Interspecific interactions : investigating the role of grey seals in the harbour seal decline

Abstract

Sympatric species can interact within and between trophic levels, having top-down and bottom-effects on the structure of ecological communities. Over the past 20 years, grey seal (Halichoerus grypus) and harbour seal (Phoca vitulina) populations around the UK have experienced contrasting population trends, with grey seals stable or increasing, and harbour seals suffering regional declines. Grey and harbour seals have the potential to interact through competition and predation, and so adverse interactions with grey seals is a potential driver to these harbour seal declines. I investigated interspecific interactions between seal species through changes in diet derived from prey hard parts analysis of scat samples (Chapter 2), individual variation in foraging behaviour from GPS tracking devices (Chapter 3), and evidence of predation in strandings reports and citizen science observations (Chapter 4). I demonstrated changes to the dietary niche of harbour seals through time, e.g. in the Moray Firth, the summer diet diversified between 1988 (D = 3.98 ± 0.61) and 2010 (7.16 ± 0.53). Over the same period, grey seal diet remained highly specialised on calorific sandeels (>98% biomass). From tracking data, there was no evidence of harbour seals working harder between 2012 and 2016/17; two points in time representing an increase in grey seal density of >50%. However, harbour seals were moderately repeatable in trip metrics and had higher levels of individual specialisation in 2012 which could be interpreted as intraspecific competition being a more dominant force than interspecific competition in this year. Finally, by analysing strandings data and citizen science reports from around Scotland, I demonstrated that grey seal predation of harbour seals is widespread and increased in the strandings data between 2010 and 2021. The highest odds for predation as cause of death were found in the East Scotland Seal Monitoring Unit (SMU), on adult harbour seals, and during their summer breeding season. I also modelled different predation scenarios for the SMU and demonstrated that observed levels of predation could be largely responsible for the observed annual declines of up to 21%. The results of these studies combined provide evidence that interactions with grey seals could have contributed to regional declines, and continued pressures imposed through competition and predation will make it challenging for depleted populations of harbour seals to recover.