Comparison of dorsal and pectoral fin denticles for grey nurse, great white, and six whaler sharks from east Australian waters

Abstract

Escalating demand for shark fins poses a major threat to shark populations world-wide and the monitoring of shark catches can be very difficult when only the fins are present. Identifying a species of shark using only its fins can be enhanced by using the dermal denticles as they maintain their structural integrity irrespective of freezing or drying. In this study, scanning electron microscopy was used to examine various denticle characteristics including: the number and persistence of ridges and cusps, posterior edge appearance, dispersion and size (length and width) at three positions (anterior margin, centre and posterior margin) on the dorsal fin and dorsal surface of the pectoral fin of sharks caught in the coastal waters off New South Wales, Australia. Samples were obtained from commercial and recreational fishers, and the shark meshing programme. Catches included two threatened species - Carcharias taurus and Carcharodon carcharias, and six carcharhinid whalers - Carcharhinus brachyurus, C. falciformis, C. leucas, C. limbatus, C. longimanus and C. obscurus. Detailed examination revealed similarities and differences among sampling positions within a fin, between fin types, and among species. Patterns of change in denticle characteristics and size across the sampling positions provided an efficacious means of unequivocally identifying eight shark species. Future studies should evaluate field-based sampling of denticle characteristics as they may provide a cost-effective alternative to genetic techniques for identifying sharks and enable much needed data on the quantity and species composition of sharks harvested for their fins in local waters.