Modelling the effects of prey size and distribution on prey capture rates of two sympatric marine predators

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.

Figure 1. Influence of changing prey size on prey capture rates for guillemots and razorbills.

Example for (A) guillemots and (B) razorbills, illustrating the influence of a decrease in 0-group sandeel size (prey quality) on prey capture rates; this scenario assumes both species still had the same proportion of prey items in their diets and other prey sizes did not decrease in size.

Figure 2. Influence of prey patch dispersion and density on energy expenditure for guillemots and razorbills.

Simulations of proportional daily time budgets and daily energy expenditure (DEE) for guillemots (A, B) and razorbills (C, D) where: (1) prey becomes more patchily distributed requiring more flight time between patches and more foraging time to meet energetic needs (A, C); and (2) prey decreases in density within patches, requiring more foraging time, but distribution is unchanged (B, D). Asterisks indicate the proportion of time activity budget which is the mean across all recoded activity budgets of birds of each species, respectively (see Appendix S4 and S5 in File S1).