Analysis of whale shark Rhincodon typus aggregations near South Ari Atoll, Maldives Archipelago

Abstract

We made surveys for whale sharks Rhincodon typus on a total of 99 d from April through June each year from 2006 to 2008 along the southern fringe of the South Ari Atoll, Maldives Archipelago. We recorded the length and sex of each shark observed and made photographs to facilitate repeated identification from their spot patterns using pattern-recognition software. We identified 64 whale sharks from digital photographs taken during 220 sightings over 3 yr. Approx. 87% of those sharks were immature males. The average length of recognisable sharks was 5.98 m (range 2.5 to 10.5 m), significantly shorter than that reported for whale sharks in other aggregations in the Indian Ocean. Our findings suggest that these sharks are either a small proportion of a local population or perhaps an even smaller component of a regional population in the western Indian Ocean. We applied a Lincoln-Petersen closed-population mark-recapture model and a Jolly-Seber openpopulation model to estimate population size, but found that neither model provided reliable results because key assumptions of each were not met.

Long-Term Photo-Identification Reveals the Population Dynamics and Strong Site Fidelity of Adult Whale Sharks to the Coastal Waters of Donsol, Philippines

“…There was significant male bias (87%) for those individuals whose sex could be confirmed, though it could not be determined for 62.5% of individuals. Such male bias is consistent with coastal aggregations from across the globe Rowat et al, 2007Rowat et al, , 2009Riley et al, 2010;Ramírez-Macías et al, 2012a,b;Himawan et al, 2015;Rohner et al, 2015b) and with others from within the Philippines (Araujo et al, 2014. Adult-dominated sites remain a rarity, namely at Darwin's Arch in the Galapagos Islands (ECU), Gorda Banks in Baja California (MEX), at an offshore aggregation in Qatar (QAT), and at St Helena Island (GBR) in the South Atlantic, and at a newly identified area in the mid-equatorial Atlantic off Brazil (Ramírez-Macías et al, 2012b;Acuña-Marrero et al, 2014;Clingham et al, 2016;Macena and Hazin, 2016;Robinson et al, 2016).…”

“…The population estimate at Donsol of 1,767 ± 41 is comparable to estimates from the Gulf of Mexico of 2,167 individuals (McKinney et al, 2017), although different methods were applied. It is considerably larger though than estimates from Ningaloo with 320-440 individuals , the Seychelles with 348-488 individuals (Rowat et al, 2009), the Maldives with 68-81 individuals (Riley et al, 2010) or Holbox Island, Mexico, with 521-809 individuals (Ramírez-Macías et al, 2012a) using the same methods. Although markrecapture approaches provide an estimate of population size, these numbers are more indicative than absolute, particularly in light of a lack of standardized methods for estimating whale shark abundance across sites.…”

“…Whale shark aggregations worldwide are mostly dominated by juvenile males (e.g., Meekan et al, 2006;Rowat et al, 2007Rowat et al, , 2009Riley et al, 2010;Ramírez-Macías et al, 2012a,b;Araujo et al, 2014Araujo et al, , 2016Himawan et al, 2015;Rohner et al, 2015b;McKinney et al, 2017). Few aggregations dominated by adults have been identified to date, namely the Galapagos Islands (Acuña-Marrero et al, 2014), St. Helena Island (Clingham et al, 2016) and the Revillagigedo Islands off Baja California (Ramírez-Macías et al, 2012b), and a large proportion of adults has been reported at an offshore aggregation in Qatar (Robinson et al, 2016) and in the mid-equatorial Atlantic off Brazil (Macena and Hazin, 2016).…”

Long-term membership of whale sharks (Rhincodon typus) in coastal aggregations in Seychelles and Djibouti

“…In the Maldives, 63 individuals were identified (95% immature males) with a mean length of 5.9 m AE 1.5 m s.d. (Riley et al 2010), while in Mozambique 300 individuals (81% males) were identified with a mean length of 6.7 m (Pierce et al 2008). By comparison, the Djibouti aggregation, while similar in sex ratio to these other aggregation areas with 85% males to 15% females, had a much smaller mean length than whales sharks in any other aggregations (3.7 m AE 0.6 m s.d.).…”

“…Most information on these sharks comes from studies conducted in coastal areas where the sharks are known to aggregate on a regular basis and where the majority of sharks have been found to be juveniles or sub-adults (,8 m), such as at Ningaloo Reef, Western Australia (Meekan et al 2006), the Maldives (Anderson and Ahmed 1993;Riley et al 2010), northern Mexico (Eckert and Stewart 2001) and the Philippines (Alava et al 1997) (Fig. 1).…”