To gain insight into whale shark (Rhincodon typus) movement patterns in the Western Indian Ocean, we deployed eight pop-up satellite tags at an aggregation site in the Arta Bay region of the Gulf of Tadjoura, Djibouti in the winter months of 2012, 2016 and 2017. Tags revealed movements ranging from local-scale around the Djibouti aggregation site, regional movements along the coastline of Somaliland, movements north into the Red Sea, and a large-scale (>1000 km) movement to the east coast of Somalia, outside of the Gulf of Aden. Vertical movement data revealed high occupation of the top ten meters of the water column, diel vertical movement patterns and deep diving behaviour. Long-distance movements recorded both here and in previous studies suggest that connectivity between the whale sharks tagged at the Djibouti aggregation and other documented aggregations in the region are likely within annual timeframes. In addition, wide-ranging movements through multiple nations, as well as the high use of surface waters recorded, likely exposes whale sharks in this region to several anthropogenic threats, including targeted and bycatch fisheries and ship-strikes. Area-based management approaches focusing on seasonal hotspots offer a way forward in the conservation of whale sharks in the Western Indian Ocean. 

Satellite tags were deployed on whale sharks in the Arta Bay region of the Gulf of Tadjoura (11.57°N, 42.77°E; Fig. 1) in January and/or December in 2012 (n = 2), 2016 (n = 3) and 2017 (n = 3; Table 2). Sharks were visually located by boat-based searches from a 6 m long skiff with a single outboard engine and then approached slowly. Sharks larger than 3.5 m were targeted (1) to satisfy a minimum size for tagging and (2) due to the assumption that only sharks of a certain size migrated from the Gulf of Tadjoura study site. Free-divers entered the water from the vessel to tag and measure sharks, as well as take photo-ID images. Tags were deployed by a pole-spear with a welded plate and rubber buffer to prevent insertion greater than 8-10 cm. 2012 tags were leadered with a ~15 cm length of 45 kg nylon filament covered with several layers of heat shrink tubing and attached via a titanium flat anchor M dart (Wildlife Computers) and placed at the base of the first dorsal fin, on the left side. 2016 and 2017 tags were connected to a large titanium anchor (Wildlife Computers) via a 50 cm stainless steel tether. Tether lengths were selected to allow the anchor to be placed 8-10 cm below the skin and leave space to let the tag lie flat against the body surface in the case of the former, and to facilitate breaking the air-surface barrier for transmission during deployment for the latter. Individual sharks were measured (total length, TL) by one of two methods; 1) by using visual observation and comparing the shark to an object of known size, and/or 2) by an intense photogrammetric laser measurement campaign using the methods as described in Jeffreys et al. (2013). Photo-ID images were also taken of the left and right flanks of tagged individuals and matched with the existing Djibouti database using the public domain pattern-recognition software I³S (Interactive Individual Identification System; Van Tienhoven et al. 2007). All fieldwork was approved by, and conducted with the knowledge of, the Ministry of Environment, Djibouti and local authorities in Arta. All procedures followed standard international guidelines for tagging whale sharks and staff were trained by experts in the field (D. Rowat and M. Meekan; Wilson et al. 2006, Robinson et al. 2017).

Pop-up satellite archival transmitting (PSAT) tags (6 x MK10, 2 x MiniPAT; Wildlife Computers, Inc., WA, USA) recorded light levels, depth and ambient temperature and were programmed to remain attached for 100 days (2012), 120 days (2016) or 153 days (2017), or programmed to detach if recording depths greater than 1800 m or a constant depth reading (± 1.0 m) for more than one week. The tags recorded depth and temperature data in predefined bins every six hours for transmission, with depth bin size varying slightly between 2016 and 2017 deployments (0-2, 2-5, 5-10, 10-25, 50-100, 100-200, 200-300, 300-400, 400-500, >500 and 0-10, 10-50, 50-100, 100-250, 250-500, >500 respectively). Histogram sampling was offset by three hours so that depth and temperature data were collected for local day (6:00-12:00 and 12:00-18:00) and night (18:00-00:00 and 00:00-6:00) periods. The 2012 tags recorded and archived depth data at five-minute intervals, which were subsequently summarized into the 2016 bins. A Wilcoxon rank sum test was used to compare median day- and night-time depths for time-series data from the two 2012 tags. 

Location data and/or processed archived data were transmitted and retrieved through the Argos satellite system when the tags detached. Detachment of the tag from the shark was identified by a combination of near-continuous high quality Argos transmissions for the first few hours of each day and depth summaries from histograms consistent with surface records (Hearn et al. 2013). The tags deployed in 2016 and 2017 also transmitted data when sharks swam at the surface, and, in addition, housed a Fastloc global positioning system (GPS) for acquiring location information.

Data were extracted from the raw tag data using the Wildlife Computers Data Analysis Program 3.0 (available at https://wildlifecomputers.com/support/downloads/). Please refer to the PDF at https://static.wildlifecomputers.com/Spreadsheet-File-Descriptions-1.pdf for more detail on each file. 

Data from sharks 104072 and 104073 are provided in one file.