Abstract

The use of indices to assess the state of fisheries and the health of ecosystems has been widely accepted. The Mean Trophic Level Index is one example, calculated as the mean trophic level of all landed biomass. Recently, marine ecosystems in the Gulf of Mexico (GOM) and US Atlantic south of Chesapeake Bay were reported to be severely overfished and food webs there badly deteriorated, on the basis of a low intercept and subsequent decline in the mean trophic level of the landed species. Here we illustrate, in a case study using landings data from the aforementioned ocean regions, that this metric is poorly suited for assessing the state of fisheries or ecosystem health because of confounding effects of selective fishing practices. This study may be relevant to all US fisheries because the US GOM exceeds all regions except Alaska in the amount and value of commercial landings. By comparing these landings data with fisheries independent data from US GOM estuaries, NMFS long-line data, and shrimp fishery by-catch data from the US GOM, we demonstrate that commercial targeting, gear selectivity, and, in the GOM, high landings of shrimps and menhaden, drive the index as previously calculated.

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