Some like it hot: Repeat migration and residency of whale sharks within an extreme natural environment

Abstract

The Arabian Gulf is the warmest sea in the world and is host to a globally significant population of the whale shark Rhincodon typus. To investigate regional whale shark behaviour and movements, 59 satellite-linked tags were deployed on whale sharks in the Al Shaheen area off Qatar from 2011-14. Four different models of tag were used throughout the study, each model able to collect differing data or quantities of data. Retention varied from one to 227 days. While all tagged sharks crossed international maritime boundaries, they typically stayed within the Arabian Gulf. Only nine sharks dispersed through the narrow Strait of Hormuz into the Gulf of Oman. Most sharks stayed close to known or suspected feeding aggregation sites over summer months, but dispersed throughout the Arabian Gulf in winter. Sharks rarely ventured into shallow areas (<40 m depth). A single, presumably pregnant female shark was the sole animal to disperse a long distance, crossing five international maritime boundaries in 37 days before the tag detached at a distance of approximately 2644 km from the tagging site, close to the Yemeni-Somali border. No clear space-use differentiation was evident between years, for sharks of different sizes, or between sexes. Whale sharks spent the most time (~66%) in temperatures of 24-30°C and in shallow waters <100 m depth (~60%). Sharks spent relatively more time in cooler (X2 = 121.692; p<0.05) and deeper (X2 = 46.402; p<0.05) water at night. Sharks rarely made dives deeper than 100 m, reflecting the bathymetric constraints of the Gulf environment. Kernel density analysis demonstrated that the tagging site at Al Shaheen was the regional hotspot for these sharks, and revealed a probable secondary aggregation site for whale sharks in nearby Saudi Arabian waters. Analysis of visual re-sightings data of tagged sharks revealed that 58% of tagged individuals were re-sighted back in Al Shaheen over the course of this study, with 40% recorded back at Al Shaheen in the year following their initial identification. Two sharks were confirmed to return to Al Shaheen in each of the five years of study.

Fig 1. The locations of all study sites and other points of interest for whale sharks within the Arabian Gulf and Gulf of Oman (wider region shown in inset).

Fig 2. Overview of all locations generated from light-level analysis.

The colour of the symbol indicates the tag ID as shown to the right.

Fig 3. The estimated straight-line 2644 km journey of a presumed pregnant whale shark tagged in Al Shaheen.

Fig 4. Overview of all locations transmitted by Argos tags from the Arabian Gulf and Gulf of Oman, split into years and season, together with Arabian Gulf bathymetry.

Fig 5. Kernel density analysis showing the pattern of transmission locations from satellite tags attached to whale sharks from the Al Shaheen area of Qatar and indicating a possible new aggregation site in Saudi Arabian waters.

Fig 6

Kernel Density Analysis and Minimum Bounding Geometry (MBG), showing the 50% and 95% Percentage Volume Contours (PVC) representing the core habitat use and total range respectively for: complete Argos locations (A), locations transmitted during summer (B), and locations transmitted during winter (C).

Fig 7. Overview of the Argos locations transmitted throughout the study period (excluding the Socotra pop-off point) for male and female whale sharks tagged in Al Shaheen.

Fig 8. Mean actual hourly depths for time-series capable tags whilst sharks were within Al Shaheen (blue), outside of Al Shaheen (green) or within the possible aggregation site in Saudi Arabian waters (red).

Fig 9

Estimated satellite tag locations produced from light level analysis throughout deployment of MiniPATs 132232 (A) & 132231 (B) including 95 and 50% location confidence areas and depth. Argos locations throughout deployment of MK10F 129823 (E) & 138516 (G). Deployment time-series depth-temperature data for MiniPATs 132232 (B) & 132231 (D) and MK10F 129823 (F) & 138516 (H). Grey spots represent where temperature data were unavailable.