Blue shark (Atlantic and Pacific populations) COSEWIC assessment and status report: chapter 7
Habitat
Blue sharks are considered epipelagic, meaning they are associated with the surface layer of the ocean. There are indications that there are latitudinal variations in depth preference but they are typically found between the surface and 350 m (Nakano and Seki 2002). They prefer offshore habitats but have been observed on occasion inshore. Blue sharks are known to occur in waters between 5.6-28º C. Their preferred temperature has been estimated based on associated catch rates and varies considerably by both region and study (Nakano and Seki 2002). Strasburg (1958) reported that 99% of blue sharks in the Pacific were caught in the water temperature range of 5.6-18.9ºC. There is evidence of a latitudinal trend in depth preference. In tropical latitudes (20ºS-20ºN) there appears to be a preference for deeper waters (80-220 m) and no apparent seasonal shift in abundance (Nakano and Seki 2002). In northern latitudes, the blue shark is found in shallower waters and there is evidence for a seasonal shift in abundance as animals move to higher latitudes in the summer (Nakano and Seki 2002). Water temperature is likely the principal factor determining the depth and latitudinal distributions of blue sharks.
Habitat preference by sex and size has been described from research surveys in the North Pacific (Nakano and Nagasawa 1996). Females were caught between 8-21º C whereas males were caught between 12-21 ºC. Larger females (>90 cm) tend to occur in cooler waters than similar-sized males (Females 8-21º C, Males 14-21ºC). Nakano (1994) proposed a general model based on temperature preferences, later supported by McKinnell and Seki (1998), describing sex and size segregation in the North Pacific. The band between 35-45ºN was identified as a parturition area, and a slightly wider band as the nursery (68-134 cm total length). Subadult females (134-199 cm TL) are typically found north of the parturition grounds while subadult males occupied waters south of the parturition grounds. Nakano’s model predicts that most of the blue sharks in Canada’s Pacific waters would be subadult females. A single study on Canada’s Pacific coast lends support to this hypothesis with 93% (n=134) of the observations being immature females (IEC Collaborative Marine Research and Development Limited 1992).
Sex and size segregation in the northwest Atlantic has also been observed (Pratt 1979; Campana et al. 2004; Beerkircher 2005). All studies have indicated the absence of mature females from the waters off the northeastern U.S. and Canada. Campana et al. (2004) examined length frequency data from summer observations in the commercial and recreational fisheries (Figure 8). These data suggest that mature males comprise a small component of the commercial catch (considered more representative) but comprise a large portion of the recreational derby fishery due to targeting of large individuals. Immature blue sharks of both sexes were found in equal proportions (Campana et al. 2004; Beerkircher 2005). Based on analyses of U.S. pelagic longline observer data from 1992-2003, Beerkircher (2005) found that mean fork length decreased with increasing latitude and as a general trend the largest mean lengths were found in areas south of 35°north latitude. Mean fork lengths in the northeast distant (NED) ICCAT region (including waters around Newfoundland) were found to be 139 cm (n=11174) and 133 cm (n=11172) for males and females respectively. Northern waters may be important nursery areas based on the higher proportion of smaller individuals (Beerkircher 2005). Across the Atlantic in the English Channel, Vas (1990) reports that only one of 73 captured sharks was male, which is similar to Stevens (1976) who reports that only 6% of the catch was composed of males.
Figure 8: Length Frequency Distributions of Blue Sharks Examined at Shark Derbies and Measured by Observers in the Commercial Catch on the Scotian Shelf Between 1993-2003 Over the Same Range of Months and Years
Vertical dash line represents the size at 50% maturity. Source: Campana et al. 2004.
There has been no research investigating trends of habitat availability for blue sharks in Canada. Based on anecdotal evidence, it appears that warm water events (i.e., El Niño) in Canada’s Pacific waters results in an increase in the abundance of blue sharks but this has not been confirmed. Commercial catch rates of blue sharks in the North Pacific have been shown to decline considerably in sea surface temperatures less than 16°C and in latitudes greater than 40°N (Bigelow et al. 1999).
Protection of habitat for blue sharks has not been considered in Canada. The loss of blue shark habitat (i.e., global epipelagic environments) is not thought to be a conservation concern at present time. Note, for the purposes of this report, fisheries interactions are not considered to be a threat to habitat.
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