Blog written by Sammy Andrzejaczek. Read the full paper here.
In the early 2000s, a local conservation organisation reported that juvenile whale sharks were aggregating in the Arta Bay region of the Gulf of Tadjoura, Djibouti, and feeding on dense patches of tiny crustaceans and baitfish. The sharks could be seen between October and February, but no one knew where they went during other times of the year. Our recent study, published in Ecology and Evolution, used satellite tags to find out!
The biggest fish in the sea
Whale sharks (Rhincodon typus) are the largest fish in the sea, growing to over 12 meters in length. Because these sharks are docile, spectacular and often found in large numbers at coastal sites, they are the target of ecotourism industries worldwide. In Djibouti, the aggregation of whale sharks is unusual in that these sharks seem to be smaller in size (on average ~3.7 metres long) and more numerous than in almost all other coastal aggregations, with over 100 encounters a day recorded by research teams.
Despite their popularity, very little is known about whale sharks outside of seasonal aggregation sites. So where do these animals go when beyond the prying eyes of tourists? This is an important question to answer, particularly as populations are declining on a global scale. Whale sharks grow slowly, taking up to 30 years to reach maturity. This along with their high vulnerability to ship strikes and bycatch in fisheries worldwide, led to this species being listed as ‘Endangered‘ on the IUCN Red List. If conservation strategies for the species are to be successful, we need to know where they are going and the places they visit on their migrations.
Whale sharks off the coast of Djibouti are no exception to these risks, being situated close to one the world’s busiest shipping lanes in the Gulf of Aden, as well as countries such as Somalia, with high estimated shark catch occurring in artisanal fisheries. Complicating efforts to understand whale shark movements from Djibouti is the remote and often inaccessible nature of the coastline in surrounding regions.
Technology to the rescue
We used electronic tags that transmit location information to Earth-orbiting satellites to track the movements of whale sharks from the Djibouti aggregation site. To deploy the tags, our team swam alongside each whale shark and attached the device to the back of the shark. Once attached, tags would record their location and diving behavior over a period of several months, and we could use these data to remotely reconstruct and analyze the sharks’ movements. We waited until the end of the season (December – January) to deploy the tags in the hope the tagged sharks would reveal to us where they moved to once they left Djibouti waters.
The first whale shark was satellite tagged in Djibouti in January 2006 as a part of the on-going whale shark monitoring programme run by the Marine Conservation Society Seychelles (Djibouti Project) in collaboration with visiting whale shark researchers from Australia, Seychelles and the United Kingdom. The 3.5m male shark was named ‘Shiraz’, and he provided the first detailed information on this species’ movement patterns in the area. Unfortunately, Shiraz’s tag fell off early after only nine days, preventing us from seeing if whale sharks migrate away from Djibouti in the early months of the year, or if they stay in the area year-round.
Fortunately, our team had more luck in 2012, 2016 and 2017, when eight sharks were tracked for up to 116 days each!
So, where did they go?
The answer to our question was not as straightforward as we may have hoped – the different sharks all showed different movement patterns! Some sharks remained local, some moved to the coastline of Somaliland, two moved north into the Red Sea, and one moved more than 1000 km away to the east coast of Somalia, outside of the Gulf of Aden.
Tagged whale sharks entered the waters of at least five nations, including Djibouti, Yemen, Eritrea, Somalia and Somaliland. Interestingly, four of the sharks spent time around the coast of Somaliland, with two of the sharks patrolling a 125 km stretch of coastline there for several weeks. Could this be another feeding area for these animals? We can’t be too sure, especially due to the remote and inaccessible region of this coastline making research efforts difficult.
Our tags also recorded the diving behaviour of sharks. We found they spent a lot of time in shallow surface waters but also dove very deep, with two sharks diving deeper than 1000 m! One shark actually did this seven times, diving to a maximum of depth of 1856 m! Why do they do this? It’s also hard to tell, but dives to this deep ‘midnight’ zone of our ocean may allow the sharks to feed on dense aggregations of tiny plankton that hide in these waters.
What does this all mean?
Collectively, the whale shark movements we recorded suggest that these animals are at high risk of exposure to human-driven threats in the Gulf of Aden and surrounding regions. Whale sharks face risks of targeted and/or incidental capture, entanglement and boat strike, and because they move through the waters of different nations, they will receive with varying levels of protection as they travel.
Thanks to the work of local environmental groups, the area around Arta in Djibouti is now designated as a Marine Protected Area to safeguard the primary feeding area for whale sharks, but management efforts in the other areas where we recorded whale shark movements are needed in order to fully protect them.
The Djibouti Project was made possible by the support and cooperation of a number of partners assisting MCSS, including the principal partner, Megaptera, the Ministry of Environment for Djibouti, and overseas partners and researchers from the Marine Conservation Society Seychelles, Australian Institute of Marine Science and Reefcare International (Australia). The costs of the first tag were sponsored largely by the Cooperation Service of the French Embassy in Djibouti (SCAC), by the local tour operator Dolphin Excursions Sarl, and by the Fondation Nature & Découvertes.