Abstract
Blue whale (Balaenoptera musculus) movements are often driven by the availability of their prey in space and time. While globally blue whale populations undertake long-range migrations between feeding and breeding grounds, those in the northern Indian Ocean remain in low latitude waters throughout the year with the implication that the productivity of these waters is sufficient to support their energy needs. A part of this population remains around Sri Lanka where they are usually recorded close to the southern coast during the Northeast Monsoon. To investigate inter-annual variability in sighting locations, we conducted systematic Conductivity-Temperature-Depth (CTD) and visual surveys between January–March 2011 and January–March 2012. In 2011, there was a notable decrease in inshore sightings compared to 2009 and 2012 (p < 0.001). CTD data revealed that in 2011 there was increased freshwater in the upper water column accompanied by deeper upwelling than in 2012. We hypothesise that anomalous rainfall, along with higher turbidity resulting from river discharge, affected the productivity of the inshore waters and caused a shift in blue whale prey and, consequently, the distribution of the whales themselves. An understanding of how predators and their prey respond to environmental variability is important for predicting how these species will respond to long-term changes. This is especially important given the rapid temperature increases predicted for the semi-enclosed northern Indian Ocean.
Highlights
Given the importance of krill to foraging blue whales, and the close relationship between physical oceanographic variables and krill distribution, we investigated the links between salinity, sea surface temperature and blue whale distribution and abundance over the years 2009, 2011 and 2012
HYCOM global hindcast model data of sea surface salinity indicated the presence of high salinity water between 33.5 and 34.5 PSU around Sri Lanka in 2009 and 2012 (Figure 2)
No anomalous rainfall was experienced during the same period in 2009 and 2012 (Figure 3)
Summary
Blue whale populations within the northern Indian Ocean are thought not to undertake polar migrations but remain in warm low latitude waters year round [16], with a part of their population remaining resident around Sri Lanka as evidenced by year-round sightings, strandings and acoustic detections [1]. That they choose to remain resident in tropical waters suggests that there is sufficient food in the area to offset the need to migrate
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