Methods for detecting and quantifying individual specialisation in movement and foraging strategies of marine predators

There is increasing realisation that individuals in many animal populations differ substantially in resource, space or habitat use. Differences that cannot be attributed to any a priori way of classifying individuals (i.e. age, sex and other group effects) are often termed ‘individual specialisation’. The aim of this paper is to assess the most common approaches for detecting and quantifying individual specialisation and consistencies in foraging behaviour, movement patterns and diet of marine predators using 3 types of data: conventional diet data, stable isotope ratios and tracking data. Methods using conventional diet data rely on a comparison between the proportions of each dietary source in the total diet and in the diet of individuals, or analyses of the statistical distribution of a prey metric (e.g. size); the latter often involves comparing ratios of individual and population variance. Approaches frequently used to analyse stable isotope or tracking data reduced to 1 dimension (trip characteristics, e.g. maximum trip distance or latitude/longitude at certain landmarks) include correlation tests and repeatability analysis. Finally, various spatial analyses are applied to other types of tracking data (e.g. distances between centroids of distributions or migratory routes, or overlap between distributions), and methods exist to compare habitat use. We discuss the advantages and disadvantages of these approaches, issues arising from other effects unrelated to individual specialisation per se (in particular those related to temporal scale) and potential solutions

Consistency in migration strategies and habitat preferences of brown skuas over two winters, a decade apart

At-sea movements and activity patterns of brown skuas Stercorarius antarcticus lonnbergi from South Georgia were analysed in 2 winters, a decade apart, to examine the degree of consistency in migration strategies and habitat preferences during the non-breeding and pre-laying exodus periods. Oceanographic habitat preferences of tracked skuas were determined using a robust model accounting for availability. At the population level, brown skuas were broadly consistent in their choice of wintering areas and habitat preferences, although the distribution extended farther east in 2012 than in 2002. Skuas preferred areas associated with static oceanography (bathymetric features) both during the non-breeding and pre-laying periods, which may explain the consistency between years in habitat use. There was no significant effect of year on departure dates from South Georgia, but birds returned earlier to the colony in 2002. Migration schedules varied according to breeding status, with failed birds departing earlier than birds that bred successfully. Although failed birds travelled farther from the colony, there was little variation in dates of return. In general the timing of movements was similar between sexes, but females were more likely than males to engage in a pre-laying exodus. Brown skuas spent a much higher proportion of time sitting on the water than other seabirds during both the non-breeding and pre-laying exodus periods, and the number of flight bouts per day was surprisingly low. The selection of static features by brown skuas may indicate that skuas may have less flexibility to track environmental changes than species that use dynamic cues

Spatial scales of marine conservation management for breeding seabirds

Knowing the spatial scales at which effective management can be implemented is fundamental for conservation planning. This is especially important for mobile species, which can be exposed to threats across large areas, but the space use requirements of different species can vary to an extent that might render some management approaches inefficient. Here the space use patterns of seabirds were examined to provide guidance on whether conservation management approaches should be tailored for taxonomic groups with different movement characteristics. Seabird tracking data were synthesised from 5419 adult breeding individuals of 52 species in ten families that were collected in the Atlantic Ocean basin between 1998 and 2017. Two key aspects of spatial distribution were quantified, namely how far seabirds ranged from their colony, and to what extent individuals from the same colony used the same areas at sea. There was evidence for substantial differences in patterns of space-use among the ten studied seabird families, indicating that several alternative conservation management approaches are needed. Several species exhibited large foraging ranges and little aggregation at sea, indicating that area-based conservation solutions would have to be extremely large to adequately protect such species. The results highlight that short-ranging and aggregating species such as cormorants, auks, some penguins, and gulls would benefit from conservation approaches at relatively small spatial scales during their breeding season. However, improved regulation of fisheries, bycatch, pollution and other threats over large spatial scales will be needed for wide-ranging and dispersed species such as albatrosses, petrels, storm petrels and frigatebirds