Evidence of predation of a grey seal by a shark in the Magdalen islands
The Réseau québécois d’urgence des mammifères marins (RQUMM)submitted a series of pictures of a grey seal (Halichoerus grypus) that was found stranded at the end of August in the Magdalen Islands, Quebec. This seal, which probably stranded alive on a beach in Pointe aux loups, was dead when a RQUMM volunteer arrived on site. The adult seal of undetermined sex, which seem to be in fair body condition, showed a large wound on the ventral aspect of its thorax (Photo 1).
This ovoid-shaped wound with well-defined margins was associated with significant loss of underlying adipose and muscle tissues. On another picture (Photo 2) we can appreciate the presence of a flap of skin and of several skin lacerations organized in a circular arc.
The ventral location and appearance of these lesions are highly characteristic of wounds caused by a shark bite.
The large diameter of the main wound and our knowledge of shark species known to swim in the Gulf of St. Lawrence lead us to believe that the lesions observed on this seal were likely caused by a great white shark (Carcharodon carcharias). Seals are in fact preferential prey for this species of large predators. For this reason, the presence of great white sharks in an area is often correlated with the presence of seals. Interesting, the presence of a great white shark was reported around the Magdalen Islands this summer. This shark, a subadult male of over 2.6 meters in length, is followed by an acoustic transmitter implanted last February by the research group Ocearch. The last location available for this shark (as of August 24th) is located just northwest of the Magdalen Islands, off the coast of the seal stranding site at Pointe aux loups (see https://www.ocearch.org/?details=327). This observation and the fact that the great white sharks are usually solitary, make us believe that this shark is responsible for this predation event. The fact that the carcass did not show more bites suggests that this seal was able to escape after the initial attack and took refuge on the shore where it died from its wounds. One of the strategies of sharks when they hunt large prey (such as this seal) is to inflict an injury and wait for the prey to die from blood loss before consuming it.
The documentation of this type of event is not very frequent in the Gulf and the St. Lawrence Estuary. In fact, although predation by sharks are often proposed as a possible cause of skin lacerations or absence of sections of a carcass of marine mammals, these presentations are likely rather caused by the effect of scavengers and decomposers, such as sea fleas, as well as putrefaction (see https://baleinesendirect.org/ces-carcasses-de-phoques-qui-intriguent/). Predation observations of harbour seals by grey seals as well as infanticides by male grey seals have also been documented. Finally, in some cases, the appearance and dorsal location of the skin lacerations are more suggestive of injuries caused by boat propeller strikes.
In our opinion, this case supports an event of predation of a grey seal by a great white shark in the Gulf of St. Lawrence. With the increasing of the population of this species of shark, it is likely that events of this type will increase in frequency over the coming years.
According to Jeffrey Gallant of the Greenland Shark & Elasmobranch Education & Research Group, the great white shark is not a newcomer to the Gulf of St. Lawrence. This species has been historically present in the Gulf, and although observations are still uncommon, there appears to be an increase in numbers most likely due to the increase in the population size. This population growth is likely a consequence of the protection measures that have been put in place for this species, as well as the significant increase in seal populations that are prey of choice in the North Atlantic. It is also possible that an increase in media coverage of these observations (social networking effect), as well as the increase in monitoring and research activities, have contributed to this apparent increase in the frequency of observations of this species. Interesting characteristic of the behavior of the great white shark: it is rather solitary. As a result, an increase in the population will be associated with an increase in its geographical dispersal because dominant individuals (such as large females) will not tolerate smaller individuals (often the males) who will then have to explore new territories. Contrary to the situation for some warmer water species, increases in water temperature associated with global climate change does not appear to be a factor favoring the presence of great white sharks in the Gulf.
Although this type of event may seem dramatic, the apparent increase in the frequentation of great white sharks in the Gulf, and thus the opportunities for predation on seals, should be seen as a positive sign of the recovery of a species that have been severely affected by human activities. The good health of predators at the top of the food chain is in fact a sign of vitality of habitat and biodiversity. Some stakeholders, who believe that there is currently an overpopulation of seals in the Gulf, will be encouraged to see this predator play its role in the ecological balance of this ecosystem.
Stéphane Lair, CWHC – Quebec
Thanks to Jeffrey Gallant of the Greenland Shark & Elasmobranch Education & Research Group for sharing information on the biology of this species.