by Clare Prebble
Mafia is the last island in the Zanzibar Archipelago off the coast of Tanzania. This is a small island that harbours some very big fish. Each year in Killindoni Bay on the eastern side of the island, whale sharks gather from November to March in abundance. Chris Rohner, MMF’s first PhD student, moved to Mafia for the 2012-2013 whale shark season after completing his PhD in Tofo. He is leading a World Wildlife Fund (WWF) funded project in collaboration with the whale shark research team from King Abdullah University of Science and Technology (KAUST) and the Tanzanian Fisheries Research Institute (TAFIRI) to find out more about this aggregation of whale sharks.
Although there is currently only a light impact from ecotourism on the sharks, factors such as boat traffic, local fisheries, water noise and pollution may put these animals at risk. For this project, Chris has been utilising the skills from his PhD thesis and collecting a diverse array of environmental data, such as sea state, water temperature, plankton composition, and salinity. The team are also using a carefully placed array of acoustic stations, and have deployed 30 acoustic tags onto resident sharks. Combining these two sets of data will let us see how the whale sharks use the area in Killindoni Bay and identify the most important factors that draw the whale sharks there.
In addition to this work, Chris had also kindly agreed to collaborate with Clare Prebble on the PhD research she is carrying out in Tofo. He has been contributing to the project by collecting tissue samples from the Mafia sharks to include in her analyses. So, earlier this month, Clare had the amazing opportunity to visit Chris on Mafia and get a feel for the island and the research going on there.
After just a short half hour plane ride from the capital, Dar es Salaam, Clare landed in the modest airport in Killindoni. The rather rudimentary camp where the research team is based is only a five minute ride from the airport by tuk-tuk. There she met with Chris, Fernando Cagua of KAUST, and Rilke Ballero the camp manager who shed some light on all things whale shark on the island and enabled comparisons with working in Tofo.
Rilke, Fernando, and Chris on the whale shark dhow
As the wide channel between the mainland and the island is relatively sheltered, this creates extremely calm conditions on the water. Conversely, such days are certainly a rarity in Tofo! As well as differences in working conditions, preliminary results from this year’s data already show some differences in the behaviour between the two shark aggregations. It seems that the Mafia population are highly resident individuals with a number of re-sightings during the season. A staggering 38,000 recordings of the 30 tagged sharks have been recorded on the acoustic stations in three months. In contrast, here in Tofo we have a fluctuating year round population where we rarely see individuals for more than a few days at a time.
As the Mafia aggregation seems to be exploiting the local seasonal food source, the sharks displayed active feeding behaviour at the surface in almost all instances. Meanwhile, in Tofo, sharks are most often seen serenely cruising along below the surface. These differences will add an interesting dimension to Clare’s PhD work, analysing the residency and movement patterns of several whale shark populations around the world.
Fernando and Clare carrying out plankton tows in a very tranquil Killindoni Bay
One of the issues that whale sharks in Mafia face is the threat of net entanglement from the local fishers. Luckily, there seems to be no current demand for the fins or meat of the sharks from the local people. So if caught, a great effort is made to release the sharks. Due to the size and strength of the sharks, they could break or swim away with the nets so it is in the best interests of all parties to release the shark. The interactions with the fishers also continue into the night, when sightings of whale sharks are often reported near fishing boats, and sometimes result in net entanglements.
With this in mind Clare and Fernando took the opportunity to join a local fishing boat during the night shift and discover more. After an enlightening but somewhat surreal experience with the team of fishers, it became clear that their methods of fishing may indeed attract the whale sharks to the boats. Two hours after sunset the fishers send out two small satellite boats with several lanterns on board. The idea behind this is to attract the night zooplankton to the light. This in turn attracts the large shoals of small planktivorous fishes. The nets are then set around the boats to catch the fish. As the whale sharks also have a penchant for plankton they are often attracted to the same areas.
Mafia fishermen preparing the nets for the night’s fishing
As it is much harder to see the sharks in the water at night the nets can be set around them which is detrimental for both the shark and the fishers. These local interactions are certainly issues which need to be considered when planning the future management of the Mafia whale shark population.
Overall, it was refreshing and interesting to experience an aggregation of whale sharks that appear to behave so differently to those in Tofo. It was also a privilege to meet Fernando and Rilke who we would like to especially thank. Without them the trip would not have been half as smooth or enjoyable. We hope to work with them again in the future.
Sunset from the camp at Killindoni