Tuesday, November 10, 2015

[Cetology / Behaviour • 2015] Omura’s Whales (Balaenoptera omurai) off northwest Madagascar: Ecology, Behaviour and Conservation Needs

Omura’s whale Balaenoptera omurai
from Cerchio et al. 2015,  DOI: 10.1098/rsos.150301
courtesy of New England Aquarium || news.NEAq.org

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.

 KEYWORDS: Omura’s whale, Balaenoptera omurai, distribution, feeding ecology, breeding ecology, acoustic behaviour

Figure 3. Images of Madagascar Balaenoptera omurai displaying details of pigmentation and external appearance.
Five different individuals are pictured: underwater video frame captures of an adult female with calf sighted on 9 November 2013 that had just completed a feeding lunge (first row), and a lone adult on 22 October 2014 (second row), and above surface photographs from three adults sighted on 13 November 2014 (third row), 9 November 2013 (fourth row) and 12 December 2012 (fifth row). Visible features: (A) asymmetrical coloration of the lower jaw, with lightly pigmented right jaw and darkly pigmented left jaw; (B) asymmetrical coloration of the gape (inferred by inner lower lip), with lightly pigmented left gape and darkly pigmented right gape; (C) leading edge of pectoral fin white from tip to shoulder; (D) the apparent absence of lateral rostral ridges, with only faint indications detectable at some angles; (E) lightly pigmented blaze originating anterior to the eye, present only on the right side, with dark eye and ear stripe, two additional dark stripes and a light inter-stripe wash; (F) lightly pigmented chevron anterior to dorsal fin, present on both sides but asymmetrical and most prominent on right where it displays a double banded pattern; (G) highly falcate dorsal fin with gradual sloping insertion into dorsum.

Figure 4. An Omura’s whale subsurface lunge feeding, displaying several of the features detailed in figure 3.

Prior to our discovery of Omura’s whales in Madagascan waters, there was a complete absence of field data on this species. Here we have presented evidence supporting the species identification of the small rorqual that we have observed as B. omurai, along with details on the external body appearance, and initial observations on habitat use, local distribution, feeding ecology, reproductive ecology and vocal behaviour. Notably, this represents to our knowledge, the first identified population tractable for field study. In the absence of a detailed description of the external appearance of B. omurai, it has been difficult for field biologists to confidently distinguish the species from congeners, particularly Bryde’s whales. We believe that this difficulty was primarily because of the lack of observations, and now with the detailed description we have provided it should be no more difficult to distinguish Omura’s whales in the field than it would be to distinguish fin whales from sei whales, or sei whales from Bryde’s whales, provided adequate observations and documentation.

Salvatore Cerchio, Boris Andrianantenaina, Alec Lindsay, Melinda Rekdahl, Norbert Andrianarivelo and Tahina Rasoloarijao. 2015. Omura’s Whales (Balaenoptera omurai) off northwest Madagascar: Ecology, Behaviour and Conservation Needs.
Royal Society Open Science. 2: 150301.   DOI: 10.1098/rsos.150301

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