Privatization and Disinvestment - Meaning, Objectives, Advantages and Disadva...
Fish farming threats: predators, pathogens, competitors
1. Predators, Pathogens & Competitors -
Fish
A wide range of organisms can reduce the health or even
kill the Finfish or Crustaceans. They can be divided into
the following groups:
● Pests can cause fouling which can be a major issue
with seacages or pump ashore tanks of Crustaceans
or Finfish.
● Predators and pathogens are often difficult to control
in open water systems such as cages, better control is
possible in on-land systems such as tanks and ponds.
● Competitors can be for space, food or oxygen.
Individuals from the same species can be competitors
(larger, faster growing individuals can out compete
smaller animals).
2. Predators, Pathogens & Competitors -
Molluscs
A wide range of organisms can also reduce the
health of, or even kill Molluscs. Pests in terms of
fouling organisms can be a major issue with
Bivalve culture and often one of the major work
tasks on mollusc farms is removing this fouling.
Predators and pathogens are often difficult to
control in open water systems, and better control is
possible in on-land systems.
Competition can also effect Molluscs in terms of
space, food or oxygen. Individuals from the same
species can be competitors (larger, faster growing
individuals can out compete smaller animals).
3. Food for Finfish & Crustaceans
All Finfish and Crustaceans require food to grow, in terms
of quality (protein, carbohydrates, fats, vitamins etc.) and
quantity of feeds. Naturally occurring live foods are
important for fry and juveniles however for growout most
species are fed formulated (artificial) diets such as
crumbles and pellets.
The quality, quantity, availability and timing of feeding will
influence the growth and survival. Therefore, feed
management is often closely managed to ensure it is not
wasted – feed costs can be as high as 50 to 60% of total
production costs. This is easy to control in on-land
systems (easy access), however, site selection is very
important for open water (cage) culture systems (weather
conditions can delay feedings).
4. Food for Finfish & Crustaceans
Phytoplankton or algae can be eaten by some
herbivorous species – in this case there needs to be
nutrients and light for photosynthesis (nutrients can
come from land-based sources, usually washed
down rivers or waterways, or from upwellings or
current flow). This is sometimes referred to as
‘greenwater’ culture.
However, the majority of juveniles eat zooplankton
(need to be small enough for the animal to ingest).
They may also eat small insects, worms, Finfish,
Crustaceans and so on. Organic matter, either
suspended or benthic (detritus) can be eaten by
some scavenger species (eg catfish).
5. Food for Finfish & Crustaceans
Nearly all modern Finfish and Crustacean
farms utilise man-made pelleted diets
formulated for specific species requirements of
the adults.
They occasionally may be given live feeds or
other wet foods. However this is more
common in the Ornamental sector and on
Tuna farms.
This is discussed in detail in Session 12
Foods and Feeding of Stock.
6. Food for Molluscs
All Molluscs also require food to grow. Naturally occurring
foods such as algae and seaweeds or formulated (artificial)
diets such as abalone pellets are required. The quality,
quantity, availability and timing of feeding will influence growth
and survival.
This is easy to control in on-land systems, however, site
selection is very important for open water culture systems
which rely on naturally occurring feed organisms for feeding
Bivalve stock.
When choosing a site for mollusc farms, it is important to
choose one that has good concentrations of suitable
microalgal species
7. Competition for Food - Fish
‘Competition’ can occur between individuals when
there are too many animals in the one area which
results in increased competition for food.
Larger more aggressive Finfish or Crustaceans will
tend to dominate the feeding, and feeding
hierarchies can be established.
This is overcome by ensuring sufficient feeding
levels and the spreading of feed around the pond.
This is overcome by regular size grading – see
Session 15 Handling and Harvesting Stock for
more information.
Careful observation is needed to ensure that all
stocks are feeding.
8. Competition for Food - Mollusc
‘Shading’ can occur when there are too many
filter feeders (Bivalves) in the one area which
results in increased competition on the lease for
food.
Food needs can vary between species, eg oysters
and razorfish (a type of clam) are okay together as
the razorfish eat more benthic materials (detritus).
However, the situation is not so good for oysters
and mussels, as mussels filter at a much higher
rate and would out compete the oysters for food.
The layout of shellfish leases in an area need to
take this into account to maximise the production
of the area.
9. Stocking Densities - Fish
Stocking density is a measure of how many fish are stocked per unit – this can be
measured or expressed in many ways e.g.
● Kg of fish per cubic metre of water
● Number of fish per tank
● Number of fish per litre
● Number of fish per square metre of floor space
Appropriate stocking densities are critical and they can provide a method to manage
or slow growth rates due to the level of competition between stock (mainly in tanks).
However, we need to ensure that Finfish are not damaging each other at high
densities, eg. fin nipping, spiking, scale loss. Understocking can be just as big a
problem as overstocking, eg. some species need a critical mass of animals to initiate
appropriate feeding behaviour such as Finfish feeding schools.
This is also discussed in Session 15.
10. Stocking Densities - Crustacean
Appropriate stocking densities are critical for good, even growth rates of
Crustaceans. Excessive levels result in slow growth rates due to the level of
competition (space, feed, oxygen) between stock. Damage to legs and antennae can
also occur at high densities with elevated stress offering an opening to disease.
Cannibalism can also increase with stocking density, with increased cannibalism
occurring during moulting.
As most crustaceans are benthic or live on the floor of the culture system, stocking
density is generally expressed in quantity per square metre.
This is also discussed in Session 15 Handling and Harvesting Stock.
11. Stocking Densities - Mollusc
Appropriate stocking densities are critical and they provide a method to control
growth rates (fast or slow) due to the level of competition between Molluscs.
