Population dynamics, behaviour, and spatial distribution of grey seals during the breeding season at the Farne Islands.
Grey Seals (Halichoerus grypus) are long lived, iteroparous marine mammals that are found in three global populations within the North Atlantic Ocean and the Baltic Sea. They are the largest of the two seal species found within UK waters and have a much larger population (Grey seals 157300, 95% CI 144600-169400, Harbour seals 43750, 95% CI 35800-58300) and comprise a large portion of the global population (SCOS 2021). Under long standing legislation (UK and EU), they are a protected species and form large aggregations during the breeding season. During this time sexually mature females typically produce a single pup, providing sole parental care until the pup weans, around 18 days after birth. Within this period a mother produces energy rich milk which strains her already limited energy budget, due to the capital breeding reproductive strategy grey seal’s exhibit. Therefore, any behaviour displayed during this time must be chosen carefully to benefit the mother or pup so finite resources are not wasted (Shuert et al 2018).
The Farne Islands are a group of islands off the coast of Northumberland, UK on which approximately 3,500 Grey Seal pups are born each year. The islands comprise of two main groups: an inner group which lies approx. 2 km from the mainland and an outer group that is located a further 2km offshore. The seals have been monitored, in terms of their numbers, since the 1950s; most recently through the use of Unmanned Aerial Vehicles (UAVs). A distinct change in the distribution and phenology of the seals has been observed over the past 10 years. Until 2011, female seals almost exclusively pupped on the outer group of islands but have now started pupping on the inner group. There has also been a movement within the outer group, as the main islands for pupping are now Brownsman and Staple Islands, instead of North and South Wamses of a few years ago. Coupled with this change in seal pupping distribution, has been a change in the phenology of the breeding seals with the mean pupping date becoming increasingly late. The phenological changes are linked to the distributional changes as the seals on the inner group are pupping later than those in the outer group. Phenological changes in lifetime events have been recorded in a variety of species, and these have often been linked to climate change and disruption of ecological interactions (e.g. Mayor et al 2017). These phenological changes may be linked to the condition of the females, as those in poorer condition, may pup only when they are in good condition.
Body condition of marine mammals is notoriously difficult to undertake, but with the advent of UAVs, it is possible to use photogrammetry to estimate a variety of morphological measurements in mammals. The technique has been used for marine vertebrates that congregate on land to breed or rest, such as the pinnipeds (Jarod et al 2020)
It is not just mother and pup pairs that are present on haul out sites during the breeding seasons, males, both tenured and untenured, are present to breed with females as they come into oestrus when they wean their pup (Boness & James 1979). Interactions between males and females during this time are typically seen as aggressive, but limited knowledge is known of their interactions post-breeding. In addition to males, juvenile seals of both sexes have often been seen to congregate around breeding sites. The seal pups move away from the natal areas to avoid interacting with the adult seals, in many cases the pups move to different islands, a journey that puts them at risk. In the past, the pups on the Farne Islands have had a higher mortality rate than other pupping locations in the North Sea, but the reasons behind this mortality have not been known.
The behaviours and distribution of seals at fine scales have not always been prioritised under legislation and in recent research, however these behaviours could explain intra-colony population dynamics and interactions between portions of the populations e.g., juvenile-adult relationships. Given the importance of the breeding season to a population’s health and survival, with the additional stress of capital breeding, these fine-scale distributions and interactions become necessary to study and understand.
This project aims to address the fine scale spatial distribution of grey seals during the breeding season to understand patterns of haul outs, island composition and behavioural interactions. The Farne Island colony will be investigated using their terrestrial behaviours, interactions and haul out site choice to inform management of the species during this crucial life stage. Post weaning behaviours, locations and movements of pups will also be assessed to address current data gaps.
Objective 1: Use Timelapse cameras to record the behaviour of female grey seals and their interactions with their pups on 4 different islands (Brownsman and Staple Islands in the outer group, and Inner Farnes and West Wideopens in the inner group)
Objective 2: Use UAV surveys to monitor the distribution of seals (females, males, and pups) and how this varies over the pupping season.
Objective 3: Use high-definition photos from UAV to monitor the body condition of the seals during the pupping season. These will investigate how the breeding season and location of the seals has an impact on the individual seals
Objective 4: Monitor the post-weaning movements of grey seal pups.