Shark and skate egg-cases cast up on two South African beaches and their rates of hatching success, or causes of death

Abstract

Collections of chondrichthyan egg-cases cast ashore at two sites along the South African coastline were identified and examined for cause of mortality. A total of 574 egg-cases collected from False Bay could be attributed to five species of scyliorhinid shark, two skates and the elephantfish or chimaera, while the 538 egg-cases collected from Port Alfred belonged to four scyliorhinid sharks and two skate species. At both sites eggs of shysharks of the genus Haploblepharus made up more than 70% of the samples, followed by skates (5% in False Bay and 19% in Port Alfred) and then catsharks of the genus Poroderma (1 and 7% respectively). Haploblepharus egg-cases generally displayed relatively high rates of hatching success of over 60% and predation rates of 18-30%. Haploblepharus fuscus was the exception with a low hatching success of 37% and a predation rate of 40%. Poroderma egg-cases suffered predation losses of 43%, and had a hatching success of 47%. Skate egg-cases displayed non-predator induced egg mortalities of up to 35%, but predation rates of less than 14%. Hatching success was moderate at 57%. The holes found in egg-cases could be categorised into five types, of which four were clearly predator boreholes. The most abundant of these was shown to be caused by whelks of the genus Bumupena.