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Fish in the
Severn Estuary
Dr Richard Seaby
Pisces Conservation Ltd.
richard@pisces-conservation.com
Our Severn Estuary Data Set is based on
regular sampling at power station intakes.
Monthly sampling commenced at Hinkley
P...
Hinkley Point B is situated
on the edge of Bridgwater
Bay.
The maximum tidal range
is about 15 m and there
are extensive a...
The main energy input is detritus and
dissolved organic carbon, mostly of
terrestrial origin.

Within Bridgwater Bay there...
Fish species accumulation curve
100 randomisations of sample order

About 80 species of fish and
15 macro-crustaceans have...
What is happening to fish species in
the Severn?
Fish populations are generally less stable in estuaries than in the open sea.
This is because the populations usually comp...
The monthly abundance of whiting Merlangius merlangus
between the years 1980 and 2011 in the Severn Estuary.
The trend lin...
The monthly
abundance of eel,
Anguilla anguilla,
between the years
1980 and 2011 in the
Severn Estuary.

An example of exp...
A 30-year study of the estuarine population of yellow eel, Anguilla
anguilla, abundance in Bridgwater Bay, Somerset, UK, s...
An example of exponential increase - the sole
Note that abundance is a log
scale, so an exponential gives
a straight line – the increase
over the last 32 years is
appro...
Annual captures of snake pipefish in
Bridgwater Bay, Somerset 1981-2013
600

The snake pipefish – a short population
explo...
Threats to marine life
There are many threats to the marine life in the Severn.
It is an important estuary that has been d...
Obstructions to movement.
These have had great effects
e.g. on Salmon, Shad and Lamprey
• Anadromous fish move from the
se...
Threats to marine life – fishing
Threats to marine life
cooling water intakes
Threats to marine life
cooling water outfalls
Threats to marine life
dredging
Threats to marine life
ports
Threats to marine life
loss of wetlands and salt marsh die-back
Future threats to marine life
tidal generators
With all the changes that are occurring in
the Severn estuary, it is important to
remember how complex the interactions ar...
Fish species recorded January-December

60

55

15
Year vs Total species number
Year vs Total species number: 1992
Year vs...
The change in the 15 most abundant species
The change in next 15 most abundant species
In part these
changes can be
related to physical
change in
temperature, salinit
y and NAO
A conclusion
Animal populations can behave in surprising ways.
We need to continue collecting and recording if we are
to c...
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2013 03 fish in the severn estuary - richard seaby

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The Severn Estuary Forum is a key annual event in its eighth year and hosted by the Severn Estuary Partnership: an independent, estuary-wide initiative, involving all those interested in the management of the estuary, from planners to port authorities, fishermen to farmers.

This year’s Forum was opened by the Lord Mayor of Gloucester and supported by CIWEM. It focussed on a number of diverse topics, including the upper estuary; renewable energy possibilities; a review of the Severn Estuary Flood Risk Management Strategy; an overview of the operations and maintenance of the Severn River Crossings; Local Enterprise Partnerships; The Bristol Deep Sea Container Terminal; proposals for a Severnside Airport and Fisheries amongst others.

These engaging and exciting events are intended for all interested in learning about the latest research and policy developments dealing with the Severn Estuary and its future, and always guarantee a lively and informative day of presentations and talks. They offer a unique opportunity to learn from others, share ideas and participate in the management of the Severn Estuary.

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2013 03 fish in the severn estuary - richard seaby

