is clear
evidence that since the 1980s, the abundance of the European eel has declined throughout its
distribution range. Hypothesis of what have led to it will be discussed below. (17)
Recently European water masses experienced negative effect of the invasive nematode
species Anguilla Crassus. Firstly it was discovered in Germany in the early eighties, however at
the moment it has reached enormous abundance and present almost in each eel stock in
Europe. Variety of studies which were focused on biological effect of nematode on host and
colonized species conclude that European eel is much more vulnerable to these parasites, and it
might be one of the main reason of continues population decline. (15)
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According to the report (16), the major causes for the decline are considered to be
habitat destruction and obstruction of migration routes by dams and other chemical or physical
obstacles. Also factors such as exploitation of glass eel in estuaries, increased predation,
barriers to upstream migration, exploitation of yellow and silver eel, impediments to
downstream migration and habitat loss needed to be taken under consideration in order to
restore the depleted stocks, included all biological stages. (15) (16) (17)
1.3. Habitat and behaviour
Distribution of European eel indicates that it can live in a wide range of temperatures
and it is extremely tolerant to low oxygen environments and poor water quality generally. (20)
(29)
As it is mentioned above, habitat loss and degradation played a major role in the decline
of the eels in the second half of the 20th century. Vegetated costal habitats on rocky bottoms
provide shelter from predation, protect against currents and rich food conditions for European
eel, both adults and juveniles. Reduction of predation risk may be especially important to eels,
because they have lower burst swimming speed compared to other fishes with fusiform
shape.(20)
From Tesch’s book we know that Yellow-stage anguillia eels generally forage
nocturnally, but spend the day in the substrate. It might both burrow soft sediments such as
sand, or burrows excavated in spaces among rocks and gravel. (6) (19)
Figure 3 Time trends in juvenile abundance of the major eel stocks of the world. The average
trend of the four longest data series is shown for the European eel. (From Dekker 2004)
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1.4. Problem description
Sediment grain size is one of the most important environmental variables determining
the distribution of other fish, e.g. flatfish (24), however there is little known about substrate
choice and eels. Loss and destruction of important habitats can have a negative effect on the
eel population, which can lead to extinction of the European eel. Studies have provided
evidence, that up to 80 % of the eel population complete their growth phase by staying in salt
or brackish water along the coast, without ever reaching fresh water. (10) (13) (18) (22) (36)
In additional investigation of feeding behaviour of glass eel by using stable isotopes in
estuarine proved the fact that such intermediate zones are important feeding habitat and not
just a place for immigration. Moreover studies conducted in French river showed preference for
brackish water. (20)
This element of population dynamics is particularly poorly understood; therefore further studies
are needed to provide a better understanding of the life condition during coastal residence time,
habitat selection of juvenile eels in this important life stage. (20)
In order to fill this gap, the project was set up by DTU Aqua for availing more
information on habitat preference and use by the elvers. The present experiments are set up to
investigate one aspect of their habitat ecology: eel juvenile preference for specific coarseness of
sediment. The goal of this study was to provide new information for the protection of eel
habitat and migration corridors, as well as development of restoration plans for eels.
To address these needs we developed specific objectives:
1. What kind of sediment do juvenile eels prefer?
2. Do juvenile eels always bury in the sediment?
3. How much time it takes for eels to choose the substrate, and how do they interact with
the water column?
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2. Method
We conducted a research on marine species, which is a member of fish order that
consists of 19 families and around 400 species, who have many common features and are
distributed from temporal to tropic aquatic environment.
There are 15 species within the genus Anguilla, most of which originate from tropical
areas in the Pacific. Our research is primarily focused on one of the most exploited temperate
species: European eel - Anguilla anguilla. There is a variety of different studies on ecology and
behaviour of other species such as American eel - A. rostrata, Japanese eel - A. japonica,
Australian and New Zealand shortfin eel - A. australis, and longfin eel - A. dieffenbachia.
Particularly European eel Anguilla anguilla have long lifespan. Its life cycle involves several
morphological shifts, which end with spawning migration of mature Silver eel back to the
Sargasso Sea. The life cycle has still not been completely revealed. European eel tend to appear
in all aquatic environment and being caught as a food source in many different parts of the
World, and particularly along the coastline of almost all of the Europe. They have been noted as
highly adaptive and environment resistant for different abiotic and biotic factors such as
temperature, salinity and many others. Also, need to take into account their environment
importance and what role do they play into whole aquatic ecosystem, being a vital food source
of food source for many other aquatic species. (6)(17)(29)
2.1. Specimen collection and husbandry
During the experiment period all eels were kept in a flow-through holding tank with
differently-sized plastic tubes that could be used as hiding places, until they were needed for
experiments. (Figures 4 and 5) They were fed with dry food once every second day, starting
three weeks before the conduction experiments. Approximately 70 eels came from a small
Danish stream from the northern part of Zealand, called Hellebækken, and additional 200 were
bought from Jupiter eel, a Danish eel hatchery. The juvenile eels were originally from France.
Water temperature was kept constantly at 17°C and salinity level was approximately 14 PSU to
simulate salinity level found in a typical Danish fjord system. (23) All experiments were
conducted indoors with a constant light and air temperature in The Blue Planet.
