Seaglider Aids in Survey of Cetaceans and their Habitat along the Kona Coast of Hawaii

Figure 1. Top: Seaglider. Bottom: Seaglider track (black dots) and planned transects (three thick black lines). Green dot shows the deployment site. Red dot shows the recovery site. Depth contours in 500m intervals from 500 — 2000m. The Seaglider completed the easternmost transect and the southern part of the central transect. Future deployments will hopefully span all three transects.

Progress in understanding the distribution, behavior and habitats of cetaceans is accelerating with the deployment of new survey instruments. One such instrument is a Seaglider launched on 9 December 2013 off the Kona coast of the island of Hawaii as part of a joint research project between PIFSC and the University of Hawaii. On January 2, 2014, the instrument was recovered. During the nearly month-long mission, the Seaglider made 135 dives spanning the entire length of the Kona coast, its acoustical sensors collecting numerous recordings of cetacean clicks and whistles (Fig. 1). In addition to the acoustic data, the glider collected environmental data such as temperature, salinity, current velocity, and chlorophyll concentration during each dive. Now the acoustic data will be examined along with the environmental data to determine where and in what environments cetaceans are detected. Ultimately, the aim is to figure out why cetaceans seem to be found in distinct "hotspots" along the Kona coast.

Another goal of the mission was to learn how to make the best use of Seagliders in future studies. After the acoustic data from this Seaglider mission are analyzed, the glider will be redeployed, putting to use what has been learned so far. Based on the depths, locations, and durations of recorded cetacean sounds, the glider's sampling, or "listening", routine can be refined. Cetacean sound data can require a lot of space on the glider's storage disk, so the glider can't record data for its entire deployment. Rather, the hydrophones that collect data turn on and off on a preset schedule. This schedule balances the need to collect enough recordings to determine how cetaceans are using their Kona habitat with details such as the hydrophones' battery life and disk space.

Full processing of the acoustic data will take time, but at this stage we can begin to make some observations about the physical environment the glider encountered during its deployment. For example, in Fig. 2 we see an abrupt shift in the current direction on dive 84, just after the glider passed south of 19.5°N. This shift suggests that close to shore the current flow was along the coast — northward north of 19.5°N and southward south of 19.5°N. We can also see that the depth at which the greatest chlorophyll concentrations were observed increased as the glider moved off the Kohala Shelf around dive 60 and that the chlorophyll concentrations decreased while the glider was at its southwestern-most points from dives 110 — 120. It will be interesting to pair these observations with the locations of cetacean clicks and whistles to determine how the animals might be making use of these differences in their Kona habitat.

This glider mission was a collaboration between the University of Hawaii SOEST Ocean Glider program and the PIFSC Kona Integrated Ecosystem Assessment program.

Figure 2. Seaglider-measured currents. Each barbed arrow shows the speed and direction of the depth-averaged current for one dive. Arrows pointing up indicate northward flow and those pointing down indicate southward flow. The currents bifurcate along the coast at approximately 19.5 N latitude (dive 84) with northward flow observed to the north of that location and southward flow to the south.