The status of harbour seals ( Phoca vitulina ) in the UK

1. The abundance of harbour seals (Phoca vitulina) in the UK as a whole has increased over the past 10 years, after a 30% decline during the preceding 10 years and two major viral epidemics. However, population trends vary greatly among regions, with those on the east coast of Scotland and in the Northern Isles experiencing dramatic declines since the early 2000s and populations on the west coast being either stable or increasing. The reasons for these differences in population dynamics are unknown.2. Determining whether there has been a change in somatic growth among populations can assist in assessing potential causes for abundance declines, as shifts in juvenile growth rates or maximum length at maturity may indicate changes in environmental conditions. Resource limitations are likely to result in slower growth and later age at sexual maturity, whereas causes of acute mortality could have the opposite effect.3. Here, analysis of the most comprehensive length-at-age dataset for UK harbour seals found no evidence for major differences, or changes over time, in asymptotic length or growth parameters from fitted von Bertalanffy growth curves, across all regions, with the exception of one pairwise comparison; males from East Scotland were significantly shorter than males from all other areas by an average of almost 9 cm. However, the power to detect small changes was limited by measurement uncertainty and differences in spatial and temporal sampling effort. 4. Asymptotic lengths at maturity across all regions were slightly lower than published lengths for harbour seal populations in Europe, the Arctic, and Canada, with females being on average 140.5 cm (95% confidence interval 139.4, 141.6 cm) and males 149.4 cm (95% confidence interval 147.8, 151.1 cm) at adulthood. 5. Reliable estimates of changes in growth over time are important for understanding environmental constraints on a population, but knowledge of the underlying drivers of change is essential for the design of robust conservation and mitigation plans.

supporting

confidence: 52%

Section: Discussionmentioning

confidence: 97%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Age–length relationships in UK harbour seals during a period of population decline

Hall

Mackey

Kershaw

et al. 2019

Three‐dimensional movements of harbour seals in a tidally energetic channel: Application of a novel sonar tracking system

Hastie

Bivins

Coram

et al. 2019

Understanding how marine predators utilize habitats requires that we consider their behaviour in three dimensions. Recent research has shown that marine mammals often make use of tidally energetic locations for foraging, yet data are generally limited to observations of animals at the water surface. Such areas are also of interest to the renewable energy industry for the deployment of tidal‐stream energy turbines; this has led to concerns about potential impacts on marine mammals. Methods for measuring animal movements underwater are limited; however, active sonar can image marine mammals and could potentially measure three‐dimensional movements in tidally energetic locations. Here, a dual 720 kHz sonar system was developed to investigate the three‐dimensional movements of harbour seals (Phoca vitulina) in a tidally energetic channel. Estimated mean depth (distance from the surface) of seals was 12.0 m (95% confidence intervals [CIs]: 11.6–12.4 m), and the majority of time was spent at the surface and at approximately 10–12 m distance from the surface. When expressed as distances from the sea bed, mean distance was 18.5 m (95% CI: 18.0–18.9 m), and the majority of time was spent at 14 m from the sea bed. Seal movements were generally in the same direction as the tidal flow with mean horizontal speeds of between 0.51 and 3.13 m s−1 (95% CIs = 1.24–1.54 m s−1). Mean vertical velocities (where negative and positive values represent a descent and ascent respectively) for each seal track ranged between −1.76 and +0.88 m s−1 (95% CIs: −0.23 to +0.03 m s−1). These results provide a basis for understanding how seals utilize a dynamic tidal environment and suggest that harbour seal behaviour can be markedly different to less tidally energetic habitats. The results also have important implications for the prediction of risk associated with interactions between diving seals and tidal turbines in these dynamic habitats.

Monitoring long‐term changes in UK grey seal pup production

Russell

Morris

Duck

et al. 2019

Self Cite

1. The population size of many species, particularly those in the aquatic environment, cannot be censused directly. Counts, during the breeding season, of one component of the population (e.g. breeding females) are often used as an index to allow investigation of trends. In species, such as grey seals (Halichoerus grypus), for which births are not tightly synchronous, single counts of pups represent an unknown proportion of the total number of pups born (pup production), and thus of breeding females (i.e. each pup born represents a breeding female).2. Grey seals pup at large colonies around the coast of the UK. Information on their populations is required under national and international legislation.3. In the UK, pup production has been monitored at some colonies since 1956. Currently, large colonies (~90% of UK pup production) are monitored either using ground (~10%; annually) or aerial surveys (~80%; annually until 2010, and thereafter biennially).4. Here, the model used to estimate pup production at aerially surveyed colonies from 1987 to 2010 is described; structured pup counts from multiple surveys are combined with knowledge of life-history parameters to model birth curves.5. The resulting trends in pup production up to 2010 (aerially surveyed colonies) and 2016 (ground surveyed colonies) are examined.6. In 2010, over 45,000 pups were estimated to be born in the UK. Pup production appeared to have reached an asymptote in the Inner Hebrides, Outer Hebrides and Orkney, whereas it is still increasing exponentially in the North Sea. Although density-dependent processes acting at sea are likely to be responsible for these regional trends, we suggest that the substantial variation in trends within regions are likely caused by processes acting at the colony level. Some long-established colonies, including Special Areas of Conservation, are exhibiting decreasing trends. 7. Special Areas of Conservation often serve as de facto monitoring sites and are the focus of management efforts. The observed temporal and spatial variability in patterns of colony growth rates highlight the potential risks of using such sites to develop wider management policies.