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2.2. Sediment preference experiments
In order to observe eel subtract preference, experiments were conducted in two
identical holding tanks, the tanks were 75 cm long, 15 cm wide and 40 cm high (total volume
45 litres), specially constructed for this experiment. Each tank was divided into four equal
quadrants and separated by 15 cm high Plexiglas walls in order to avoid mixing of habitat. The
establishment of boxes with different substrates can be seen on the Figure 6. Quadrants were
proposed for fixed habitat along the bottom, each containing one of the following substrata:
sand with artificial vegetation, small gravel (GS 0-8 mm), medium gravel (GM 12-32 mm) and
large gravel size (GL 32-64 mm). Substratum size composition is presented in Table 1.
Figure 5 - Differently sized tubes used in
experiment
Figure 5 - Flow –through holding tank
Figure 6 - Establishing boxes with different
substrates
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The types of substrate were chosen from earlier observations and experiments (Pers.
Comm. Christoffersen, M), the set-up of the distribution of different substrate types can be seen
on the Figure 7. Sand was collected from Amager Strand, gravels were collected from a mineral
resources company and vegetation was simulated with bundles of plastic ribbon-shaped to
imitate shoots of macro algae or eelgrass.
Table 1 - Sediment size (mm) used in the experiment
Sediment type Sediment size(mm)
Sand and Vegetation(SV)
Half area of sand and half area of
artificial eelgrass
Small Gravel (GS) 0 - 8 mm
Medium Gravel (GM) 12 - 32 mm
Large Gravel (GL) 32 - 64 mm
Figure 7 - Distribution of different habitats into their
permanent place in both tanks
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During the experiments artificial vegetation were above sand. To improve techniques of
handling substrate and for taking it out of the experimental tank to count the eel after each
experiment, eight handmade boxes were made from two type of nets, one with small fraction
spaces for sand with vegetation and small gravel, and another for medium and large gravel, the
boxes can be seen on the Figure 9. During experiment both tanks were held in one big plastic
box which was provided by The Blue Planet (Figure 8). Aquariums were 30 cm deep . There
was also water in the plastic box (25 cm) cm to reduce water pressure on aquarium walls.
2.3. Experiment design
The plan for the experiment consisted of different parts: three weeks of experiments,
including six trials of releasing 20 randomly chosen elvers into the two aquariums (10 in each);
observations of eel behaviour in substrate, week of preparation for the experiment (getting
elvers from South of Zealand, changing water salinity and cleaning aquariums, making boxes
for different substrates), and week of calculation of the results, planning and discussion
experiment method, cleaning and maintaining. Trial days were held scheduled and followed by
feeding of elvers.
Each eel was released from 10 cm above the aquarium into random habitat with its
head facing towards the tank. While releasing the eels we tried to randomly distribute them
approximately equally to exclude bias on habitat selection. Some of the eels chose to rest on
the sides of the boxes in the water column. After the end of the experiment they were also
included in the analysis (all were in the small gravel box).
To let the eels acclimate from handling it were decided to include first 30 minutes for
the juvenile eels before starting of the experiment. The example of scheduled work sheet which
were used at each experiment day could be observed in Appendix (Table 2).
Figure 9 - Handmade boxes which we tried for our
experiment
Figure 9 - Yellow plastic box for both
experiment tanks
20. Anastasija Martjanova
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2.4. Observation strategy
Each of the six trials was done using the similar technique. Before beginning of the
experiment, salinity and water temperature were measured. Then a group of 20 elvers was
collected randomly from the large holding tank, and then the first 10
individuals were released into the Tank 1. Then, after 30 minutes of
acclimatization, first observation was made and counting of elvers
that present in water column or bury into the sediment. The next l 10
elvers were again randomly released, but now into the Tank 2 (Figure
10). Next step included observation and counting elvers and their
behaviour features such as presence in a specific water column,
change of substrate during the experiment or bury into sediment. It
was done every 30 minutes during the next two hours. After each
short observation, plastic box were covered in order to increase their
activity, knowing their nocturnal lifestyle. (6) (19)
The term “bury activity” (burial activity) means that the
individual is partly submerged in the substrate while partly present in the water column. To be
present in the water column means that 100% of the body is outside of any substrate type and
the individual is freely swimming between different habitats. While being in habitat means that
the body of the individual is completely submerged in the substrate.
Figure 10 - Releasing
the eels into random
habitat in each tank
with 30 minutes
interval in between
Image 1 – Behaviour Explanation
21. Anastasija Martjanova
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2.5. Technique of summarizing sediment preference result
After the experiments, the quadrants of Tank 1 were separated with additional Plexiglas
walls to eliminate the probability that juveniles could change their substrate due to disturbance.
To calculate how many elvers choose particular habitat, each box with juveniles and sediment
was taken outside the experiment tank and placed into a plastic bucket. Then each eel was
counted and removed from the box with substratum. (Figure 10) It was made as fast as
possible in order reduce the stress of elvers and to fit into the time frame of 30 minutes to start
with tank 2. At the end, when all calculations and observations were done, all eels were
released into another holding tank, which was separated from the main holding tank, and were
fed right after experiment. Each eel was used only once in the experiment in order to avoid any
behaviour acquired.
Experiments were conducted to gain knowledge of elvers habitat preference and to
observe their behaviour in the sediment during the experiment.
Figure 11 - The method used for
collecting and counting eels after the
experiment
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3. Results
3.1. Morphological measurements
Size and weight of each individual were measured in millimetres and grams respectively,
the results of experiments are presented in Table 3 and Table 14 in the appendix. In total, 127
eels were measured. Eels’ sizes vary from 7 cm to 14 cm with an average of 10.9 cm. Table 3
and Figure 12 show that the most abundant group of individuals - with 69 eels - is with length
slightly above average, length varied from 11cm to 13cm. Followed by 44 juvenile eels which
compose the group of individuals between 9cm and 11cm. Least populated group contains eels
who are much smaller than average, lower than 9 cm with the number of 8 individuals, as well
as the adults who grew up to 13 cm and longer, with only 6 individuals.
