BOTTOM OF THE BARREL:
Increasing catches from large-scale fisheries are being found to impact ecosystems in a variety of ways including changes in species community structure. Food webs, or the flow of biomass through the ecosystem, are being changed through removal of predatory fish by the industry. The general trend is to take highly desirable top predators, and as these populations decline, move to take lower level prey species. This fundamentally changes the way in which predators and prey interact within the ecosystem. Taking lower trophic level fish creates competition between humans and their natural predators.
Research by Pauly et. al. (1998) shows a shift in the past 45 years of fishing from large piscivorous fish to smaller invertebrates or planktivorous fish. Their results come from analysis of FAO landings data and trophic level characterization of 220 species of fish. Using trophic level data they characterized landings in terms of the trophic levels. For example, primary producers in the ecosystem are low on the trophic level and are given a value of 1, while larger fish that feed higher in the trophic level such as snapper are given a value of 4.6. They conclude that globally, fisheries landings are decreasing at a rate of 0.1 trophic levels per decade. Larger species populations characterize low trophic levels, so there is an expected increase in the amount of fish landed in these lower trophic levels. However, the researchers show that the landings seem to first increase, then stagnate and eventually decline, which raises doubt about the sustainability of current fishing practices.
One example of this trend is on the east coast of Canada,
where the cod fishery collapsed and was replaced by a shrimp fishery.
Shrimp are a food source for cod and this could lead to a more difficult
recovery of cod populations.