Killer whale predation of seals in the inshore waters of Shetland : investigating the ecological drivers and consequences of an apex predator-prey interaction

Abstract

The impact of predation has important consequences for individual fitness and the potential to culminate in population level effects. Understanding the causes and consequences of predator-prey interactions is therefore a mainstay in ecological research. This thesis explores the predator-prey interactions of killer whales (Orcinus orca) and seals in inshore Shetland. Data was derived from dedicated land-based field research and community-led monitoring efforts. A multivariate approach was used to explore potential environmental, ecological, spatial, and temporal drivers of predator and prey co-occurrence and species-specific capture probability. Killer whale diet composition with associated uncertainties in prey identification were quantified through an expert elicitation process and used to inform species- and age-class consumption estimates. Using a bioenergetic approach, the energetic requirements of inshore-foraging killer whales were estimated for the year 2021. Uncertainty in parameter estimates were explored through a Monte Carlo uncertainty analysis, with plausible ranges of killer whale consumption estimates incorporated as a harvest-type parameter in an age and sex-segregated Leslie Matrix population model. Population trajectories for both harbour (Phoca vitulina) and grey (Halichoerus grypus) seals were simulated under different assumptions of input parameters (e.g., changes to vital rates and composition in killer whale) over a 35-year prediction period. Findings from this thesis highlight the potential importance of near-shore areas for killer whale foraging effort and prey species refuge. In comparison to grey seals, harbour seals were found to have a higher probability of capture and consumption given an observed encounter with a killer whale. For harbour seals, populations trajectories typically resulted in a decline despite assumptions as to input parameters, whereas grey seal trajectories exhibited an increase. Overall, findings suggest killer whale predation may have implications for the depleted harbour seal population in Shetland, and demonstrate the capabilities of community-led monitoring to complement dedicated sampling efforts and inform research objective.