Basking shark, Pacific (Cetorhinus maximus) COSEWIC assessment and status report: chapter 7
Habitat
The habitat requirements for basking sharks in Pacific Canadian waters have not been investigated. In other areas, basking sharks are known to be associated with oceanographic events that concentrate zooplankton, including fronts off headlands, around islands and in bays with strong fluctuation of water masses from tidal flow (Sims et al. 1997; Sims and Quayle 1998; Wilson 2004). Although they appear to prefer shallow coastal waters, basking sharks have been recorded in the epipelagic zone by aerial surveys, pelagic driftnet fisheries, and have been caught in bottom trawls off the St. Lawrence River, Scotian Shelf and Scotland (Compagno 2001; this study). Data from the Newfoundland Observer Program (NOP) indicate that basking sharks have been taken in trawl nets fishing in depths up to 1370 m with 15% of the records (n=414) from waters deeper than 1000 m.
Sub-surface diving behaviour is known from only seven animals which dived to depths well over 200 m and on one occasion to a depth of over 750 m (Sims et al. 2003; Skomal et al. 2004; Skomal 2005). Water column utilization varied considerably among individuals and is likely influenced by patterns of prey distribution varying by depth, location, and season. Skomal (2005) found that two basking sharks captured at the water surface, tagged and released in the same northwest Atlantic summer location (see Genetics section) moved to very different wintering habitats. One individual wintered off Florida and spent most of its time at the surface whereas the other individual wintered off of Jamaica and spent most of its time at depths below 480 m. With so few animals tagged it is difficult to characterize diving and water column utilization patterns except to note that all sharks showed considerable vertical movement.
Habitat availability for this species is not likely to have changed. New evidence from basking sharks studied off England suggest that the sharks target areas of high zooplankton concentrations associated with both large and small scale oceanographic conditions that change quickly (lasting hours to days) (Sims and Quayle 1998). Longer-term trends in climate may influence prey availability but recent theoretical work suggests that basking sharks can achieve a net energy gain under moderate (0.48-0.70 g m-3) concentrations of prey (Sims 1999). For the purposes of this report, fisheries interactions (i.e., entanglement) and vessel collisions are considered as direct threats (in a later section) rather than as degradation of aquatic habitat.
All habitat of basking sharks in Canada falls under federal jurisdiction managed primarily by Fisheries and Oceans Canada (DFO). At present, there is no intentional protection for basking shark habitat. In Pacific Canada, waters adjacent to Pacific Rim National Park (Broken Group and West Coast Trail components) are areas where basking sharks were sighted historically. Present restrictions in these waters would not afford much protection against perceived threats (i.e., vessel collisions, entanglement in fishing gear and salmon farming net pens).
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