Status of Baltic grey seals: Population assessment and extinction risk

Abstract

The grey seal (Halichoerus grypus) population in the Baltic Sea is recovering after a century of bounty hunting and 3 decades of low fertility rates caused by environmental pollution. A conservative estimate of the population size in 2003 was 19,400 animals, and available data suggest an annual rate of increase of 7.5% since 1990. The growing population has led to increased interactions with the fishery, and demands are being raised for the re-introduction of the hunt. We provide a demographic analysis and a risk assessment of the population, and make recommendations on how to decrease the risk of overexploitation. Although hunting increases the risk of quasi-extinction, the risk can be significantly reduced by the choice of a cautious hunting regime. The least hazardous regimes allow no hunting below a 'security level' in population size. Obviously, to implement such a hunting regime detailed knowledge of the population size and growth rate is required. It is not possible to estimate "true" risks for quasi-extinction, but we used an approach where the relative difference for different scenarios can be compared. With a security level at 5,000 females, the population quasi-extinction risk increases 50 fold at an annual hunt of 500 females compared with a scenario with no hunting. The risk of quasi-extinction is very sensitive to declines in the mean growth rate and to increased variance in growth rate. The variance in the population estimates over the last 14 years imply that it would take 9 years to detect a decline from 1.075 to 1.027 in the rate of population increase. We also show how the age composition of killed animals influences the impact of the hunt. The overall recommendation is that hunting should be kept to a minimum, carefully documented and accompanied by close population monitoring.Harding, K.C., Härkönen, T., Helander, B. and Karlsson, O. 2007. Status of Baltic grey seals: Population assessment and extinction risk. NAMMCO

Detecting Density Dependence in Recovering Seal Populations

“…1). We assume even sex ratios for all age classes (Harding et al 2007). The mean age at first parturition is termed age m (Fig.…”

“…The number of seals in each age (i) older than 1 can be obtained by multiplying the number of seals in the previous year class (i -1) the year before their survival N i(t?1) = pN i-1(t) , where p is either sub-adult or adult survival (Table 1). We parameterize the model with age-specific survival and fertility rates for Baltic gray seals (Harding et al 2007) (Table 1). K e (Eq.…”

INCREASEDCONTRACAECUM OSCULATUMINFECTION IN BALTIC COD (GADUS MORHUA) LIVERS (1982–2012) ASSOCIATED WITH INCREASING GREY SEAL (HALICHOERUS GRYPHUS) POPULATIONS

“…During the last 30 yr, the Baltic grey seal population has recovered from a few thousand to more than 20,000 (Harding et al 2007). Following this increase, grey seal distribution has expanded from a core area in the central Baltic during the last decade and increasing numbers have been observed in the southwestern Baltic, where several hundred animals are now observed at haul outs in Danish and Swedish waters (Sveegaard et al 2012).…”

Anthropogenics: Human Influence on Global and Genetic Homogenization of Parasite Populations

“…This is contrary to the phocids of the Arctic-Boreal region, whose population size has decreased dramatically from human activity, i.e., hunting, bycatch, viral diseases, etc. (Hauksson, 2002;Barron et al, 2003;Pastor et al, 2004;Di Guardo et al, 2005;Andersen et al, 2006;Harding et al, 2007). Similarly, Antarctic fish species belonging to the Channictiydae, Bathydraconidae, and Nototheniidae, and that function as intermediate hosts for these nematodes, have not been subject to the population reduction from overfishing that has occurred in host fish species, i.e., Gadus morhua, in the Boreal region found infected by the Arctic members of the C. osculatum complex (Cook et al, 1997;Masood, 1997).…”