Another effect of overfishing is the progressive decrease of the trophic level (Box 7) of the catch. Over exploitation of the top of the food chain and the targeting of more abundant species lower in the food chain is called ‘fishing down the food chain’ (Pauly and Watson 2003). Overfishing has shortened the chain and sometimes removed one or more of the links, increasing the system's vulnerability to natural and human induced stresses, as well as reducing the supply of fish for human consumption (Figure 2).

BOX 7: What is a trophic level?

Two alternative marine food chains. One way to understand the structure of ecosystems is to arrange them according to who eats who along a food chain. Each link along the chain is called a trophic level. Levels are numbered according to how far particular organisms are along the chain from the primary producers at level 1, to the top predators at the highest level. Within marine systems, large predators such as sharks and saithe, are at a high trophic level, cod and sardines are in the middle, and shrimp are at a low trophic level with microscopic plants (mainly phytoplankton) at the bottom sustaining marine life. The arrows in the figure indicate who is eating who.In this example, cod are at trophic level 5, whereas the large saithe which eat them are on trophic level 6. The small saithe, which feed on smaller prey, are at a lower trophic level. A single species may consume prey from several different levels. In this case, its trophic level is calculated according to the proportion of its diet that comes from the various trophic levels it feeds on, and will not necessarily be a whole number.

Source: Pauly and Watson 2003

Figure 2: Fishing down marine food webs

The arrow indicates how, over time, humans have depleted stocks of larger fish found closer to the surface and are now fishing smaller fish from deeper water or on the bottom of the sea. When fish are taken from the bottom the habitat usually changes from one rich in various plants and bottom-dwelling organisms, to a near-lifeless muddy substrate.

Source: Daniel Pauly, Fisheries Centre, University of British Columbia, Canada

Monitoring the average trophic level of fish catches can show when stocks of larger predatory fish are beginning to collapse. A study of fisheries off the west coast of Newfoundland, for instance, described how the average trophic level of the catch decreased markedly over a 43-year period. The level went from a maximum of 3.65 in 1957 to 2.6 in 2000 (Pauly and Watson 2003). This meant that, toward the end of this period, fewer fish such as cod were being caught, the average size of fish caught was smaller and other species, which the cod may have preyed upon, made up a greater proportion of the catch, and were therefore also being removed from the food chain. As well as the catch having less economic value, there are important long-term implications as, with less food, it may take cod stocks longer to recover. Overall, it is clear that managing target species without considering other species and the ecosystem that supports the fisheries is not an effective approach. Appropriate mechanisms are needed to effect ecosystem-based management that encompasses multi-species management.