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).
7: What is a trophic level?
|Two alternative marine food chains.
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.
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
2: Fishing down marine food webs
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.