How Do Fisheries Affect Marine Food Webs?

Fisheries have affected food webs in a variety of ways, including the depletion of species at all trophic levels from abalone to sharks. Pauly et al. have proposed that "fishing down marine food webs" is ubiquitous, based on declining trends in mean trophic level (MTL) in global catches. However, updated analyses in 2010 show that catch MTL has been increasing for two decades, and that catch MTL trends are often opposite to ecosystem MTL trends obtained from trawl surveys and fisheries stock assessments. Furthermore, catch MTL trends are greatly influenced by a handful of species with large catches. For instance catch MTL for species above trophic level 3.5 either increase or decrease over time depending on the assumed trophic level estimate of Atlantic cod. Careful examination of fishing trends reveals that fishing down is not ubiquitous, trophic level is uncorrelated with price, and fishing proceeds from the most profitable to the least profitable species, with profitability a function of price, population size, and accessibility. The discussion over the ubiquity of fishing for profits vs. fishing for predators is further evaluated in this talk, based on greatly expanded assessment compilations, updated catch data and revised trophic level estimates for key species.