January and February are traditionally peak months for giant manta aggregations around the southern Mozambican coast. While routinely working offshore in the Bazaruto Archipelago National Park (BANP) monitoring marine life, BCSS scientists spotted large aggregations of giant manta rays (Manta alfredi) in different locations on several days from one of the BCSS research boats. On the 31st of January, aggregations exceeded 20 individuals sighted on the surface. The mantas were feeding on the plankton trapped on a “front” on the surface following a water current line. The scientists of BCSS were able to record the spatial data of the mantas for several days, as well as monitoring their behaviour from the boat. (text continues after photo)
There were giant mantas in various locations in the Bazaruto Archipelago, all surrounding Benguerra Island, the second largest island of the archipelago. Most mantas were roaming offshore waters, yet close to the islands; shallow waters that were no deeper than 30 meters, east from Benguerra Island. A few mantas were also spotted in deeper waters in an area called “The Canyon”, as well as in the southern pinnacles of San Sebastian Peninsula. Yet, the largest aggregations occurred near Benguerra Island.
Giant manta rays primarily source of food is plankton. Plankton blooms occur regularly off the Bazaruto Archipelago, making parts of the water’s surface colour light green/brown, which can easily be seen from the surface. A few days before the expeditions, the wind was shifting from south to north, which potentially trigged an upwelling event causing the water column to change composition, also driving convection cells. This created a very defined “front” event, which traps plankton, concentrating it into high densities. BCSS scientists suggest that a type of plankton (unidentified for now) attracts the mantas in such high quantities, which also exploit the high plankton concentrations created by the convection cells of the “front”.
Giant manta activity is common in the Bazaruto Archipelago from December to March. However, it is quite rare to see them in large numbers like this. It is a good sign in regard of the health of the Bazaruto Archipelago ecosystems to see so many mantas. BCSS maintains a time-series database detailing marine animals sightings on a weekly basis, which will be soon accessible through the online platform and website for the Ocean Observatory.