The Common Blacktip Shark, Carcharhinus limbatus, and the Australian Blacktip Shark, C. tilstoni are the primary chondrichthyans targeted by the Northern Territory Offshore Net and Line Fishery (ONLF). These sharks are similar in appearance and until recently could only be distinguished through slow and/or destructive means. Despite their similar appearance, these sharks have very different life histories. The Australian endemic, C. tilstoni breeds annually, matures faster and at a smaller size, than C. limbatus, which breeds biennially and is found in tropical and warm coastal waters world-wide. Our aim was to develop a method that would enable accurate field identification of these two species.

Ninety seven morphological measurements were collected from 112 sharks (95 C. tilstoni, 17 C. limbatus) identified by precaudal vertebrae counts and genetic analyses. Multivariate and machine learning techniques were used to identify six key measurements (fork length, caudal-fin peduncle height, interdorsal space, second dorsal-fin height, pelvic-fin length and pelvic-fin midpoint first dorsal fin insertion) that can be used to distinguish between the two species. There were also significant differences in pelvic fin markings between species: C. limbatus had a distinct black mark greater than 3% of the total pelvic fin area, while C. tilstoni had either smaller markings, markings with diffuse edges, or no markings at all.

The morphological differences identified here will become an important part of a multi-faceted approach to C. tilstoni/C. limbatus identification in the ONLF and will inform the management and conservation of these commercially important sharks.