Callan Duck

Organochlorine contaminants in Caspian and harbour seal blubber.

Organochlorine contaminants (pesticides and individual polychlorinated biphenyl congeners, PCBs) were measured in Caspian (Phoca caspica) seal blubber and compared with concentrations in Harbour (Phoca vitulina) seal blubber. Concentrations of total dichlorodiphenyltrichloroethane (DDT) and its metabolites were significantly higher in Caspian seal than Harbour seal samples, whereas PCB congeners were significantly higher in Harbour than Caspian seals. Our finding suggests that Caspian seals are exposed to high levels of DDT pesticides. In contrast, PCBs are circulating in this population at much lower levels than in marine mammals from the North Sea.

Uncovering the links between foraging and breeding regions in a highly mobile mammal

1. The annual cycle of many animals is characterized by the need to satisfy different life history priorities, often requiring seasonal movements. For such species, investigating carryover effects (such as the year-long drivers of breeding success) and managing protected areas effectively, relies on quantifying these movements. Here, we model the seasonal movements of the UK population of grey seals Halichoerus grypus and show how insights from the model can improve its management. 2. We fit a hidden process model to two types of information – regional population redistribution and individual movements – to estimate the seasonal transition probabilities of breeding female grey seals among four regions around the UK. 3. We found that between 21% and 58% of females used different regions for breeding and foraging. 4. For our study period, we detected an increase in the breeding performance of animals that foraged in the Hebrides and South-East Coast. 5. Grey seal Special Areas of Conservation (SACs) were designed to encompass a significant proportion of the UK breeding population: ~ 40% of the breeding females in our study area. Of the females breeding on SACs, only 15% breed in Northern Scotland, but up to 50% forage there. Our results indicate that, by only considering the breeding distribution of females that breed in SACs, the impact of anthropogenic activities on nearby SACs may be overestimated, whereas impacts on remote SACs may be underestimated. 6. Synthesis and applications. By quantifying the link between the foraging and breeding distributions of grey seals, management of breeding populations can be focused on the foraging regions where the resources necessary for reproduction are acquired. The construction of marine developments is dependent on demonstrating that they will not have an adverse effect on the integrity of Special Areas of Conservation (SACs), and we have shown that this requires consideration of the seasonal transition probabilities estimated in this study. Our specific results provide support for management strategies that jointly consider SACs and Marine Protected Areas (MPAs). More generally, we prescribe combinations of data on population size, breeding performance and individual movement that can enable our framework to be applied to seasonally migrating species.

An Approximate Bayesian Method Applied to Estimating the Trajectories of Four British Grey Seal (Halichoerus grypus) Populations from Pup Counts

For British grey seals, as with many pinniped species, population monitoring is implemented by aerial surveys of pups at breeding colonies. Scaling pup counts up to population estimates requires assumptions about population structure; this is straightforward when populations are growing exponentially but not when growth slows, since it is unclear whether density dependence affects pup survival or fecundity. We present an approximate Bayesian method for fitting pup trajectories, estimating adult population size and investigating alternative biological models. The method is equivalent to fitting a density-dependent Leslie matrix model, within a Bayesian framework, but with the forms of the density-dependent effects as outputs rather than assumptions. It requires fewer assumptions than the state space models currently used and produces similar estimates. We discuss the potential and limitations of the method and suggest that this approach provides a useful tool for at least the preliminary analysis of similar datasets.

Grey seal (Halichoerus grypus) pup mortality not explained by local breeding density on North Rona, Scotland

Pre-weaning pup mortality of grey seals Halichoerus grypus in a breeding colony (North Rona, Scotland, U.K.) was examined in relation to the density of local adult females over two breeding seasons. Accurate, fine-scale maps of daily seal locations within a GIS database permitted the extraction of measures of female density determined at a range of spatial scales from 5 to 50 m around each pup on the date of death. Adult densities around live pups provided the null distribution of adult densities, which were tested against the distribution of densities where deaths occurred. Pre-weaning pup mortality at this colony was not related to any of our density measures. The proportion of pups dying each day showed no seasonal trends. Analysis of topographic features from a sub-meter digital terrain model for locations where deaths occurred suggested that there were fewer mortalities in prime pupping habitats. This could have been a direct effect of habitat or the relative quality of mothers in these habitats. The distance separating mothers and pups tended to be greater on the periphery of the colony compared to more central areas. Pups in the peripheral areas may be more exposed to predation by gulls.

Pup mortality in a rapidly declining harbour seal (Phoca vitulina) population

The harbour seal population in Orkney, off the north coast of Scotland, has reduced by 65% between 2001 and 2010. The cause(s) of this decline are unknown but must affect the demographic parameters of the population. Here, satellite telemetry data were used to test the hypothesis that increased pup mortality could be a primary driver of the decline in Orkney. Pup mortality and tag failure parameters were estimated from the duration of operation of satellite tags deployed on harbour seal pups from the Orkney population (n = 24) and from another population on the west coast of Scotland (n = 24) where abundance was stable. Survival probabilities from both populations were best represented by a common gamma distribution and were not different from one another, suggesting that increased pup mortality is unlikely to be the primary agent in the Orkney population decline. The estimated probability of surviving to 6 months was 0.390 (95% CI 0.297 – 0.648) and tag failure was represented by a Gaussian distribution, with estimated mean 270 (95% CI = 198 – 288) and s.d. 21 (95% CI = 1 – 66) days. These results suggest that adult survival is the most likely proximate cause of the decline. They also demonstrate a novel technique for attaining age-specific mortality rates from telemetry data.

The status of harbour seals ( Phoca vitulina ) in the United Kingdom

The UK holds approximately 40% of the European harbour seal population, with the majority found around the coasts of Scotland. Harbour seal populations in the UK have been monitored through a series of repeated aerial surveys of animals hauled out during the annual moult in early August. This moult count is used as a consistent index of population size. Survey methods and frequencies vary. The Scottish and English east coast populations mainly haul out in tidal estuaries and are surveyed annually, using fixed wing aircraft and digital photography. Populations in north and west Scotland often haul out on rocky shores and are surveyed less frequently, using helicopters fitted with thermal imagers. Overall, the most recent minimum estimate of the UK harbour seal population is 24,250 seals of which 19,800 are in Scotland, 3,200 in England and 1,250 in Northern Ireland. The results show that the number of harbour seals in eastern England was increasing before the 1988 and 2002 phocine distemper (PDV) epizootic but has not increased since the end of the 2002 epizootic. There is also evidence of a general decline in most of the large harbour seal colonies around Scotland. The populations along the north and northwest mainland coast were an exception, with numbers appearing to be stable. Between 2001 and 2008, the population in Orkney declined by 67% and Shetland declined by 40%, indicating harbour seals in these areas experienced substantially increased mortality or very low recruitment over this period. The widespread declines, ranging from Shetland to The Wash, suggest that the causes may have been present over a large part of the North Sea and waters off western Scotland.