Changes in haematological parameters in relation to prey switching in a wild population of harbour seals

Abstract

1. Previous studies have found marked inter‐annual variation in winter food availability, diet composition and body condition in a population of harbour seals (Phoca vitulina L.) in Northeast Scotland. This study aimed to determine whether there were other physiological consequences of prey switching by comparing haematological parameters in years when the clupeids herring and sprat dominated the diet and in years when seals switched to alternative prey. 2. There were significant differences in leukocyte and erythrocyte parameters in relation to diet composition. In contrast, indices of body condition did not explain the variation in haematological parameters, suggesting that the observed changes did not result from differences in the energetic content of the prey. 3. Leukocyte counts were significantly higher after ‘good’ clupeid years, although the differences in mean counts were small. Such differences could have resulted either from immuno‐suppression, for example because of differences in prey nutrient or contaminant levels, or from differences in the pathogen challenge resulting from geographical variations in water quality. 4. The differences in erythrocyte parameters were more marked, and there was evidence of widespread macrocytic anaemia when seals switched from clupeids to alternative prey. Such differences could result either from acclimation, as a result of prey‐specific foraging strategies, or from differences in the nutritional quality 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 consider the long‐term physiological effects of variations in food availability on the population dynamics of generalist marine top predators. In particular, it is hypothesized that fish‐induced anaemia may be responsible for declines in certain pinniped populations.

Contaminant Loads And Hematological Correlates In The Harbor Seal (Phoca vitulina) Of San Francisco Bay, California

“…The relationships observed here between hematological parameters and contaminants may be best seen as indicative of sentinel responses to contaminant exposure, responses that were not necessarily pathologic. Hematological parameters of marine mammals also can be affected by parasite load, diving behavior, stress, dietary shifts, and other individual differences (Bryden & Lim, 1969;Roletto, 1989;Geraci & Medway, 1993;Thompson et al, 1997). If such factors covaried with contaminant levels, it would not be possible to distinguish their relative contributions or causal proximity to alterations in hematological parameters.…”

Seasonal variation in the diet of harbour seals in the south-western North Sea

“…However, nutritional quality and diet composition may also be important. Thompson et al (1997b) reported that in years when harbour seal diet in the Moray Firth was dominated by gadoids and cephalopods, health and body condition indices in adults and yearlings were significantly lower, compared to years when clupeids were the most important prey species. These nutritional requirements may also be important factors affecting prey selection and foraging in harbour seals.…”

Satellite tracking reveals unusual diving characteristics for a marine reptile, the olive ridley turtle Lepidochelys olivacea

“…As such, species with generalist diets, variable foraging behaviours and many reproductive strategies tend to be buffered from catastrophic environmental shifts that put specialist species at a higher risk of extinction when population sizes are low (Pimm 1991, McKinney 1997, Kotze & O'Hara 2003. In the marine environment, plasticity in foraging behaviour and diet has been demonstrated for many different higher vertebrate taxa including seabirds (Votier et al 2004), marine mammals (Thompson et al 1997, Harcourt et al 2002 and some fish species (Ellis et al 1996, Garrison & Link 2000, Bellwood et al 2006.…”