The needle in the haystack: Searching for a whale shark aggregation on the Great Barrier Reef
Over three years ago, Run for the Reef approached us to undertake a project to investigate the occurrence and habitat use of whale sharks on the GBR. Our initial reaction was that was akin to looking for a needle in a haystack. Whale sharks are sighted on the GBR and in the Coral Sea, but not that often and most of the time not in large numbers. All whale shark research around the world is conducted at known aggregation sites (Map 1), it has to be, because without a predictable aggregation, we would be spending a small fortune in time and resources to be in the air and on the water long enough to find the occasional shark to tag and collect samples. Which makes studying these large highly mobile animals outside of known aggregations almost impossible.
Map 1: Showing whale shark aggregations around the world where research is conducted. Highlighting Australia’s east coast is a hole in the global whale shark knowledge
So determining if the GBR has a predictable aggregation site became the initial focus of the study. To do this we collated a range of data and information, including mapping historical sightings, recent social media posts and aerial survey data, examining oceanography and bathymetry information and marlin movements to identify productive areas on the GBR. We layered all this information, and the information we had pointed to Wreck Bay as the most logical area to investigate, and the most likely time to investigate being November-December. The bathymetry of Wreck Bay is similar habitat to other aggregation sites around the world, November-December the area is productive due to currents and upwellings, it is the time that Marlin will move to this area, and most importantly, we did receive reports of over 50 whale sharks being sighted in this area during that time period on multiple occasions.
Map 2. Sightings of whale sharks in Queensland. Wreck Bay the site we need to investigate.
Further evidence came from the first whale shark satellite tagged on the GBR. On the 24th of October 2018 Dan McCarthy from Big Fish Down Under deployed our first satellite tag on a whale shark. The shark swam to Wreck Bay and remained there for 2 weeks in November, the location and time we predicted would be the best time to undertake an expedition search for whale sharks.
In November 2019 we undertook the Far Northern GBR expedition to Wreck Bay on the vessel Argo with a camera crew and a team of researchers from Oceans Foundation, Ecosearch and Project Manta.
Success and Frustration
We spent five days in Wreck Bay searching for whale sharks, and after a slow start (no sharks in first 2 days), we found up to 13 individuals over three days (13 individual sightings, so could be some repeats over multiple days), and we satellite tagged four individuals.
Given that we were told it was ambitious to expect to tag any whale sharks on the expedition, we were pleased that this exploratory expedition found some sharks, and we got some tags deployed. After the satellite tags were attached, the four sharks all started sending messages, and remained in the Wreck Bay region for the next month.
However, the expedition was trialing a new clamp system for attaching satellite tags to the dorsal fin, and unfortunately, despite the initial success in sending data, the sharks all stopped sending data over the next few months. Given the battery life of the tags is 4 years, we have to assume the attachment method failed and the clamps have come off.
The longest track was from 17th November 2019 to 13th February 2020. The track showed that the whale shark remained in the Wreck Bay region until mid-January before moving out into the Coral Sea. All shark tracks can be followed on OCEARCH and CitizensGBR marine life tracker – reeftracks.org
This was the first expedition to study whale sharks in the northern GBR, and we really did not know if we would even find a shark, so as an exploratory expedition/pilot study, we had some success. But we also learnt some valuable lessons that should help us maximize our time in the north this year and manufacture a better attachment for the satellite tag, as we will be heading to Wreck Bay again later in 2020 to search for whale sharks again, and hopefully be more successful with tagging. Check live satellite tracking on OCEARCH
Have we found the needle in the haystack, i.e. an aggregation site? It is too early to say, but the movement information gained from future satellite tagging will be crucial to determining aggregation sites and understanding the importance of the GBR for whale shark ecology.