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COSEWIC Assessment and Status Report on the Basking Shark (Pacific population) in Canada

Biology

Biological information has been obtained primarily from the work by Compagno (2001) and from a United Kingdom proposal to list basking shark under Appendix II of CITES (United Kingdom 2002). Both reports provide a comprehensive review of basking shark biology.


Life Cycle and Reproduction

The life cycle and reproduction of basking sharks are poorly understood but likely similar to other lamnoid sharks. Pairing is thought to occur in early summer based on observed courtship behaviour (nose to tail circling) and scarring (Matthews 1950; Sims et al. 2000). Gestation period has been estimated at 3.5 years by Parker and Stott (1965) and, more recently, at 2.6 years by Pauly (2002) who assumed a length at birth of 1.5 m and a von Bertalanffy growth coefficient (K) of 0.062/yr. Information about pregnancy is based on a single basking shark with a litter of six young estimated to be between 1.5 and 2 m in length (Compagno 2001). Like other lamnoid sharks, the basking shark may exhibit embryonic ovophagy, which supplies nutrients to the developing embryos (Compagno 2001). Time between successive litters may be two to three years (Compagno 2001).

Longevity is presumed to be approximately 50 years and age at maturity is estimated at 12 to 16 years in males and 16 to 20 years in females (UK 2002). Length at maturity is estimated at 4.6 to 6.1 m for males based on clasper development (Bigelow and Schroeder 1948); females are presumed to mature at a larger size than males as in many other shark species. Estimates of annual productivity (rmsy) range from 0.013 to 0.023 based on the methodology of Smith et al. (1998) using age at maturity, maximum age and average fecundity (UK CITES proposal 2002). This suggests that the potential for recovery (rebound rate) is lower for basking shark than for any of the 26 species of Pacific shark examined by the Smith et al. (1998). Pauly (2002) calculated the natural mortality (M) to be 0.068. Based on an age of maturity of 18 years for females (midrange of 16-20 years), the generation time can be estimated as 18+1/0.068=33 years. In contrast, the UK CITES proposal (2002) reports the generation time as 22 years.


Herbivory/Predation

At birth, basking sharks are between 1.5-1.7 m in length, large enough to escape predation by most marine species. Very large predators, such as the white shark and killer whale may kill basking sharks but no such kills have ever been documented.


Physiology

Basking sharks have been recorded in surface waters ranging from 8 to 24 °C, with most observations from 8 to 14 °C (Compagno 2001). Four sharks tagged with temperature data loggers in the northeast Atlantic were typically found in waters between 9 and 16 °C (Sims et al. 2003).

Basking sharks periodically shed their gill rakers and are presently thought to cease feeding while they regenerate new ones (4-5 months) (Compagno 2001). Their massive livers may act as a metabolic store that maintains energetic requirements while not feeding (Compagno 2001). Recent tagging has largely disproved the longstanding theory that basking sharks ‘hibernate’ in deep water over the winter (Sims et al. 2003).


Dispersal/Migration

Very little is known regarding the dispersal and migratory patterns of individual basking sharks. There has been only one conventional tagging study in the North Atlantic and none of the156 individuals tagged was recaptured (Kohler et al. 1998). Seasonal migrations are suspected to occur from deep to shallow water or from lower to higher latitudes based on seasonal changes in abundance on both the Atlantic and Pacific coasts of North America.

In the northeast Pacific, basking sharks were visibly most abundant in spring and summer off British Columbia and Washington, and off California in autumn and winter. It has been inferred from these observations that there is a single northeast Pacific population that migrates seasonally (Compagno 2001).

Similarly, off the U.S. Atlantic seaboard, seasonal appearances of basking sharks moving from south to north between spring and summer suggest an annual latitudinal migration. Recent tracking studies of three basking sharks in the northwest Atlantic provide evidence for strong latitudinal movements southward associated with a change in seasons from late summer to winter (Skomal et al. 2004; Skomal 2005). However, three satellite-tagged sharks in the northeast Atlantic (U.K.) tracked for 162, 197, and 198 days did not exhibit any strong latitudinal migration between seasons but rather horizontal movements associated with the continental shelf (Sims et al. 2003).

