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

Abstract

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.

Underlying environmental factors and geomagnetic fields linked to 45 pilot whalesGlobicephala macrorhynchusstranding in Modung white beach, Indonesian coast on 19thFebruary 2021

“…For whale shark, coastal areas of shallow bathymetry in close proximity to the reef slope and deeper water were preferred habitats and whales were common found in this area (De la Parra Venegas et al 2011, Donati et al 2016, Diamant et al 2016). This whale behavior is in accordance with the stranding case of pilot whale in this study where the whale aggregations were found in shallow water.…”

Does bathymetry drive coastal whale shark (Rhincodon typus)aggregations?

“…spawn in Belize ( Heyman et al, 2001 ). However, Donati et al (2016) describe the aggregation at South Ari atoll as a yearlong phenomenon with no seasonal peak and there is a current lack of understanding as to the main cause of this aggregation if it is not seasonally prey driven. Nonetheless, these whale sharks will feed on plankton and various spawn in the shallow coastal waters where they are regularly observed ( Hoffmayer et al, 2007 ; Motta et al, 2010 ).…”

“…Studies have shown almost all R. typus aggregations occur in coastal areas of shallow bathymetry in close proximity to the reef slope and deeper water ( De la Parra Venegas et al, 2011 ; Donati et al, 2016 ; Diamant et al, 2016 ). Past studies have illustrated increased zooplankton availability in areas of steep bathymetry, which R. typus preys upon ( Bouchet et al, 2015 ).…”

Analysis of the temporal and spatial variability of whale shark (Rhincodon typus) aggregation in the South Ari Marine Protected Area, Maldives, Indian Ocean

“…In the coastal waters of the Republic of Maldives, whale sharks are seasonally present and show a semi-annual residency pattern. They move along the western atolls during the northeast monsoon (from December to April), and along the eastern atolls during the southwest monsoon (from May to November) (Anderson & Ahmed 1993;Riley et al 2010;Anderson et al 2011;Cagua et al 2014;Donati et al 2016). Movements of whale sharks along the double chains of atolls of the Maldives, appear to be affected principally by the seasonal variability in productivity (Anderson et al 2011) and by ocean currents (Anderson & Ahmed 1993;Wilson et al 2001).…”

“…The south region of Ari Atoll (Alif Dhaalu Atoll) represents an exception compared to the other atolls in the Maldives. Indeed, although the driver of aggregation has not to be defined yet, this area hosts year-long aggregations, mainly composed of juvenile males (Riley et al 2010;Donati et al 2016;Perry et al 2018;Allen et al 2021;Harvey-Carroll et al 2021). The high re-sighting rate of some individuals suggests a high site fidelity over the years, and the absence of adult, neonates, and female indicates that this area might be a "secondary nursery area" and a developmental habitat for juvenile males (Perry et al 2018;Allen et al 2021;Harvey-Carroll et al 2021).…”