Fine-scale determinants of female grey seal (Halichoerus grypus) pupping site and habitat preferences at North Rona, Scotland.

Abstract

The grey seal (Halichoerus grypus) is widespread in the Northern Hemisphere and throughout its range hauls out to breed terrestrially on a variety of substrates. A major breeding site in the eastern North Atlantic is the remote island of North Rona, Scotland, which is characterised by undulating grassy terrain, with limited access to the sea, punctuated by irregularly spaced fresh to brackish water pools of variable size. Previous long term research at North Rona has suggested that the distribution of breeding females is influenced by key habitat features including proximity to pools of water and to access from the sea. Using distributional information available for the North Rona colony in conjunction with a set of ecologically relevant environmental predictors within an extensive GIS database, the ecological niche of the grey seal at North Rona was modelled using Ecological Niche Factor Analysis. This was used to determine the distribution of suitable habitat at the North Rona colony and to elucidate the environmental determinants of female pupping site, and subsequent habitat, preferences over multiple years spanning 1998-2010. The environmental predictors utilised were chosen based on the conclusions of previous research. Following the delineation of basic environmental preferences, the influence of social interactions was considered within this modelling approach to further help explain the distribution of pups of various stages.

Adult female grey seals show a preference for both pupping sites and subsequent habitat near to pools of water of low salinity at intermediate distances to access points to the sea, though appear to exhibit stronger preferences regarding the characteristics of their habitat than pupping site. It is concluded that these preferences are a result of a requirement for proximity to pools for thermoregulation and for drinking water to avoid a negative water balance. However, females do not typically choose sites directly next to or within pools, this is a result of a trade-off between proximity to pools and proximity to their pup, which is at greater risk when separated from its mother, or close to pools in areas of high adult density. It also appears likely that females choose sites at intermediate proximity to access points as sites directly next to access points experience greater disturbance from other seals arriving to, or leaving, the colony. The widespread availability of apparently suitable habitat suggests that the North Rona colony is not restricted in size by limited availability of suitable pupping sites; other potential drivers of the decline of the North Rona colony are therefore discussed, with recommendations for future research. A parallel analysis investigating pup habitat use showed that weaned pups, unlike neonates, appear to avoid locations near to adult female grey seals. It is concluded that this is a result of social interactions driving weaned pups into areas abandoned, or not yet colonised, by adult females, which are aggressive towards conspecifics during lactation. Overall, the ENFA has provided an excellent means to assess the terrestrial pupping site and subsequent habitat preferences of the grey seal, though alternative approaches are suggested for also assessing social influences on space use.