Demographic analysis of the Port Jackson shark Heterodontus portusjacksoni in the coastal waters of eastern Australia

Abstract

Demographic analyses are used to assess the status and vulnerability of elasmobranchs but their accuracy is often affected by limited basic biological information. Although the Port Jackson shark Heterodontus portusjacksoni (Meyer) is currently not considered at threat, there is insufficient data for eastern Australia to assess this rigorously. The present study determined vital demographic rates of adult and juvenile H. portusjacksoni at four locations on the central and southern coast of New South Wales, Australia from January 2002 to December 2005 using underwater visual census, tag–recapture and samples obtained from a commercial fishery. Natural mortality was low in adults (0.063–0.074 year–1) and juveniles (0.225 year–1), but substantial at the embryonic stage (0.783–0.896 year–1). Adult growth rates (31.4–32.7 mm year–1) were slightly less than that of juveniles (36.8–37.5 mm year–1). Males at both stages grew slightly faster than females. However, H. portusjacksoni had slower growth rates than many other elasmobranch species. Having a low intrinsic rate of increase (r = 0.069 year–1), long generation times (μ1 = 22.5 year) and a low rebound potential, adults are the stage with the greatest impact on population growth. Hence, their life history strategy makes them susceptible to serious decline under exploitation, and management should strive to maintain the adult reproductive stock as a priority.