There is evidence that basking shark populations may segregate spatially and seasonally by sex and/or maturity. Watkins (1958) found that most basking sharks caught in Scottish (95%) and Japanese (65-70%) surface fisheries were female. Compagno (2001) reported that in fisheries off the United Kingdom, basking sharks were mostly females (97.5%) when encountered frequently in summer but mostly males (unknown %) and uncommon in winter. Lien and Fawcett (1986) reported that more males than females were caught incidentally in the inshore waters of Newfoundland. Globally, there is an absence of pregnant specimens reported, which might indicate a spatial or bathymetric segregation of breeding and non-breeding members of the population. Alternatively, the absence of records of pregnant females may simply reflect the low reproductive capacity of the species. In Clayoquot Sound, Darling and Keogh (1994) identified two males by the presence of large white claspers hanging from the pelvic region. Basking sharks are rarely encountered until they have reached 3 m in length. There is only one confirmed account of a juvenile basking shark (1.7 m) and it was observed off the British Isles (Compagno 2001).


Interspecific Interactions

The presence of basking sharks on the ocean surface in areas of high zooplankton concentrations, combined with the anatomical adaptation of specialized gill rakers, suggests that they are primarily planktivores. Stomach content analyses confirm that zooplankton is the preferred prey, but these analyses are based primarily on basking sharks that were active at the surface when they were captured in commercial fisheries. Deepwater pelagic shrimps have been found in the stomach of one basking shark from Japan suggesting that mesopelagic food sources may be important too. Compagno (2001) mentions an anecdotal report of basking sharks preying upon small schooling fishes such as herring. Similarly, a gillnet fisherman from British Columbia reported catching a 7.8 m (26 ft) basking shark which when hoisted by the tail with a crane, was found to be full of 20 cm (8 inch) herring (Gisborne pers. comm. 2004a). Thus, a wider range of prey sources, aside from zooplankton, may be utilized.

Basking sharks have been found to actively seek out areas of high zooplankton concentrations (Sims et al. 1997; Sims and Quayle 1998). Sims (1999) calculated that a minimum prey density of between 0.55 and 0.74 g m-3 would be required for net energy gain and corroborated his estimate with field observations. This implies that basking sharks can survive and grow in conditions where prey concentrations are lower than previously thought necessary (Parker and Boeseman 1954).


Behaviour

Basking sharks are known for their tendency to appear seasonally in large aggregations in particular localities where they are observed intermittently over several months before disappearing again (Darling and Keogh 1994; Compagno 2001). In British Columbia, anecdotal and newspaper accounts also indicate that several bays and small inlets were noteworthy for the regular occurrence of high densities of basking sharks. These aggregations may reflect some unknown breeding or foraging behaviour (Harvey-Clark et al. 1999; Sims et al. 2000).

An aggregation of basking sharks in Pachena Bay (west coast of Vancouver Island) was described firsthand by a journalist on board a fisheries patrol vessel as “literally crawling with sharks. There were dorsal fins [basking shark]everywhere we looked” (Vancouver Sun, May 16 1956). Densities of basking sharks in the Alberni Canal (1921) (Barkley Sound, west coast of Vancouver Island) were described as being in the thousands by the owner of a whaling company (Port Alberni News, August 31, 1921). Similarly Gisborne (2004b pers. comm.) describes how “one day, somewhere between 1960 and 1962, I was boating up Effingham Inlet (Barkley Sound, west coast of Vancouver Island) in my 16’ boat; when I got near the head of the inlet, all I could see were dorsal fins [basking shark].” Anecdotal reports of aggregations in Clayoquot Sound are also reported in Darling and Keogh (1994).


Adaptability

All known or inferred life history parameters imply that basking shark populations cannot recover quickly following a reduction in abundance. They may respond to changes in the environment by shifting their distribution to more favourable areas. Aquaculture or artificial captive breeding is not a feasible option to promote recovery.