A blacktip's black tip: The reliability of using dorsal-fin patterns for photo identification of blacktip reef sharks (Carcharhinus melanopterus)

Photo identification (photo ID) has increasingly become a valuable technique serving not only to identify individual animals but also to monitor populations, track migration patterns and assess wildlife health, among others uses. Various species of sharks are amenable to photo ID, among which the blacktip reef shark (Carcharhinus melanopterus) is a particularly suitable subject. Their distinctive pigmentation and dorsal-fin patterns serve as potential key identifying features. This study focuses on the variation in dorsal-fin patterns among individual neonatal and juvenile C. melanopterus around Moorea, French Polynesia. We employed a Gower distance matrix to assess dissimilarities in dorsal-fin patterns and conducted an elliptical Fourier analysis (EFA) to characterize shape variations. The results from the EFA were further summarized using principal component analyses. Additionally, we explored the potential symmetry between the left and right sides of the dorsal fin. To assess the long-term reliability of using pigmentation patterns for photo ID, dorsal-fin patterns of recaptured individuals were compared using regressions of log-transformed dorsal-fin measurements over log-transformed pre-caudal lengths. Recaptures occurred over varying time frames, ranging from as short as 2 weeks to as long as 9 months. The diverse range allowed us to evaluate the temporal stability of dorsal-fin patterns across different intervals. The analyses revealed that each individual shark's dorsal-fin pattern is unique, but the left and right sides are asymmetrical. Regarding the analysis of recaptured individuals, the ontogenetic changes in dorsal-fin size were not significant enough to alter the dorsal-fin patterns, thus ensuring their temporal stability. The application of photo ID techniques, as demonstrated in this study, underscores its indispensable role in conservation strategies, promoting a deeper understanding of elasmobranch species.