Monitoring long‐term changes in UK grey seal pup production

Abstract

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.

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

“…show different dynamics to those of harbour seals (Russell, Morris, Duck, Thompson, & Hiby, 2019;Thomas et al, 2019). Both pup production and population estimates for grey seals in the West Scotland and the Western Isles SMUs were increasing until the mid-1990s…”

From pup to predator: generalized hidden Markov models reveal rapid development of movement strategies in a naïve long‐lived vertebrate

“…Although predation is a limiting factor on survival of many newly-independent animals, such as cheetahs Ancinonyx jubatus (Laurenson 1994) and passerine birds (Sullivan 1989), and predation events on weaned grey seal pups have been recorded in the UK (Brownlow et al 2016), there is little evidence that predation has a population-level impact on first-year survival in UK grey seals. Indeed, an agestructured Bayesian model revealed that the grey seal population in Orkney reached apparent carrying capacity in the early 2000s, driven by density-dependence acting on pup survival (Thomas et al 2019), likely through processes at-sea related to food availability (Russell et al 2019). Furthermore, it is estimated that newly-weaned pups have an average of 36 days to feed successfully before their protein reserves are critically depleted (Bennett et al 2007).…”

The diet of harbour and grey seals around Britain: Examining the role of prey as a potential cause of harbour seal declines

“…The abundance of grey seals around Britain has been increasing since the 1960s and the total population was estimated at 141,000 (95% CI 117,500-168,500) in 2016 (SCOS, 2017). Regionally, numbers have been more or less stable on the west of Scotland since the 1990s and in Orkney since the 2000s, albeit with some interannual variation; however, numbers continue to increase in the North Sea (Thomas, Russell, Duck, Morris, Lonergan, Empacher, Harwood, 2019;Russell, Morris, Duck, Thompson & Hiby, 2019).…”