Quantifying competition between two demersal fish species from spatiotemporal stomach content data
Competition is challenging to quantify in natural systems and inference is often made on indirect patterns of potential competition, such as trends in population trajectories and overlap in spatiotemporal distribution and resource use. However, these indicators are not direct measures of fitness, nor do they say if the contested resource is limited in supply, which are key features of competition. Here we combine stomach content and biomass density data from scientific bottom trawl surveys to evaluate if competition is occurring between two dominant demersal fish species in the southern Baltic Sea: Atlantic cod (Gadus morhua) and flounder (Platichthys spp.). We use multivariate generalized linear latent variable models (GLLVMs) to quantify diet similarities across the domain, diet overlap indices on relatively small spatial scales to test if predator density drives diet overlap, and spatiotemporal GLMMs fit to prey weights in individual predators to evaluate the effects of local biotic