The DTAG

This afternoon, we finally found an animal that was approachable and attached a tag programmed to release in 6 hours, so we hope to retrieve it before midnight tonight. While we wait, watch, and listen for the signal that it is on the surface, here is some background about the tags.

Our oceans are not only vast, but also deep. Some marine mammals have evolved the means to exploit resources and ecological niches far beneath the surface.

This is no mean feat, as the deep ocean is an inhospitable place, with cold temperatures and extreme pressures. As an object descends in the water column, the pressure it experiences increases by one atmosphere with every 10 meters. Thus, an animal that dives to 1,000 meters experiences 100 times more pressure than when it as at the surface.

To study deep diving odontocetes—toothed whales such as sperm whales, beaked whales and pilot whales—that forage at depths in excess of 1,000 meters (where no scientist can easily follow), novel techniques are needed to document their behaviour. One such method is to attach data logging devices capable of surviving the rigours of deep ocean exploration. Engineers at WHOI developed just such a device, known as a DTAG, that is able to digitally record movement and acoustic data (Johnson & Tyack 2003).

Three-dimensional dive profile

The DTAG is a non-invasive, cell-phone-sized tool that contains a suite of sensors that includes either a single or a pair of hydrophones (underwater microphones), three magnetometers, three accelerometers, and instruments that record temperature and pressure. These sensors are able to record data 50 times per second, the result being incredibly fine-scale, high-resolution data that can shed a detailed light on a tagged animal’s three-dimensional movements while carrying the tag. The hydrophone in the DTAG is able to sample at a rate of up to 192 kHz and records all the sounds the tagged animal makes and the sounds that it hears while moving through its environment.

Attaching a DTAG. (Luis Lamar, Advanced
Imaging & Visualization Lab, WHOI)

In order to attach the DTAG to a whale, it is placed in a fairing that contains floatation foam, a VHF transmitter, and four soft suction cups and attached to the end of an eight meter carbon fiber pole. A team approaches slowly and quietly in a small boat to minimize disturbance to the whale at the surface. As the body of the animal rises up out of the water the team member on the bow of the boat attaches the tag to the dorsal surface of the animal. The tag is attached to the dorsal surface of animals. After a pre-programmed length of time, the suction cups release automatically.

The capabilities of the DTAG have enabled scientists to shed new light on the diving, fine-scale foraging, and acoustic behaviour of many marine mammal species. It is now helping us to investigate the behaviour of the large male sperm whales of Kaikoura.

1,400 meter dive

The sperm whales here in the Kaikoura Canyon begin making echolocation clicks soon after they "fluke up" to start a foraging dive. As they descend into the depths they produce loud clicks. By interpreting the echoes that return, the sperm whales are able to navigate beneath the surface and detect prey in the darkness beyond the reach of sunlight. So far, our tagged whales have dived to depths of almost 1,400 meters to feed.

The echolocation clicks that the whales produce have a rhythmic nature to them and can be heard for many miles. When potential prey, such as squid, are detected the sperm whales home in on their quarry and then switch from the regular echolocation clicks to a rapid sequence of clicks known as a "buzz" as they attempt to catch their prey. After foraging for almost an hour, the sperm whales return to the surface to breath before heading back to the larder of the abyss once more.

Frequency spectrogram of a
click-buzz sequence.

By attaching the DTAG and analyzing the natural sounds made by the whales, scientists are able to provide previously unavailable perspectives on the three-dimensional nature of their foraging behaviour. Such data can be used to fill information gaps about diving behaviour that can, in turn, be linked to foraging. In particular, how whales exploit the water column during foraging events and how animals respond to the ever-increasing presence of human-produced noise in the oceans. 

Reference
Johnson, M. and Tyack, P. L., "A digital acoustic recording tag for measuring the response of wild marine mammals to sound," IEEE J. Oceanic Eng, 28 (2003), 3-12.