Weight of observed individuals varies from 0.3 g to 3.13 g, with an average of 1.6 g.
From Table 4, Table 14 from the Appendix and Figure 13 you can distinguish that most
abundant group is within range of weight from 1.5 g o 2 g with 55 individuals, followed by
group of juvenile eels weighting from 1 g to 1.5 g. Both groups are close to the average value
and together compose 75% from all measured individuals. Next group consists of eels that had
a body length within the range from 2 g to 3 g. Last group of eels were who had highest
weight, 3 g and more, with just 4 eels.
0
10
20
30
40
50
60
70
80
l<9 9<l<11 11<l<13 13<l
Nofeels
Lenght fraction
Figure 12 - Results of measurements of eel lenght
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3.2. Behaviour observation of presence in water column and bury
activity
A total of 129 European eels were used in experiments. Results at the end of all
experiments showed that the majority of the juvenile eels preferred to stay totally or almost
covered in the habitat; (Table 13 in Appendix) with only 4 juvenile eels who remained in the
water column, therefore we will not include their subtract type preference due to insignificance.
(Table 5) (Table 13) However, the observations at different time showed that at the start of
experiment there were 7 eel in total that stayed in water column.
After, it followed an increase and reached its peak with 11 eels at 30 minutes after the
start of the experiment (Figure 14) (Table 6 and Table 7). After that, at 1 hour after eels where
released, we observed the same number off eels in the water column, as it was at the start of
0
10
20
30
40
50
60
w<1 1<w<1.5 1.5<w<2 2<w<3 3<w
Nofeels
Weight fraction
Series1
Figure 13 - Results of measurements of eel weight
0
2
4
6
8
10
12
Start 30 min Hour Hour and 30
min
Two Hours
Nofeels
Time
Sum of N eels in water
column
Figure 14 - Summarized number of eels which presented in different time frame their
presence into water column and do not entering any chosen habitat
24. Anastasija Martjanova
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experiment. The number continued decreasing to just 4 juvenile eels at the end of experiment.
The numbers were summarized after conduction of all experiments.
From the total number of individual, more than 10%, which is 14 individuals, showed
bury activity (Table 8) (Table 13). Those individuals showed that among all sediments the most
popular was medium gravel (GM) and large gravel (GL), with 5 eels in each at the end of
experiment days. The observations at different time showed that at the start of experiment
there were 7 eels that were present partly in water column and partly in sediment. It followed
with a slight increase and sharply reached its peak with 17 eels in an hour after the start of
experiment (Figure 15) (Table 9) (Table 10). Then it decreased to just 11 buried eels, and at
the end of experiment we observed intermediate number with 14 buried eels in a different
subtracts. All numbers were summarized after all the experiments.
3.3. Subtract preference of juvenile eels
Table 11 presents data from experiment including trial dates, and numbers of eels
presented in different habitat by the end of the experiment. Summarized data, which is a part
of the bootstrapped confidence intervals (to include the extra uncertainty); it is shown in Figure
16. Counts were normalized and plotted into bar charts function (GM+GL/GS+SV). Distribution
can be visually compared in the Figure 16. We observe that there are two groups (GL + GM)
and (GS+SV) and those two groups are significantly different from each other with GL+GM
dominance, with 72% from total eel juvenile number (Table 12). Tendency of GM is slightly
higher than GL; however more data is needed to conclude that statement.
0
2
4
6
8
10
12
14
16
18
20
Start After 30 min Hour 90 min Two hours
Nofeels
Time
N of eels bury sediment
Figure 15 - Summarized number of eels which presented in different time frame
their bury behaviour (means partly presented in substrate and in the water
column)
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Table 12 Results of eel habitat Preference
Habitat Significance Number of eels observed in habitat
GL 0.302326 39 (30%)
GM 0.426357 55 (42%)
GS 0.124031 16 (12 %)
SV 0.147287 19 (15%)
Total: 129 (100%)
Figure 16 - Comparing observed and theoretical proportions (P- proportions in each habitat with
95% ci) Normalized counts (GM+GL/GS+SV)
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4. Discussion
4.1. Behaviour observation of presence in water column and bury
activity
Burial activity is one of the most important aspects of eel lifestyle. Despite the fact, that
in our experiment most of eels decided to stay in substrate, 14 eels at the end of trials were
partly present outside of preferred habitat. (Table 8) Most of them give preference to medium
and large size gravel, both with 5 eels respectively. Suitable size of holes between gravel might
be a reasonable explanation why eels choose especially this type of habitat.
It is reasonable to assume that we did not observe any of burying eel in small gravel
because of subtract tight composition, which did not allow eels to bury in it. Explanation why
there is such as small number of eels burying eels in sand and vegetation habitat can happen
due to the fact that artificial vegetation was spread among all boxes with subtract, therefore
when we tried to observe them and count they, hided or tried to escape and bury completely.
Also artificial vegetation might be not as good and qualitative as natural, which also might be
an explanation. (20)
If we take a look on time frame (Figure 15) we observe that highest burying activity was
shown after one hour from the start of the experiment. That might be due to their
acclimatization and time for exploration for suitable habitat. After this peak, the number of eels
who were actively burying in subtract decreased continuously supposedly because of the fact
that eels have made their choice of habitat and just bury into it completely.
