The isotopic niche and diet of St. Lawrence Estuary beluga over two periods of contrasting environmental conditions revealed insights into their adaptability to ecosystemic changes

Jory Cabrol, Véronique Lesage, Ève Rioux

Despite the implementation of protective measures to reduce the risks of extinction, the St. Lawrence Estuary (SLE) beluga (Delphinapterus leucas) population is predicted to decline within a few generations unless more aggressive mitigation of current threats are undertaken. Most of those threats seem related to their foraging ecology, including among others the change in prey abundance, quality and availability due to major changes in the trophic structure under the synergetic influence of anthropogenic and climatic perturbations. Nonetheless, the current paucity of information available on their trophic ecology precludes us from determining its current capacity to face changing resources, but also to identify current key food resources. Thus, the main objective of this study was to determine: how the diet of SLE beluga has changed at the population and individual level in relation to prey environmental variations over the past three decades. To do so, we used a stable isotope (δ13C and δ15N) approach based on muscle tissue from 98 beluga found dead, and over >1000 specimens of their potential prey collected over two periods of contrasting environmental conditions: 1996—2003, which is post-groundfish collapse, and; 2015—2020, which is characterized by generalized low prey biomass, extreme warmth and low sea-ice conditions. Results revealed that the realized trophic niche width of the SLE population increased due to the emergence of extreme generalists and an increased variability in resources selection among conspecific. Quantitative analyses of diet indicate that although diet is still dominated by small pelagic fish (> 40%), several species such as smelt, sandlance and the functional group of tomcod/striped bass that were relatively unimportant in beluga diet in 1996—2003 increased in importance in the most recent years, leading to a diversification in resource use. Our results demonstrate that ecosystem shifts can profoundly reshape the diet of species, and that SLE beluga were able to adapt to some extent to these changes. Whether the observed changes in niche width and diet allowed SLE beluga to fully adapt to these environmental changes are uncertain; unusually high calf mortality and pregnancy-related female mortality reported since 2010 suggest that this might not be the case.