Studying spatial distribution of the whale shark in the Gulf of Tadjora, Djibouti
Authors: Leblond Savinien T. R. , Rowat David R. L. 
Published: July 2019

Background The Marine Conservation Society Seychelles, in partnership with Dolphin Excursions and NGOs Decan and Megaptera, have been monitoring the whale shark population off Djibouti during the peak aggregation season since 2006. The first formal scientific study found a substantial aggregation of very small juvenile whale sharks off the Arta coastline and in subsequent years a significant number of photo-identified individuals were resighted and appeared to be preferentially using certain areas. Approach This study explored the spatial utilization of the site during the annual two week monitoring trip in January 2014. Monitoring was done on two surveys each day for two six-day periods. We focused mainly on the known aggregation areas around Arta and Acacia coastlines, with a one-day survey to Ghoubet al Kharab (20 km to the West) each week. A GPS waypoint was recorded for every shark encountered which was identified by the spot pattern using the I3S Photo Identification software. Geographic Information System pattern analyses (Getis Ord Gi* tool, with a 10 m nearest neighbour band) were carried out to assess whether there was any significant clustering or over-dispersion in the spatial distribution of sharks during the morning and afternoon. The encounter data were further analysed using SOCPROG to define movements between the areas and any tendency for individual associations. Results During the two week study, 782 encounters were recorded with 127 individual whale sharks. These sharks were seen along the 3 km of Arta and 4 km of Acacia coastline in the survey area. During the morning 13% of sharks clustered in significant “hot spots' off Acacia and 32% of Arta, while 13% and 1% were found to be over-dispersed in “cold spots' in both Acacia and Arta respectively (95%C.I.). When examining the spatial distribution in the afternoon no significant cold spots areas of overdispersion were found, while 33% of sharks where found in hot spot clusters off Acacia and 28% in Arta (95%C.I.). Morning aggregations at Acacia spanned a length of 3 km along each coast of Acacia and Arta while afternoon aggregations were more compact only spanning 2 km at Acacia and 1.5 km at Arta. Clusters were located up to 500 m off the shore lines in the mornings compared to 200 m in the afternoons. Movement models indicated transitions both between Acacia and Arta and also into and out of the study area. 17 sharks associated with another specific individual 6 or more times (max 46) during the study period with a Coefficient of Variation of 1.23 between sampling periods. A hierarchical cluster analysis defined 6 clusters with a Cophenetic Correlation Coefficient of 0.79. Conclusion The juvenile whale shark aggregation in the Gulf of Tadjora occurs mainly along the coastlines of Acacia and Arta with sharks forming more significant/dense clusters in the afternoons. The hot spots occur over areas of healthy coral reef with many branching and tabulate acropora corals while the cold spots form near the 45 degree rubble slopes with little or no coral cover. The westerly winds in the gulf drive plankton into the coves of Acacia and Arta which accumulate around coral reefs intensifying whale shark feeding behavior in these areas. While associations between individuals were identified between daily sampling periods this was found for only a small number of individuals over a relatively short time period; further studies are needed to see if these associations remain stable over longer periods.