A 28-year record of the isotopic niche of baleen whales in the Gulf of St. Lawrence, Canada: a perspective on ecosystem changes and potential for food competition

Fin whales ( Balaenoptera physalus ), humpback whales ( Megaptera novaeangliae ) and minke whales ( Balaenoptera acutorostrata ) seasonally coexist in sympatry in the Gulf of St. Lawrence (Canada) where they feed to replenish their energy reserves. Over the past decades, these three species have experienced significant shifts in resource availability as the St. Lawrence ecosystem encountered major trophodynamic changes due to climatic and anthropogenic perturbations. This study aimed to understand how the realized trophic niche of these rorqual species has changed over time. To achieve this objective, stable nitrogen and carbon isotope ratios from 1110 whale skin biopsies sampled between 1992 and 2019 were used to define the isotopic niche of each species, quantify their diet using Bayesian isotopic mixing models, and assess the degree of individual diet specialization. Resource partitioning among these three sympatric species increased during the 2011–2019 period, as highlighted by the limited overlap observed among their isotopic niches. A recent dietary shift toward an increased reliance on pelagic fish (capelin, herring and/or mackerel) in fin whale and minke whale and a reduced contribution of krill suggests a possible reduction in krill abundance in the Gulf of St. Lawrence in recent years. These findings provide a unique insight into the ability of three generalist species to coexist through partitioning food resources, and adapt to ecosystem changes. Given the climatic context, knowledge of preferred prey is crucial for the conservation of these species.