The ecology of whale sharks Rhincodon typus within the Arabian Gulf and Gulf of Oman

Abstract

Historically, whale sharks, Rhincodon typus, have been infrequently documented from the Arabian Gulf and the northern Gulf of Oman. This thesis documents the population and movement ecology of whale sharks within this region between 2011 and 2014. The Al Shaheen Area of Qatar and the Musandam region of Oman were the main hotspots for whale shark occurrence. The Al Shaheen oil field is located 90 km off the coast of Qatar in the Arabian Gulf. Platform workers reported 4351 individual whale sharks between 2011 and 2014 based on daily cumulative sightings. Peak numbers of whale sharks were observed between May and September from the west of Al Shaheen. Density estimates of up to 150 sharks within an area of 1 km2 were recorded. Most animals observed were actively feeding on surface zooplankton, consisting primarily of mackerel tuna Euthynnus affinis eggs. A total of 420 individual sharks were identified from their spot patterns from the entire region, with the majority (81%, n=341) encountered at Al Shaheen. Population size for the Arabian Gulf and Gulf of Oman combined was estimated using the program SOCPROG at 2837 sharks ± 1243.91 S.E. (95% C.I. 1720-6295) with 120 sharks present on any given day. A mean residency period of 17 days within this area and 31 days outside the study area was estimated. A model representing a fully mixed population showed the best goodness of fit to investigate Lagged Identification Rate (LIR) of occurrence between two areas, inside and outside of the Arabian Gulf, suggesting a degree of interchange between the Arabian Gulf and Northern Gulf of Oman populations. Inter-annual re-sighting rates of sharks at Al Shaheen varied between 22 and 88%. The estimated total length (TL) for all sharks assessed in Al Shaheen was 6.9 m (± S.D. 1.24, N=296), with males (7.25 m ± 1.34, N=171) being larger than females (6.44 m ±1.09, N=78; t = 4.68, df = 247, P = 0.0001). This aggregation had a significant male bias (69% N=248), with the majority of males assessed as mature (63%, N=81). This is the first whale shark aggregation site dominated by mature animals. Length at which 50% of the animals were mature (TL50) was 7.29 m with all animals at 9 m being mature based on clasper morphology. Movements of individual sharks between the Qatari, Omani and UAE waters were confirmed by individual spot pattern recognition. Satellite tracking of whale sharks within the region showed that whale sharks prefer depths in excess of 40 m and rarely made dives deeper than 100 m. Temperature preferences were between 27 °C and 33 °C. Sharks were seen to aggregate in the summer and disperse widely throughout the region in the winter months outside of the tuna-spawning season. Genetic analysis of whale sharks from Al Shaheen determined that the whale sharks found in this area were genetically similar to the wider Western Indian Ocean populations. Stable isotope analysis showed distinct differentiation in feeding for sharks from Qatar, Tanzania and Mozambique. Anthropogenic threats from the region were found to include boat impacts, accidental and direct capture in nets and a target fishery using tail ropes. Species-specific protection should be considered within countries hosting regional whale shark hotspots and, in addition, whale sharks should be afforded protection within Al Shaheen.