Health and age and sex distributions of Baltic grey seals (Halichoerus grypus) collected from bycatch and hunt in the Gulf of Bothnia

Abstract

Bäcklin, B-M., Moraeus, C., Roos, A., Eklöf, E., and Lind, Y. 2011. Health and age and sex distributions of Baltic grey seals (Halichoerus grypus) collected from bycatch and hunt in the Gulf of Bothnia. – ICES Journal of Marine Science, 68: 183–188. Age distribution, sex ratio, and season of death were investigated in grey seals taken as bycatch and hunted in the Gulf of Bothnia for the period 2002–2007. In seals 1–20 years old, certain health variables, earlier reported as parts of a disease complex related to persistent organic pollutants, were also investigated. Most of the hunted group were female (60%), but there were more males (62%) in the bycatch group. In spring, there were more bycatch than hunted yearlings. Seals taken as bycatch were leaner than hunted animals, and in hunted seals, there was a significant seasonal difference in blubber thickness that was not observed in seals taken in the bycatch. The prevalence of intestinal ulcers was 18–50% in bycatch and 24–74% in hunted animals. Hunted males 4–20 years old had a significantly greater prevalence of intestinal ulcers than bycatch males in the same age group. Adrenocortical thickening and mean weight of the adrenals increased with age. No lesions were observed in female reproductive organs, and in autumn, nearly all mature females were pregnant. The main differences between hunted and bycatch grey seals were in the season when death occurred, the prevalence of yearlings, the sex ratio, the blubber thickness, and the prevalence of intestinal ulcers.

Age, Sex and Body Condition of Baltic Grey Seals: Are Problem Seals a Random Sample of the Population?

“…The means of the measurements did not differ significantly (42.4 ± 9,6 vs. 42,2 ± 10,4) between the hunters and SMNH. This indicates that the mean measurements of blubber thicknesses were comparable (Bäcklin et al 2011). …”

“…Stranded seals often show a thin blubber layer (starvation due to disease or old age) and by-caught seals are often thinner than seals received from hunt (Bäcklin et al , 2011. Therefore, these groups are suggested to be presented separately (Figure 1) since their proportions will influence the GES determination.…”

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

“…Unfortunately, little data on the age structure of seals caught as bycatch or stranded are available. According to a fairly small data (n = 48 females and 78 males) from Sweden from 2002(Bäcklin et al 2011, 77% of females caught as by-catch were < 11 years old, which is a higher value than the proportion of females < 11 years in the Finnish hunting bag (72% for [2003][2004][2005][2006][2007]. If the number of females caught as by-catch is high, then the true mortality rate of females (< 10-11 years) may be even greater than the values given in our life tables.…”

“…During a study on the Norwegian coast, even 25% of tagged grey seal pups were caught as by-catch in fishing gear (Bjørge et al 2002). In Sweden, 62% of seals caught as by-catch were males (Bäcklin et al 2011). It is thus possible that the proportion of pups (especially males) is slightly underestimated in the catch when individuals caught as by-catch are not taken into account.…”

“…Most likely due to environmental toxins, Baltic grey and ringed seal (Phoca hispida botnica) females suffered from severe reproductive disturbances in the 1960s and 1970s (Bergman & Olsson 1986, Bergman 1999, Harding & Härkönen 1999, Nyman 2000, Bäcklin et al 2003, Nyman et al 2003, Bergman 2007) and productivity of the seal populations was low during several decades. Due to the decreased levels of organochlorines in the Baltic Sea during recent decades reproductive health of the seals improved, and today the pregnancy rate of grey seals is supposed to be normal (Bäcklin et al 2010(Bäcklin et al , 2011. Consequently, the population increased at an annual rate of 7.5%-8.5% since 1990 until recent years (Harding et al 2007) and the population size is at present (2010) at least 23 000 individuals (Kunnasranta 2010).…”