Basking shark, Pacific (Cetorhinus maximus) COSEWIC assessment and status report: chapter 6

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Distribution

• Global Range
• Canadian Range

Global Range

Basking sharks are found circumglobally in temperate coastal shelf waters but are characterized by localized occurrences (Figure 2), occurring off the coast of fifty countries (Froese and Pauly 2005). In the North Atlantic, basking sharks are observed in waters off countries as far south and east as Senegal, through to Europe (including the Mediterranean Sea), Norway, Sweden, Russia, westward to Iceland, Canada (Newfoundland, Nova Scotia, New Brunswick), along the eastern seaboard of the United States and into the Gulf of Mexico. In the North Pacific, they are observed as far south and west as Japan, through to China, along the Aleutian Islands, Alaska, British Columbia, along the western seaboard of the United States and Mexico (Baja California and northern Gulf of California) (Compagno 2001). Basking sharks have not been observed in equatorial waters.

Figure 2: Global Distribution of Basking Sharks

Dark grey areas represent known basking shark distribution and light grey areas represent possible distribution based on temperature preferences. Map source: Compagno 2001.

Canadian Range

The current distribution of basking shark in Canada’s Pacific waters is largely unknown. There have only been six reliable sightings on the Pacific coast since 1994 (two confirmed live sightings and four reported as bycatch) (Figure 3). Historical records dating back to the late 1700s have been used to identify several discrete bays and inlets where basking sharks were once regularly found (Table 1; Wallace and Gisborne 2006). Large aggregations of basking sharks were found repeatedly in three areas: (1) Rivers Inlet/Queen Charlotte Sound; (2) Clayoquot Sound; and (3) Barkley Sound (Figure 3). Unconfirmed or smaller groups have been observed historically in numerous other locations in the Strait of Georgia (Cape Lazo, Texada Island and Saanich Inlet) and off the southern banks of Vancouver Island (Table 1).

Figure 3: Known Occurrences of Basking Sharks off British Columbia Between 1996 and 2004 from Trawl Catches

(the PacHarTrawl database, n=4, circles denote individual weights) and opportunistic visual surface sightings (N = 2; stars). Location names (Queen Charlotte Sound, Rivers Inlet, Clayoquot Sound, and Barkley Sound) refer to areas with large historical aggregations of basking sharks.

Over the time period of three generations (66-99 years), basking sharks have all but disappeared from all areas where they were historically abundant. This disappearance cannot be attributed to a reduction in search effort. The three areas identified as historically most important are still used intensively by mariners. Moreover, the coastline of BC is increasingly well-travelled by tourism operators and recreational users. Being large and conspicuous at the surface, basking shark would certainly have been reported in other areas if the distribution of basking shark had merely shifted. The historical extent of occurrence in Pacific Canada includes the area of the continental shelf at depths <200m, an area of 80 000 km². The current extent of occurrence is unknown but much reduced.