What roles do marine mammals play in St. Lawrence ecosystems?
Marine mammals are basically predators, but their importance in marine ecosystems is often not well known. Information concerning marine mammal feeding habits and the structure of the food web they belong to is necessary to better understand their roles in the St. Lawrence.
To go through the looking glass
The study included four species of seals (harbour seals, grey seals, hooded seals and harp seals) and one cetacean; the beluga whale. Between 1995 and 1997 muscle or blood samples were taken from animals from the St. Lawrence Gulf and Estuary. These samples came from carcasses or, in the case of seals, from live, captured animals. The position of each species in the food web was evaluated by measuring the abundance of certain stable isotopes, 13C et 15N, in relation to their more common forms, 12C et 14N. The isotopes 13C et 15N accumulate in a predictable way at each level of the food web and enable researchers to obtain information on the type of prey ingested.
Marine mammals examined occupy the highest positions in the food webs of the Gulf and Lower St. Lawrence Estuary. The harbour seal and the hooded seal occupy the highest trophic levels, followed by the grey seal, the harp seals of the Gulf and male beluga whales. Harp seals that visit the Estuary in winter and female beluga whales occupy lower trophic levels.
Marine mammals that feed in the Gulf do not have the same ratio 13C/12C as those that feed in the Estuary. The technique employed for this study can therefore be used to detect seasonal movements. For example, the harbour seals studied likely resided in the Lower Estuary over the course of the winter preceding the sampling, while the grey seals captured in the Lower Estuary had visited the waters of the Gulf before they were captured.
Although certain species studied sometimes consumed the same type of prey, seasonal movements or habitat preferences differed from species to species. Competition for food resources is likely not heavy between these species.
This study served to construct a frame of reference to better understand the complex ecosystems of the maritime portion of the St. Lawrence River.
Véronique Lesage, Maurice Lamontage Institute/Fisheries and Oceans Canada and University of Waterloo, Mike O. Hammill, Maurice Lamontagne Institute/Fisheries and Oceans Canada, and Kit M. Kovacs, Norwegian Polar Institute and University of Waterloo.
Department of Fisheries and Oceans Canada — St. Lawrence Vision 2000, Parks Canada — Green Plan, University of Waterloo, St. Lawrence National Institute of Ecotoxicology (SLNIE), Fonds pour la formation de chercheurs et l’aide à la recherche (FCAR), Canadian Wildlife Service