New insights into the South Ari atoll whale shark, Rhincodon typus, aggregation

Background Examples of aggregations of large Chondrichthyes are well known worldwide and occur mostly in response to seasonal increases in prey abundance. In the Maldives, whale sharks, Rhincodon typus, are thought to have a semi-annual residency pattern, moving west from December to April and east from May to November. However, an important aggregation of predominantly immature male whale sharks seems to be persistent all year round throughout the South Ari atoll region. Understanding residency patterns of this aggregation as well as inter-atoll movements is fundamental to determine the drivers of aggregation, and to design effective conservation management plans at a regional and national scale. Approach The present study used mark-recapture photo identification of whale sharks sightings collected from 2006 to 2015 in the Republic of Maldives. For the aggregation analysis, the Maldives Whale Shark Research Programme (MWSRP) in collaboration with 52 stakeholders and citizen scientists, members of the Big Fish Network (BFN), provided over 3000 sightings, across the South Ari Marine Protected Area (S.A.MPA). Generalised linear models were used to determine seasonality patterns and the effect of environmental variables. Maximum likelihood population models were fitted to establish the size and residency patterns of the aggregation. Results Through the close collaboration between MWSRP and the BFN, 271 individual whale sharks were successfully identified. At a regional scale, as in most whale shark aggregations worldwide, the South Ari aggregation is mainly composed of immature males (86% of the sexed individuals). However, despite a large seasonal variation in sea surface temperature, primary productivity, wind speed and direction, the lag identification rate did not show any significant seasonal pattern. Population models revealed that the aggregation could be characterised as an open population, with an average of 15 sharks present at any given time. Our findings also imply that whale sharks visit the aggregation for a mean of four years and during that time stay for around two of every six weeks. Conclusions South Ari atoll aggregation shows unprecedented levels of residency and site fidelity. Despite showing size and gender structures similar to other aggregations around the world, this aggregation is remarkable because there is no seasonal peak in sightings. It remains to be investigated if this is a due to aperiodic ocean climate phenomena (as in Tofo, Mozambique) or if sharks are aggregating in response to drivers different than feeding. For instance, exploitation of an undetected food source in adjacent deep waters, and subsequent thermoregulation might account for the observed residency patterns. In conclusion, our findings have important implications for the improvement of the conservation management of this symbolic species in the Republic of Maldives, where it is largely targeted by an unregulated tourism industry.