Thursday, August 6, 2009

Listening to beaked whales

Beaked whales are very difficult to sight at sea. Their blows are seldom visible, they can hold their breath on a dive for 1 - 1.5 hours, and when they do surface it is for a short time. When beaked whales were first tagged with acoustic recording tags in early part of this decade, we learned that they make deep dives to almost 2 km deep, over one mile below the sea surface to feed on deep prey. Once they reach a depth of a few hundred meters, they start to produce echolocation clicks that are so high in frequency that they are barely audible to our ears. These clicks are short upsweeps that are different from the sounds of any other marine animals, so when we hear their clicks, we can know their is a beaked whale around. The whale directs most of the sound energy forward, like a flashlight beam, but some of the energy goes in all directions. When you are several kilometers from a clicking beaked whale, when the whale sweeps its beam past the hydrophone, the phone detects several clicks getting louder as the beam points towards the phone and then fainter as the beam points away. The photos show the hydrophones, or underwater microphones, that CIBRA from the University of Pavia provided for this project. They are designed to be towed from the ship.

In the past few years, scientists have learned to tow hydrophones and use these short scans to detect beaked whales on surveys. But Med09 needs to be able to listen to most of the clicks from when a whale starts clicking to when it stops. The Pavia towed hydrophone system uses 4 hydrophones that can tell us whether the whales are forward or behind us, left or right, so we can steer towards them. Today was the day that we used this ability to follow beaked whales during a dive and to link sightings with acoustic detections. Once we heard faint scans from a group of beaked whales, we approached it and started circling the group. Once the clicks stopped, we notified the visual observers to get ready for the whales to surface. The observers then sighted a group of whales where we expected them to come up, and we picked up clicks on the next dive. By circling the whales with a radius of about 500 m, we were able to hear clicks more or less continuously until the whales stopped to surface again. This capability was an important part of our study design, so we were very happy to show how well it could be achieved with the towed hydrophones.

The weather conditions were ok but not excellent for tagging. We put the tag boat out twice to follow and attempt to tag beaked whales, but the taggers were not able to approach whales close enough to do so. Tagging beaked whales requires a small boat being able to approach to within about 12 feet to attach the tag using a pole. We are hoping for better luck tomorrow.