Demographic Structure and Mortality Rate of a Baltic Grey Seal Population at Different Stages of Population Change, Judged on the Basis of the Hunting Bag in Finland

Abstract

We examined the demographic structure and mortality rate of the Baltic grey seal (Halichoerus grypus) population from the early 2000s when the population increased rapidly to the late 2000s when the growth rate slowed down. We calculated life tables based on the age structure of hunted grey seals in the Finnish sea area. The catch was treated as a sample of the dying part of the population. The catch was male biased and the proportions of female pups and mature males in the catch increased from the early to late 2000s. Annual mortality rate of the youngest age classes was high and higher among males than females, which resulted in low sex ratio (males to females) of the population. Sex ratio was, however, higher in the latter years due to the increased mortality rate of females, especially those < 10 years of age. Accordingly, the proportion of mature females in the population decreased and relatively fewer pups were produced during the late 2000s than in the early 2000s.

A Novel Tool to Mitigate By-Catch Mortality of Baltic Seals in Coastal Fyke Net Fishery

“…Our study indicates that by-catch particularly increases juvenile mortality, which may not reduce population growth and viability as much as a similar increase in adult—especially female—mortality would [ 32 ]. Nevertheless, by-catch mortality may constitute another significant source of anthropogenic mortality in the grey seal population, whose annual hunting pressure in the Finnish sea area was around 5% of the total population [ 33 ], and should therefore be taken into account in the population management. By-catch mortality also increases total mortality of the ringed seal population and mitigating it may become increasingly important for population management, as the population growth of this ice-dependent species is predicted to decrease due to climate change [ 12 , 34 ].…”

Quantifying Contemporary Organic Carbon Stocks of the Baltic Sea Ecosystem

“…For mammals, we multiply the number of sighted individuals per area by a mean weight per individual. We calculate this mean weight based on age-and sex-distribution data of the Baltic Sea for gray seals [∼94 kg based on Lundstedt-Enkel et al (2008) and Kauhala et al (2012)] and harbor seals [∼42 kg based on Härkönen and Heide-Jørgensen (1990) and Härkönen et al (1999)]. To approximate the mean weight of ringed seals (∼58 kg), we use data on sex-and agedistribution from the northwestern coast of Hudson Bay and from Svalbard (Holst et al, 2006;Krafft et al, 2006).…”

Comparison of muscle and hair stable isotope ratios in three phocid seals

“…The tissue samples had been 4 collected from bycaught or stranded individuals (n=30) during the years 2002-2010 from freshwater Lake Saimaa. In Baltic ringed seals and gray seals molting time is in April-May (Härkönen et al, 2008;Kauhala, Ahola, & Kunnasranta, 2012). Baltic ringed seal samples included both pre-molt (April) and post-molt hair (May-June) while all gray seal samples were from the post-molt period (May-October).…”