Predation of grey seals by great white sharks in the Gulf of St. Lawrence: an increasing cause of mortality?
During the summer of 2023, the Réseau québécois d’urgence des mammifères marins (RQUMM) received reports of several suspected cases of predation by sharks on marine mammals. Indeed, for the months of July and August alone, at least 14 incidents, likely associated with shark bites, were reported among marine mammals. Eleven of these cases involved grey seals, one involved a harbour seal, and one involved a seal of undetermined species. An observation of a harbour porpoise attacked by a white shark was also photographed by fishermen. Among the 11 reported grey seals, 10 were found dead, and one was still alive at the time of the initial observation.
The injuries on the grey seals are characterized by large oval or crescent-shaped wounds, sometimes spaced apart, often with significant loss of muscle and underlying fat depending on the success of the attack. These wounds have more or less distinct margins, with or without flaps of skin and fat. Multiple curved lacerations, typical of shark bite marks, are also frequently observed on both sides of the main wounds (picture 1). Some of these carcasses were submitted by the RQUMM for examination at the CWHC Quebec regional centre (CQSAS). The observations made so far suggest that the vast majority of these attacks occurred antemortem (while the animal was still alive). The large size of these bites, the tooth marks, and our knowledge of the distribution of different shark species in the Gulf lead us to believe that these predation cases are caused by attacks by great white sharks.
The number of documented predation cases by sharks seems to have increased in the last two years. This increase could be due to an increase in the presence of this species in the Gulf (both in terms of the number of sharks and the number of days they spend there). It can also be proposed that sharks are becoming more familiar with the locations used by the seals. Furthermore, it should be noted that the geographical distribution of these predation cases appears to be expanding. In previous years, the vast majority of cases were observed around the Magdalen Islands. However, in 2022 and 2023, several cases were also documented in Gaspésie, specifically in the Percé and Chandler regions (picture 2). This suggests that white sharks are venturing further west into the Gulf.
The presence of this shark species in the Gulf is monitored by various organizations, including the Department of Fisheries and Oceans Canada (DFO), which has equipped some white sharks with acoustic transmitters. Other types of transmitters also sporadically track the movement of tagged sharks (see https://www.ocearch.org/tracker/). It is interesting to note that several of the grey seal predation cases coincide geographically and temporally with the presence of white sharks in the vicinity. In past years, DFO had detected the presence of white sharks in the waters of the Gaspé Peninsula (for the first time in 2014, then in 2020, 2021, and 2022) and in the waters of the Magdalen Islands (since 2019).
Several hypotheses can be proposed to explain the increase in the presence of white sharks in the Gulf of St. Lawrence. Firstly, the grey seal population in the Gulf has increased in recent decades, which is likely to attract a greater number of predators like sharks. The recovery of the great white shark population, an endangered species in the northwest Atlantic Ocean, facilitated by various conservation measures, potentially promotes the species’ dispersion over larger territories and its return to certain ecosystems. Indeed, since white sharks are solitary hunters, as the population grows, some individuals must find new hunting grounds, including further north, such as in the waters of the Gulf. Finally, it is worth mentioning the possible effect of warming waters in the Gulf due to climate change on the species’ presence. Warmer water temperatures, better suited to this species, could potentially increase its presence and the duration of its summer stays in the Gulf, thereby increasing the risk of seal predation. It has to be mentioned that his is only a hypothesis at this time, which is currently investigated by scientists from DFO as part of a joint research program on the great white shark and the grey seal.
The impact of this increase in predation pressure on seal populations is difficult to determine and is currently being studied by DFO. The latest surveys conducted by DFO in 2021 indicate that the grey seal population in the Gulf consists of 366,000 individuals. It will be interesting to see if grey seals adapt their behaviour and habitat use to reduce the risks associated with potentially closer cohabitation with this top predator in the ecosystem.
Although the risk of accidents remains low, the Atlantic White Shark Conservancy proposes some preventive measures to limit the risk of interaction with white sharks during aquatic activities, including being aware of the species’ presence and avoiding swimming near seal haulouts or in murky waters https://www.atlanticwhiteshark.org/white-shark-public-safety
Ysanne Michaud-Simard, Stéphane Lair, RCSF-Québec
With the participation of RQUMM and Xavier Bordeleau (DFO)