Grey seal tracking reveals different behaviours of resident and transient population components

Grey seals recently recolonised the Dutch coasts. The exponential population growth is fuelled by immigrating grey seals from the UK, who subsequently breed in the Netherlands. In addition, population survey data suggested that a number of grey seals use the Dutch area to forage, but return to the UK to breed. Although such population surveys can provide insight into discrepancies and regional imbalances in the number of seals observed on land, the data are insufficient to measure the actual underlying migration processes. Using an extensive data set on individually tracked grey seals, we study the movement of grey seals in relation to where they forage and breed. The tracked seals of different age and sex classes were recognised to be either resident or transient based on where they went during and outside the breeding season. The grey seals breeding in the Netherlands also only haul-out in the Netherlands throughout the period between the moult and next breeding season. Only animals that bred elsewhere (respectively 50% of the adult females and 67% of the adult males tracked in this study) visited regions other than Dutch waters during the rest of the year. Some however, remained in Dutch waters continuously between the breeding trips to other countries. The tracking data suggest a higher number of grey seals might be visiting the Dutch waters than was estimated in a population analysis based on aerial surveys. These results might also have consequences for the population modelling used to estimate the grey seal population size in the UK. As eastern Atlantic grey seals in the North Sea are a mixed meta-population, coordinated, international collaborations are needed to assess their status and trends. Since transient seals might be driven by intraspecific competition, ecological studies on for example, the role of grey seals as a top predator, need to consider the entire North Sea grey seal population