Understanding shifts in body size distributions - a comparative study of the impacts of fishing and climate on North Sea demersal fishes

The North Sea is a huge economic resource for millions of people and is undergoing dramatic changes due to overfishing and climate change. The extent to which fishing and climate are causing changes in the North Sea demersal fish community status and ecosystem health is difficult to quantify. This thesis implements newly combined empirical methods and adaptive modelling techniques in an attempt address this area of uncertainty. The first study develops a statistical framework for understanding variations in size-based indicators (SBIs), metrics for describing ecosystem health, across space and time simultaneously, as well as finding the most likely driver of observed changes. SBIs declined in the central North Sea due to fishing, but increased in the Norwegian trench region due to changing sea bottom temperature. This highlights the need to consider environmental and fishing impacts on the change in body size distribution of fish communities. The remainder of the thesis improves an existing multispecies, size-structured model by incorporating empirical relationships between growth and temperature and oxygen. Including temperature or temperature and oxygen leads to a smaller body size distribution under warmer, deoxygenated waters. When oxygen alone is included no significant effects on either the individual size or the community size structure are found. The temperature-oxygen size-structured model is subsequently used alongside extracted environmental data from two global climate models (GCMs) to project a number of size-based indicators for 2100. The most striking result is the huge difference in size-based indicators depending on which GCM is used. Corrections to the GCMs must therefore be made before the use in ecological projections. The common result, however, is that temperature and oxygen will cause changes in the body size distribution. Although these effects are much smaller than fishing effects, they demonstrate that ignoring impacts of climate change could cause considerable overestimates in the biomass of North Sea demersal fish.