Population Structure, Abundance and Movement of Whale Sharks in the Arabian Gulf and the Gulf of Oman

Abstract

Data on the occurrence of whale sharks, Rhincodon typus, in the Arabian Gulf and Gulf of Oman were collected by dedicated boat surveys and via a public-sightings scheme during the period from 2011 to 2014. A total of 422 individual whale sharks were photo-identified from the Arabian Gulf and the northern Gulf of Oman during that period. The majority of sharks (81%, n = 341) were encountered at the Al Shaheen area of Qatar, 90 km off the coast, with the Musandam region of Oman a secondary area of interest. At Al Shaheen, there were significantly more male sharks (n = 171) than females (n = 78; X2 = 17.52, P < 0.05). Mean estimated total length (TL) for sharks was 6.90 m ± 1.24 (median = 7 m; n = 296). Males (7.25 m ± 1.34; median = 8 m, n = 171) were larger than females (6.44 m ±1.09; median = 7 m, n = 78; Mann-Whitney U test, p < 0.01). Of the male sharks assessed for maturity 63% were mature (n = 81), with 50% attaining maturity by 7.29 m and 100% by 9.00 m. Two female sharks of >9 m individuals were visually assessed as pregnant. Connectivity among sharks sighted in Qatari, Omani and UAE waters was confirmed by individual spot pattern matches. A total of 13 identified sharks were re-sighted at locations other than that at which they were first sighted, including movements into and out of the Arabian Gulf through the Strait of Hormuz. Maximum likelihood techniques were used to model an estimated combined population for the Arabian Gulf and Gulf of Oman of 2837 sharks ± 1243.91 S.E. (95% C.I. 1720–6295). The Al Shaheen aggregation is thus the first site described as being dominated by mature males while the free-swimming pregnant females are the first reported from the Indian Ocean.

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

“…The size at which 50% of the population reached maturity was c. 6.8 m, considerably smaller than that previously reported in the western Indian Ocean using laser photogrammetry (9.2 m, Rohner et al, 2015b) and visually estimated at Ningaloo Reef, Western Australia (8.1 m, Norman and Stevens, 2007). However, it is similar to the size at maturity from the Gulf of Mexico and Qatar, where 50% maturity in males was estimated at 7.0 m and 7.3 m, respectively (Ramírez-Macías et al, 2012b;Robinson et al, 2016). In the present study we used visual estimates, which have inherent errors (Rohner et al, 2011;Sequeira et al, 2016), and thus our results are indicative more than absolute.…”

“…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). Whale sharks at Donsol are uncharacteristically larger than those found elsewhere in the Philippines (e.g., 5.2 m mean total length in the Bohol Sea, Authors, unpub.…”

“…This is considerably longer than that reported elsewhere. Using the same methodology, whale sharks at Panaon Island, Southern Leyte spent c. 27 days in the area (Araujo et al, 2016), and at an offshore aggregation in Qatar, whale sharks spent c. 29 days there (Robinson et al, 2016). In Ningaloo Reef, Western Australia, Holmberg et al (2009) estimated whale sharks spent 33 days in the area using different methods.…”

“…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). Some evidence exists that adults spend most of their time in the open ocean (Ramírez-Macías et al, 2017), which might explain why coastal sites tend to be juvenile dominated.…”

“…Norman and Stevens (2007) reported size at maturity (TL 50 ) of 8.1 m at Ningaloo Reef in Western Australia using a rope of known length, whereas Rohner et al (2015b) reported 9.2 m in Mozambique using paired-laser photogrammetry. Other studies have also used photogrammetry and visual estimates to determine maturity in males (Qatar, 7.3 m, Robinson et al, 2016;Gulf of Mexico, c. 7.0 m, Ramírez-Macías et al, 2012b, respectively), yet concerns remain about the inaccuracy of such approaches (see Sequeira et al, 2016). Caution should therefore be taken when using these approaches to determine the demographics of whale shark aggregations.…”