Freefall in global fish stocks.

Fishers today are capturing species that are significantly lower on the marine food chain than species they caught just 10 years ago, according to a recent study in the journal Science, co-authored by fisheries scientists from the University of British Columbia and the International Center for Living Aquatic Resources Management in the Philippines. Since the early 1970s, marine fish stocks have been in a global freefall: dozens of individual species have been fished to commercial extinction, the rate of growth in marine harvests has plummeted to near zero, and the composition of global catches has downshifted to smaller, bonier, oily fish that eat low on the food chain.

Although global harvests have increased dramatically since 1950, the average trophic level (position within the marine food chain) of that harvest has dropped 10 percent, from 3.4 to 3.1, signalling a loss of complexity and biodiversity. These trends indicate a pattern of resource exhaustion.

The study is based on reports from all over the world including the U.N. Food and Agriculture Organization's annual global catch data for 220 fish and aquatic species. From these, the authors determined the average trophic level of each fish species - based on what the fish eat and what their food items eat - and then calculated the average trophic level of annual world harvests from 1950 to 1994. This study marks one of the first attempts to designate fractional trophic levels for every commercial fish species based on their interactions in the marine food web, rather than a whole number designation which ignores the fact that many fish feed at different levels. A high trophic level fish, such as tuna (4.2) or cod (3.8), for example, eats many smaller fish such as herring (2.8) or pout (3.1) which in turn eat tiny fish, zooplankton, and phytoplankton (2.2-2.5). Most of the fish consumed by people occupy levels 2.4 through 4.

As fishers deplete large, long-lived predatory species, such as cod, tuna, shark, and snapper that occupy the highest levels of the food chain, they move down to the next level - to species that tend to be smaller, shorter-lived, and less valuable. As a result, fishers worldwide now fill their nets with plankton-eating species such as squid, jacks, mackerel, sardines, and invertebrates including oysters, mussels, and shrimp. The new findings support the conclusion that fishers are working harder to capture less valuable species - they are essentially fishing down the marine food web.

Initially, this transition brings new bounty: at higher trophic levels, fish are larger but there are fewer of them. At lower levels, the species are smaller and more plentiful. In addition, higher-trophic level species exert a top-down control on lower-trophic level species in food webs. Relieved of predatory pressures and competition for food, the smaller species are free to fill in the empty niche once occupied by their predators. This is one reason why global fish catches have not declined dramatically in the past 25 years, despite severe overfishing.

When fishers deplete large apex predators, in a sense they select for smaller species, be they juvenile predators or altogether different species. But unless fishing effort is reduced, the cycle of overfishing will soon repeat itself, triggering abrupt declines in these lower-level species. Fishers may soon be only a step away from harvesting jellyfish, krill and plytoplankton.

The track record from the heavily exploited northern Atlantic and Pacific oceans and the Mediterranean and Black Seas foreshadows the fate of other fishing grounds: the catch composition of each of these regions peaked by the early 1970s and has dropped steadily since then. Fisheries off the coast of Atlantic Canada and New England have suffered a double blow - a 20 percent loss of trophic level and a 120-percent decline in total landings. Where fish of high trophic levels remain relatively abundant, as has Alaskan pollock (3.8 trophic level) in the Bering Sea and Gulf of Alaska, its influence on catch composition is outweighed by the over-exploitation of numerous other species, which causes a downward drag on the marine food web.

In contrast, the trophic level of catches in the eastern Pacific has fluctuated widely, reflecting the high variability of small species such as anchovies, mackerel, and sardines. in the southern Atlantic, trophic levels have increased slightly, most likely due to the exploitation of previously untapped stocks of squid and hake on the Patagonian shelf. Indeed, species that were once considered too expensive to harvest and unappealing to consumer tastes are now landed as a matter of course.

Redirecting fishing efforts to lower trophic levels can shift the populations of predator and prey alike. In the North Sea, for example, fishing for Norway pout benefited its prey, krill, but at the expense of copepods - a low level zooplankton species that is consumed by krill, pout, and commercially valuable cod and saithe. Cod also consumes pout directly, so by targeting pout, and indirectly harming copepods, fishers threaten the recovery of the cod and saithe fishery. As this example shows, once the slide to lesser quality fish begins, enabling the fishery to regain a higher trophic level is difficult, if not impossible.

An aspect of overfishing the study does not quantify directly is the subsequent loss of individual species' role in the ecosystem, which can have deleterious effects on nutrient flows and energy balance in the marine food web as a whole. Similarly, questions remain as to the degree to which these changes arc reversible, how changes in population structure affect the overall productivity of the marine ecosystem, and whether severe over fishing of lower-level species threatens to undermine the ability of all fisheries to recover.

The authors caution that continuing down the current path will lead to more fisheries collapses, lower trophic level species, and ultimately lower catches. But the cycle of fishing down the marine food web can't go on forever. (At lower trophic levels, the species are so small and diluted that it is no longer economically feasible to fish.) At the current rate of descent, it will take only 30 to 40 years to fish down to the level of plankton.

The authors point to the strong leverage policymakers have with no-take marine reserves. Setting aside strategically located areas where fishing is prohibited can help rebuild fish populations and restore the marine ecosystem. But until fisheries are managed as an interactive ecosystem comprised of dynamic and fluctuating multiple species, rather than a series of single species managed for optimum yield, any recovery in fish populations will be fleeting at best.