During the experiments every 30 minutes eels were checked for their presence in the
water column. Results showed insignificant number of eels at the end of experiment, probably
due to fact that there had not been enough space to swim between habitats.
Another explanation might be because of too short time of experiment and too much
disturbance during opening and closing of the aquarium. The time observation diagram 2
showed that the highest number off eels that were present in the water column after 30
minutes of the experiment (Figure 14). Reasonably to presume this is the time for them to
decide, so they swam around and tried to find most preferable habitat. Experiment proves fact
that elvers like to hide into subtract or at least bury in it partly, therefore a very small number,
only 4 individuals, were present in a water column during observation and at the end of the all
experiments.(Table 5)
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4.2. Subtract preference of eel juveniles
Numerous studies on flatfish proved that there is a direct link between burial capability
and fish defence mechanism. Sediment might increase the survivorship of vulnerable flatfish
juveniles. In addition, good habitat choice is advantage to have higher prey density. Therefore,
in nature those two mechanisms are taken first into the consideration when it comes for choice
the habitat. (24)
In contrast with halibut and flounder, no studies of substratum preference of eel
juveniles were conducted. Some studies used field observations for adult eels, but their habitats
and life is very poorly understood.
Nevertheless, at the end of each experiment we had a chance to see the results of our
main project objective - what substrate young elvers favoured the most. We could clearly see
that medium gravel and large gravel highly outperform small gravel and sand vegetation, with
94 and 35 elvers respectively. (Table 12) Our results fully match our expectations which were
based on various researches on eel subtract preference all over the world. According to
research of New Zealand short-finned eel habitat preference, juvenile eels were mainly found
on a gravel and/or mud substrate, but larger eels (>300 mm) preferred sandy substrates.(25)
Another study was conducted on Anguilla japonica eels by Tsukamomo in 2003 which
results also support the idea of preferred tube-design habitat to sand, and can be explained
that in tubes it is easier to bury than in sand. (19)
In addition to this, in the book which studied habitat off eels and two other species, it is
mentioned that eels mostly preferred coarse and gravel substrata among others. (26)
Moreover, according to the results of research on Anguilla Reinhardtii adult eels were
most abundant in rocks, fine gravel, and sand; whereas juveniles and sub-adults were occurring
in cobbles and gravel and rocky subtract. Another eel species - short-finned eel (Anguilla
australis) mostly preferred coarse and fine gravel and sand. (27)
Despite the positive results, those researchers made field observations of influence of
biotic and abiotic factors. Absence of biological factors such as food, predation and conspecifics,
in the laboratory experiments, the habitat choice of eels may be more strongly determined by
the physical attributes of available habitats. Therefore if observation would be conducted at the
field, data may vary.
We believe that habitat choice was based on different aspects of eel behaviour. As
claimed by different researches the main tactic for eel is to find safe place where it is able stay
and hide from disturbances and predators. (6)
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Good substrate, with fine coarse holes is a perfect habitat which helps young eels be
less vulnerable and increase their chances for surviving. That also might explain why we
observed that juvenile eels had a lower preference for sediment substrata with grain sizes. It is
because they simply could not bury in it (GS). However, in our results there are some
individuals who were found in this habitat, and it might confuse the reader. Those eels were
found between the box with substrate and plastic quadrant. Especially small eels showed this
behaviour due to their tiny size, where they were probably hiding from larger elvers.
Least chosen habitat was sand and vegetation. There are possible few reasonable
explanations why this type of substrate was less popular compared to medium and large gravel.
In our experiment we used artificial vegetation, which probably might have affected choice of
habitat. Also, as it is claimed in mentioned studies, smaller eels prefer coarse, rough stones and
gravel, while larger eel prefer mud and fine sand. Larger eels seem stronger and powerful,
while elvers need a hard substrate such as gravel to feed and spend time at their migration
corridors .Above all, we suggest that the ability to bury and be fully covered by substrate was a
primary (leading) factor influencing substratum preference.
4.3. Sampling techniques
Various factors had affected our results on observation of eel presence in water column,
burring behaviour, and habitat choice.
Firstly, it is fact that it was an indoor experiment in a controlled environment. Predation,
prey availability, noise and other disturbances might effect on eel presence in water, burring
activity or preferred habitat in natural environment. (6) (8)
Secondly, our observation time might be too short for eels to find most suitable habitat
for living. Each 30 minutes we had to open the large aquarium tank and disturb them with light,
and noise. We know that they are nocturnal species, meaning they are more active during the
night; however our experiment were carried during the day, which also might have affected on
their performance. Although, during experiment we carry them in a closed tank, and tried to be
accurate and quiet while making time observations and notes. (6) (8) (19)
Even though, at the end of experiment, when we checked our first holding tank we had
taken all boxes with substrate out to count eels who preferred to stay in this habitat. During
this we unintentionally disturbed other eels in the second holding tank because both of them
were in the same aquarium. Therefore, experiment techniques could be improved if we had two
separate tanks where we could place holding glass tanks.
