Quantifying competition between two demersal fish species from spatiotemporal stomach content data

Inference on competition is often made on indirect patterns of potential competition, such as population trends and spatiotemporal overlap in diet and distribution. However, these indicators do not test if the contested resources are limited in supply, nor if they decline as competitor biomass increases. Using stomach content and biomass data, we evaluate food competition between Atlantic cod ( Gadus morhua ) and flounder ( Platichthys spp.) in the Baltic Sea. We quantify diet overlap and fit geostatistical mixed models to evaluate effects of local-scale covariates on stomach contents. The dietary overlap is low and does not decline with predator density. We find that cod feed less on the isopod Saduria entomon at high flounder densities. However, the total prey weight in cod is not affected by flounder densities. This suggests interspecific food competition is not limiting the overall feeding of cod, but affects its diet composition. In addition, we find support for intraspecific food competition in large cod and flounder. Our study illustrates the importance of local-scale processes when inferring competition from stomach content data.