Factors influencing blue whale aggregations off southern Sri Lanka

[Truncated] Blue whales (Balaenoptera musculus) inhabiting the Northern Indian Ocean are thought to comprise a distinct population or even potentially a different subspecies. Elsewhere, blue whales migrate long distances between lower latitude wintering grounds and higher latitude summer feeding grounds. Northern Indian Ocean blue whales, however, are thought to reside in tropical waters year-round including Sri Lankan waters. They have been documented engaging in courtship displays and as mother-calf pairs. They are also observed defaecating and in aggregations typical of feeding areas. The presence and colouration of the scats is good indication that these whales feed on euphausiids in these low latitude waters. Blue whales are found relatively close to the southern coast of Sri Lanka, an area subject to seasonally reversing monsoonal currents, and where a deep submarine canyon incises the narrow, steep continental shelf.

A combination of satellite imagery, field data and numerical modelling were used to examine the physical processes and their potential influence on the distribution of blue whales. The study defined the circulation patterns around Sri Lanka in order to provide an explanation for the year-round, elevated chlorophyll levels along the southern coast. The numerical model also elucidated a previously undocumented shorter period of sporadic upwelling off southern Sri Lanka during the Northeast monsoon. The major upwelling region is located along the south coast and is shown to be due to flow convergence and divergence associated with offshore transport of water. The location of the upwelling centre was dependent on the relative strengths of wind driven flow along the east and west coasts.