Linking predator diet and prey availability: common murres and capelin in the Northwest Atlantic

We examined the predator-prey interaction between an apex seabird predator, the common murre Uria aalge, and capelin Mallotus villosus, the primary forage fish in the Northwest Atlantic. Sampling of parental deliveries to murre chicks was carried out during the breeding season on Funk Island, located off northeast Newfoundland, Canada. Concurrent vessel surveys were conducted throughout the murre's diving and foraging range around the colony to characterize the prey field. Results indicated that in years when capelin was abundant in the size range consumed by murres (suitable capelin), murres delivered large and small fish in similar proportions, whereas they delivered more large fish when suitable capelin abundance was low. Considering the relative abundances of small and large suitable capelin, these observations suggest negative prey switching by the predator. Using foraging theory, we derived a model which estimates the probability of delivering a specific prey type (large or small capelin or other prey) to the chick based on prey availabilities. This quantitative model was capable of reproducing the general patterns in the observations. It also allowed estimating the shape of the common murre's multispecies functional response (MSFR) which indicated that this would conform to the definition of prey switching, and could then be classified as a Type 3. From an applied perspective, our results support the use of predator diets as indicators of their food base, but also highlight the need for understanding the shape of the predator's MSFR for quantitative development of these types of applications.