Geographical Variations in Reproductive Parameters of Shortspine Spurdog in the North Pacific.

Abstract

Reproductive parameters of Squalus mitsukurii were compared for four localities in the North Pacific, i.e. off Choshi (app. 36°N, 141°E), Masseiba (app. 30°N, 139°E), Ogasawara Islands (app. 27°N, 142°E), and on the Hancock Seamount (app. 30°N, 180°E). The size at maturity and the maximum size for both sexes were larger in Choshi than in Ogasawara and Hancock by 160-280mm. Sexual dimorphism in the two parameters was also found for each locality. Clasper lengths and testis weights showed a rapid increase with the onset of maturation. Fecundity expressed as litter and clutch sizes showed large differences among the Choshi, Masseiba, and Ogasawara specimens. Fecundity increased with latitude. Both litter and clutch sizes increased with the length of the parent and were described as regression lines for these three localities. As a result, we concluded that this shark has a common reproductive strategy but that each population manifests a fecundity schedule according to the local environmental conditions. The annual increment of growth was estimated to be 18 mm after maturation on the assumption that the shark produced pups every two years.