Mean Trophic Level
In fisheries, the mean trophic level for the fisheries catch across an entire area or ecosystem is calculated for year y as:
where is the catch of the species or group i in year y, and is the trophic level for species i as defined above.
It was once believed that fish at higher trophic levels usually have a higher economic value; resulting in overfishing at the higher trophic levels. Earlier reports found precipitous declines in mean trophic level of fisheries catch, in a process known as fishing down the food web. However, more recent work finds no relation between economic value and trophic level; and that mean trophic levels in catches, surveys and stock assessments have not in fact declined, suggesting that fishing down the food web is not a global phenomenon. However Pauly et al. note that trophic levels peaked at 3.4 in 1970 in the northwest and west-central Atlantic, followed by a subsequent decline to 2.9 in 1994. They report a shift away from long-lived, piscivorous, high-trophic-level bottom fishes, such as cod and haddock, to short-lived, planktivorous, low-trophic-level invertebrates (e.g. shrimps) and small, pelagic fish (e.g. herrings). This shift from high-trophic-level fishes to low-trophic-level invertebrates and fishes is a response to changes in the relative abundance of the preferred catch. They argue this is part of the global fishery collapse.
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