Marine mammals and the community structure of the Estuary and Gulf of St Lawrence, Canada: evidence from stable isotope analysis

The trophic relationships of both the benthic and pelagic communities in th e Estuary and Gulf of St Lawrence regions were examined, with a special foc us on the trophic position (TP) and relationship(s) among harbour, grey, ho oded and harp seals and beluga whales. A multiple stable isotope and multip le tissue approach, used in:conjunction with conventional dietary informati on, suggested that marine mammals occupied the highest trophic positions in the food webs of both communities and that they overlapped with one anothe r to some extent trophically. Harbour seals Phoca vitulina and hooded seals Cystophora cristata occupied the highest TP, grey seals Halichoerus grypus , Gulf harp seals Phoca groenlandica, and male beluga whales Delphinapterus leucas were intermediate, and Estuary harp seals and female beluga whales were at the lowest TP. A general pattern of increasing enrichment of C-13 o r N-15 with age was observed in marine mammals las well as fishes), althoug h yearlings showed a decreased enrichment compared to both younger and olde r age classes. Sex also influenced delta N-15 values. Males were more N-15- enriched than females, with the difference between the sexes increasing wit h age, and being most pronounced in species that are sexually dimorphic wit h respect to body size. Geographical location also influenced isotope abund ance. Estuary organisms were generally C-13-enriched relative to Gulf anima ls. delta C-13 values were on average lower in short-term diet integrators (blood serum) than in longer-term diet integrators (red blood cells) of har bour seals captured in April to June in the Estuary, which suggests that th ey probably did not move outside the Lower Estuary during the winter. Grey seals captured in the Lower Estuary did, however, show evidence of having b een in the Gulf region some weeks or months before capture.