Aquatic Species at Risk - Basking Shark (Pacific Population)

Species - Details

Basking Shark - Photo Credit: Chris Gotschalk

Scientific name: Cetorhinus maximus
Taxonomy:
Fishes (marine)
SARA Status:
Endangered (2010)
COSEWIC Status:
Endangered (2007)
Region: Pacific Ocean

Related Information

SARA Status - Basking Shark

COSEWIC assessment and status report on the basking shark (2007)

Did You Know?

Basking sharks periodically shed their gill rakers (bristle-like structures at the back of their mouths used to filter food from the water) and are presently thought to cease feeding while they regenerate new ones over a 4-5 month period. Their massive livers, which can weigh one tonne, may act as a metabolic store that maintains energetic requirements while not feeding. Recent tagging has largely disproved the longstanding theory that basking sharks ‘hibernate’ in deep water over the winter.

Description

The basking shark is the second largest fish in the world, with a maximum recorded size of 12.2 m. This filter-feeder is named after its conspicuous behaviour of ‘basking’ (more accurately feeding) at the surface. The basking shark is typically blackish to grey-brown. It has an extremely large mouth with minute teeth, elongated gill slits, a pointed snout, and a crescent-shaped caudal fin. Gill openings have prominent gill rakers. It is the only species in its family, Cetorhinidae. The earliest fossil basking shark is 29 to 35 million years old.

Basking Shark - Photo credit: NOAA

Credit: NOAA

Distribution and Population

Basking sharks are found around the world in temperate coastal shelf waters and exist in Canada in both the Pacific and Atlantic Oceans. Along the North American Pacific coast, basking sharks were historically found off California in winter and spring and in particular areas off British Columbia in summer and fall, suggesting a seasonal north-south migration. Basking sharks have rarely been seen in North-American Pacific waters over the last fifteen years. Historically, however, large goupings were observed in nearshore waters along the west coast of Vancouver Island and in one location along the central mainland coast of British Columbia.

Canada’s Pacific population of basking sharks has virtually disappeared. There are only six confirmed records of basking sharks in the Canadian Pacific since 1996, four of which are from trawl fishery observer records. Historical records indicate a widely spread population. The minimum historical population reconstructed from documented kills was at least 750 individuals, and possibly a few thousand, whereas the current population is extremely low. It is estimated that their rate of decline has exceeded 90% within about sixty years, or two to three generations.

Habitat

Basking sharks are planktivores, and areas with high concentrations of zooplankton (small crustaceans and fish larvae drifting in the water column) appear to be their favoured habitat, typically including fronts where water masses meet, headlands, and around islands and bays with strong tidal flow. They spend much of their time near the surface, although there is recent evidence that basking sharks may also use deepwater habitats greater than 1000 m.

Basking Shark - Photo Credit: Chris Gotschalk

Credit: Chris Gotschalk

Biology

Basking sharks are slow-moving filter-feeders. Longevity is likely about 50 years, while maximum reported length is 12.2 m. The species can be found alone or in groups and appears to be segregated based on sex.

Size at birth probably ranges between 1.5 and 2 m. Males are thought to reach maturity at between 12 and 16 years and females between 16 and 20 years. Gestation has been estimated at 2.6 to 3.5 years, the longest gestation known for any animal, with time between litters estimated at 2 to 4 years. Litter size is approximately six pups. The estimated annual reproduction rate is the lowest of any shark known.

Basking sharks periodically shed their gill rakers (bristle-like structures at the back of their mouths used to filter food from the water) and are presently thought to cease feeding while they regenerate new ones over a 4-5 month period. Their massive livers, which can weigh one tonne, may act as a metabolic store that maintains energetic requirements while not feeding. Recent tagging has largely disproved the longstanding theory that basking sharks ‘hibernate’ in deep water over the winter.

Adult basking sharks have no known predators but young individuals are most likely vulnerable to other large shark species.

As an interesting oddity, on the Pacific coast basking sharks are considered by some scientists as the most plausible explanation for the repeated sightings of sea serpents, sea monsters, and the mythical Cadborosaurus (Caddy).

Threats

Historical targeted killing for liver oil (1941-1947), bycatch, and a directed eradication program by Canada’s Department of Fisheries and Oceans between 1955-1969, including the ramming of sharks with a blade-equipped boat, are believed to be responsible for basking sharks’ disappearance. At the time of this eradication program, basking sharks were considered a nuisance to commercial salmon fishing operations, including gillnetting and trolling. Eradication was aimed to reduce the nuisance factor.

Currently, mortality incurred from fishing operations and vessel collisions are thought to be the largest threats to basking shark populations. Of all shark species, the basking shark appears to be the most vulnerable to human impacts. Characteristics making them vulnerable include late age of maturity, long gestation period, long periods between gestations, low productivity, sex segregated populations, overlapping habitats with commercial fisheries, nearshore/coastal habitat, surface behaviour, fearlessness around boats, and naturally small populations.

The high value of basking shark fins has also promoted a lucrative trade to Asian countries.

Protection

In Canada the species receives protection by broad regulations that prohibit finning of any shark species. They also receive protections under the Fisheries Act. Basking shark are being monitored using regular over-flights that visit areas where they have been abundant in the past.

Given that there is no market for other parts of basking sharks in Canada, there is no directed exploitation. Directed kill of basking sharks is prohibited by European Community countries, United States, and New Zealand. Internationally, the IUCN Red List assessment has categorized basking sharks as Vulnerable globally and Endangered in the northeast Atlantic and north Pacific and even Critically Endangered in the case of Barkley Sound, B.C.

References:

COSEWIC 2007. COSEWIC assessment and status report on the basking shark Cetorhinus maximus (Pacific population ) in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vii + 34 pp.