  1. 1. Fish in the Severn Estuary Dr Richard Seaby Pisces Conservation Ltd. richard@pisces-conservation.com
  2. 2. Our Severn Estuary Data Set is based on regular sampling at power station intakes. Monthly sampling commenced at Hinkley Point B in 1980, and is still continuing. Fish and macro-crustaceans are monitored on the power station filter screens and plankton nets are placed in the intake. The Severn Estuary Data Set
  3. 3. Hinkley Point B is situated on the edge of Bridgwater Bay. The maximum tidal range is about 15 m and there are extensive areas of inter-tidal mud. This macrotidal system has suspended sediment loads as high as 3 g per litre. Salinity ranges between 18 and 32 parts per thousand. The Habitat
  4. 4. The main energy input is detritus and dissolved organic carbon, mostly of terrestrial origin. Within Bridgwater Bay there is little planktonic or benthic primary production, because of the turbidity of the water and the instability of the substrate. For the Bristol Channel including Bridgwater Bay, Joint & Pomroy (1981) estimated annual primary production to be only 6.8 g C m-2 y-1. In comparison their estimate for the outer Bristol Channel in the vicinity of Lundy Island was 164.9 g C m-2 y-1. Very little primary production
  5. 5. Fish species accumulation curve 100 randomisations of sample order About 80 species of fish and 15 macro-crustaceans have been recorded. Fish Species Accumulation Curve Species Number 100 ransomisations of sample order 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 Species acquisition curves, historical records and published reports all suggest that our Hinkley sampling records all the resident and most of the migratory fish and macro-crustaceans present between 1980 and now. 50 100 150 200 Sample 250 300 350 The almost linear increase in species after 100 samples is due to the capture of occasional migrants. The larger mobile species
  6. 6. What is happening to fish species in the Severn?
  7. 7. Fish populations are generally less stable in estuaries than in the open sea. This is because the populations usually comprise the younger age classes, and species that only utilise the estuary for a proportion of their life. Shown here are two common species, whiting and flounder, fish populations that are observed to be fluctuating around reasonably constant levels. These examples lend support to the view that density-dependent control is operating. Long-term stability in fish populations
  8. 8. The monthly abundance of whiting Merlangius merlangus between the years 1980 and 2011 in the Severn Estuary. The trend line is a 12-month moving average. The monthly abundance of flounder Platichthys flesus between the years 1980 and 2011 in the Severn Estuary. The trend line is a 12-month moving average. Whiting and flounder dynamics
  9. 9. The monthly abundance of eel, Anguilla anguilla, between the years 1980 and 2011 in the Severn Estuary. An example of exponential decline - the eel
  10. 10. A 30-year study of the estuarine population of yellow eel, Anguilla anguilla, abundance in Bridgwater Bay, Somerset, UK, shows that the population number has collapsed. Since 1980, the decline has averaged 15% per year. The abundance of eel in 2009 is estimated at only 1% of that in 1980. Henderson, P., Plenty, S., Newton, L. and Bird, D. (2011) Evidence for a population collapse of European eel (Anguilla anguilla) in the Bristol Channel. Journal of the Marine Biological Association of the United Kingdom . pp. 1-9. ISSN 0025-3154 An example of exponential decline - the eel
  11. 11. An example of exponential increase - the sole
  12. 12. Note that abundance is a log scale, so an exponential gives a straight line – the increase over the last 32 years is approximately exponential. An example of exponential increase - the sole
  13. 13. Annual captures of snake pipefish in Bridgwater Bay, Somerset 1981-2013 600 The snake pipefish – a short population explosion – hardly present for 2 decades Annual number caught 500 400 300 200 100 0 1980 1990 2000 Year Dangers of short-term sampling 2010
  14. 14. Threats to marine life There are many threats to the marine life in the Severn. It is an important estuary that has been developed and industrialized over a long period, and this development is unlikely to stop in the foreseeable future. Now I will briefly outline some of the major impacts and the threats they produce to the fish of the Severn Threats to marine life
  15. 15. Obstructions to movement. These have had great effects e.g. on Salmon, Shad and Lamprey • Anadromous fish move from the sea up rivers to spawn. Obstructions to their movement have been particularly disastrous. • For example: In the River Severn “Lampreys too, which were formerly considered of more importance than salmon, and were caught in the upper Severn, have altogether ceased to visit it since the erection of the first weir in 1843”. A particular estuarine problem
  16. 16. Threats to marine life – fishing
  17. 17. Threats to marine life cooling water intakes
  18. 18. Threats to marine life cooling water outfalls
  19. 19. Threats to marine life dredging
  20. 20. Threats to marine life ports
  21. 21. Threats to marine life loss of wetlands and salt marsh die-back
  22. 22. Future threats to marine life tidal generators
  23. 23. With all the changes that are occurring in the Severn estuary, it is important to remember how complex the interactions are between the many species of fish in the estuary and the environment in which they live. A changing world
  24. 24. Fish species recorded January-December 60 55 15 Year vs Total species number Year vs Total species number: 1992 Year vs Ave temp 14 50 13 45 12 40 11 35 10 30 1980 1990 2000 9 2010 Average seawater temperature January-December Temporal variation in species number and average seawater temperature Species richness has been increasing from about 35 to greater than 40 per year. Over the same period average temperature has also increased. Year Changes in fish species richness
  25. 25. The change in the 15 most abundant species
  26. 26. The change in next 15 most abundant species
  27. 27. In part these changes can be related to physical change in temperature, salinit y and NAO
  28. 28. A conclusion Animal populations can behave in surprising ways. We need to continue collecting and recording if we are to create the data sets that will lead to the predictive science we desire, that allow us to predict the impacts we have on the aquatic environment.

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