Listening for Large Whales in the Offshore Waters of Alaska

摘要

In 1999, the first phase of a multiyear program was initiated at the National Oceanic and Atmospheric Administration's National Marine Mammal Laboratory and Pacific Marine Environmental Laboratory to advance the use of passive acoustics for the detection and assessment of large whales in offshore Alaskan waters. To date, autonomous recorders have been successfully deployed in the Gulf of Alaska (1999–2001), the southeastern Bering Sea (2000–present), and the western Beaufort Sea (2003–2004). Seasonal occurrences of six endangered species (blue, fin, humpback, North Pacific right, bowhead, and sperm whales) have been documented on the basis of call receptions in these remote ocean regions. In addition, eastern North Pacific gray whale calls were detected in the western Beaufort Sea from October 2003 through May 2004. Here we provide an overview of this suite of research projects and suggest the next steps for applying acoustic data from long-term recorders to the assessment of large whale populations.nnLarge whales were severely depleted by commercial whaling from the 18th through the late 20th century, to such an extent that all were included in the initial US listing of endangered species in 1973. Targeted species included blue, fin, humpback, right, Bryde's, and sperm whales (box 1), and although precise numbers are unknown, the removal of at least two million whales over roughly 200 years is well documented (Clapham et al. 1999). While this intensive exploitation underlies the recent controversial hypothesis of top-down ecosystem forcing—the so-called “megafaunal collapse” hypothesis (Springer et al. 2003, Mizroch and Rice forthcoming)—it also sets a challenge to scientists and resource managers charged with estimating current population sizes and habitat protection for these endangered species.nnWith the end of the cold war and the subsequent willingness of the US government to allow dual use of some military assets (Nishimura and Conlon 1993), a unique opportunity arose to use the US Navy's SOSUS (Sound Surveillance System) underwater hydrophone network to detect and track whales. Biologists welcomed this opportunity, and found in SOSUS an unprecedented tool to detect blue and fin whale calls over long distances in the North Atlantic and North Pacific basins (Clark 1995, Watkins et al. 2000, Stafford et al. 2001) and to track individual whales that produced atypical calls (Watkins et al. 2004). In the North Pacific, the seasonal detection of endangered blue and fin whale calls, using SOSUS, provided a means to correlate call occurrence with habitat features in remote areas off the Kamchatka Peninsula (Moore et al. 2002) and to investigate whale response to ocean climate variability off California (Burtenshaw et al. 2004).nnThe success of research that used SOSUS to track seasonal occurrence patterns in whale calls fostered the development of autonomous recorders that could be deployed virtually anywhere in the world's oceans (Fox et al. 2001, Wiggins 2003). Two types of recorders have been used off Alaska: (1) autonomous hydrophones developed by the National Oceanic and Atmospheric Administration's (NOAA) Pacific Marine Environmental Laboratory (PMEL) (Fox et al. 2001; www.pmel.noaa.gov/vents/acoustics/whales/bioacoustics.html) and (2) acoustic recording packages (ARPs; Wiggins 2003, http://cetus.ucsd.edu). The PMEL hydrophone consists of a watertight titanium pressure case containing alkaline batteries, a data logging system with one to six hard disk drives, and a hydrophone outside the case. The ARPs, developed by the Marine Physical Laboratory of the Scripps Institution of Oceanography, consist of a frame that holds the batteries, hard disk drives, and ballast, with a hydrophone suspended about 7 meters (m) above the frame. The recording bandwidth for both recorders ranges from 230 to 880 hertz (Hz) (sample rates 500 to 2000 Hz) depending on the unit, with hard disk drive storage capacity of 36 to 160 gigabytes. They are thus capable of continuous recording during 200- to 400-day deployments. Both types of instruments need to be recovered in order to retrieve the acoustic data.nnThe principal difference between the instruments is that the PMEL recorder is moored with the hydrophone suspended up into the deep sound channel, while the ARP's hydrophone samples sound at roughly 10 m above the seafloor (figure 1). Both instruments have proved to be flexible tools for acoustic observations of large whales in remote areas of the world's oceans, as demonstrated by deployments along the mid-Atlantic ridge (Nieukirk et al. 2004), the eastern tropical Pacific (Stafford et al. 1999a), and the Antarctic Peninsula (Sirovic et al. 2004). As such, they are especially suitable for cetacean detection in the offshore waters of Alaska, where standard visual surveys are often hampered by darkness and bad weather.