Tagging Data Reveal Movements and Offshore Habitats of Melon-Headed Whales

Although melon-headed whales are often sighted in Hawaiian offshore waters, little is known about their travels around the archipelago. Recent research by PIFSC scientists has provided new information about the cetacean's movements and habitat.

During cetacean surveys in 2008, eleven melon-headed whales were tagged off the coasts of the Hawaiian Islands by Dr. Robin Baird (Cascadia Research Collective) and his research team. Data transmitted via satellite by the tags provides a record of the animals’ location over time. Of the 11 whales tagged, five left Hawaii's coastal waters and traveled offshore. PIFSC researchers Jeffrey Polovina and Phoebe Woodworth and their colleagues examined the tracks of the five offshore excursions along with output from a Hawaii region oceanographic model, a configuration of the Hybrid Coordinate Ocean Model (HYCOM). Their analysis showed that when the melon-headed whales traveled offshore, they moved into distinct areas of both cyclonic and anticyclonic eddies, as well as a non-eddy-associated area southwest of the island of Ni'ihau.

Both cyclonic (cold core) and anticyclonic (warm core) eddies concentrate nutrients and prey in specific regions marked by specific currents. Cyclonic eddies concentrate nutrients and prey around their edges, which are marked by strong currents. Conversely, anticyclonic eddies concentrate nutrients and prey in their centers, which are marked by weak currents. By examining tracks of the melon-headed whales' location in conjunction with ocean current data output by the HYCOM Hawaii regional model, Polovina's team found that when the whales were offshore, they moved into these areas of concentrated nutrients and prey. Statistical analysis confirmed that the habitats selected by the whales are indeed significantly different than the surrounding area. It is hypothesized that melon-headed whales use these eddy regions for foraging. In addition to selecting eddy-associated foraging habitats, the three melon-headed whales that moved offshore and traveled westward all transited through a small area southwest of the island of Ni'ihau - an area termed the "Ni'ihau Box" in this study. Of these three whales, two spent at least half their offshore time in the Ni'ihau Box, one deviating from the edge of a cyclonic eddy to do so. No clear motivating factor such as seasonality, currents favorable for foraging, or bathymetry could be found to explain why the whales moved through this area.

The accompanying diagram shows the multi-day track of one melon-headed whale overlaid on output of the HYCOM Hawaii regional model coinciding with one day of the track. Over much of the recorded track, the whale spent time in offshore waters associated with a cyclonic eddy and the Ni'ihau Box. This whale was tagged off the Kona coast of Hawaii on 19 April 2008 and traveled northwestward. On 2 May 2008 it moved offshore west of Oahu into the edge region of a cyclonic eddy, depicted as the blue and purple circle of low sea surface heights and high surface currents in the HYCOM output. The whale then left the eddy and spent 4–9 May 2008 in the Ni'ihau Box region before returning to the eddy's periphery for the remaining three days of its recorded track.

Analysis of the tracking data jointly with the HYCOM model output has provided a first look at the offshore behavior and habitat of the melon-headed whale, a little known member of Hawaii’s pelagic ecosystem.

Track of a melon-headed whale in inshore (dotted black) and offshore (solid black) waters. Downward 
            pointing triangle marks the tag deployment site off the Big Island of Hawaii, and the upward pointing 
            triangle marks the end of the recorded track.  HYCOM sea surface height (SSH, color) and surface currents 
            (white arrows) are shown for 9 May 2008.  The red circle indicates the position of the melon-headed whale 
            on the day coinciding with the HYCOM output. The rectangle encompassing the island of Ni'ihau indicates 
            the Ni'ihau Box region.
Track of a melon-headed whale in inshore (dotted black) and offshore (solid black) waters. Downward pointing triangle marks the tag deployment site off the Big Island of Hawaii, and the upward pointing triangle marks the end of the recorded track. HYCOM sea surface height (SSH, color) and surface currents (white arrows) are shown for 9 May 2008. The red circle indicates the position of the melon-headed whale on the day coinciding with the HYCOM output. The rectangle encompassing the island of Ni'ihau indicates the Ni'ihau Box region.