Modeling trophic interactions between parental common murres and capelin off the northeast Newfoundland coast

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This presentation focuses on trophic interactions between capelin (Mallotus villosus) and its primary avian predator, the common murre (Uria aalge) on Funk Island during the breeding season. Diet is evaluated through parental deliveries to the chicks and prey availability is estimated from pelagic trawl data within the murre's foraging range. Diet composition is assessed using percentage by number (%N), with its confidence limits obtained by bootstrapping. Since the common murre is a capelin specialist and feeds on capelin larger than 100mm (suitable capelin), three prey categories were considered: small capelin (100-140mm total length), large capelin (>140mm total length) and others (prey species other than capelin). Considering the densities of these prey groups as explanatory variables and assuming a multinomial probability distribution for the individual prey deliveries, the common murre's diet is being modeled in two different ways. One is purely statistical and uses a standard multicategory logit model. The second one, derived from ecological theory, estimates the probabilities of consuming different prey categories from a generalized form of the multispecies Holling functional response. Both models describe well the observed diets, but the model with ecological roots has a better fit than the purely statistical one. Furthermore, in years when the abundance of suitable capelin was high the proportions of large and small capelin consumed were not significantly different, while they were in years of low suitable capelin abundance.