World Fisheries

source FAO:

Maximum Sustainable Yield

The total catch of ocean fish increased from 18.5 million metric tonnes in 1950 to 73.5 million metric tonnes in 1996, an increase of close to 400%. The rapid increase in catch began to level off in the 70's and the catch per capita peaked at 14.4 kg per person per year. The total fish harvest has continued to increase, due to the increase in fish farm production, however, there are limitations on that source also. While researcher do not agree on the ultimate upper limit of harvest, it seems clear both from the actual catch, and the evidence of overfishing in individual species that we are very near that limit if we have not already passed it. Looking at individual species, 69% are considered to be either fully exploited or over exploited and on the decline. Only 9% are considered to have potential for increased fishing. Some species have declined dramatically, the Atlantic Cod in particular, but the stocks of many high value groundfish have decreased by 70 to 80 % in the last 20 years. To keep production up, there has been an increase in fishing for lower value fish, and fish from more distant oceans. Fish landings in the Northwest Atlantic have decreased by 36.6% since 1970, while the landings in the Southwest pacific, and East Indian Oceans have increased by 800 % and 312% respectively, however, these areas are now fully exploited and can no longer be counted on to counter balance the loss of production in traditionally fished areas. (Garcia & Newton)

"The tragedy of the commons" Declining species and habitat destruction
The tragedy of the commons a dilemma as old as civilization. Basically it implies that if there is open access to a common resource, the ocean in this case, the tendency to extract resources from the common source will lead to destructive over use. In the case of fishing, as the fish are depleted, fishermen are forced by economic necessity to increase the efficiency and the intensity of their efforts which leads to more rapid depletion and more destructive means of obtaining fish. Examples of the latter range from the increased use of explosives and poisons, to the increase in size and range of ocean fishing craft and factory ships.
Four main factors contribute to the decline of species and their poor recovery.
1. The depletion of large mature fish has led to an increase in the intensity of fishing and to an increase in the number of juveniles and non food fish caught. The overall discard rate is 25%, but in some species the rate is much higher, reaching a peak in shrimp trawls where 84% of the catch by weight is discarded. Fish unlike mammals continue to increase in weight after maturity, and egg production increases in proportion to weight placing a premium on older fish for maintaining populations. So the increase in fishing pressure not only lowers the rate of reproduction, but the killing of ancillary species, the "bycatch" disrupts the entire food chain.
2. Pollution, both chemical, and silt, from land based sources is destroying spawning grounds and inshore species. 70% of commercial species spend at least some part of their life cycle in shallow saguaros which is highly vulnerable to siltation.
3. Fishing techniques such as the use of heavy trawls destroys the ocean floor ecosystem on which the fish depend. Studies in Australia indicate that even 15 years after closing an area to all fishing, the sea floor habitat had not recovered from the effects of trawling. Dynamiting coral reefs, and using poisons is obviously tremendously destructive to the ecosystem and may prevent the local recovery of fish for decades or longer.
4. Climate changes: Cooler air and water temperatures off the coast of Labrador during the early 70's and mid 80's have reduced the productivity of Cod. The effect of El Nino on the Peruvian fisheries is well documented. Fish catches range from 12.3 million tonnes in 1970 to as little as 100,000 tonnes in a El Nay year. What is not so well known, but sheds light on the potential for loss of fish production as the oceans warm, is the increase in carbon and decrease in oxygen from a high of 1.0 ml/liter to less than 0.1ml/liter over the 10 year period ending in 1979. Because of the reduction of anchovies, the primary producers, plankton and copepods, ended up as detritus rather than food increasing the sedimentation rate by a factor of 10. (Cushing, 1982) One of the implications of this finding is the slow rate at which the ecosystem can adapt to perturbations. A change in temperature may have greater long term effects than a shifting of fish populations to a new zone with more compatible temperatures. It may mean a displacement of the producer and consumer so the overall production of the system is greatly reduced. Shifts in wind patterns, caused either by El Nay or other climate cycles reduces the upwelling along the coast from California to Washington, causing drastic reductions in the rate of survival of salmon. As the food disappears, other fish move farther offshore, but the salmon smolt coming out of the rivers have no choice but to stay in the coastal waters. Not only do they suffer from lack of food, but birds deprived of the normal variety of coastal fish prey exclusively on the salmon.
The critical question is how much of a rise in temperature can the earth' circulatory system tolerate before the wind patterns that block the upwelling become dominant, or ocean currents shift their location or direction permanently.
Collapse can be precipitous
The collapse of the fisheries in the Black sea is an example of the rapid decline that can happen once conditions deteriorate past a critical threshold. In the late 80's 1 billion metric tonnes of fish were taken annually, but by 1992 the catch was down to 100,000 tonnes due to overfishing and nutrient pollution (eutrophication).(Garcia & Newton)
Fish Farming - Solution or just another problem
Fish Farm Production in Tonnes and Dollars
Source: FAO
Fish farming has dramatically increased in the last 15 years and has more than made up for the loss in production of ocean catch. At first it seems like a solution, and it is in part, but there are limitations. The pollution caused by the large populations of fish and shrimp raised in pens in estuaries is causing significant damage to the local ecosystem, particular in Thailand where there are heavy concentrations of shrimp farms. There is also the issue of food. At present a significant amount of food for raising fish consists of ground fish meal. To the extent that wild fish are caught and deliberately turned into meal to feed farmed fish, the system is counterproductive. It takes roughly 5lb. of fish meal to gain one pound of farmed fish, plus a lot of fuel and expense. At the same time the ocean fish are deprived of their food fish. Better to leave the fish in the ocean to be consumed by their natural predators and catch the larger fish. It is true that some of fish meal is made from byproducts of the fishing industry, but that is near its limit, so additional fish food will have to come from agriculture which is facing its own limitations.  
Web Sites:
1.United Nations FAO The FAO is one of the best data sources for fish production and food.
2.Cushing, D.H., Climate and Fisheries, Academic Press, New York, 1982.
3. Garcia, S., and Newton, C.,1997 Current situation, trends, and prospects in world capture fisheries. In Pikitch, E.L., Huppert, D.D. and Sissenwine, M.P. ed. Global Trends: Fisheries Management. American Fisheries Society Symposium 20, Bethesda, Md.
McGoodwin, James R., Crisis in the World's Fisheries, Stanford University Press, Stanford, 1990.
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