South West to Eastern Sri Lanka IMMA

Criterion A – Species or Population Vulnerability

The Northern Indian Ocean blue whale Balaenoptera musculus indica, occurring within the IMMA, is a currently recognised subspecies of Balaenoptera musculus, which is listed as Endangered in the IUCN Red List of Threatened Species. Therefore, without additional descriptions of the subspecies status specifically within the Northern Indian Ocean, it is assumed that this subspecies meets IMMA Criterion A on Species or Population Vulnerability. In addition the sperm whale Physeter macrocephalus is listed in the IUCN Red List as a Vulnerable species and therefore meeting Criterion A of the IMMA selection criterion.

Criterion B: Distribution and Abundance

Sub-criterion B2: Aggregations

Primary productivity is high within the area of the IMMA, with monsoon related continental shelf-edge upwelling and nutrients brought in from numerous river outfalls creating ideal conditions for cetacean aggregations. Blue whales that are normally solitary are seen feeding in loose aggregations along the shelf edge off the south and east coast and in the Trincomalee canyon (Alling et al., 1986; Ilangakoon 2009, 2012a, Ilangakoon and Sathasivam, 2012). Large numbers of sperm whales also aggregate around Trincomalee canyon and sometimes super pods numbering in the hundreds are known to move along the deeper waters just off the continental shelf edge in southern Sri Lanka and up the western coast towards the Gulf of Mannar. Meanwhile, smaller cetaceans such as the long snouted spinner dolphin form large schools numbering between 500-1000 individuals especially off the south coast. Mixed-species feeding aggregations of smaller cetaceans are also frequent off the south-southwest coast (Ilangakoon 2012b). These aggregations may sometimes involve Stenella spp., Tursiops truncatus and some species of “black fish” which often associate with large whales. Such mixed aggregations usually occur in shelf edge upwelling zones within the IMMA.

Criterion C: Key Life Cycle Activities

Sub-criterion C1: Reproductive Areas

Adult blue whales accompanied by young calves are frequently recorded both off the east coast in the vicinity of the Trincomalee canyon and off the south coast (Alling et al., 1991; Ilangakoon and Sathasivam, 2012) within this IMMA. Adult blue whales have also been observed engaging in breeding related behavioural activities such as high-speed chasing, breeching and synchronized diving and surfacing (Ilangakoon and Sathasivam, 2012). All this indicates that this habitat is important not only as a feeding area but also in relation to the breeding cycle of these Endangered northern Indian Ocean blue whales. Likewise, the area around Trincomalee canyon is inhabited throughout the year by groups of female sperm whales with young calves and even newborn animals with foetal folds have been frequently observed within the last three decades (Whitehead et al., 1983, Ilangakoon 2012). The more common species of small cetaceans such as the spinner dolphin, bottlenose dolphin, spotted dolphin and Risso’s dolphin that appear to have resident schools in the area are also frequently accompanied by calves when observed during sighting surveys and calves of these species are also frequently recorded in gillnet bycatch of the coastal fisheries in the southwest, south and east of Sri Lanka (Leatherwood and Reeves, 1989; Ilangakoon, 1997).

Sub-criterion C2: Feeding Areas

The waters around Sri Lanka are among the few feeding areas for blue whales that have been identified in the Indian Ocean (Alling et al., 1991; Gill, 2002; Ilangakoon, 2009; Ilangakoon and Sathasivam, 2012). Blue whales are observed feeding (sub-surface skimming as well as feeding dives) along the continental shelf edge and in shelf waters along the entire continental shelf within this IMMA, as well as within the Trincomalee canyon in the north eastern extremity of the IMMA (Alling et al., 1991; Ilangakoon and Sathasivam, 2012). The blue whales undertake extremely localized movements within a small but highly productive, monsoon-driven feeding area (Ilangakoon and Sathasivam, 2012). The northeast monsoon affects the northeast coast of Sri Lanka and adjacent waters from November to March, the time when blue whales are abundant in the waters around Trincomalee, as reported by Alling et al. (1991), who suggested that these waters are an important feeding ground. The outfall of Sri Lanka’s largest river, the Mahaweli, is located in Trincomalee and deposits the largest volume of nutrient-rich waters at this time of the year, enhancing productivity in the area. Re-sightings of individually identified animals in consecutive years also indicate site fidelity to these productive feeding grounds. Meanwhile, the southwest monsoon creates upwelling and nutrient enrichment through river outfalls off the south and west coasts from May to September and localized upwelling occurs between the southern tip of India and Sri Lanka (Rao et al., 2006). Both sightings and strandings of blue whales have been recorded from the area during this period of the year (Ilangakoon, 2002; 2006a; 2006b; 2009; Ilangakoon and Perera, 2009; Ocean Alliance, 2003) and again re-sighting of identified individuals in multiple years indicates site fidelity. However, sightings of feeding off the south coast are not limited to this monsoon period but have also been recorded during the north-east monsoon (Ilangakoon, 2009; Ilangakoon and Perera, 2009; Ilangakoon and Sathasivam, 2012), indicating that feeding opportunities are not limited to monsoon-related upwelling and that blue whales in these waters exploit seasonal upwelling-related food sources and at least a part of the population is able to find enough sustenance here throughout the year. This species, which is normally migratory, remain in these waters year-round, indicating the importance of the area as a feeding ground.

Sperm whales feed primarily in and around the Trincomalee canyon (Whitehead, 1983) but regularly move along the shelf edge of the south/southwest coast too. Some of these movements include super-pods numbering in the hundreds of animals and it is not well documented if these are movements between food patches or for other reasons. Spinner dolphins while being the most common species of small cetacean in the IMMA, are seen feeding in waters of the continental shelf throughout the day but most often from mid-morning to early afternoon. Large aggregations of feeding spinner dolphins can be observed during the tuna fishing seasons and in fact fishers usually follow these dolphins in order to find the tuna.

Criterion D: Special Attributes  

Sub-criterion D2: Diversity

Diversity is one of the primary criteria for this IMMA. While Endangered B. m. indica is a primary species in the area throughout the year other baleen whale such as Bryde’s whales (Balaenoptera brydei) and more recently the Omura’s whale (Balaenoptera omurai) have been recorded in the area (Leatherwood and Reeves,1989; Ilangakoon 2012; de Vos et al., 2016). Likewise P. macrocephalus and S. longirostris are primary species in this IMMA. Additionally small cetaceans ranging from four species of blackfish (G.macrorhynchus, O. orca, P. crassidens, P. electra, F. attenuata), two more Stenella species (S. attenuata, S. ceoruleoalba), T. truncatus, G. griseus, L. hosei, S.bredanensis, K. sima, K. breviceps are known to use these IMMA waters as habitat. Such a diversity of species concentrated in an area that is relatively small makes this IMMA somewhat unique in terms of species richness alone.