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The effect of prey quality and ice conditions on the nutritional status of Baltic gray seals of different age groups

Abstract

We analyzed a long-term data set of the body condition of Baltic gray seals (Halichoerus grypus) over time and investigated how average subcutaneous blubber thickness of different age groups of seals corresponds to environmental factors. Blubber thickness of pups declined until 2010. The decreasing weight of 5–6-year-old herring (Clupea harengus), the main prey fish for Baltic gray seals, explained well the decline. In the Gulf of Finland, the blubber thickness of pups declined also in recent years (2011–2015) with declining number of days with permanent ice cover. In other regions, the blubber thickness of pups increased during recent years with increasing weight of herring. The blubber thickness of sub-adults in Baltic Proper and that of hunted adult females in the Bothnian Bay also increased during recent years, and the weight of age 6+ or 7-year-old herring best explained the increase. The blubber thickness of all age groups of seals was thinnest in the Bothnian Bay where also herring weight was lowest. There was a negative correlation between blubber thickness of seals and herring catch size (an index of herring abundance) suggesting that herring quality, not the quantity, is important for the nutritional status of Baltic gray seals. Nutritional status of gray seals may thus reveal changes in the marine food web which affect herring quality. Marine food web, in turn, may be affected, e.g., by climate change. The warming climate also has an impact on ice cover and thus body condition of seal pups.

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Acknowledgements

We are grateful to J. Vainio from the Finnish Meteorological Institute for providing data of the ice conditions in the Baltic Sea. We wish to thank all hunters and fishermen who sent us samples of seals, and C. Moraeus and P. Timonen who helped us in the laboratory.

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Correspondence to Kaarina Kauhala.

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Communicated by: Cino Pertoldi

Appendix

Appendix

Table 6 The interactions (regression analysis) between the mean weight of herring or ice conditions (independent variables) and the blubber thickness of gray seals. Results with p < 0.10 and F > 4.0 are included. All interactions are positive. Data for 2002 (for sub-adults also 2003) were excluded due to small sample sizes. Ice conditions = the number of days with permanent ice cover between 15 February and 15 March. BP = Baltic proper, GF = the Gulf of Finland, BS = the Bothnian Sea, BB = the Bothnian Bay. Subscript in the right column: S = southern regions (BP + AS + GF). Pups: < one year old, sub-adults: 1–3 years old, adults: > three years old
Table 7 Correlations between herring catch size and blubber thickness of gray seals. Results with p < 0.10 are included. BP = Baltic proper, GF = the Gulf of Finland, BS = the Bothnian Sea, S = southern regions (BP + AS + GF). Pups: < one year old, sub-adults: 1–3 years old

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Kauhala, K., Bäcklin, BM., Raitaniemi, J. et al. The effect of prey quality and ice conditions on the nutritional status of Baltic gray seals of different age groups. Mamm Res 62, 351–362 (2017). https://doi.org/10.1007/s13364-017-0329-x

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  • DOI: https://doi.org/10.1007/s13364-017-0329-x

Keywords

  • Baltic food web
  • Blubber thickness
  • Climate change
  • Halichoerus grypus
  • Herring