Abstract
The size of the year 2000 summer population of grey seals in the Baltic Sea was estimated using identification of individual seals from photographs taken over a period of 6 years. Photos were taken at haul-out sites within all major grey seal areas in the semi-closed Baltic Sea. The point estimate is 15,631, based on a value for annual survival of identification markings of 0.904, which was also estimated using the photo-id data, with 95% confidence limits from 9,592 to 19,005. The estimate is subject to an unknown, but probably small, upward bias resulting from the risk of failure to identify all individuals in the photographs used for the analysis. An estimated minimum of 15,950 seals were counted at moulting haul-outs in 2003, which thus provides a lower bound on the population size in that year and represents 80% of the photo-id point estimate.
Highlights
Grey seals (Halichoerus grypus) were once numerous in the Baltic Sea
The results presented below were based on solution of the following set of equations that incorporate correction factors for losses from the marked population and for selection in favour of well-marked seals by the photographer but not for the risk of missing released animals in the recaptures samples: Σj (E[ΣvΣst Rij,st,v] / Q ΣvΣst Smd(s,t)Ni,s,vN’j,t,v) Pj,2000 = 1 (4)
We used that value to calculate the year 2000 population size point estimate, as the data were restricted to a subset of the Swedish haul-outs we can not be certain that this value is applicable to the whole Baltic
Summary
Grey seals (Halichoerus grypus) were once numerous in the Baltic Sea. A mere 100 years ago the population was estimated, based on hunting statistics, to comprise approximately 88,000 to100000 animals (Harding and Härkönen 1999). The current monitoring strategy was developed during the 1970s when the situation for Baltic grey seals was critical and management was directed to saving the few seals that remained. The main focus of monitoring at the time was towards the detection of population trends while the actual size of the population was of lesser interest. With a growing grey seal population and a management strategy that includes culling as a way to mitigate seals/fisheries conflicts, there is an increased need to estimate the size of the population
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