29. Anastasija Martjanova
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Subtract were collected accurately and no eel suffered any injury. However artificial
vegetation, which was handmade cut and designed, can be too sharp or dark, meaning the eels
just did not like it. Finally, when experiments were finished we placed all eels in one holding
tank and results of observation showed that 129 eels were sampled, however when after the
experiment we measured them we counted only 127, which means two small eels were eaten
by larger sized eels. (6) (28) (29)
4.4. Implication on management
A significant part of population is economically dependent on eel fisheries. Only in
Europe about 25000 tons are consumed annually. However, since early eighties eels faced a
dramatic decline not only the European eel population, but also all eel species around the
world. (Figure 3) (30)
Due to the strong human influence, such as: overfishing, water pollution, degradation
and habitat loss through engineering works, hydro-electric power stations there is a negative
effect on the whole aquatic system, as well as there is species decline. (15)
Spawning migration is crucial driving mechanism in eel life cycle, but unfortunately due
to wide range of threats there is no longer an easy access between inland waters and the
ocean. Anoxic condition in brackish water, tidal currents, physical barriers like dams, port
activity such as dredging and extraction of substrate could be some of possible reasons of eel
natural occupancy destruction. Of course, there are other biological factors such as predation
and diseases caused by viruses and invasive species, they have a great impact on population
decline, however we do not discuss them in this report. (15) (31)
4.5. Habitat destruction
Marine aggregate extraction operation can potentially have a negative influence on sand
eel, making them vulnerable. It is due to the fact that sand eels possess loyal attention to their
habitat therefore loss of resources targeted sands and gravels could be one of key factor
affecting their survivability. (32)
There was one research which activity was conducted on 3 exploration areas and 3
extraction areas in Øresund. Results of mapping showed that sand, gravel and course to fine
Holocene sand were major type of coastal substrate. The environmental effects of extraction of
raw materials showed direct effects on flora and fauna caused by suctions hopper dredging and
trailer dredging in the areas. (33)
Another one, The INDICANG1 project, highlighted that most preferred habitats for eels
were vastly ruined. (34)
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According to OSPAR overview, it probably is the main reason of recent eel population
decline and switching the sex ratio which negatively affects the population growth rate. They
suggest that mortality which is caused by habitat destruction has the same level, as mortality
level caused by fishing or turbines. (34)
Report from FISRWG also showed that eels prefer pools with deep waters and large
substrate, making them highly vulnerable to human activities which lead to habitat degradation
such as logging and land development. It is strongly recommended to avoid that activation to
protect stream environment for eel natural habitat. (35)
Thus, in light of the results of this study, it can be assumed that excavation and habitat
destruction observed in recent years has probably reduced the quantity and quality of suitable
habitats for juvenile eel, which has negative effect on both ecology and economy.
4.6. Further studies
Various observations of European eel life history can describe eels not as completely
catadromous, but "facultative or semi-catadromous". Due to the fact that the European eels
were found in different water types, such as freshwater, brackish, and coastal area. Therefore,
the fact that freshwater phase is not an essential part of their life history, makes them not
absolute catadromous species. (10) (13) (18) (22) (36)
This is the only experimental study until now, as far as we know, that contribute to the
understanding the habitat choice of eel juveniles in brackish salinity water. Therefore further
studies should be taken in order to contribute to a better understanding of substrate selection
of all Anguillae group.
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5. Evaluation of the Project
Aim of this project was to find answer for our research question from experimental
studies related to overall problem formulation "Is there a preference choice for a specific type
of substrate for elvers"?
The project has been a challenge due to the fact that problem has a very specific
formulation; there were very limited amount of information sources, combined with not very
favourable and suitable laboratory conditions. Theory was based on biological knowledge, which
includes topics such as life cycle analysis and general eel behaviour, mainly provided by Tesch
2003 book (6) and Dekeer (17) and Deeler (8) reports, those sources fully covered all our basic
knowledge topics of eel ecology. Another topic of our theoretical background was focused on
conservation status of particular species, and was considered by Red List as critically
endangered (14). By this and other researches we could highlight factors which have most
negative effect on European eel population, who recently experience a rapid decline.
However, there is a lack of information about such important life style aspect like
European silver eel habitat preference; neither there is any information about elver habitat
preference. It might be due to the fact that this specific field is still very novel, because just few
decades ago scientist have noticed that not all eels perform catadromous behaviour. Therefore
our expectations were based mainly on biological knowledge from above-mentioned books,
researchers report from other eel species, such as A. japonica, A. australis, and A. rostrata,
means we have not been fully confident in our expectations.
In addition to that, it was quite difficult to find an acceptable information about habitat
destruction and it effect on population decline. Some information was performed in various
countries; however we did not find nearly descriptive enough research or report about Danish
coastal habitat destruction, which highlight the issues of marine excavation and it negative
effect on flora and fauna, focused on eel habitat destruction.
Despite on the fact, that various researcher has proved of unnecessary part of
freshwater in ell life cycle, there is no any report from Danish board on how many of eel
juveniles enter Danish estuarine annually. Also, there is no studies which observe their further
migration to fresh water, or how many decide to stay in brackish water, as well as what factors
might affect that behaviour.
Taken into consideration and reviewing project we need to agree that six experiment
trial were enough to find that there is a preference for subtract. In order to achieve results of
sufficiently high significance level, in order to confirm confident answer for our problem
formulation. To provide more significant data on the habitat preference, we strongly
32. Anastasija Martjanova
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recommend increasing the number of trials. In addition, observation with time frame has
showed that two hours, which were given to elvers to select an a appropriate habitat among
given substrates is not enough. This is based on observation of eels that were present in water
column, and on their burying activity. If elvers show such behaviour pattern that means that
they did not find a right habitat to stay in. Moreover, due to strict time frame we could not use
another box to separate holding tanks during the experiment. That lack lead to continues eel
distribution during the experiment trials, especially at the end of the trial when first holding tank
has to be cleaned up for eel counting, while another holding tank was stayed too close, so eels
from this holding tank were continuously interrupted. This fact, probably, might have an effect
on eel behaviour and preference of particular subtract. All this experimental concerns might be
avoided and improved in further studies.
