CHANGES IN HEMATOLOGICAL PARAMETERS IN RELATION TO PREY SWITCHING IN A WILD POPULATION OF HARBOR SEALS

1. Previous studies have found marked inter-annual variation in winter food availability, diet composition and body condition in a populatio n of harbour seals (Phoca vitulina L.) in Northeast Scotland. This stu dy aimed to determine whether there were other physiological consequen ces of prey switching by comparing haematological parameters in years when the clupeids herring and sprat dominated the diet and in years wh en seals Switched to alternative prey. 2. There were significant diffe rences in leukocyte and erythrocyte parameters in relation to diet com position. In contrast, indices of body condition did not explain the v ariation in haematological parameters, suggesting that the observed ch anges did not result from differences in the energetic content of the prey. 3. Leukocyte counts were significantly higher after 'good' clupe id years, although the differences in mean counts were small. Such dif ferences could have resulted either from immune-suppression, for examp le because of differences in prey nutrient or contaminant levels, or f rom differences in the pathogen challenge resulting from geographical variations in water quality. 4. The differences in erythrocyte paramet ers were more marked, and there was evidence of widespread macrocytic anaemia when seals switched from clupeids to alternative prey. Such di fferences could result either from acclimation, as a result of prey-sp ecific foraging strategies, or from differences in the nutritional qua lity of prey. 5. These results indicate that generalist predators such as the harbour seal may exhibit physiological responses to changes in the composition of their diet, These data highlight the need to consi der the long-term physiological effects of variations in food availabi lity on the population dynamics of generalist marine top predators. In particular, it is hypothesized that fish-induced anaemia may be respo nsible for declines in certain pinniped populations.