Distribution, seasonal abundance and bycatch of basking sharks (Cetorhinus maximus) in New Zealand, with observations on their winter habitat
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- Francis, M. & Duffy, C. Marine Biology (2002) 140: 831. doi:10.1007/s00227-001-0744-y
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Basking sharks occur throughout New Zealand, but are most common in cool temperate latitudes of 39–51°S. Inshore records from miscellaneous sources peaked in spring–summer, with few winter records. Two records were of sharks observed in a large brackish water lake. About 203 basking sharks were observed caught by commercial trawlers between 1986 and 1999. Multiple captures were common, including 14 in one tow. Most trawl-caught sharks were taken near or outside the 250 m depth contour, and 91% came from three small regions – East Coast (EC) and West Coast (WC) of South Island and Snares–Auckland Islands (SA). The highest catch (93 sharks) and catch rate (58 sharks per 1,000 tows) were from EC, where sharks were caught only in spring–summer. In SA, sharks were caught mainly in summer, and in WC all were caught in winter. The modal seabed depths for shark tows were 300–400 m at EC, 700–800 m at WC and 150–250 m at SA. Sharks were therefore caught in the deepest water in winter at WC. It was impossible to determine the actual depths of capture, but circumstantial evidence indicates that most sharks were caught on or near the bottom. The capture of some sharks in midwater in winter argues against hibernation, because hibernating sharks are unlikely to hover in midwater. Males dominated catches in all regions, particularly in WC and SA. In WC and SA, most sharks (94%) were 7–8 m long, whereas in EC most sharks (73%) were <7 m. Based on their lengths, many of the WC and SA males could have been mature, but most EC males were probably immature. Few of the females would have been mature. This study provides support for the hypothesis that basking sharks over-winter in deep water on the continental slope.