Distribution and Abundance of the Blue Whale (Balaenoptera musculus indica) off Sri Lanka during the Southwest Monsoon 2018

Abstract

Blue whales Balaenoptera musculus are the most abundant and widely distributed cetacean species in Sri Lankan waters. A vessel-based opportunistic line transect survey focusing on marine mammals was conducted in Sri Lankan waters between 24 June to 12 July 2018, while an ecosystem survey was performed by the R/V Dr. Fridtjof Nansen. The entire transect length was 2090 km within an area of 3895 km2. Overall, 72% (n = 57) of blue whale observations were group assemblages ranging between 1 and 5 individuals out of a total of 79 sightings. The largest aggregations of blue whales were recorded at the intersection of the submarine canyon off the coastline of Mirissa and busy shipping lanes between Dondra Head and Galle, where previous ship strikes were recorded. Overall, the average observed group size was 2.64 (CV, 12.34%; 95% CI, 2.07–3.38), the average group density was 0.0029 km−2 (CV, 35.96%; 95% CI, 0.00145–0.00610), and the total blue whale abundance within the survey area was 513 individuals (CV, 38.02%; 95% CI, 243–1083). These survey results fill vital knowledge gaps regarding the abundance and distribution of blue whales in Sri Lanka, which is essential for the establishment of management and conservation strategies.

Seasonal Occurrence of the Indian Ocean Blue Whale (Balaenoptera musculus indica) off South Coast of Sri Lanka

“…Meanwhile, in a cetacean survey in the Maldivian waters in the 2003-2004 northeast monsoon season, no BWs were recorded [33]. The Fridtjof Nansen ecosystem survey in the 2018 SW monsoon revealed large BW aggregations on the south and west coast of Sri Lanka [15]. However, the opportunistic sighting data for this study in 2015 revealed that the highest probability of monthly sightings and the mean number of sightings per day were highest during the northeast monsoonal and first intermonsoonal seasons (November to April), although this comparison may not be comprehensive, as the major part of the southwest monsoon (early March to the end of July) was not covered by the survey.…”

“…Moreover, sightings were distributed mainly along the continental shelf margin from Dondra Head to Galle. Previous studies also revealed that the highest frequency of BW sightings in Sri Lanka transpired between Dondra Head and Galle [4,14,15] and off Mullaitivu to Batticaloa [3] on the east coast. The highly productive upwelling of nutrients and high plankton bloom productivity are both influenced by the strong southwest monsoon and the bathymetric features of Dondra Canyon, which provide a critical foraging habitat for dense aggregations of BWs [31,32].…”

“…However, spatial and temporal variations might be expected regarding prey availability. Mirissa on the south coast, Mullaithivu and Trincomalee on the east coast and Kalpitiya on the northwest coast have been identified as three major BW hotspots in Sri Lanka [15], although BWs on the coast of Mirissa are faced with multiple threats in comparison with the other sites [16,17]. Within the past two decades, an amalgamation of anthropogenic threats, such as increased vessel traffic, ship strikes, acoustic pollution, maritime activities, and commercial WW operations, have significantly increased in the area [4,[17][18][19].…”