First records of Balaenoptera edeni (Cetartiodactyla: Balaenopteridae) in the coast of Oaxaca, Mexico

Abstract

To date, few studies have focused on the Bryde’s whale, Balaenoptera edeni, one of the nine species comprising the family Balaenopteridae. This species can be distinguished from other rorquals by the presence of three parallel longitudinal ridges on top of the rostrum, whereas other rorquals possess only one. The potential distribution of this species in the Mexican Pacific is thought to extend south from 26° N, along the coast of Mexico from Baja California to the border with Guatemala. Our knowledge about this species in the Mexican South Pacific is limited, particularly off the coast of Oaxaca where no previous sightings have been reported. The sightings were recorded during oceanic trips to monitor humpback whale (Megaptera novaeangliae) migrations off the central coast of Oaxaca, Mexico, between December 2011 and January 2012. The observation platforms were 6 m fiberglass boats with a 60 hp outboard engine. The geographical coordinateswere registered with a handheld GPS; also recorded were sea surface temperature and the behavior of rorqual specimens. The initial species identification was based on the presence of three parallel longitudinal ridges on the head, and was subsequently corroborated by comparing the photographs and video taken during the sightings with field guides. Four separate rorqual sightings were documented between 1.3 and 7.1 km from the coast of Estacahuite, Zipolite, and Ventanilla, Oaxaca, Mexico. Two sightings involved groups with calves; during one sighting, three killer whales, Orcinus orca, were also observed, although neither predatory behavior nor other interaction occurred. The behaviors observed included breaches by calves, surface activity, and slow and erratic movements with sudden changes of direction in response to the presence of the research boat. During surface exhalations, one individual produced vocalizations that were audible at the surface. The presence of B. edeni off the central coast of Oaxaca, Mexico, may be associated with the oceanographic characteristics and high productivity of the Gulf of Tehuantepec. All behaviors observed during the sightings were typical of the species, including evasive maneuvers and sudden changes of direction with respect to the boat. The fortuitous recording of audible vocalizations clearly recognizable from the research boat during surface exhalations is remarkable. These sightings are the first reports of B. edeni for the state of Oaxaca and the Mexican South Pacific. Thus, there is a clear need to increase intensive, long-term monitoring efforts in order to better understand the ecological implications and other aspects related to the presence of the Bryde’s whale and other marine mammals in the region. Future research should focus on assessing the spatial and temporal occurrence of B. edeni in this region.