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Recovery Strategy for the Basking Shark (Cetorhinus maximus) in Canadian Pacific Waters

3. Description of the Species and its Needs

3.1 Species Description

The Basking Shark is the world’s second largest fish growing up to 12.2 m. Current understanding of the life history of these sharks suggests they are long lived (~50 years), slow growing, and slow to mature, with a probable low fecundity, and overall low productivity (McFarlane et al. 2009). They are thought to feed primarily on zooplankton in the water column by filtering their prey using a specialized adaptation called gill rakers. There is some information to suggest that other prey sources may also be utilized (COSEWIC 2007). The Pacific population of Basking Shark is thought to belong to a single seasonally migrating population. This is based on historical sightings that found that the disappearance of Basking Sharks from California waters in the spring and early summer (Squire 1967, 1990) coincided with the appearance of Basking Sharks in British Columbia waters (Darling and Keogh 1994). Recent satellite tagging information from the north Atlantic has found that Basking Sharks are capable of very large migrations across ocean basins and hemispheres and spend significant periods well below the surface (Gore et al., 2008; Skomal et al. 2009). Basking Sharks have very low genetic diversity and such little differentiation between ocean basins that it is not possible to designate distinct populations based on genetic differences (Hoelzel et al. 2006). To date, no genetic analyses have been completed for the North Pacific population of Basking Shark.

Figure 1: Illustration of the Basking Shark

Illustration of the Basking Shark

3.2 Needs of the Basking Shark

Given their large size and their planktivorous feeding strategy, Basking Sharks require oceanographic conditions that concentrate prey. The necessary conditions may change over relatively small spatial and temporal scales (Sims and Quayle 1998). For example, a local area that is suitable feeding habitat one day may not be on the next. Based on experience elsewhere, specific geological structures (i.e., headlands, banks) that concentrate prey are used regularly by Basking Sharks (Sims and Quayle 1998; McFarlane et al. 2009; Gore et al. 2010). In the northeast Atlantic it was calculated that Basking Sharks require a minimum prey density of between 0.55 and 0.74 g·m-3 for net energy gain (Sims 1999).

In Canada’s Pacific waters, no specific areas have been identified for reproduction, pupping, or rearing (COSEWIC 2007). Historically there are some areas that were frequented by Basking Sharks on a regular basis (Barkley Sound, Clayoquot Sound, and Rivers Inlet). Some habitat characteristics that attract particular life stages such as high seasonal food availability are known, but these features vary over temporal and spatial scales. For example, Basking Sharks tend to aggregate in the transition zones of coastal shelves where there is enhanced zooplankton abundance (Sims et al. 2006).  Further, zooplankton abundance (the preferred prey) and community structure have been observed to vary on decadal-scales, as seen in the prolonged period of relative low copepod abundance in some areas of the northeast Pacific from 1989-1997 (King, 2005).  Globally, Basking Sharks have been observed in surface waters ranging from 8-24° C with a preference for waters between 9 and 16ºC (Sims et al. 2003).

The key factors limiting Basking Shark recovery and survival are their slow growth and maturation and low fecundity which lead to overall low productivity. Even in the absence of human mortality, Basking Shark populations grow very slowly. Global experience has shown that when Basking Shark populations are exposed to even low levels of human caused mortality, their populations decline (Gore et al. 2010). Basking Sharks often utilize surface waters making them particularly susceptible to human caused mortality. There is evidence from other regions that females are more often than males to be on the surface which further adds risk to the populations (COSEWIC 2007). While on the surface Basking Sharks can become entangled in fishing gears, be directly harvested, disturbed, harmed, or can collide with vessels. The Pacific population of Basking Shark is endangered primarily due to human caused mortality that occurred between 40 and 70 years ago. While on the surface, Basking Sharks are primarily engaged in feeding (COSEWIC 2007), however putative mating behaviour has also been recorded (Harvey Clark 1999; Gore et al. 2010).