Changes in the Diet of St. Lawrence Belugas

Jory Cabrol, a researcher at the Maurice Lamontagne Institute (MLI) specializing in feeding behaviour ecology and lead author of the study, explains that pelagic fish (article in French) – including capelin – were still amongst the most frequently consumed prey by individual belugas between 2015 and 2020. However, their proportion in the diet is lower compared to data collected between 1997 and 2003.

Dr. Cabrol mentions that striped bass, rainbow smelt and Atlantic tomcod (webpages in French) have become more important in the beluga’s diet at the expense of capelin and herring, two lipid-rich pelagic species that are currently in decline. In the early 2000s, significant differences were observed between the diets of males and those of female-calf herds due to spatial segregation during the summer months. These dietary differences were less pronounced during the second study period (2015-2020).

Belugas “eat different proportions [of prey], but are similar in terms of specialization, regardless of sex. There is less diversity between individuals, as all individuals are tending to become a little more generalist,” summarizes Jory Cabrol. In 2021, a DFO study reached a similar conclusion for fin whales in the St. Lawrence Estuary: a marked trend toward a more generalist diet, believed to be due to changes in the ecosystem.

To arrive at these results, muscle tissue was sampled from 96 beluga carcasses. Recovered from the shores of the St. Lawrence between 1997 and 2020, only individuals over two years old were used to avoid dietary biases related to nursing. With these samples, DFO scientists performed what is called stable isotope ratio analysis. These isotopes are different forms of the same carbon, oxygen, sulphur or even nitrogen atom. Stable isotope analysis was carried out to determine whether belugas had a more general or specialized diet.

Samples of prey from the stomachs and digestive systems of the beluga carcasses were also taken. Prey abundance surveys and observations of feeding behaviours also provided insight into the diets of these marine mammals.

In 1944, biologist Vadim Vladykov noted that pelagic fish were an integral part of the St. Lawrence beluga’s diet. Today, DFO scientists agree that the species that once filled the belugas’ stomachs are no longer the same. The conclusions drawn from this new study raise several questions, particularly regarding the beluga’s chances of recovery. Will it be able to adapt to changes in its environment? Will it be able to find enough lipid-rich food to ensure its survival?

In September 2024, the DFO published a report entitled Recovery Potential Assessment for the St. Lawrence Estuary Beluga (Delphinapterus leucas) Population. The report stated that the risk of near-extinction of the St. Lawrence beluga in conditions where water temperatures rise by another 0.5°C over the next 100 years would be extremely low (0.3%). On the other hand, there is a 61% chance that the population would fall to a critical level, i.e. fewer than 1,600 individuals.

As the species preyed upon by the St. Lawrence beluga are more numerous and diverse than they were before, however, protection is becoming complex, adds feeding behaviour ecology specialist Jory Cabrol. “It is essential to have continuous data over very extended periods to fully understand and quantify the influence of ecosystem changes on beluga populations and their prey, as well as on potential competitors like seals. Answering these questions takes decades,” he concludes.