Harbour seal population trends in Scotland
The Sea Mammal Research Unit (SMRU) has been conducting surveys since the late 1980s to monitor the populations of harbour seals in Scotland. The aerial surveys occur in August during the annual moult, when the greatest and most consistent numbers of seals are found ashore. These counts provide with a minimum number of harbour seals in each area, which is an index of population size.
SMRU aerial survey counts have shown a decline in the numbers of harbour seals around the east and north coast of Scotland and in the Northern Islands since around 2000. For example, harbour seal numbers have declined by around 93% in the Tay estuary (East Coast) since 2002, by approximately 76% in Orkney since 1997, and by around 40% in 2002, although counts have remained stable since then. In contrast, populations on the West Coast and in the Western Isles are either stable or increasing (see map on the right).
SMRU has been funded by Scottish Government to investigate the causes of these declines in harbour seal numbers. The current project focuses on estimating the survival and the birth rates for harbour seals at sites that show contrasting population trajectories. These results will be combined with information on potential drivers of population change that have not been excluded as factors affecting harbour seal survival and birth rates. These include:
- prey quality and availability
- grey seal population size and mortality caused by grey seals
- the occurrence and exposure of seals to toxins from harmful algae
The current research aims to get a better understanding of vital rates and population dynamics of harbour seal populations, including the spatial overlap between grey and harbour seals, and ultimately improve our understanding of the main (potential) drivers of the decline.
The project will focus in sites with contrasting population trajectories. Data will be collected mostly during spring and summer, focusing the effort during the breeding season, in June and July. In August, when the harbour seals moult and spend more time ashore, we will run aerial surveys to do a moult count. In each of the study sites we will collect the following data:
- Photo-identification – Harbour seals can be individually identified based on the pattern of their pelage (see image on the right). By visiting certain sites regularly over consecutive years and taking photographs of individual seals we can construct sighting histories of individuals. These can be then used to estimate vital rates. The photographs and observations will also help us determine which females have pups every summer, to estimate birth rates.
- Ground counts – the number of seals on shore and in the water will be counted at the visited haulouts, which will give us information on the haulout usage during the breeding season and which haulouts females choose to have their pups.
- Individual covariates – A limited number of individuals will be captured and released in selected sites to collect individual covariates (e.g. sex, age, condition, pregnancy status, health status, toxin exposure), and investigate their effect on vital rates estimates. These live captures will also provide an opportunity to radio tag a sample of seals using telemetry tags developed by the SMRU Instrumentation group . The deployed tags will inform on the animals use of haulout site, which will help us direct subsequent photo ID effort.
- Seal scat samples – where possible and without causing undue disturbance at the haulout sites, we will collect scat samples, which can inform us about the seals’ diet and their exposure to toxins.
- Seal carcasses – any seal carcasses found will be examined post-mortem to estimate cause of death in different regions. This work will be coordinated with the Scottish Marine Animal Stranding Scheme.