David Mark Powter

Dietary studies are essential for an understanding of elasmobranch ecology and their role in marine ecosystems. The diet, head morphology and dentition of Heterodontus portusjacksoni, an abundant, epibenthic shark in the coastal waters of temperate eastern Australia, were examined in 2004–2005. The stomach contents of the juvenile, subadult and adult stages of 136 males and 100 females were examined. Diets were broad (32 prey taxa), but dominated by molluscs, teleosts and cephalopods. Analyses of stomach contents data demonstrated that diet differed significantly by ontogenetic stage, but not by sex. Juveniles and subadults consumed mainly benthic infauna and epifauna, with subadults ingesting greater amounts of diogenid crustaceans, and adult diets dominated by demersal/pelagic prey. Trophic level differed ontogenetically, from secondary consumers as juveniles and subadults to tertiary consumers as adults. The mainly tricuspidate juvenile dentition changed with maturity to a greater proportion of large molariform distal teeth, whereas the snout and jaw lengthened and broadened. Adult males retained a greater proportion of anterior S-family teeth than females, which was most likely related to copulation. The ontogenetic variation in dietary composition, facilitated by differences in dentition and mouth morphology, demonstrated that dietary resources were partitioned ontogenetically.

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.

The reproductive biology and ecology of the Port Jackson shark Heterodontus portusjacksoni was investigated at three locations on the central and southern coast of New South Wales (NSW), Australia from January 2002 to December 2005 using underwater visual census surveys and samples obtained from a commercial fishery. Adults displayed sexual dimorphism in total length (L_T) at sexual maturity, with males maturing between 762 and 772 mm L_T and females between 902 and 905 mm L_T. The mean ovarian fecundity was estimated at 16 offspring per female but was unrelated to female L_T. Male gonado‐somatic (I_G) and hepato‐somatic (I_H) indices and female I_G declined from July to November as did maximum ovarian follicle diameter and the diameter of the three largest follicles. Adults were absent from inshore reefs between December and July. Hence, H. portusjacksoni has a synchronous annual breeding season in NSW, which occurs between July and November (the austral winter to spring), with a peak in oviposition from August to October. Heterodontus portusjacksoni copulatory and ovipository behaviour are reported for the first time. Copulation was observed and involved oral grasping of the female’s pectoral fin by a single male, which wrapped his body around hers to insert one clasper. Ovipositing females appeared to search crevices in the reef prior to delivering a single capsule, which was washed into the crevice by water movement, with the female departing very soon after oviposition. This study represents the first rigorously quantitative analysis of H. portusjacksoni reproductive biology and ecology in NSW waters.

Egg capsules of the Port Jackson shark Heterodontus portusjacksoni were examined during underwater visual census surveys at two sites in New South Wales, Australia, during the 2002–2005 reproductive seasons (austral winter). Embryonic mortality was high (0·783–0·896 per annum) with the majority (99·2%) resulting from predation. The crested horn shark Heterodontus galeatus and common tent shell Astralium tentoriformis (Gastropoda, Turbinidae) were positively identified as egg predators. Gastropod predation was insignificant, accounting for only 2·0 and 2·1% of total mortality at Terrigal Haven and Dent Rock, respectively. Most capsules appeared to have been depredated by large predatory fishes with the eastern blue groper Achoerodus viridis (Labridae) and the black stingray Dasyatis thetidis (Dasyatidae) as possible candidate predators. The rate of embryonic mortality in H. portusjacksoni is higher than that reported for other oviparous elasmobranchs. This high level of mortality has significant consequences for the conservation and management of this species, especially when combined with an understanding of their low fecundity, late maturity and protracted reproductive life.