oa On their way to pupping grounds?: Unprecedented parade of large female whale sharks at Darwin Island, Galapagos Marine Reserve

Background The Galapagos Whale Shark Project is a multi-institutional effort initiated in 2011 with the aim to characterize the presence, population structure and movement patterns of whale sharks within and around the Galapagos Marine Reserve. This specific study aimed to understand their occurrence, residency and habitat use around Darwin Island, located at the northern tip of the Galapagos Archipelago, where large individuals had been reported to be seasonally abundant. Approach This study followed a diversified methodology approach that included: analysis of a local underwater visual census database of pelagic species (2007–2013) to establish seasonality in their occurrence; specific whale shark surveys (2011–2013) to collect information about shark's size (laser photogrammetry), sex, behavior and signs of potential pregnancy; photo-ID records (2011–2013) obtained during these surveys to determine residency and abundance; and the deployment of acoustic tags for continuous tracking around Darwin Island to assess habitat use at the study site. Results Whale shark presence at Darwin Island follows a seasonal pattern. During the cool season (July–December), a strongly female-biased whale shark population, composed mostly (91.8%) by large individuals (11.35 m ± 0.12 m (TL ± SE)), pass through the study site. The great majority of these individuals show clear distended bellies, which could be a sign of a potential pregnancy. Population dynamics models for these apparently pregnant sharks estimated the presence of 3.76 ± 0.90 (mean±SE) sharks in the study area per day with an individual residence time of 2.09 ± 0.51 (mean±SE) days. Assuming constancy in these rates for the entire cool season, we can estimate a net abundance of 695 ± 166 (SE; 95%CI 442–1110) apparently pregnant whale sharks per season. Movement patterns analysis of four apparently pregnant individuals revealed an intense use of Darwin's Arch, where no feeding or specific behavior has been recorded or could be inferred from their dive profiles, together with periodic excursions around the island's vicinity. Sharks showed a preference for intermediate depths (20–30 m) with occasional dives mostly to mid-water, remaining the majority of their time at water temperatures between 24–25°C. Conclusions The lack of evidence of specific behavior observed at Darwin Island, together with the short residence time and strong intra-seasonal abundance and high turnover rate, indicate that this location is not an aggregation site but an important stopover in a migration. In the case of adult R. typus individuals observed, this migration might involve reproductive purposes, as all but one were apparently gravid.