Stocks of some small ocean fish exploited for fish meal are over-fished, and their populations fluctuate sharply during the climate shifts brought on by El Nino-Southern Oscillation events.
In seasons when these stocks are depleted, available food supplies for commercially valuable marine predators such as cod and also marine mammals and seabirds decline.
In the North Sea, for example, over-exploitation of many capelin, sandeel, and Norway pout stocks, largely for production of fishmeal, has been linked to declines of other wild fish such as cod and also changes in the distribution, populations sizes, and reproductive success of various seal and seabird colonies.
Similarly, off the coast of Peru, scientists have documented a strong interaction between anchoveta stocks and the size of sea bird and mammal populations.
Introduction of Non-Native Fish and Pathogens:
Aquaculture can also affect stocks of wild fish by allowing escapes of non-native species and by spreading diseases among both farmed and wild fish. Scientists call these introductions of non-native organisms “biological pollution.”
Atlantic salmon — the dominant salmon species farmed worldwide — frequently escape from net pens. In some areas of the North Atlantic Ocean, as much as 40 percent of Atlantic salmon caught by fishermen is of farmed origin. In the North Pacific Ocean, more than a quarter million Atlantic salmon have reportedly escaped since the early 1980s, and Atlantic salmon are regularly caught by fishing vessels from Washington to Alaska.
Increasing evidence suggests that farm escapees may hybridize with and alter the genetic makeup of wild populations of Atlantic salmon, which are genetically adapted to their natal spawning grounds.
This type of genetic pollution could exacerbate the decline in many locally endangered populations of wild Atlantic salmon. In the Pacific Northwest, there is evidence that escaped Atlantic salmon now breed in some streams, perhaps competing for spawning sites with beleaguered wild Pacific salmon. Movement of captive fish stocks for aquaculture purposes can also increase the risk of spreading pathogens.
The relationships between farmed and wild fish and disease transfer are complex and often difficult to disentangle. In Europe, however, serious epidemics of furunculosis and Gyrodactylus salaris in stocks of Atlantic salmon have been linked to movements of fish for aquaculture and re-stocking.
Since the early 1990s, the Whitespot and Yellowhead viruses of shrimp have caused catastrophic, multimillion-dollar crop losses in shrimp farms across Asia. Both pathogens have recently appeared in farmed and wild shrimp populations in the United States, and the Whitespot virus has been reported in several countries in Central and South America.
In Texas shrimp farms, the Whitespot virus has caused high mortalities, and the disease may also kill wild crustaceans. This virus is thought to have been introduced into a Texas shrimp farm by release into nearby coastal waters of untreated wastes from plants processing imported Asian tiger shrimp, and also by shipping of contaminated white shrimp larvae throughout the Americas.
Nutrient Pollution from Aquaculture Wastes Untreated wastewater laden with uneaten feed and fish feces may contribute to nutrient pollution near coastal fish ponds and cages, especially when these are situated in or near shallow or confined water bodies. Such pollution also can be severe in regions where intensive aquaculture systems are concentrated.
In many such areas, buildup of food particles and fecal pellets under and around fish pens and cages interferes with nutrient cycling in seabed communities. And when quantities of nitrogen wastes such as ammonia and nitrite are greater than coastal waters can assimilate, water quality can deteriorate to a level that is toxic to fish and shrimp.
Aquaculture managers clearly have a stake in regulating nutrient pollution since poor water quality and high stocking densities often promote outbreaks of disease and lead to declines in farmed fish production. While waste problems have been widely discussed, however, current management solutions are largely limited to controlling the intensity of fish production by reducing stocking and feeding levels rather than treating wastes.
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