Age, Growth, and Reproduction of the Finless Porpoise, Neophocaena Phocaenoides, in the Coastal Waters of Western Kyushu, Japan

Abstract

In the coastal waters of western Kyushu, Japan, a total of 97 incidentally taken or stranded finless porpoises, Neophocaena phocaenoides, was collected for studying age, growth and reproduction. An additional 17 specimens from the Inland Sea were used for a comparison of life history. Mean neonatal body length was 78.2 cm. Both males and females grew to around 140 cm by 5 yr of age. The maximum body lengths of males and females in western Kyushu were 174.5 cm and 165.0 cm, respectively, which were smaller than those recorded in other Japanese waters. Females probably attain sexual maturity at ages of 6–9 yr and at body lengths of 135–145 cm. Males probably mature sexually at ages of 4–6 yr, at body lengths of 135–140 cm and at weight of testis of 40–150 g. The lack of females aged 5–6 yr and males aged 4–5 yr precluded firm conclusions on ages at sexual maturity. Parturition in western Kyushu was estimated to be prolonged from autumn to spring, whereas in the Inland Sea and Pacific waters it was restricted from spring to summer with a peak in April. These geographical differences and available information on distribution implies that the finless porpoises in western Kyushu constitute a local population.

Effects of bycatch on the population viability of the narrow-ridged finless porpoises in Ariake Sound and Tachibana Bay, Japan

“…In this study, we investigated the effects of by catch mortality caused by gill nets. Because bycatch by other fishing methods might occur (Shirakihara et al 1993) and other human-induced effects may exist, our predicted risk is an underestimate in terms of conservation of the porpoise population. Our predictions can be applied to evaluate the impact of all human-induced mortality in place of gill net bycatch mortality.…”

“…The value of each parameter for narrow-ridged finless porpoises was obtained from the literature. Values for females only were incorporated, because the sex ratio at any given age was assumed to be 1:1. k was given as 29, because the longevity of the porpoises is approximately 30 yr (Shirakihara et al 1993, Kasuya 1999, Jefferson et al 2002b, and the estimate of λ would not be sensitive to k (Bradford et al 2008).…”

Finless porpoise bycatch in Ariake Sound and Tachibana Bay, Japan

“…1) were collected during a survey for the studies of life history and food habits of the porpoises in 1987 to 1992. Entangled individuals were provided by fishermen as biological specimens (M. Shirakihara et al 1993Shirakihara et al , 2008. The data included the type of gear used, net length/height, mesh size, distance from shore and depth at the location of net setting, the time of net setting and retrieving, target fish species, fish species caught, months when the net was used and months when the fishermen sighted the porpoises.…”

“…1). This population, whose size in 1993/1994 was estimated at 3093 individuals (Yoshida et al 1997), suffers mortalities in bottom-set and drift gillnets (M. Shirakihara et al 1993Shirakihara et al , 2008. To clarify the magnitude of bycatch in gillnet fisheries for this population, we conducted interview-based surveys among fishermen in 2007 and 2008.…”

Age and reproduction of the finless porpoises,Neophocaena asiaeorientalis, in the Yellow Sea, Korea

“…Most of the studies on reproduction of the finless porpoises have been conducted in Japan and China. In Japanese studies, the largest specimen was 175-194 cm; minimum TBL at sexual maturity was 135-140 cm in females, and 135-145 cm in males; minimum age at sexual maturity was 4-5 years in females and 3-4 years in males; the calving season was spring-summer or winter (Kasuya et al 1986;Furuta et al 1989;Shirakihara et al 1993). In Chinese studies, the largest specimen was 164-206 cm in females and 168-227 cm in males; minimum TBL at sexual maturity was 132-145 cm in females and 132-150 in males; minimum age at sexual maturity was 5-5.5 years in females and 4-6 years in males; calving season was springwinter (Gao & Zhou 1993;Chang & Zhou 1995;Jefferson et al 2002).…”