MMF Principal Scientist Chris Rohner has led a groundbreaking study into preferred whale shark habitats using ‘robot sharks’ and comparing their digital movements to the movements of real whale sharks.
Chris and his team found that the real whale sharks preferred the coastal waters of southern Mozambique (between Zavora, Tofo and Pomene), where the water was cooler, richer in chlorophyll-a and shallower than in the semi-randomly moving robot sharks' habitat. The study also showed that in that same preferred area, the use of gill nets is on the rise and posing a significant threat to the whale sharks.
Real whale sharks preferred the coastal waters of southern Mozambique, where the water was cooler, richer in chlorophyll-a and shallower than in the semi-randomly moving robot sharks' habitat
The whale shark is an endangered, highly migratory species with a wide, albeit patchy, distribution through tropical oceans. Ten aerial survey flights along the southern Mozambican coast, conducted between 2004–2008, documented a relatively high density of whale sharks along a 200 km stretch of the Inhambane Province, with a pronounced hotspot adjacent to Praia do Tofo.
To examine the residency and movement of whale sharks in coastal areas around Praia do Tofo, where they may be more susceptible to gill net entanglement, Chris and the team tagged 15 juveniles with SPOT5 satellite tags and tracked them for 2–88 days as they dispersed from this area.
Chris and the team tagged 15 juvenile whale sharks with SPOT5 satellite tags
While several individuals left shelf waters and travelled across international boundaries, most sharks stayed in Mozambican coastal waters over the tracking period. The team tested for whale shark habitat preferences, using sea surface temperature, chlorophyll-a concentration and water depth as variables, by computing 100 random model tracks for each real shark based on their empirical movement characteristics.
Whale sharks spent significantly more time in cooler, shallower water with higher chlorophyll-a concentrations than model sharks, suggesting that feeding in productive coastal waters is an important driver of their movements. To investigate what this coastal habitat choice means for their conservation in Mozambique, the team mapped gill nets during two dedicated aerial surveys along the Inhambane coast and counted gill nets in 1,323 boat-based surveys near Praia do Tofo.
The study suggests that feeding in productive coastal waters is an important driver of whale shark movements
This study shows that, while whale sharks are capable of long-distance oceanic movements, they can spend a disproportionate amount of time in specific areas, such as along the southern Mozambique coast. The increasing use of drifting gill nets in this coastal hotspot for whale sharks is likely to be a threat to regional populations of this iconic species.
To read about the study in more detail, please click here.