Acoustic Survey Investigates Cetacean Foraging "Hotspot" on the Kona Coast of Hawaii

In March 2014, the NOAA Ship Oscar Elton Sette served as a research base for a team of PIFSC scientists exploring the habitat of cetaceans in coastal waters of the island of Hawaii along the Kona Coast region. The research, designated as project SE1401, was carried out in partnership with scientists from the Joint Institute for Marine and Atmospheric Research at the University of Hawaii at Manoa (JIMAR), Hawaii Institute of Marine Biology at the University of Hawaii (HIMB), University of Hawaii at Manoa, Department of Oceanography (UHM-OCEAN), University of Hawaii at Manoa, Marine Biology Program (UHM-MB), Monterey Bay Aquarium (MBA), and the NOAA Teacher at Sea (TAS) program.

Figure 1.  Operational area of research project SE1401 conducting cetacean forage research in the Kona IEA region.  Symbols A-F mark 
        locations the standard Kona IEA grid stations, and symbols H1 and H2 indicate location of HARP deployments and/or recoveries.  
        Targeted research operations were focused around the nearshore stations (A & C) where deep cetacean foraging "hot spots" have been 
        identified and characterized on previous missions.
Figure 1. Operational area of research project SE1401 conducting cetacean forage research in the Kona IEA region. Symbols A-F mark locations the standard Kona IEA grid stations, and symbols H1 and H2 indicate location of HARP deployments and/or recoveries. Targeted research operations were focused around the nearshore stations (A & C) where deep cetacean foraging "hot spots" have been identified and characterized on previous missions.

The project conducted operations in the waters around the NOAA Kona Integrated Ecosystem Assessment (IEA) region (Fig. 1) during 17-28 March. During the 12-day mission, the research team, led by Chief Scientist Dr. Donald R. Kobayashi from the PIFSC Ecosystems and Oceanography Division, worked on two objectives. First, the standard Kona IEA oceanographic and active acoustics survey was conducted to add another year's data to this valuable time series of observations (started in 2011 and continued in 2013) and provide new seasonal coverage for winter months. Second, a deep nearshore "hotspot" for cetacean foraging was investigated using a variety of methodologies. Previous surveys using a Simrad EK60 acoustic sounder at a frequency of 70 kHz have delineated a large daytime and nighttime signal of biological activity (presence of organisms) at 400-600m depth nearshore. Tagged cetaceans have been recorded feeding within this layer and the water mass slightly above and below the layer. Physical and biological characteristics of the layer were studied using a variety of tools: a CTD unit and rosette of water sampling bottles; active acoustic surveys; a remotely operated vehicle (ROV) deployed from the Sette; a BlueView sonar imaging unit attached to the ROV; a Didson sonar imaging unit lowered on the CTD rosette; a GoPro camera on the CTD rosette; baited deep handline gear and squid-jigging gear; and a Cobb midwater trawl.

This report presents some of the acoustics data collected around the foraging hotspot along multiple transects using the Simrad EK60 acoustic sounder at 38 kHz and 70 kHz. Acoustics Team leader Adrienne Copeland from HIMB collected, processed, and summarized the data to construct both daytime and nighttime vertical profiles of reflective biomass from 0-700m depth (Fig. 2). The results showed that the strong scattering density observed in the 2011 and 2013 summer cruises, primarily at the 70 kHz frequency, also was present in March 2014 with very comparable densities. These results provide continued support for the hypothesis that a relatively abundant island-associated mesopelagic boundary layer is the basis for a food web that supports insular cetacean stocks in the region. These acoustic data are used in our cetacean habitat modeling project in collaboration with Erin Oleson (PIFSC Protected Species Division) and Robin Baird (Cascadia Research Collective) under the Kona IEA research initiative.

Daytime vertical profile of reflective biomass from 0-700m depth over the 
        cetacean foraging "hot spots" from deployment of the Simrad EK60 acoustic sounder at 38 kHz and 70 kHz during winter and summer 
        surveys. Data are aggregated from transects between stations A and C for each season. Nighttime vertical profile of reflective biomass from 0-700m depth over the 
        cetacean foraging "hot spots" from deployment of the Simrad EK60 acoustic sounder at 38 kHz and 70 kHz during winter and summer 
        surveys. Data are aggregated from transects between stations A and C for each season.
Figure 2. Daytime (upper panel) and nighttime (lower panel) vertical profiles of reflective biomass from 0-700m depth over the cetacean foraging "hot spots" from deployment of the Simrad EK60 acoustic sounder at 38 kHz and 70 kHz during winter and summer surveys. Data are aggregated from transects between stations A and C for each season.