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Fishing Down the Food Web

The Problem

  • After decades of over-exploiting fish stocks, the commercial fishing industry is now threatening the very basis of the marine food chain.
  • Having exhausted catches of larger, longer-lived species (e.g., tuna, cod, snapper), fishing fleets are increasingly concentrating on catching smaller, shorter-lived, plankton-eating species (e.g., squid, mackerel and sardines, and invertebrates such as oysters, mussels, and shrimp), which are nearer the bottom of the food chain.
  • As predatory fish are selectively removed from the ocean, they must increasingly rely on lower trophic level species for food. However, the seasonal abundance of these smaller, fast-reproducing species fluctuates greatly and the remaining larger fish are thus exposed to greater variability in their food supply. When commercial fisheries then target these vital species at the base of the food chain, adding to this already delicately balanced situation, they push the entire ecosystem to the brink of collapse.
  • A classic example of such a situation recently occurred in Europe’s North Sea. Norway cod were so overfished that fisherman focused on catching pout. Pout, however, feed on copepods and krill. Krill feed on copepods but so do juvenile cod. As pout were removed, krill populations increased and copepod numbers declined. With the decline of copepods, young cod lost a food source making their path to recovery even more difficult.
  • Ultimately, then, fishing down the food web causes two problems:
    • Lower-level competitors can take advantage of the removal of top-level predators as predatory pressure and competition for food is reduced;
    • As fisheries then target lower-level organisms (i.e., prey fish and invertebrates) predators are increasingly deprived of much of the very food source necessary for population re-establishment.

The Causes

  • On one level, the cause is relatively simple: For decades the world’s commercial fishing fleets have been taking unsustainable amounts of fish and marine life, to the extent that many fish stocks are now declining or even collapsing.
  • The more underlying causes are somewhat more complex. Although it is frequently argued that increases in the global fish catch are the result of a “growing world population,” little of the increased catch from the world fishing fleet finds its way to those peoples where population is increasing most dramatically and where hunger is most marked. Indeed, foreign fishing fleets generally remove fish from the marine waters of those countries for export to the industrialized world.
  • In essence, over-fishing is largely caused by a combination of factors, of which growing human populations is but one. Others include:
    • Consumer demand for more and different fish;
    • Increased technological efficiency in terms of location and catching methods;
    • The globalization of much of the fishing industry and the markets for fishery products;
    • The continuing failure on the part of elected officials, fishery managers and the public to heed scientific warnings about recommended catch levels or the impact of fisheries policies;
    • Government subsidies that encourage the continued building and deployment of greater numbers of larger fishing vessels with ever-more destructive gear;
    • Illegal, unreported and unregulated (IUU) fishing.

The Context

  • The problems of fishing down the food web are but the latest manifestations of a global commercial fisheries industry that has been responsible for over-exploitation of fish populations for decades.
  • According to the Food and Agriculture Organization (FAO) of the United Nations, 47 per cent of major fish stocks are “fully exploited,” with no possibility for expansion; 18 per cent are “over-exploited,” and 10 per cent are “depleted.”
  • A 2003 paper in the prestigious science journal Nature stated that the biomass of predatory fish -- both open ocean species including tuna, swordfish, marlin and large groundfish such as cod, halibut, skates and flounder -- has been reduced to a mere 10% of pre-industrial levels.
  • In addition to direct impacts on target species, commercial fishing fleets continue to damage or obliterate habitat (e.g., by bottom trawling), catch non-target species of marine wildlife (i.e., bycatch or bykill), including non-target fish, sea turtles, seabirds, and marine mammals, and, as explained above, cause breakdowns in broader marine ecosystems.

Further Reading

Pauly, D. et al. 1998. Fishing down marine food webs. Science 279: 860-863.

Pauly, D. and Watson, R. 2003. Counting the last fish. Scientific American, July: 42-47.

Sala, E. et al. 2004. Fishing down coastal food webs in the Gulf of California. Fisheries, March: 19-25.