Regardless of all this points of question and challenges, it should be mentioned the
advantages of the project in terms of handling real experiment and participation of highly
experienced scientists, relative literature search, evaluation and analysis. Overall, by doing this
particular project author have not only improved her qualifications within the fields of fish
biology, ecology and environmental conservation, but also strongly confirmed her desire of
professional career in marine conservation ecology as a future career choice.
33. Anastasija Martjanova
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6. Conclusion
According to the results of this research project, we confirm that there is a pattern of
elvers substrate preference. Major part (72% of all sampled elvers) had favoured gravel with
medium and large coarseness. Whereas the number of eels that preferred to stay in gravel with
small coarseness, and sand and vegetation was very small: only 12% and 14 %, respectively.
During the observation, eels showed moderate burial activity, with only 14 individuals observed
by the end of experiment. Such behaviour feature like swimming in the water column was
insignificant, with just 4 eels at the end of all experiment trials. This study is part of ongoing
effort to fully understand the population dynamics and life cycle of eel species, and European
eel in particular, therefore the research will continue in the field or in laboratory, but with more
trials and number of individuals, this will expand our knowledge and confidence about juvenile
eel subtract preference, and will have a significant positive improvement on conservation
techniques of all European eel population.
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7. Glossary
Catadromous species - fish live in fresh water, breed in the sea.
Amphihaline - Aquatic species which pass periodically, at well-defined stages of their
life cycle, from salt to fresh water and vice versa
Leptocephalus - the leaf-shaped marine larval stage
Glass eel - marine larvae when they reach brackish water, before developing into the
pigmented, growth phase
Elver - referred to a migration stage when during feeding in the coastal environments,
the glass eels become pigmented.
Yellow eel - major growing stage, which starts after eels' migration to fresh water, it
continues for 2-20 years.
Silver eel – last stage of yellow eel maturation, when body fat and summer water
temperature promotes European eel maturation and get it ready for migration to the sea.
Major difference between freshwater, brackish and marine water - from
ecological point of view for eel life cycle there is different salinity level, with highest at marine
water, followed estuarine waters and with the lowest salinity level at freshwater ecosystem
Marine snow - small particles of organic material that fall from upper waters down to
the sea bed. It includes dead animals and plants, fecal matter, sand, soot, and other inorganic
dust.
The Kattegat (in Danish, English) or Kattegatt (in Swedish) is a sea which is
surrounded by Denmark and Sweden.The sea area is a continuation of the Skagerrak.
The Skagerrak - is a strait running between the three Scandinavian countires, which
also connect the North Sea and the Kattegat sea area, finally leads to Baltic sea.
Nocturnal life style - active mainly during the night.
Sex ratio - the relative number of males and females in a population.
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8. Reference list
1) Schmidtt J. 1906. Contributions to the life of the eel (Anguilla(Anguilla vulgaris FLEM).
Rapports et Proces-Verbaux dess Reunions du Conseil International pour1'Explorationn
de la Mer 5:137-264.
2) Schmidt, J. (1922-1922) The breeding places of the eel. Philosophical Transactions of
the Royal Society of London Series B 211: 179–208.
3) Oceanic fronts in the Sargasso Sea control the early life and drift of Atlantic
eels. / Munk, Peter; Hansen, Michael Møller; Maas, Gregory E.; Nielsen, Torkel Gissel;
Castonguay, Martin; Riemann, Lasse; Sparholt, Henrik; Als, Thomas Damm; Aarestrup,
Kim; Andersen, Nikolaj G.; Bachler, Mirjam.
In: Royal Society of London. Proceedings. Biological Sciences, Vol. 277, No. 1700, 2010,
p. 3593-3599.
4) Harden Jones FR (1968) Fish migration. Edward Arnold Ltd, London, p 325
Rev Fish Biol Fisheries (2005) 15:367–398 DOI 10.1007/s11160-006-0005-8 123
RESEARCH ARTICLE The European eel (Anguilla anguilla, Linnaeus), its lifecycle,
evolution and reproduction: a literature review
5) Tsukamoto, K., Aoyama, J. and Miller, M.J. 2002. Migration, speciation, and the
evolution of diadromy in anguillid eels. . Canadian Journal of Fisheries and Aquatic
Sciences 59: 1989-1989)
6) The eel: biology and management of anguillid eels. Chapman and Hall, London. Tesch,
F.W. (2003). The Eel, 5th ed. Blackwell Publishing, Oxford 408pp
7) Otake T., Nogami K., Maruyama K.Dissolved and particulate organic matter as possible
food sources for eel leptocephali. Marine Ecology Progress Series1993;92:27-34.
8) Deelder, C. 1970. Synopsis of biological data of the eel Anguilla anguilla (Linnaeus,
1758). FAO Fish. Synop., 80: 68.
9) Van Ginneken, V., G. Van Den Thillart. 2000. Physiology: Eel fat stores are enough to
reach the Sargasso. Nature, 403: 156-157.
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10) Nature 396, 635-636 (17 December 1998) | doi:10.1038/25264 Do all freshwater eels
migrate?Katsumi Tsukamoto1
, Izumi Nakai2
& W-.V. Tesch3
11) Swedish Board of Fisheries. Fakta om svenskt fiske [Facts about Swedish fisheries].
