Short-term fluctuations in hammerhead shark numbers at an aggregation site based on visual census and telemetry data

The application of mark-resight experiments in elasmobranch science has been sparse due to constraints imposed by challenging underwater conditions. The recent development of more general mark-resight estimators provides the framework to test the use of alternatives to sighting methodologies, such as acoustic telemetry. Here we explored the use of acoustic telemetry in combination with underwater visual surveys (UVS) to assess the population size of the endangered scalloped hammerhead shark (Sphyrna lewini). During September 2011, September 2012, and October 2013 field trips deploying acoustic tags and conducting UVS were conducted at Darwin Island, Galapagos Marine Reserve. Resident population and super-population sizes were modeled using the Immigration-Emigration Logit Normal Mark-Resight Estimator. Resident population size was estimated at 500 individuals for 2011, 670 ind. for 2012, and 400 ind. for 2013. Super-population was estimated at 600 ind. for 2011, 1740 ind. for 2012, and 590 ind. for 2013. We found strong evidence of individual heterogeneity and time- and session-dependent resighting probabilities. Our analysis produced a reliable approximation of the population size of hammerhead sharks in this ecological hotspot, supporting the use of combined approaches in underwater mark-resight experiments. Based on the low resighting probability observed, these results suggest Darwin Island is inhabited by a high number of transient individuals (mostly females) dynamically moving between the study site and offshore (feeding or breeding) grounds or unmonitored aggregation sites at other islands and seamounts. Further work should explore the inclusion of environmental covariates with variable numbers of tags to improve estimations of resighting probability and population size in case of temporary emigration of marked and unmarked individuals.