The foraging ecology and conservation of the Great Black-backed Gull Larus marinus

Abstract

The deterioration of marine ecosystems due to anthropogenic activities has multilayered consequences for seabird populations. Seabirds may suffer direct impacts such as reduced resource availability due to competition with fisheries, but deterioration in marine resources may also trigger indirect impacts such as the destabilisation of trophic relationships between predators and prey, leading in turn to predator-prey conflicts within seabird communities. Monitoring the abundance of seabird species and advancing our knowledge of their foraging ecology is therefore fundamental to assess the health of populations, identify research priorities, and guide conservation efforts. Within this thesis, I use a range of analytical and practical approaches to garner information about a seabird that is often involved in seabird predator-prey conflicts, but for which there is little information about its conservation status and foraging ecology; the Great Black-backed Gull Larus marinus. I firstly use a stochastic population model to project changes in Great Black-backed Gull populations under IUCN Red List criteria at a global, continental, and national scale over 36 years. I demonstrate Great Black-backed Gulls have experienced severe declines at a global scale, meeting the requirements for a “Vulnerable” listing on the IUCN Red List. I then use long-term seabird demographic data from the Isle of May, Scotland, to quantify the impacts of Great Black-backed Gull predation on the viability of an Atlantic Puffin population. I demonstrate that whilst current predation levels are not sufficient to trigger a decline in the Atlantic Puffin population, predation significantly impacts their population growth; this can have consequences for the management of seabird populations of the Isle of May and impact assessments of renewable energy developments in the Firth of Forth that could interfere with those seabird populations. Next, I investigate the year-round foraging ecology of a Great Black-backed Gull population using stable isotope analysis, demonstrating that the varying pressures seabirds face throughout the annual cycle may result in changing foraging strategies. Indeed, I highlight shifts in trophic position, trophic niche, and resource use in Great Black-backed Gulls between breeding and non-breeding periods, whilst dentifying that marine resources are key for the population year-round. Furthermore, I use bio-logging to examine individual-level differences in the foraging movements of ten Great Black-backed Gulls in response to shifting prey availability. I describe extreme differences in area use and foraging effort among individuals, as well as contrasting individual responses to an increase in prey availability, demonstrating heterogeneity in individual-level foraging behaviour within the population. Finally, I quantify and report deleterious impacts from bio-logging devices on the breeding performance of Great Black-backed Gulls, demonstrating that the species appears to be sensitive to harness-mounted devices. Through population modelling, diet analysis, and bio-logging, I have addressed knowledge gaps regarding the conservation and management of Great Black-backed Gulls populations, including predator-prey conflicts. Furthermore, I have provided new insights into the trophic dynamics of Great Black-backed Gulls throughout the annual cycle, quantifying their reliance on different resources and describing their foraging strategies at the individual level.

Date of Award	8 May 2024
Original language	English
Awarding Institution	University of the Highlands and Islands
Sponsors	Super DTP
Supervisor	Elizabeth Masden (Supervisor), Nina Jayne O'Hanlon (Supervisor) & Benjamin Williamson (Supervisor)