Fish Ecology and Fisheries - Freshwater Ecology: Concepts and Environmental Applications - page 444

Many fish have been stocked in habitats in which they are not native
under the impression that they would improve the fishery. Unfortunately,
the ultimate biological effects of such introductions usually have not been
carefully considered or researched in advance. Precautionary approaches
should be taken for new species introductions (Bartly, 1996) to avoid neg-
ative impacts in the future. These approaches include assuming that a
species will have a negative impact until proven otherwise and putting the
burden of proof of impact on the agency, company, or individual that re-
leases the fish.
Several problems can arise when fish are stocked, including competi-
tion with or predation on native fish species, hybridization with native fish
stocks, lack of genetic diversity in fish populations that may lead to poor
fitness, and transfers of parasites and disease (Moyle et al., 1986). For ex-
ample, whirling disease entered salmonid assemblages of the western United
States through stocked trout. This disease has negative economic impacts
and may endanger native fishes. Introduction of whirling disease might
have been avoided by careful assessment of stocking programs and more
cautious aquaculture procedures.
The size and type of fish to be stocked are often of concern. Large fish
survive better, but small fish are less expensive to rear. Eggs are stocked
only rarely because of poor survival. Fish should also be stocked when and
where they are most likely to survive and grow. If too many fish are
stocked, interspecific and intraspecific competition may lead to poor
growth. Thus, it is crucial to be aware of growth, mortality, and recruit-
ment characteristics associated with the environment to be stocked, in ad-
dition to the ecology and the life history of the species used.
AQUACULTURE
The culture of aquatic organisms is called aquaculture . Fishes, crus-
taceans, and some algae are grown for consumption, stocking, and the
aquarium trade. Fish culture may be an important source of protein in de-
veloping countries; world aquaculture production is dominated by pro-
duction of carp. The amount of carp produced is about 10-fold greater
than that of any other fish (Fig. 21.12), with most production in Southeast
Asia. Culture of fishes and crustaceans is generally more profitable if tech-
niques are based on an understanding of the ecology of aquatic ecosystems.
The trick in aquaculture is to maximize productivity but avoid the nega-
tive effects of eutrophication. For example, to grow at maximum rates, fish
need a significant amount of food. Excessive food additions can lead to in-
creased biochemical oxygen demand, anoxia, and death of fish. In addi-
tion, aquaculturists grow organisms at high densities, which provides ideal
conditions for propagation of diseases and parasites and attracts predators
(Meade, 1989).
The diseases of cultured aquatic organisms include protozoa, fungi,
bacteria, and viruses. Stressed and crowded fish are more prone to infec-
tions. Bacteria are more commonly problematic in cold-water species of
fish, but antibiotics are generally effective in controlling these infections.
Several species of viruses also cause problems for different fishes. These
diseases cannot be treated once stock is infected. Rather, strict handling