Trophic Changes in Marine Ecosystems

To understand changes in the health of marine ecosystems, we need reliable indicators. One of the most widely used indicators is mean trophic level in fishery landings, which has been used to detect shifts in landings from high-trophic-level predators to low-trophic-level prey. Declines in mean trophic level occur when predators collapse (“fishing down marine food webs”) or more commonly when low-trophic-level fisheries expand (“fishing through marine food webs”). We examined whether this catch-based indicator tracks changes in marine ecosystems using collections of ecosystem models, landings, stock assessments, and trawl surveys from around the world. By examining ecosystem models, assessments and surveys, we were able to avoid many of the problems that make it problematic to infer ecosystem status from trends in catch data, such as geographic expansion, changes in regulations, market forces, and technological innovations. We found some surprising results. Our models predicted that catch mean trophic level would often be negatively correlated with ecosystem mean trophic level, a result that was confirmed by ecosystem-level comparisons and by comparisons of catch and biomass data from stock assessments. We also found that mean trophic level in catches, assessments, and surveys have actually increased in recent years, contrary to earlier findings of precipitous declines in catch mean trophic level. Additionally, the pattern of fishing differs substantially from the “fishing down the food web” hypothesis, which predicts that top predator catches are declining. Instead, we found nearly continuous increases in catches of top predators, with the exception of Atlantic cod. Our ecosystem models demonstrate that this pattern of evenly increasing landings across marine food webs will result in stable mean trophic levels in both landings and ecosystems, masking the likely outcome of increasing numbers of fisheries collapsing over time. While our compilations are the most comprehensive yet assembled, these kinds of data are available mostly from developed countries and especially from areas with a long history of heavy exploitation, such as the North Atlantic. To better understand trends in marine ecosystems, we need to increase efforts to conduct trawl surveys and stock assessments in the less-studied areas of the globe, particularly in the developing world. In the meantime, we should avoid using mean trophic level in fishery landings as an index of ecosystem well-being.