Basking shark, Pacific (Cetorhinus maximus) COSEWIC assessment and status report: chapter 10

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Limiting Factors and Threats

Basking sharks are particularly vulnerable to any human-induced mortality because of their late age of maturity, low fecundity, long gestation period, long periods between gestations, low productivity, sex segregated populations, use of habitat that supports commercial fisheries, lack of fear of vessels, and current small population size.

Human-induced mortality in Pacific Canadian waters is primarily from continued interactions with fishing gears as well as collisions with vessels (although there are only anecdotal records to verify this). Records indicate that basking sharks are readily caught by trawl (bottom, midwater, and shrimp), and easily become entangled in longlines, gillnets, prawn traps, cod traps, and even herring seines.

Historical fisheries and the directed eradication program appear to be the most likely causes for the low abundance of basking sharks observed today in Canada’s Pacific waters. There have been few confirmed sightings or catches in recent times. Only four basking sharks have been captured (and presumably killed) since 100% observer coverage of the groundfish trawl fishery began in 1996 (Table 2). Basking sharks are susceptible to entanglement or capture by other gear types as well. In Clayoquot Sound there have been three reports of entanglement, one with prawn gear (1988), one in a herring seine (1992), and one in a salmon gillnet (1992) (Darling and Keogh 1994). Despite the few recent instances of mortality, fisheries interactions remain a concern because of the decline in the number of basking sharks. In fact, most basking sharks sightings are now associated with incidental capture.