COSEWIC Assessment and Status Report on the Basking Shark (Pacific population) in Canada
- Assessment Summary
- Executive Summary
- COSEWIC History, Mandate, Membership and Definitions
- Lists of Figures, Tables and Appendices
- Species Information
- Population Sizes and Trends
- Limiting Factors and Threats
- Special Significance of the Species
- Aboriginal Knowledge
- Existing Protection or Other Status Designations
- Technical Summary
- Acknowledgements, Authorities Consulted, and Information Sources
- Biographical Summary of Report Writers
- Appendix 1: Headlines and Titles of Articles Pertaining to Basking Sharks from Non-scientific Sources
Basking sharks are named after their conspicuous behaviour of ‘basking’ (more accurately feeding) at the surface. The basking shark is distinguished from other sharks by its large size (second largest fish in the world), elongated gill slits, pointed snout, a large mouth with minute teeth, and a crescent-shaped caudal fin. Colouration is typically blackish to grey-brown. Gill openings have prominent gill rakers.
Although there has been no research on the population genetics or population structure of basking sharks, due to its biogeographic separation from those in the Atlantic, the Pacific population of basking sharks is treated as a single COSEWIC Designatable Unit (DU). There is no evidence from either the North Pacific or North Atlantic of transoceanic movements.
Basking sharks are found circumglobally in temperate coastal shelf waters but are characterized by localized occurrences. Canadian records from Pacific waters indicate they utilize virtually all coastal temperate waters. Conventional tagging studies have yielded no tag returns. Along the Pacific coast, basking sharks were observed historically in discrete locations off California in winter and spring and in particular areas off British Columbia in summer and fall suggesting a latitudinal migration. There are very limited observer data on bycatches of this species in Pacific waters because of the rarity with which basking sharks have been seen over the last thirty years. Historically, however, large aggregations were observed in nearshore waters along the west coast of Vancouver Island and in one location along the central mainland coast of British Columbia.
Areas where oceanographic events concentrate zooplankton appear to be the favoured habitat of basking sharks, typically including fronts where water masses meet, headlands, and around islands and bays with strong tidal flow. There is recent evidence that basking sharks may also utilize deepwater habitats greater than 1000 m. The quality of foraging habitat changes over short spatial and temporal scales based on oceanographic conditions.
Information on the early life history of this species is poorly known. Animals less than 3 m in length are rarely encountered. Size at birth probably ranges between 1.5 and 2.0 m. Litter size is known from only one animal with six young. Males are thought to reach maturity at between 12 and16 years and females between 16 and 20 years. Pronounced sex segregation is evident in data from surface fisheries. Gestation has been estimated at 2.6 to 3.5 years, the longest known for any animal, with time between litters estimated at 2 to 4 years. Longevity is likely about 50 years, while maximum reported length is 12.2 m. The estimated annual productivity is the lowest of any shark known. Generation time is estimated between 22 and 33 years.
Adult basking sharks have no known predators but young individuals are most likely vulnerable to other large shark species. Basking sharks are primarily planktivores, seeking out areas of high zooplankton concentrations. A modelling study suggests that they should be able to obtain a net energy gain by feeding in areas of moderate zooplankton concentrations.
Population Sizes and Trends
No abundance estimates exist for basking shark populations in Canada’s Pacific waters. Our understanding of population size and trends comes primarily from historical catch records.
Canada’s Pacific population of basking sharks has virtually disappeared. Historical records indicate an abundant and widely spread population. Historical targeted killing (commercial and recreational), bycatch, and a directed eradication program by Canada’s Department of Fisheries and Oceans, including the ramming of sharks with a blade-equipped boat, are believed to be responsible for their disappearance. There are only six confirmed records of basking sharks in the Canadian Pacific since 1996, four of which are from trawl fishery observer records. Overall, the population is but a small fraction of its relatively large historical abundance and distribution.
Limiting Factors and Threats
Ongoing 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, low fecundity, long gestation period (apparently the longest of any vertebrate), long periods between gestations, low productivity, sex segregated populations, overlapping habitats with commercial fisheries, nearshore/coastal habitat, surface behaviour, no fear of vessels, and naturally small populations.
Special Significance of the Species
The basking shark is the only species in its family. The earliest fossil basking shark is 29 to 35 million years old. It qualifies for the category “charismatic megafauna” by virtue of its large size (second largest fish in the world) and conspicuous surface activity. On the Pacific coast basking sharks are the most plausible explanation for sea serpents, sea monsters, and the Cadborosaurus (Caddy). The high value of basking shark fins has promoted a lucrative trade to Asian countries. The recent inclusion of basking shark under Appendix II of CITES is intended to regulate this trade. The basking shark may be more vulnerable to human impacts than any other marine fish.
In Canada the species receives de facto protection by broad regulations that prohibit finning of any shark species. 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. They fall under a variety of status designations. 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”.
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