Demographic analysis of the scalloped hammerhead, Sphyrna lewini, in the northwestern Pacific

Abstract

Population status of Sphyrna lewini in the northwestern Pacific was estimated by demographic analyses using the best available life history parameters. Demographic tables were constructed using estimates of natural mortality (M) of 0.558/year for age 0 and 0.279/year for ages 1-15 with maximum age of 15. A fixed fecundity was set as 12.9 female embryos per litter and an age-specific fecundity was from N=(-26.105+0.179L)/2. Females mature at age 5 and reproduce every two years. The population increase rate was estimated to be 22.8%/year, and the generation time was 7.57 years without exploitation. The net reproductive value, generation time and intrinsic rate of natural increase decreased with the increase of fishing mortality. At F=0.3 and fishing started at age 5, the population increase rate was 12%/year, the generation time was 6.26 years and population doubled every 6.1 years. Sphyrna lewini population had strong resilience as long as fishing started at an age older than age at maturity but would decline when intensive fishing started at an age younger than age at maturity.