Oceanographic influences on a global whale shark hotspot in southern Mozambique
Published on 04. December 2014
Oceanographic influences on a global whale shark hotspot in southern Mozambique
Christoph A Rohner, Scarla J Weeks, Anthony J Richardson, Simon J Pierce,
Marites M Magno-Canto, Gene Carl Feldman, Geremy Cliff, Michael J Roberts
ABSTRACT:
Coastal aggregations of whale sharks Rhincodon typus around the world are generally seasonal and driven by prey availability. At a major aggregation site in southern Mozambique, whale sharks are, somewhat unusually, present and seen feeding throughout the year. We investigated potential oceanographic mechanisms that may regulate prey availability on the narrow regional shelf and hence account for this year-round whale shark hotspot. We used regional aerial surveys to show that the highest density of whale sharks (29 sharks 100 km-1) was near Praia do Tofo (23.85˚S, 35.55˚E). To investigate how the regional oceanography influences the enrichment of shelf waters, we used 5- and 9-year time series of hourly underwater temperature, and 10-year time series of remotely-sensed sea surface temperature, chlorophyll-a concentration and sea surface height anomaly data. We found that upwelling of cool, nutrient-rich water was common in the region throughout the year and describe three mechanisms, all of which are likely to stimulate productivity: (1) Shelf-edge upwelling and subsequent elevated plankton biomass in coastal waters north of Praia do Tofo, driven by the interaction of southward-propagating mesoscale eddies with the narrow shelf. In situ temperature data show that this interaction frequently leads to intense upwelling, up to 7.5˚C daily amplitude, throughout the year, but is most pronounced in spring/summer. (2) Divergent upwelling south of Praia do Tofo driven by the current flow along the shelf edge as it diverges from the coastline. This upwelling can occur throughout the year and is similar in intensity, but less frequent, than the shelf-edge upwelling. (3) Vortex-driven upwelling by the Delagoa Bight lee-eddy, which may increase phytoplankton biomass in the Bight and also force a northward coastal current that transports recently-upwelled water towards Praia do Tofo. We hypothesise that whale sharks aggregate in coastal waters around Praia do Tofo throughout the year because these upwelling mechanisms contribute to year-round productivity in the region.
PeerJ PrePrints 2:e661v1