Omura’s whales (Balaenoptera omurai) off northwest Madagascar: ecology, behaviour and conservation needs

Abstract

The Omura’s whale (Balaenoptera omurai) was described as a new species in 2003 and then soon after as an ancient lineage basal to a Bryde’s/sei whale clade. Currently known only from whaling and stranding specimens primarily from the western Pacific and eastern Indian Oceans, there exist no confirmed field observations or ecological/behavioural data. Here we present, to our knowledge, the first genetically confirmed documentation of living Omura’s whales including descriptions of basic ecology and behaviour from northwestern Madagascar. Species identification was confirmed through molecular phylogenetic analyses of biopsies collected from 18 adult animals. All individuals shared a single haplotype in a 402 bp sequence of mtDNA control region, suggesting low diversity and a potentially small population. Sightings of 44 groups indicated preference for shallow-water shelf habitat with sea surface temperature between 27.4°C and 30.2°C. Frequent observations were made of lunge feeding, possibly on zooplankton. Observations of four mothers with young calves, and recordings of a song-like vocalization probably indicate reproductive behaviour. Social organization consisted of loose aggregations of predominantly unassociated single individuals spatially and temporally clustered. Photographic recapture of a female re-sighted the following year with a young calf suggests site fidelity or a resident population. Our results demonstrate that the species is a tropical whale without segregation of feeding and breeding habitat, and is probably non-migratory; our data extend the range of this poorly studied whale into the western Indian Ocean. Exclusive range restriction to tropical waters is rare among baleen whale species, except for the various forms of Bryde’s whales and Omura’s whales. Thus, the discovery of a tractable population of Omura’s whales in the tropics presents an opportunity for understanding the ecological factors driving potential convergence of life-history patterns with the distantly related Bryde’s whales.

First record of Omura’s whale, Balaenoptera omurai, in Sri Lankan waters

“…As described by Cerchio et al (2015), this individual showed evidence of asymmetrical colouration of the lower jaws, with a darkly pigmented left jaw (Fig. 2 a) and lightly pigmented right jaw (Fig.…”

“…4. Asymmetrical chevron on both right and left sides Lightly pigmented chevron anterior to the dorsal fin evident on both sides but more pronounced on right side of the body, displaying a double-banded pattern also described by Cerchio et al (2015).…”

“…The detailed description of the external features of this species provided by Cerchio et al (2015) were useful for the identification of this individual within Sri Lankan waters. The key features of interest that enable us to discern between Omura's and Bryde's that are often recorded in these waters include the prominent dorsal ridge on the rostrum (Bryde's whales often have three head ridges; the central ridge is flanked by two lateral rostral ridges), body colouration (Bryde's whales are dark throughout their bodies), and the shape of the dorsal fin (which is small and falcate in relation to that of the Bryde's whale) (Yamada 2009).…”

“…3. Presence of right side blaze As described by Cerchio et al (2015) white pigmentation is more extensive on the right side of the body compared to the left. This individual also showed evidence of a lightly pigmented blaze anterior to the eye only present on the right side and multiple dark stripes bisecting the blaze.…”

“…The dorsal fin was pointed, strongly falcate and backswept (Wada et al 2003;Cerchio et al 2015;Jefferson et al 2008;Ranjbar et al 2016) Apart from the distinguishing characteristics, this individual also possessed a mark that resembled a tyre mark on its left dorsal flank and an entanglement scar on the tip of its left rostrum. Both these markings appear to be made by external interactions and are not characteristics of this species.…”