Abstract
Understanding how prey capture rates are influenced by feeding ecology and environmental conditions is fundamental to assessing anthropogenic impacts on marine higher predators. We compared how prey capture rates varied in relation to prey size, prey patch distribution and prey density for two species of alcid, common guillemot (Uria aalge) and razorbill (Alca torda) during the chick-rearing period. We developed a Monte Carlo approach parameterised with foraging behaviour from bird-borne data loggers, observations of prey fed to chicks, and adult diet from water-offloading, to construct a bio-energetics model. Our primary goal was to estimate prey capture rates, and a secondary aim was to test responses to a set of biologically plausible environmental scenarios. Estimated prey capture rates were 1.5±0.8 items per dive (0.8±0.4 and 1.1±0.6 items per minute foraging and underwater, respectively) for guillemots and 3.7±2.4 items per dive (4.9±3.1 and 7.3±4.0 items per minute foraging and underwater, respectively) for razorbills. Based on species' ecology, diet and flight costs, we predicted that razorbills would be more sensitive to decreases in 0-group sandeel (Ammodytes marinus) length (prediction 1), but guillemots would be more sensitive to prey patches that were more widely spaced (prediction 2), and lower in prey density (prediction 3). Estimated prey capture rates increased non-linearly as 0-group sandeel length declined, with the slope being steeper in razorbills, supporting prediction 1. When prey patches were more dispersed, estimated daily energy expenditure increased by a factor of 3.0 for guillemots and 2.3 for razorbills, suggesting guillemots were more sensitive to patchier prey, supporting prediction 2. However, both species responded similarly to reduced prey density (guillemot expenditure increased by 1.7; razorbill by 1.6), thus not supporting prediction 3. This bio-energetics approach complements other foraging models in predicting likely impacts of environmental change on marine higher predators dependent on species-specific foraging ecologies.
Highlights
The foraging ecology of marine higher predators has been the subject of intensive research in recent decades
An alternative approach is to construct a bioenergetics model where information on daily energy requirements is used in conjunction with data on time activity budgets and diet to derive estimates of prey capture rate and estimate how such rates vary with prey availability [6,7,8]
We developed a bio-energetics model and estimated prey capture rates, expressed as energy gain over the time spent foraging and items caught per dive, for guillemots and razorbills breeding at a major colony in the North Sea
Summary
The foraging ecology of marine higher predators has been the subject of intensive research in recent decades. Quantifying prey capture rates is challenging because most marine predators feed out of sight of a land-based observer and many catch their prey underwater. Prey capture rates can be assessed using animal-borne cameras that record foraging behaviour [9,10,11], or with gastric, oesophageal or magnetic loggers that record prey ingestion [12,13,14]. Such methods are not readily applicable to small-sized species and/or those that eat small prey items. An alternative approach is to construct a bioenergetics model where information on daily energy requirements is used in conjunction with data on time activity budgets and diet to derive estimates of prey capture rate and estimate how such rates vary with prey availability [6,7,8]
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