Abstract
The Department of Energy and Climate Change (DECC) Offshore Energy Strategic Environmental Assessment (SEA) programme requires robust evidence to inform DECC policy development and execution.
Funded by multiple agencies, the Sea Mammal Research Unit, in collaboration with others, has deployed around 600 telemetry tags on harbour (Phoca vitulina) and grey seals (Halichoerus grypus) in the UK over the last 25 years. The objective of this project was to consolidate these data to allow combined analyses for the production of a high quality information base on the at-sea distribution, movements and behaviour of UK seals in the form of peer-reviewed papers.
To this end, all data were collated and consolidated into a managed database. Protocols were developed to clean all historical and incoming telemetry data in a consistent and effective manner. Protocols are also in place that ensures telemetry tags are monitored in real time. These procedures facilitate the use of telemetry data in a standardised format for various projects.
Three main areas were addressed in the subsequent data analyses. First the foraging distribution of grey seals was related to their breeding distribution. Second, using a state-space model we improved on current methods for defining activity budgets in seals by categorising four states: hauled out, resting at-sea, diving and foraging, and we related these budgets to intrinsic (sex, age), time invariant (region) and time variant (day of the year, time of day) covariates. Finally, we investigated the foraging habitat preference of both species.
Grey seals are capital breeders; they accumulate resources for breeding during the majority of the year and then do not forage while suckling their pups. Thus understanding where the effects of any given at-sea impact may be reflected in a breeding population ashore (especially at European and other conservation sites) is critical. This requires quantification of the movement of female grey seals between the foraging and breeding seasons. Along with survey count data in both seasons, telemetry data on regional transitions of individuals allowed quantification of regional transition rates between foraging and breeding seasons. We found that between 21 and 58% of females used different regions for foraging and breeding. For these animals any impacts of their environment, including anthropogenic effects, on reproductive success will not be apparent in the region of impact. Taking the estimated transition probabilities into account is particularly important when assessing the potential impact of off-shore developments on animals which breed on conservation sites such as Special Areas of Conservation.
The activity budgets of both grey and harbour seals in the UK were quantified using telemetry data from 63 and 126 individuals, respectively. Complete activity budgets, encompassing activity at-sea and on land, had never been quantified for these species. Previous investigations of at-sea activity budgets in grey seals elsewhere have defined only two activities: foraging and travelling. However, both species dive to forage and travel and we found that prolonged periods of non-diving behaviour was evident in both species, possibly related to food digestion. Without taking into account such non-diving behaviour, the proportion of time spent foraging and travelling may be incorrectly estimated and subsequently important foraging areas may be incorrectly defined. Thus in this study, behavioural data were used to define activities: resting behaviour (hauled out on land or at-surface in the sea) and diving. Movement data was then used to split diving into either foraging behaviour (which was defined by slow tortuous movements) or travelling (faster, more direct movement).
We found that both species spent a similar proportion of time in the four states (hauled out, at surface, foraging and travelling) but that their drivers differed. For example, the proportion of time resting in harbour seals, but not in grey seals, varied regionally. In grey seals the probability of foraging varied seasonally with both sex and age. To allow inclusion of data from both ARGOS and phone tags, the interval for which we defined states was 6 hours. To permit further investigation of activity budgets and increase the use of the results for other projects such as the building of energetic models, these models should be rerun on a finer time resolution with data from phone tags. Most phone tag data are from harbour seals and defining activity budgets on a finer temporal resolution is particularly important for this species as they engage in shorter trips than grey seals. It should be noted that for grey seals it is currently not possible to define high temporal resolution activity budgets (and thus determine the impact of the temporal resolution of the data on grey seal actively budgets) due to a lack of recent and thus high (temporal and spatial) resolution telemetry data. The activity budget data will be used to delineate the at-sea usage developed in the Scottish Government Project by activity state.
Habitat preference analyses allow an understanding of the environmental drivers of a species' distribution. In turn, this enables management to be concentrated in the most appropriate areas. Previous studies on habitat preference in UK seals have focused on grey seals only and were based on all at-sea locations. However, as central place foragers seals travel between land and foraging sites, and so preference for certain environments is likely to be related to activity state. For instance foraging habitat preference is likely to be related to the influence of the environment on distribution or catchability of prey species. Environmental drivers may include sediment type, depth and sea temperature. Such preference may not be evident when travelling. Thus in this project we examined habitat preference for both species using two types of data: (1) all at-sea locations and (2) only foraging locations. In both species and using both data types, all covariates were retained to explain at-sea distribution: distance from haul-out, depth, sediment type and sea temperature. However the shape of the relationship between the covariates and at-sea distribution differed between species and in grey seals differed with data type. In harbour seals which have shorter trips, there was little difference in results with data type. However, this result may be partly because of the temporal resolution of the activity data and thus should be reinvestigated once activity budgets are defined on a 2 hour resolution