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

Abstract

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.

Allo-suckling occurrence and its effect on lactation and nursing duration in harbour seals (Phoca vitulina) in Orkney, Scotland

“…Lack of maternal experience has been suggested as a contributing factor to fostering and allo-suckling in pinniped species, including harbour seals (Boness et al 1992). It was not possible to distinguish young, inexperienced harbour seal mothers from older females based on their body size in the photo-identification data, because female harbour seals will reach 90% of their asymptotic length (indicative of sexual maturity ) by the age of 3.2 years (Hall et al 2019). However, 11 of the breeding females included in this study could be aged from counts of growth layer groups in the incisors (Hall et al 2019) when captured and sampled as part of a broader research project in and 2017(Arso Civil et al 2018).…”

“…It was not possible to distinguish young, inexperienced harbour seal mothers from older females based on their body size in the photo-identification data, because female harbour seals will reach 90% of their asymptotic length (indicative of sexual maturity ) by the age of 3.2 years (Hall et al 2019). However, 11 of the breeding females included in this study could be aged from counts of growth layer groups in the incisors (Hall et al 2019) when captured and sampled as part of a broader research project in and 2017(Arso Civil et al 2018). These included 4 females aged 4 to 14 (mean = 10.1) in years observed suckling their own pups only, and seven females aged 5 to 18.5 (mean = 10.7) in years observed allo-suckling other pups.…”

Increasing numbers of harbour seals and grey seals in the Solent

“…This included the following: moving surveys to an entirely boat‐based platform; conducting surveys concurrently at both harbours to remove the chance of individuals being counted twice; undertaking surveys at least once a month between May and September to capture the harbour seal pupping season (with additional months surveyed on an opportunistic basis); and recording the approximate age of individuals (adult, subadult/juvenile, or pup) where possible. It can be challenging to assign an age cohort from observation alone (except for newborn pups), especially for harbour seals; despite females having lower overall lengths at maturity, they display higher early growth rates than males (Hall et al., 2019). Hence, subadults and juveniles were combined into one category.…”

Evolutionary history of a Scottish harbour seal population

“…Archived blood samples were collected from 93 harbour seals that had been captured, sampled and released as part of earlier ecological studies (Thompson et al, 1997b;Hall et al, 2019). Samples were collected between 1992 and 1995 (see Thompson et al, 1997b for details of capture and sampling methods), and constituted 47 females and 46 males (48 juveniles, 37 adults, 8 sub-adults, data are available at https://doi.org/10.15454/AOZ7JI); representing approximately 6% of the population.…”