State-space analyses indicate experience, prey availability, competition, and reproductive status drive foraging behaviour in a top marine predator

Radio and satellite tracking of animal movement has greatly improved our understanding of the behaviour and ecology of many marine and terrestrial species. However, statistical methods for analyzing such data, especially those that can objectively identify behaviour, have been lacking. State-space models (SSMs) are promising new statistical tools for analyzing a range of time series data, including animal movement data. Here I use a state-space method to identify and understand that at-sea movement behaviour of grey seals (Halichoerus grypus) in the Northwest Atlantic. Movement data was collected between 1995 and 2005 by deploying Argos satellite tags on 134 animals hauled out at Sable Island, NS (44.0°N 60.0°W). The data set includes 104 adults, 24 young of year, and 6 2-3 yr old juveniles. The data were analyzed by fitting a correlated random walk model (CRW) that switched between 2 behavioural states (nominally foraging and travelling) in a SSM framework. The approach produced regular location estimates and classified behaviour, allowing for a series of additional analyses that would have otherwise been impossible. SSM results indicate that grey seals concentrated foraging effort over a few small banks on the Scotian Shelf, but these locations change through the year and differ between sex and age classes. Foraging behaviour also differed considerably between sex and age classes, due to size differences, differing reproductive strategies of males and females, and non-reproductive status of juveniles. In addition, a strong seasonal effect was present. All age classes foraged nearer to shore in the summer and summer foraging effort was generally lower. Foraging effort increased in the fall, especially for adults as they prepared for the Dec-Jan breeding season. By mid-winter all age classes increased foraging effort, with longer foraging trips to more distance foraging areas, suggesting seals are tracking prey migrations and prey availability. There is also evidence that young-of-year were competitively excluded from foraging in areas near Sable Island in summer, when all other age classes foraged in close proximity to the island. This is in line with other evidence that suggests this large population of grey seals is approaching carrying capacity.