What's Trending in the Gulf of Mexico?

The health of fisheries ecosystems and food webs are assessed via a metric coined the mean trophic level index– calculated as the mean trophic level of all landed biomass. Based on this index, marine ecosystems in the Gulf of Mexico and US South Atlantic Ocean were deemed to be severely overfished and food webs badly deteriorated. Here is illustrated, in a case study using landings data from the Gulf of Mexico, that this metric may be poorly suited for assessing overfishing or ecosystem health because of confounding effects of selective fishing practices. By comparing indices based on landings data and two fisheries-independent sets of survey data, the cause for the low index value of the Gulf of Mexico is made evident. Commercial targeting and high landings of low trophic level species such as shrimp and menhaden drive the index as previously calculated. Discussed as well is that while the index is higher than previously reported, and increasing rather than decreasing, overfishing could still occur.