Trends in the Structure of Marine Ecosystems: An Analysis of Trawl Survey Data

Marine ecosystems have been heavily impacted by fishing pressure which can cause large changes in total biomass and biodiversity.  Fishing directly removes species and causes secondary effects such as changing predatory and competitive interactions and altering energy pathways, all of which affect the functional groups and size distributions of marine ecosystems.  We conducted a meta-analysis of nineteen trawl surveys from around the world to identify consistent changes in functional and size based groups across ecosystems.  Changes in the ratio of pelagic taxa to demersal taxa were variable across the surveys. Pelagic species were not uniformly increasing, but did show periods of increase in certain regions.  In the western Atlantic, the pelagic-to-demersal ratio increased across a number of surveys in the 1990s and declined in the mid 2000s.  Similarly, changes in the size structure of organisms in the different surveys were variable across ecosystems. Trends in the biomass of taxa whose maximum length was less than 100 cm were typically steady or slightly increasing, while trends of larger species ranged from increasing to decreasing.  The trawl survey data suggest there has been considerable variability over time and region, but not consistent trends of fishing down the food web across all ecosystems.