Industrial fishing over the past half-century has noticeably depleted the topmost links in aquatic food chains
Roll on, thou deep and dark blue Ocean–roll!
Ten thousand fleets sweep over thee in vain;
Man marks the earth with ruin–his control
Stops with the shore
When Lord Byron wrote these lines, nearly 200 years ago, exploitation of the seas was already well under way, and some marine species, such as right whales, were on the path to extinction. Yet the poet's verse supposes that the sea is immune from environmental degradation, a common misconception then and now. People can, in fact, ruin the sea as surely as they can ruin the land. The only difference is that ecological destruction in the ocean is harder to see, particularly when the damage is inflicted on the delicate and largely invisible web of marine life.
If one could don magic spectacles—with lenses that make the murky depths of the ocean become transparent—and look back several centuries to an age before widespread abuse of the oceans began, even the most casual observer would quickly discover that fish were commonly much more abundant. And many now-depleted species of marine mammals (a list that includes not just right whales but Hector's dolphins, Caribbean monk seals and Steller sea lions, among others) would, by comparison, appear plentiful. But without such special glasses, the differences between past and present oceans would indeed be hard to discern.
The opacity of the sea thus accounts, in part, for the disregard that most people display toward the warnings that we and others have been trying to convey. But the remoteness and impenetrability of this habitat are not the only impediments to our efforts. We have also been frustrated by the very richness of the ocean, in that it provides a seemingly never-ending supply of seafood. Indeed, with only rare exceptions, the total catch from global fisheries justs keep going up.
The premier source for such statistical information about world fisheries is the Food and Agriculture Organization, one of the technical organizations of the United Nations. Surveys compiled by the FAO are perhaps not as accurate or as detailed as many scientists would like, but they are in most cases all that is available. Two years ago, we examined a large set of FAO statistics on fisheries and showed that they do in fact portend disaster, despite the continuing growth in total amount being taken from the sea each year. The alarm rings when one realizes that the kinds of animals being caught have been changing in a fundamental way: In essence, rather than being satisfied with the big fish, people are now routinely going after many of the little ones too. Although the situation is not exactly analogous to eating one's seed corn, the result may be equally catastrophic.
Little fish serve the marine ecosystem in various ways, notably as prey for bigger fish. One straightforward way to describe the feeding hierarchy uses the notion of trophic level. By convention, marine plants (such as the tiny, drifting phytoplankton) and various forms of organic detritus make up the first trophic level; herbivores and detritivores are assigned to the second trophic level; and the first and higher-order carnivores are said to occupy trophic levels ranging from three to five.
More precisely, the trophic level of such predators can take on non-integral values, because the diet of these animals is commonly somewhat mixed. For example, an adult jack swimming around the Caribbean might eat equal amounts of herbivorous zooplankton (tiny plant-eating animals, which have a trophic level of two) and small fish that have a trophic level of three (say, because they consume only the local herbivorous zooplankton). This jack would then belong to trophic level 3.5.
Unfortunately, the trophic level of a captured fish is not stamped on its side. So it is not an easy task to track the average trophic level of all the fish caught each year. But after modeling the food webs of many marine ecosystems, we realized that we had accumulated a great deal of knowledge about the approximate trophic levels of commonly fished species. All that was needed was to combine these estimates with FAO catch statistics collected since the 1950s.
Doing so uncovered a systematic shift in the composition of global capture fisheries, with the average trophic level showing a slow slide downward over the past half-century. To judge the significance of our result requires an understanding of the procedures we employed, of the statistics underlying our analysis and of the various concerns that have been raised since our study was first published.
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