Processed data is only available for grey seals, which will be evaluated in the following. Results are based on combined Swedish and Finnish data. Future evaluations will include German, Estonian and Polish data.
Figure 12. Baltic grey seals do not attain GES both with regard to nutritional status, since observed data fails the GES limits 40mm for hunted seals and 35mm for by caught seals. However, current data suggest that grey seals are approaching the carrying capacity of the system, where the GES boundary of 25 mm is applicable.
A strict Bayesian analysis has not been carried out yet, but it is evident that such an analysis will support GES for time periods 1993-2001 and 2002-2004 for data in Figure 13, whereas it would fail for data 2008 and later for both hunted and by-caught grey seals, when tested against GES boundaries (40 and 35 mm for hunted and by-caught seals respectively) for populations experiencing exponential growth. (However, it is suggested that GES would be attained in both cases when testing against a prior at 25mm, which is applicable in populations close to carrying capacity.)
Figure 13. Grey seals. The mean fall/winter blubber thickness ± SD in examined 1–3 years old non-pregnant by-caught (1993–2011) and hunted (2002–2010) grey seals in Sweden. All were by-caught or shot between August and February. The decrease is significant (p<0.002). N is the number of investigated animals. Green denotes the GES boundary for populations at carrying capacity.
Decreasing blubber thickness in both hunted and by-caught grey seals are statistically significant.
Although data still is too scarce to establish a GES-boundary for ringed seals, data indicate that also the nutritive condition of ringed seals is deteriorating (Figure 14). Decreasing blubber thickness is seen both in juveniles and adults (Figure 14)
Figure 14. Ringed seals. The mean fall/winter blubber thickness ± 95% CI in examined 1–3 and 4–20 years-old animals (bycaught or shot). GES boundary has not been agreed but suggested as 35.6 mm and 51.4 mm for young and adult, respectively. Number of samples is given beside the means.
Considerable and sufficient material is collected annually for grey seals in Finland and Sweden, so confidence is high for this particular species in the central and northern part of the Baltic. The samples also include Swedish material from the southern Baltic, but it would be desirable to include samples from Denmark, Germany and Poland.
High confidence for grey seals is supported by earlier studies which have shown that the autumn/winter blubber thickness has decreased significantly in Baltic grey seals since the beginning of 2000s, especially in 1-4 year-old seals from by-catch and hunt (Bäcklin et al., 2010). This decreasing trend has also been observed in young Baltic ringed seals (Kunnasranta et al., 2010). There could be several reasons for a thin blubber layer in the autumn/winter season e.g., disease, contaminants, decreased fish stocks and change in diet, or a change in the quality of the diet. The reason for the decreasing trend in blubber thickness in seals is unknown but so far no correlations to disease have been found. Data is still scarce for ringed seals in both management units, resulting in low confidence in this species. For harbour seals material is collected annually, but needs to be compiled and analyzed further.