Mean trophic level

 

Mean trophic level

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The mean trophic level of the world fisheries catch has steadily declined because many high trophic level fish, such as this tuna, have been overfished.

In fisheries, the mean trophic level for the fisheries catch across an entire area or ecosystem is calculated for year y as:

${\displaystyle T{L}_y=\frac{\sum _i(T{L}_i\cdot Y_{iy})}{\sum _i{Y}_{iy}}}$

where ${\displaystyle Y_{iy}}$ is the annual catch of the species or group i in year y, and ${\displaystyle T{L}_i}$ is the trophic level for species i as defined above.^[8]

Fish at higher trophic levels usually have a higher economic value, which can result 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.^[20] However, more recent work finds no relation between economic value and trophic level;^[21] 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.^[22] 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., shrimp) and small, pelagic fish (e.g., herring). 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 consider that this is part of the global fishery collapse,^[17][23] which finds an echo in the overfished Mediterranean Sea.^[24]

Humans have a mean trophic level of about 2.21, about the same as a pig or an anchovy.^[25][26]