A Visual Sighting and Acoustic Detections of Minke Whales, Balaenoptera acutorostrata (Cetacea: Balaenopteridae), in Nearshore Hawaiian Waters

Minke whales, Balaenoptera acutorostrata (Lacépède), have been considered a rare species in Hawaiian waters due to limited sightings during visual and aerial surveys. However, our research suggests that they are more common than previously considered. In spring 2005, a combined visual-acoustic survey of cetaceans in Hawaiian waters resulted in the sighting of a minke whale within 22 km of Kaua‘i. Minke whale vocalizations were also detected at several other locations near Kaua‘i and O‘ahu. These 2005 reports are the first from nearshore (<50 km) Hawaiian waters despite years of previous shipboard and aerial surveys. The lack of historical sightings is likely due to misidentification or the inability to detect these animals during poor sighting conditions. We recommend that future cetacean surveys in Hawaiian waters include a passive acoustic component to increase the likelihood of detecting minke whales. Minke whales, Balaenoptera acutorostrata (Lacépède), are the smallest baleen whale and are typically solitary animals, occasionally occurring in small groups of two or three (Horwood 1990). There are few low-latitude sightings of minke whales in the North Pacific Ocean, and the species is considered rare in Hawaiian waters (Shallenberger 1981). Shallenberger (1981) reported that minke whales occurred seasonally within the Northwestern Hawaiian Islands; however, aerial surveys extending 46 km (25 nmi) from the main island chain conducted during the years 1993–1998 (Mobley et al. 2000) and 2003 (Mobley 2004) failed to detect any minke whales. Shipboard sightings offshore of the Hawaiian Islands include a juvenile minke whale riding the bow wave of a Navy ship south of Honolulu (Balcomb 1987) and four confirmed and one possible sighting by Hawaiian longline fishery observers (Carretta et al. 2005). A pilot study consisting of a combined visual-acoustic cetacean survey in the deep waters surrounding the Hawaiian Islands was conducted from 17 to 24 February 2005 aboard the R/V Dariabar (Norris et al. 2005). The Dariabar is a 26.5-m (87-ft) schooner outfitted for visual and passive acoustic detection of cetaceans. The survey track (Figure 1) included waters offshore of O‘ahu and Kaua‘i, with focused effort in Kaulakahi Channel between Kaua‘i and Ni‘ihau. In this paper, we present the visual and acoustic records of minke whales detected during this survey. materials and methods Visual effort consisted of two experienced visual observers rotating between two elevated observation platforms (3.8 m above sea level) and a crow’s nest platform located 7.4 m above sea level on the mast. Observers scanned the water for cetaceans using 7 50 Pacific Science (2007), vol. 61, no. 3:395–398 : 2007 by University of Hawai‘i Press All rights reserved 1 Manuscript accepted 28 September 2006. 2 Southwest Fisheries Science Center, 8604 La Jolla Shores Drive, La Jolla, California 92037 (e-mail: [email protected]). 3 Cetos Research Organization, P.O. Box 885, Bar Harbor, Maine 04609. 4 Pelagikos, 499 Embarcadero, Oakland, California 94606. 5 Naval Facilities Engineering Command, Pacific, 258 Makalapa Drive, Suite 100, Pearl Harbor, Hawai‘i 96860. reticulated binoculars (Fujinon) and unaided eye. Sighting data were recorded to a computer with a real-time Global Positioning System input. Passive acoustic detection and localization of cetacean vocalizations were accomplished using a two-element hydrophone array (frequency response 500 Hz 40 kHzG 10 dB) towed 300 m behind the vessel. The signal conditioning and acquisition system consisted of a mixing board (Behringer Europro) for signal conditioning (amplification, equalization, and filtering) and a PCMIA data acquisition card (National Instruments model 6062E). Bearing angles to sound sources were determined using Ishmael signal analysis software (Mellinger 2001), which computes the time delay between hydrophones via a crosscorrelation algorithm to determine bearing angles to the sound source. Bearing angles were plotted using Whaltrak, a plotting and data-logging program developed by Southwest Fisheries Science Center. Localization of sound sources was determined by the convergence of sequential bearings over time. results and discussion At 1315 hours on 21 February 2005, a minke whale was sighted 20.7 km (11.1 nmi) southwest of Kaua‘i (Figure 1). The whale surfaced approximately 100 m off the port stern of the Dariabar (21 50.18 0 N, 159 51.91 0 W ) within a group of 15 pilot whales and near a group of three humpback whales. Species identity was confirmed by several experienced marine mammal observers during multiple surfacings. Diagnostic characteristics included the animal’s small body size, falcate dorsal fin, and triangular rostrum. Soon after the sighting, a small inflatable boat was launched from the Dariabar; resighting and behavior information was relayed between an observer on the crow’s nest and the scientist in the small boat until 1615 hours. Our results provide the first confirmed visual detection of a minke Figure 1. Map of the surveyed tracklines, with minke whale sighting (black star) and the six localized acoustic detections of minke whale boing sound sources (white stars) made during the spring 2005 R/V Dariabar cruise. 396 PACIFIC SCIENCE . July 2007