Embryonic mortality and predation on egg capsules of the Port Jackson shark Heterodontus portusjacksoni (Meyer)

Abstract

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.