Freefall in global fish stocks.

• Author: McGinn, Anne Platt

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...