An unusual (and fatal) prey for a beluga

As part of the monitoring program on the causes of mortality in the St. Lawrence beluga population, CWHC-Quebec received the carcass of a young female beluga (Delphinapterus leucas) found stranded near Sainte-Luce on March 21st.

At necropsy, the most surprising observation was the presence of a black dogfish (Centroscyllium fabricii) lodged in the oral cavity, obstructing the animal’s laryngeal opening.

The tail of a black dogfish (asterisk) can be seen sticking out of the oral cavity of this beluga. The shark’s body blocked the larynx opening, which caused death by asphyxiation.

Cetaceans have a respiratory system adapted to aquatic life, which includes a tubular larynx (arytenoepiglottic tube or “goose beak”) that connects the trachea to the blowhole by passing through the centre of the esophagus. Belugas do not chew their preys but swallow them whole, and in some cases must “disengage” their epiglottis (the terminal part of the larynx) from the palatopharyngeal muscle to allow the swallowing of larger preys. If during this “disengagement” the prey becomes stuck, it can prevent the larynx from returning to its normal position and thereby obstruct the airway, leading to death by asphyxiation.

This type of incident has been previously reported in certain toothed whale species (dolphins, porpoises, and pilot whales) that attempted to swallow fish with spines on their fins or flatfish. A case of asphyxiation following the ingestion of a starry flounder (Platichthys stellatus) has also been reported in a Cook Inlet beluga, Alaska. However, the event described here is a first for the St. Lawrence beluga population.

The presence of gastric contents around the black dogfish suggests the beluga attempted to regurgitate this oversized prey, but was unsuccessful. The shark’s rough skin and the two large spines on the dorsal fins likely prevented it from being “spit out.”

St. Lawrence Estuary belugas typically feed on various species of fish and invertebrates that are usually less than 30 cm in length. To our knowledge, predation on the black dogfish has not been documented in this species before. The black dogfish is a small shark, about 60 to 80 cm in length, living in the deep cold waters of the Atlantic basin, including the St. Lawrence Estuary. It feeds on various invertebrates (cephalopods, crustaceans, and jellyfish) as well as small redfish.

Black dogfish that caused the beluga’s death by asphyxiation. The presence of gastric nematodes (parasitic roundworms) around the shark indicates that the beluga tried to vomit to dislodge the trapped shark but was unsuccessful.

The reasons why this beluga preyed upon an unusual and ultimately fatal species remain uncertain. However, it can be suggested that a decrease in the availability of preferred preys may push belugas to consume riskier, atypical prey. Ecosystem changes, such as a marked increase in the population of redfish (a prey of the dogfish) could also boost the black dogfish population in the estuary, thus increasing the chances of encounter by belugas. While these are currently only hypotheses, it’s essential to monitor the impact of ecosystem changes on the availability of beluga preys in the St. Lawrence Estuary to better understand how these changes affect the conservation of this threatened population.

References:

Rouse, N., Burek-Huntington, K., Shelden, K. (2017). Asphyxiation of an endangered Cook Inlet beluga whale, Delphinapterus leucas. Marine Fisheries Review. 79. 10.7755/MFR.79.2.3.

Stolen, M., St Leger, J., Durden, W.N., Mazza, T., Nilson, E. (2013). Fatal asphyxiation in bottlenose dolphins (Tursiops truncatus) from the Indian River Lagoon. PLoS One. 19;8(6):e66828.

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