Insights from life history traits of Risso's dolphins (Grampus griseus) in Taiwanese waters: Shorter body length characterizes northwest Pacific population

Risso's dolphins (Grampus griseus) are widely distributed throughout temperate to tropical pelagic waters of the world and are one of the most frequently encountered cetaceans in eastern Taiwanese coastal waters. Because their life history is poorly known, the goal of this study was to investigate the relationship between age, body length, and sexual maturity of Risso's dolphins in Taiwanese waters. Ninety-two carcasses of dead-stranded or fisheries bycaught dolphins (1994-2008) were measured and dissected (total body length, TBL 125-290 cm); sexual maturity was assessed in 33 dolphins; and age was estimated by counting dentinal growth layer groups in routine histologically prepared tooth sections of 28 dolphins. Sexual dimorphism in TBL was not detected. The onset of sexual maturity occurred at 240-255 cm in females and 253-265 cm in males, which was at about 10 yr of age for both sexes. Our stranding, bycatch, and previous boat survey records suggest that Risso's dolphins occur year-round and likely have a summer-fall calving season in Taiwanese waters. The similar life history parameters and calving season in dolphins from Taiwanese and Japanese waters suggest a common population in the northwest Pacific, which has a noticeably shorter body length than in other regions.