Cornell Bioacoustics Research Program director Christopher Clark has a close encounter with a bowhead whale during the 1984 bowhead census.

Each spring, several thousand bowhead whales (Balaena mysticetus) migrate north from their wintering area in the Bering Sea, through the Bering Stait and Chukchi Sea, then east past Point Barrow, Alaska to their summer range in the Beaufort Sea. In the fall, the whales reverse this path, migrating west, and then south back to the Bering Sea.

The spring bowhead migration has been extensively studied from shore-based visual stations and from the air by the North Slope Borough. Additionally, in 1984, 1985, 1986, 1992, 1993, 2000, and 2001, arrays of underwater micrphones (hydrophones) have been used to detect and locate vocalizing bowheads during the spring migration, by researchers from the Bioacoustics Research Program. These efforts represent the largest known research project on the vocal behavior of a migratory species. The primary objective of this acoustic effort has been to augment the visual survey effort and produce a reliable estimate of the size of the population with the eventual aim of deriving a population trend. These efforts have been largely successful and have clearly indicated that the combination of the two methods outperforms either method alone.

The bowhead census is conducted during the spring from mid-April through May, sometimes extending into June. The dual-mode census requires close coordination between the visual survey and acoustic monitoring efforts. The visual observation site, or "perch", is typically located on a grounded ice ridge that is many miles from the shore line, and the ice edge extends in both directions parallel to the shore line.

Bowheads will continue to migrate even during heavy ice conditions with nearly 100% ice cover and swim under the ice even when there are areas of open water. These dynamic, dangerous and often fatal conditions present a migratory challenge that is unique to the Arctic, and the bowhead is the only mysticete species that is adapted for living in such a formidable habitat.

In the spring of 1993 from mid-April through May, a very successful census was conducted off Point Barrow, Alaska. Conditions during that year were unusual in that during the majority of the field season there was little to no ice obstructing the visual census effort, and for most of the time there was open water along the nearshore edge of the shorefast ice from which acoustic arrays were deployed.

The quantity of the 1993 acoustic array data processed was nearly twice that of any previous year due to improvements in the acoustic field collection and processing techniques (for a description, see Clark , Charif., Mitchell, and Colby, 1996, PDF). Figure 1. (below) shows a scatterplot of all 6047 revised location fixes from the 1993 bowhead acoustic census. The visual observation perch is located at coordinates (0,0), and the positive Y axis points in the direction of 0 degrees magnetic.

Figure 1

The 2000 survey included the use of pop-up hydrophones, deployed perpendicular to the traditional ice-edge array of hydrophones-- requiring both a helicopter and large quantities of ingenuity. It was hoped that these instruments might extend acoustic coverage of migrating animals out to 20-30 km.

The latest successful visual and acoustic survey, conducted in 2001, consisted of 1,043 hours of acoustic array data. Due to improved processing techniques, the analysis was completed in record time. We will post results here when available.

Listen to vocalizations of bowheads as they migrate past Point Barrow