Fiskeriverket, Göteborg. [In Swedish] 2006.
12) Coastal habitat support to fish and fisheries on the Swedish west coast Johan Ståla ,
Sandra Paulsenb , Leif Pihla , Patrik Rönnbäckc , Tore Söderqvistd and Håkan
Wennhagea
13) Daverat F, Limburg KE, Thibault I, Shiao JC and others (2006) Phenotypic plasticity of
habitat use by 3 temperate eel species, Anguilla anguilla, A. japonica and A. rostrata.
Mar Ecol Prog Ser 308:231–241
14) Jacoby, D. & Gollock, M. 2014. Anguilla anguilla. The IUCN Red List of Threatened
Species 2014:e.T60344A45833138. http://dx.doi.org/10.2305/IUCN.UK.2014-
1.RLTS.T60344A45833138.en.)
15) Laffaille, Pascal and Caraguel, Jean-Marie and Legault, Antoine Temporal patterns in the
upstream migration of European eels (Anguilla anguilla) at the Couesnon estuarine dam.
(2007) Estuarine, Coastal and Shelf Science, Vol. 73 (n° 1-2). pp. 81-90. ISSN 0272-
7714
16) ICESS 2002. International Council for the Exploration of the Sea.. Report of the
ICES/EIFAC Working Group on Eels.. ICES CM. 2002/ ACFM: 03
17) Willemm Dekker [2004] Slipping through our hands - Population dynamics of the
European eel
18) Tzeng WN, Wang CH, Wickstrom H, Reizenstein M. Occurrence of the semicatadromous
European eel Anguilla anguilla in the Baltic Sea. Marine Biology 2000;137(1):93-9
19) Dou SZ, Tsukamoto K (2003) Observations on the nocturnal activity and feeding
behavior of Anguilla japonica glass eels under laboratory conditions. Environ Biol Fish
67: 389−395
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20) How American eels Anguilla rostrata construct and respire in burrows J. P. N. Tomie1 ,
D. K. Cairns2,*, S. C. Courtenay1
21) A Review of the Population Dynamics, Migrations and Management of the European eel
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freshwater and seawater 215 Aquat Biol 1: 205–216, 2008 habitats. Mar Ecol Prog Ser
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juveniles of two Alaska flatfishes Journal of Experimental Marine Biology and Ecology,
Vol. 282, No. 1-2. (2003), pp. 85-101, doi:10.1016/S0022-0981(02)00447-1 by Allan W.
Stoner, Michele L. Ottmar
25) Habitat preferences of shortfinned eels (Anguilla australis), in two New Zealand lowland
lakes. N. Z. J. Freshw. Res. 33, 233–248. Jellyman, D.J., Chisnall, B.L., 1999
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27) Freshwater Fishes of North-Eastern Australia; Brad Pusey;Mak Kennard;Angela
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28) Jessop, B. M. 2000. Estimates of population size and instream mortality rate of American
eel elvers in a Nova Scotia river. Transactions of the American Fisheries Society 129:
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29) Scientific Opinion of the Panel on Animal Health and Welfare on a request from the
European Commission on Animal Welfare Aspects of Husbandry Systems for Farmed
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Reproduction: A Literature Review;Reviews in Fish Biology and Fisheries, 2006, Volume
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& Consultancy Ltd. ABP Marine Environmental Research Ltd. Report No. R2188TN.
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____________________________________________________________________
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Frontpage Source : “Glass Eel” Chris Bowser NYSDEC
Figure 1 Distribution patterns of eel larvae with the size of the larvae in mm (source: Schmidt
1923)
Figure 2 Life Cycle of European Eel from Rob Slaupkaukas (proposal FOR THE INCLUSION OF
THE European EEL (Anguilla anguilla) On CMS Appendix II)
Figure 3 Figure 3 Time trends in juvenile abundance of the major eel stocks of the world. The
average trend of the four longest data series is shown for the European eel. (From Dekker
2004)
41. Table 2 Sample of worksheet from one of experiment day
Date
Salinity of water in Big holding aquarium
N of experiment
Holding Tank 1
Presence in water column Time
N of eels in water
column
SV GS GM GL
Releasing time
Start of experiment(after 30 minutes of acclimatization)*
30 minutes after start of experiment
One hour later
90 minutes after
End of experiment ( two hours long )
Substrate preference by the end of experiment
*time to release elvers to Holding tank 2
Bury activity Time N of eels who bury** SV GS GM GL
Releasing time
Start of experiment(after 30 minutes of acclimatization)
30 minutes after start of experiment
One hour later
90 minutes after
End of experiment ( two hours long )
Substrate preference by the end of experiment
**can see a part of eel present out of substract
42. Anastasija Martjanova
Page 41 of 52
Holding Tank 2
Presence in water column Time
N of eels in water
column
SV GS GM GL
Releasing time
Start of experiment(after 30 minutes of acclimatization)
30 minutes after start of experiment
One hour later
90 minutes after
End of experiment ( two hours long )*
Substrate preference by the end of experiment
*should be after 30 minutes of the end of experiment in holding
Tank 1
Bury activity Time N of eels who bury** SV GS GM GL
Releasing time
Start of experiment(after 30 minutes of acclimatization)
30 minutes after start of experiment
One hour later
90 minutes after
End of experiment ( two hours long )
**can see a part of eel present out of substract
43. Table 3 Results of measurements of eel length
Table 4 Results of measurements of eel weight
Table 5 Summarized results of observation on eel presence in water column after two hours versus
staying in a preferred subtract type
Table 6 Summarized results of observation on eel presence in water column during all experiment days
versus a each time scale
Length fraction
(cm)
N of eels
% from
total
SortedLength[cm]
<9 8 6
9,1-11 44 357
11,1-13 69 54
13,1< 6 5
Total 127 100
Weight Fraction (g) N of eels % from total
SortedWeight[g]
<1 11 9
1.1-1.5 39 31
1.6-2 55 43
2-3 18 14
3< 4 3
Total: 127 100
Type of substrate Amount of replicates
Sand & Vegetation 1
Gravel S 0
Gravel M 1
Gravel L 2
Time Amount of replicates
Start 7
30 min 11
Hour 7
After 90 minutes 6
Two Hours 4
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Table 7 Results from all experiment days with detailed observation on eel presence in a water column.
