subject: Catching Big Fish Eventually Leads to a Smaller Dish [print this page] Catching Big Fish Eventually Leads to a Smaller Dish
We may think of them as part of a tasty meal, something that goes well with chips or a salad, but in the wild, cod, salmon and tuna are large predatory fish located towards the top of the food web and over reliance on their consumption leads to many problems in the ecosystems they inhabit.
Food webs are often extremely complex, but will always consist of organisms at differing trophic levels (position in the food chain). Primary producers, those that utilise an outside source of energy (usually that of the sun, although there are some notable exceptions) are found at the bottom. Organisms that feed upon the primary producers (typically herbivorous organisms) are found at the next level. The levels of complexity will increase until an apex predator, the organism at the top of the chain, is found.
This is a huge generalisation; there are a huge number of intra and interspecific interactions within food webs. A species preyed upon at an early stage of its life may in turn grow and prey upon the species that it was once preyed upon by. However, one thing holds true within any ecosystem and that is that the balance of a food chain is a very fragile thing, and the disproportionate removal of species of key importance (keystone species) to the balance of a food chain often results in drastic changes in the abundance of species at different trophic levels. This often has dire consequences for the ecosystem.
Keystone species are often those found at the highest and lowest trophic levels. As already stated; cod, salmon and tuna are located towards the top of their food web (at a high trophic level). Their presence in the ecosystem serves to keep mid-level species, such as anchovies, sardines and mackerel at sustainable levels with regard to organisms further down the chain. In the last century the level of overfishing of these fish has been astronomical, and this has resulted in the proliferation of the mid-trophic level species they previously kept in check. This, in turn has further consequences lower down the web. An explosion of mid-trophic level species can lead to the eventual exhaustion of an ecosystem's primary producers, due to overgrazing. An easier to understand example would perhaps be the effects seen upon the removal of big cats and hyenas from a biodiverse savannah region in Africa. Populations of grazing animals such as wildebeest would proliferate and place increasing pressure on the availability of vegetation, which could eventually result in a population crash.
Cod, salmon and tuna species are typically among the largest species in their ecosystem, and display low rates of reproduction in comparison to many smaller fish species. This makes it harder for populations to recover when they become overfished. Long term overfishing often leads to adaptations detrimental to the health of these fish.
In the case of the North Atlantic cod (Gadus morhua), which is the species of cod most commonly harvested, big individuals will often be fished out first, leaving smaller individuals in the population. However, it is a slow growing species, and in the population, larger more mature individuals have the highest success rate at spawning juvenile fish that survive to adulthood. The removal of larger individuals thus results in a population in which fewer juveniles survive to maturity. The selection pressures placed upon populations by overfishing will also favour individuals that reach sexual maturity earlier and have a higher rate of fecundity per body size. However, these adaptations come at a cost to the health of the individual and they will often not be able to weather fluctuations in environmental conditions as well as individuals that reach sexual maturity later and are less fecund.
It's an extremely complex problem and one for which many possible solutions have been proposed. At a recent conference for the American Association for the Advancement of Science (AAAS), Professor Villy Christensen of the University of British Columbia, a leading expert on fisheries, advocated the consumption of smaller species of fish (species such as anchovies and sardines that are traditionally used for fishmeal) in place of tuna, cod and salmon. Much was also the focus of Hugh's Fish Fight, a recent series of programs on Channel 4.
Spreading the burden of the ever increasing demand for fish across a number of species would appear to make sense. However, there is always the worry that these alternative species would also become overexploited, and their removal from an ecosystem would mean less food for large predatory species such as cod and tuna. As with any new practice in the commercial fishing industry, the implementation of the catch of a wider variety of species is something that needs to be handled very carefully, but could be very good news for the overexploited species that we know only too well.
