Blue shark (Atlantic and Pacific populations) COSEWIC assessment and status report: chapter 5

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Species Information

• Name and Classification
• Morphological Description
• Genetic Description

Name and Classification

The blue shark (Prionace glauca) is the only species in the genus Prionace and belongs to the family Carcharhinidae collectively known as requiem sharks (Compagno 1984). The family has 12 genera and 48 species including the tiger shark (Galeocerdo cuvier) and commonly encountered reef sharks (Carcharhinus sp.). The blue shark is the only species in this family regularly found in temperate waters, the others being largely restricted to tropical environments. In French this species is called requin bleu.

Morphological Description

The blue shark is easily recognized by its distinctive coloration. The dorsal region is a vibrant dark blue, the sides bright blue, then abruptly white ventrally (Nakano and Seki 2002). The body is long and slender reaching 3.8 m in length with the most obvious anatomical features being a long pointed snout and long sickle-shaped pectoral fins (Figure 1) (Mecklenburg et al. 2002). The pectoral fins are as long as the distance between the tip of the snout to the last gill slit. The caudal fin bears a distinctive notch characteristic of carcharhinids just below the end of the upper lobe. The eyes are large with a nictitating lower eyelid (i.e., able to open and close). The spiracle (an opening between the eye and first gill slit) is absent or very small. There are five moderate sized gill openings with the middle one the largest and the last two positioned over the pectoral fin. The teeth on both the lower and upper jaw are triangular with smooth or finely serrated edges with the upper teeth overlapping at the bases (Mecklenburg et al. 2002).

Figure 1: Blue Shark (Prionace glauca)

Source: Hart 1973

Genetic Description

There has been no population genetic work undertaken on blue sharks. There were no differences in morphometric comparisons between blue sharks caught in equatorial western Atlantic and these in the northwestern Atlantic and in the Pacific (Hazin et al. 1994).

Blue sharks are the only species out of 33 shark species investigated by the National Marine Fisheries Service (NMFS) Cooperative Shark Tagging Program (Atlantic) to show trans-equatorial movement, but most of the recoveries of tagged individuals were from the hemisphere in which they were released (Kohler et al. 1998) suggesting that the populations are largely separated by hemispheres. Tagging studies in the northwestern Atlantic conducted in Canadian, American, and international waters indicate considerable movement with many sharks tagged in Canada being recovered from the central and eastern Atlantic. These studies strongly suggest there are no physical barriers to gene flow throughout the North Atlantic, but that does not rule out the potential for population structuring based on behavioural preferences. Similar to the Atlantic, movement in the North Pacific is widespread with an apparent northward seasonal movement in late spring and summer (Nakano and Nagasawa 1996). Tagging studies undertaken by the California Department of Fish and Game have shown trans-Pacific movements (CDFG 2003).

Models from both the North Atlantic and Pacific suggest that there may be north-south seasonal movements within each hemisphere (Nakano and Seki 2002).

Designatable Units

The North Atlantic and North Pacific populations of blue shark occupy two distinct biogeographical units, the Atlantic and Pacific Oceans. They are geographically isolated by the continental landmass of North America: there is no evidence of movement between ocean basins. For the purposes of this report, blue sharks in the North Pacific and North Atlantic are considered to be two designatable units.