Basking shark, Pacific (Cetorhinus maximus) COSEWIC assessment and status report: chapter 5

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Species Information

• Name and Classification
• Morphological Description
• Genetic Description
• Designatable Units

Name and Classification

The basking shark (Cetorhinus maximus Gunnerus, 1765) is the sole member of the family Cetorhinidae belonging to the order Lamniformes. Other common names include sun shark, bone shark, and elephant shark. In French this species is known as Pèlerin. In Pacific Canada, the basking shark was also commonly but incorrectly referred to as mud shark in early historical accounts.

Morphological Description

This animal is most readily distinguished in the field from other sharks by its large size (maximum reported 12.2 m), elongated gill slits which extend almost to the mid-dorsal of the head, pointed snout, a large subterminal mouth with minute hooked teeth, caudal peduncle with strong lateral keels, and crescent shaped caudal fin (Compagno 2001, Figure 1). Colour is typically blackish to grey-brown, grey or blue-grey above and below on body and fins, undersurface sometimes lighter, often with irregular white blotches on the underside of the head and abdomen (Compagno 2001). Internal gill openings have prominent gill rakers formed from modified dermal denticles.

Figure 1: Basking Shark (Cetorhinus maximus)

Source of figure: Compagno 2001.

Genetic Description

The population structure of basking sharks is poorly known. There has been no population genetic work done on this species. Inferences about population structure are based on records of seasonal occurrence and limited observations from tagging studies. In Canada, basking shark populations in the North Atlantic and North Pacific are geographically disjunct and are considered to be reproductively isolated from one another due to their preference for temperate waters that would preclude migration through the Arctic Ocean.

In the North Pacific, basking sharks were observed historically in discrete locations off California in winter to spring and regularly in particular areas off British Columbia in summer and fall (Squire 1967, 1990). This information, combined with recent satellite tracking information from the North Atlantic (Sims et al. 2003; Skomal et al. 2004; Skomal 2005), suggests the possibility of a single panmictic population along the west coast of North America. On the other hand, throughout their global range, basking shark aggregations have been reported to occur repeatedly in discrete areas where they are typically found in large numbers and for only part of the year (Compagno 2001). Thus, philopatry and more complicated genetic population structure may exist.

Designatable Units

Given that basking sharks are found exclusively in temperate oceans and that there is no connection through the Arctic or from the south, the species in Canadian Pacific waters is considered to be separate from that in Canadian Atlantic waters and to comprise a separate designatable unit.