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

Recovery Feasibility Summary

Basking Sharks are particularly vulnerable to any human-induced mortality because of their late age of maturity, low fecundity, long gestation period, long periods between gestations, low productivity, sex segregated populations, use of habitat that supports commercial fisheries, lack of fear of vessels, and current small population size (COSEWIC 2007). It is difficult to accurately assess the feasibility of recovery for the Pacific population of Basking Shark due to the lack of understanding of the factors affecting the survival and productivity of the species. The recovery feasibility is also linked to recovery efforts undertaken in the southern portion of their range (i.e., within the U.S. and Mexico). Nevertheless, based on the best current available information, recovery of the Pacific population of Basking Shark is determined to be feasible.

1. Are individuals capable of reproduction currently available now or in the foreseeable future to sustain the population or improve its abundance?

Answer: Unknown

It is unknown whether individuals capable of reproduction are currently available to improve the population growth rate or population abundance in Canadian waters. Population trajectory models were used by McFarlane et al. (2009) to determine the recovery potential of the Pacific population of Basking Shark. If a breeding population exists in Canadian Pacific waters, and no further human-induced mortality and changes to existing habitat occurs, it was estimated that it will take approximately 200 years for the population numbers of Basking Sharks to return to their unexploited state if human induced mortality is zero (McFarlane et al. 2009).

2. Is sufficient suitable habitat available to support the species or could it be made available through habitat management or restoration?

Answer: Yes

The current distribution, migration, reproduction, pupping and rearing grounds of the Basking Shark in Canadian Pacific waters is unknown. 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). There are historical areas that were regularly visited by large numbers of Basking Sharks (e.g., Barkley Sound, Clayoquot Sound, and Rivers Inlet); however, a recovered stock may not return to these areas. Habitat availability for this species is not likely to have changed. Basking Sharks are often associated with both historic and current distribution of the humpback whale (Megaptera novaeagliae) (Wallace and Gisborne 2006; Newton pers. comm. 2007). As humpback whale distribution has not changed and abundance has recently increased, it seems reasonable to conclude that suitable habitat is available for Basking Sharks in Canadian Pacific waters (McFarlane et al. 2009).

3. Can the primary threats to the species or its habitat be avoided or mitigated through recovery actions?

Answer: Yes

Threats that could potentially affect Basking Sharks in Canadian Pacific waters have been identified (see section 4 ‘Threats’). Current threats, listed in order of significance, include entanglement, collision with vessels, harassment and prey availability. The first step in mitigating significant threats is the improvement of knowledge of Basking Shark ecology and biology. Threats could then be managed to minimize impacts. As trends in prey availability (identified as a threat with ‘low’ level of concern) in the context of Basking Shark recovery have not been thoroughly studied, it is unknown whether or not this threat could be mitigated. Actions in Canada to mitigate known threats include modification of fishing and aquaculture practices and development of a Code of Conduct for public viewing and other public awareness tools (for a full list, see section 6.2recovery planning table’). Parallel to these actions, Canadian collaboration with U.S. and Mexico governments will promote recovery throughout the species’ range.

4. Do the necessary recovery techniques exist to achieve the population and distribution objectives, or could be developed within a reasonable timeframe?

Answer: Yes

Despite no directed fishery or eradication program for over forty years, it is estimated that the decline from pre-exploitation numbers of Basking Sharks in Canadian Pacific waters still exceeds 90%. This may be in part due to current mortalities from entanglements and incidental catch and/or in part due to the lack of knowledge of the population and distribution of this species (McFarlane et al 2009). As noted above, Basking Sharks are particularly vulnerable to any human-induced mortality because of their late age of maturity, low fecundity, long gestation period, long periods between gestations, low productivity, sex segregated populations, use of habitat that supports commercial fisheries, lack of fear of vessels, and current small population size. The necessary recovery techniques do exist and are assumed to be effective. For example, modification of fishing and aquaculture practices, and Canadian collaboration with U.S. and Mexican governments; however, it is estimated that some 200 years are required before population numbers will return to their unexploited state, assuming human-induced mortality is reduced to zero (McFarlane et al. 2009).