Understocking can be just as big a problem as overstocking, eg not enough oysters
in a basket can lead to excessive rumbling which damages the growing margin of
the shell.
Stocking densities should take into account numbers of other filter feeders that
would compete with Molluscs for food.
This is also discussed in Session 15 Handling and Harvesting Stock.
12. Type of Culture or Holding Unit - Fish
The overall traits of the species to be cultured or held must be taken into account
when selecting the type of unit or structure. The type of culture system can influence
a wide range of factors including:
• Access to food and oxygen;
• Culture unit fouling levels;
• Stocking densities;
• Ease of handling, stocking, harvesting.
ith most species there is often a range of technologies that can be chosen, eg both
Rainbow Trout and Barramundi are grown in freshwater ponds, raceways and
cages, seawater cages, recirculating tanks and flow-through tanks and raceways,
whilst Yabbies are mainly grown in earthen ponds. The type of system will depend
on the biological needs of the culture species and the economics of providing this in
a way that generates a profit for the farmer.
13. Type of Culture or Holding Unit - Mollusc
The types of culture units can influence a wide range of factors including:
● Access to food and oxygen;
● Rumbling or fouling levels;
● Stocking densities;
● Ease of handling, stocking, harvesting.
Single seed or individual Bivalves can utilise a range of different culture units, whilst
attached Bivalves are usually only on sticks or a similar substrate. Abalone need a
substrate to move around on, either cages, tanks or raceways.
14. Type of Culture or Holding Unit
We have already discussed Extensive Production systems, these are generally
Farm ponds and dams and are similar to natural conditions.
● Stocking densities are low (close to natural levels)
● Little or no water quality maintenance.
● Stock uses naturally occurring feed.
● Cost of production is low but so are yields.
● Usually hobby ventures or small scale production in non-specialised waterways
such as irrigation ditches, gully dams, natural soaks etc.
Limited food and control over the environment limit productivity and can lead to
stress and compromises growth and survival on the culture species in extensive
production systems.
15. Types of Culture Unit
Semi-intensive – commercial ponds & dams is usually undertaken
in specially constructed ponds, tanks, dams or areas of enclosed
water.
● Stocking densities are moderate to high
● Moderate to high water quality maintenance (aeration & exchange)
● Stock uses naturally occuring and supplemented feed.
● Cost of production is raised but so are yields.
● Ponds can be used for controlled breeding, commercial growout,
holding and stock manipulation
The ability to have some control over critical factors such as oxygen
levels and other environmental factors, along with provision of
artificial feeds improves the stress management of the culture species
and increases growth and survival on the culture species in semi-
intensive production systems.
16. Type of Culture or Holding Unit
Intensive – Battery culture generally occurs in indoor tanks, raceways or ponds.
These are often highly engineered systems with the capacity to precisely control the
culture environment.
● Stocking densities are high to very high.
● Rigid control of water quality parameters and maintenance.
● Specially formulated feed for maximum growth rates and water quality control.
● Cost of production is high due to capital (equipment) and operating costs (labour,
power, feed etc) but results in very high productivity.
● System often used to fatten or purge stocks before market and in the
manipulation of brood stock.
The ability to precisely control the environment ensure the culture species receives
the conditions for good growth and survival meaning productivity is highest in
intensive culture systems.
17. Protection from Elements - Fish
The local climatic conditions can impact fish and
the sun, wind and rough weather are the main
considerations for Finfish. The sun can dry out
skin and lead to sunburn if the culture unit is too
shallow, this can lead to stress or death. Wind can
cause jerking of the cages which causes stress to
fish and wears equipment or lead to erosion in
earthen ponds.
In a previous training module, we discussed the
issue of climate change and increased extreme
weather events which needs to be considered -
farms in cyclone prone areas need to be
constructed appropriately.
18. Environmental Elements - Crustaceans
The local climatic conditions can impact fish
and the sun, wind, lunar cycles and rain are
the main considerations. Sunlight can
influence algal blooms and water
temperature. Wind can cause surface
turbulence in the pond waters assisting in
aeration and mixing. Lunar cycles coincide
with moulting, and may play a role in
reproduction.
Rainfall and runoff may lower pond salinity
also affecting moulting cycles.
19. Protection from Elements - Molluscs
The local climatic conditions can impact fish and
the sun, wind and rough weather are the main
considerations. Sun can dry out fouling and other
external pests, but too much can lead to stress or
death of the Mollusc being grown. Wind can
hasten the drying process and can be a major
problem in warm climates.
Rough weather can lead to increased rumbling
within the culture units or stripping or losses of
Molluscs from the culture units.
20. Genetics
Genetics plays a major role in the performance of all living animals.
Agriculture and increasingly aquaculture relies heavily on selective
breeding of varieties of animals that grow faster, have higher yields,
better survival or disease resistance.
With wild caught seedstock (eg Mussels, Tuna) the grower doesn’t
know much about the genetic stock they came from, and has no
capacity to choose stock that will perform better under culture
conditions. With hatchery bred and reared seedstock, the grower
has much better control over the genetics, eg can undertake
selective breeding to enhance particular traits such as fast growth,
disease resistance and so on.
Selective breeding programs are already being undertaken by many
organisations to improve productivity, this along with Genetic
Modification will play a major role in the future of aquaculture
21. Stress
Stress is a major issue with production of any living organism. In aquaculture,
stress can be caused by a number of different factors, including:
● Poor handling
● Overstocking
● Suboptimal environment (water quality)
● Suboptimal feeds
● Presence of pests, predators, competitors.
Stress is discussed in more detail in the next section.
