Linking foraging behaviour of female grey seals (Halichoerus grypus) to population size, diet, and reproductive success

Abstract

Understanding the movement patterns and foraging behaviour that marine predators use to navigate and adapt to patchy and unpredictable prey availability has important implications for individual fitness. In capital breeding animals where the acquisition and storage of energy is critical to financing the costs of reproduction, these species offer interesting model systems to study the relationships between foraging behaviour, diet, and reproductive success. Northwest Atlantic grey seal (Halichoerus grypus) females are wide-ranging, long-lived, capital breeders, that are known to undergo extensive pre-breeding period foraging trips where they gain body mass prior to expending roughly a third or more of parturition body mass to support a single precocial pup during a short lactation period. Using hidden Markov Model estimates of foraging behaviour derived from satellite telemetry data obtained between 1995 and 2018, this study first uses generalized linear models (GLMs) to explore the potential effects of age and population density-dependent regulation of foraging behaviour in a grey seal population experiencing decelerating population growth. Results from 62 female deployments suggest that foraging behaviour leading up to the breeding season is impacted by increasing population size, likely because of intraspecific competition for access to the main prey during this foraging period. Although the impact of female age was less than population size, I found a dome-shaped pattern where the total distance travelled and the number of foraging locations used peaked in prime-age females. To test whether these differences in foraging tactics relate to variation in female diet, GLMs were then used to relate estimates of foraging behaviour from 50 deployments to diet estimated at instrument recovery through stable isotope (SI) signatures and quantitative fatty acid signature analysis (QFASA). The total distance travelled was linked to the spatial distribution of the primary prey species found in the diet, where individuals with more redfish (Sebastes sp.) in the diet travelled greater distances than females with sand lance (Ammodytes dubius)/mixed diets. Females that travelled greater distances and spent more time in area-restricted search also consumed relatively energy dense prey species. Finally, foraging behaviour and diet estimates were related to maternal postpartum mass (MPPM) and pup weaning mass to evaluate the fitness consequences associated with different foraging tactics and diets. The foraging tactics used appeared to depend on female size, with those estimated to be using more benthic foraging tactics having a higher MPPM. At the same time, diet diversity was negatively related to MPPM, but energy density was not, suggesting that larger females may have been foraging more efficiently or using a “quantity over quality” foraging tactic, although future studies using simultaneous estimates of the quantity of prey consumed would be required to confirm this hypothesis. Regardless, there was no detectable relationship between estimated foraging effort and diet metrics and pup weaning mass once MPPM and maternal age were included in models. With these data collected on an increasing grey seal population, evidence suggests that the environmental conditions and prey abundance were sufficient to support multiple foraging tactics leading to high population-level reproductive rates. This work demonstrates that individual variation in foraging behavior and diet contributes to individual fitness and population dynamics in free-ranging marine predators.