Article

A minimally invasive technique to assess several life-history characteristics of the endangered great hammerhead shark Sphyrna mokarran

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Abstract

A dorsal-fin photo-identification technique paired with a non-invasive parallel laser photogrammetry technique was used to non-invasively identify individual Sphyrna mokarran over time. Based on the data collected over a duration of 59 days, 16 different S. mokarran (mean±s.d. pre-caudal length: 220⋅82±13⋅66 cm; mean±s.d. cephalofoil width: 71⋅38±7⋅94 cm) were identified using dorsal-fin photo-identification, with a mean±s.d. shark re-sighting frequency of 4⋅05±3⋅06 at-sea days. The results illustrate a high S. mokarran sighting rate and therefore, the utilization of parallel laser pho- togrammetry and dorsal-fin photo-identification may be a plausible multi-year approach to aid in non-invasively determining the growth rate and inter-annual site fidelity of these animals.

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... Interestingly, a handful of observations have documented great hammerhead sharks' use of extreme shallow water (< 2 m) habitats in Andros Island and Eleuthera (Roemer et al., 2016), as well as two predation events in Bimini on a southern stingray (Dasyatis americana) and spotted eagle ray (Aetobatus narinari) (Strong et al., 1990;Chapman and Gruber, 2002). However, more recently, great hammerheads have been reliably encountered off the west of Bimini, in shallow sand bottom waters during the winter months (O'Connell and Leurs, 2016). ...
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The formulation of conservation policy for species that are rare and migratory requires broad cooperation to ensure that adequate levels of standardized data collection are achieved and that the results of local analyses are comparable. Estimates of apparent survival rate, relative change in abundance, and proportions of newly marked and returning individuals can inform local management decisions while highlighting corresponding changes at other linked research stations. We have applied computer-assisted photo-identification and mark-recapture population modeling to whale sharks Rhincodon typus at Ningaloo Marine Park (NMP), Western Australia, to create a baseline trend for comparison with other regional aggregations of the species. We estimate several ecological parameters of interest, including an average apparent survival rate of 0.55 yr(-1) for sharks newly marked (new) and 0.83 yr(-1) for sharks captured in multiple seasons (philopatric). The average proportion of philopatric sharks is found to be 0.65 of the total population, and we derive an average population growth rate of 1.12 yr(-1) for them. Our analysis uncovered significant heterogeneity in capture and survival probabilities in this study population; our chosen model structures and data analysis account for these influences and demonstrate a good overall fit to the time-series data. The results show good correspondence between capture probability and an available measure of recapture effort, suggesting that unmodeled systematic effects contribute insignificantly to the model fits. We find no evidence of a decline in the whale shark population at NMP, and our results provide metrics of value to their future management. Overall, our study suggests an effective approach to analyzing and modeling mark-recapture data for a rare species using computer-assisted photo-identification and opportunistic data collection from ecotourism to ensure the quality and volume of data required for population analysis.
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