Baltic grey seals (Halichoerus grypus) alternate between land and ice breeding, depending on ice conditions. We show that the fitness of grey seal females in terms of pup mortality and quality is reduced when breeding on land as compared with ice. The mean preweaning mortality rate on land was 21.1% (range 0% to 31.6%), and correlated with birth density (range 0.5–5.2 pups 100 m−2). The mean mortality rate on ice was 1.5%, where the highest density was 0.2 pups 100 m−2 in particularly dense breeding groups. Mean weights of pups born on ice were significantly greater (48.3 ± 8.1 kg) at the onset of moult as compared with pups born on land (37.4 ± 7.8 kg). Because indices of life-time net reproductive rate (pup survival) and pup quality (weaning weight and health) were more auspicious on ice as compared with land, diminishing ice fields will lower the fitness of Baltic grey seal females and substantially increase the risk for quasi-extinction.
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