Each section is referred to time scale from start of experiment and till the end.
Start of experiment
Type of substrate N of eels
Sand & Vegetation 3
Gravel S 0
Gravel M 2
Gravel L 2
After 30 minutes
Type of substrate N of eels
Sand & Vegetation 2
Gravel S 2
Gravel M 5
Gravel L 2
After one hour
Type of substrate N of eels
Sand & Vegetation 1
Gravel S 1
Gravel M 2
Gravel L 3
After 90 minutes
Type of substrate Amount of eels
Sand& Vegetation 1
Gravel S 0
Gravel M 2
Gravel L 3
After two hours
Type of substrate Amount of eels
Sand & Vegetation 1
Gravel S 0
Gravel M 1
Gravel L 2
45. Anastasija Martjanova
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Table 8 Summarized results of observation on eel bury activity at the end of experiment versus a
preferred subtract type
Type of substrate Amount of Eels
Sand & Vegetation 2
Gravel S 2
Gravel M 5
Gravel L 5
Table 9 Summarized results of observation on eel bury behaviour during all experiment versus each
Table 10 Results from all experiment days with detailed observation on eel bury behaviour. Each
section is referred to time scale from start of experiment and till the end
Start of the experiment
Type of substrate Amount of replicates
Sand & Vegetation 2
Gravel S 0
Gravel M 3
Gravel L 2
After 30 minutes
Type of substrate Amount of replicates
Sand & Vegetation 1
Gravel S 0
Gravel M 6
Gravel L 3
After one hour
Type of substrate Amount of replicates
Sand & Vegetation 3
Gravel S 0
Gravel M 12
Gravel L 4
Time N of eels bury in
sediment
Start 7
After 30 min 10
Hour 19
90 min 11
Two hours 14
46. Anastasija Martjanova
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After 90 minutes
Type of substrate Amount of replicates
Sand & Vegetation 1
Gravel S 0
Gravel M 7
Gravel L 3
After two hours
Type of substrate Amount of replicates
Sand & Vegetation 2
Gravel S 2
Gravel M 5
Gravel L 5
Table 11 Results from all experiment days with detailed observation on eel subtract preference
Date of
experiment
Tank # Habitat
Experiment
#
Replicants at
the end
29/09/2015 I SV 1 1
29/09/2015 I Gr S 1 3
29/09/2015 I Gr M 1 5
29/09/2015 I Gr L 1 0
29/09/2015 II SV 2 6
29/09/2015 II Gr S 2 1
29/09/2015 II Gr M 2 5
29/09/2015 II Gr L 2 1
30/09/2015 I SV 3 2
30/09/2015 I Gr S 3 0
30/09/2015 I Gr M 3 3
30/09/2015 I Gr L 3 5
30/09/2015 II SV 4 3
30/09/2015 II Gr S 4 1
30/09/2015 II Gr M 4 2
30/09/2015 II Gr L 4 5
2/10/2015 I SV 5 2
2/10/2015 I Gr S 5 1
47. Anastasija Martjanova
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2/10/2015 I Gr M 5 4
2/10/2015 I Gr L 5 3
2/10/2015 II SV 6 0
2/10/2015 II Gr S 6 2
2/10/2015 II Gr M 6 4
2/10/2015 II Gr L 6 4
5/10/2015 I SV 7 1
5/10/2015 I Gr S 7 0
5/10/2015 I Gr M 7 6
5/10/2015 I Gr L 7 5
5/10/2015 II SV 8 0
5/10/2015 II Gr S 8 0
5/10/2015 II Gr M 8 5
5/10/2015 II Gr L 8 5
6/10/2015 I SV 9 0
6/10/2015 I Gr S 9 2
6/10/2015 I Gr M 9 5
6/10/2015 I Gr L 9 3
6/10/2015 II SV 10 0
6/10/2015 II Gr S 10 0
6/10/2015 II Gr M 10 8
6/10/2015 II Gr L 10 2
7/10/2015 I SV 11 0
7/10/2015 I Gr S 11 2
7/10/2015 I Gr M 11 6
7/10/2015 I Gr L 11 3
7/10/2015 II SV 12 4
7/10/2015 II Gr S 12 4
7/10/2015 II Gr M 12 2
7/10/2015 II Gr L 12 3
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Table 13 Results of experiment observation on number of eels present in water column at the end of experiment
compared to eels who were fully covered in substrate and who present bury activity.
At the end of experiment Amount of replicates % from total N
Stayed in a water column 4 3.10%
Bury activity in sediment 14 10.85%
Present in substrate 111 86%
Total : 129(100%)
