This project investigates the vocalizations of humpback whales (Megaptera novaeangliae) that come to the wintering grounds around the four-island region of Maui, Molokai, Lanai and Kaho'olawe each year between December and April. Humpback whales produce "songs" that are broadband transient signals. A typical song last about 10-12 minutes and consists of 4-6 themes, these themes consisting of 2-4 repeated phrases containing 4-6 units that last about 1-2 seconds. The song structure is as follows:

• Unit (1-2 sec)

• Phrase (~4-6 units with 1-2 s gaps)

• Theme (~2-4 repeated phrases)

• Song (~4-6 themes)

The function of humpback whale song remains unclear although several explanations have been offered over the years. Very little is really understood about sound productions mechanisms or propagation of song. One interesting feature that has been observed is that humpbacks position themselves at about 25-20 meters depth at an angle of about 45 degrees head canted down while singing for about 10-12 minutes.

This peculiar positioning seems to be linked to the singing behavior and has not been observed otherwise. One possible hypothesis is that this position allows for best propagation of the produced sound.

In order to test this hypothesis the sound field and the source levels for individual whales have to be measured to see whether the main transmission direction is pointing at a 45 degree angle upwards. Preliminary tests towards these goals were done in 2002 and the following method was established: singing whales where located with an "aquahead" - a set of two hydrophones mounted on a horizontal bar and acoustically insulated from each other. While the device was held in the water, the operator listened to the whale song and was able to pinpoint the azimuth of the source. Through repeated measurements while maneuvering the vessel closer we were able to find the singer in less than 10 min in most of the cases. Thus the number of trackable whales was greatly increased. Once a singer was located a surface snorkler was deployed to measure the size of the animal. Simultaneously two rebreather divers were deployed equipped with two Sony video cameras and handled depth sounders.

The two divers would approach the singing whale and record its position and vocalizations on the video while taking distance measurements from the divers to the whale and displaying it on the video. One of the divers would stay in a fixed location and distance to the whale to serve as a reference recording. The second diver would move slowly around the whale in either the vertical plane or the horizontal plane and record song and ranges. The data obtained from these tapes was transferred to the computer. For each range measurement the azimuth and elevation of the diver in respect to the whale was estimated by comparing the video with a computer model of a humpback whale.

Once the viewing angle of the model matched the video the azimuth and elevation was noted. A path was then fitted through the data points.

This in turn allowed calculating and correcting for range for each recorded song unit. By comparing the power spectral density of the two recordings it is then possible to estimate the sound field and source level of a singing humpback whale.