Using multibeam sonar to track the underwater behaviour of grey whales

The study of marine mammal behaviour is made difficult by our inability to observe the animals underwater. This is particularly so of the large whales, which, it could be argued, are more surfacing mammals than diving mammals. Very generally, whales tend to spend 20-30% of their time near the surface, and 70-80% underwater - for example, feeding grey whales typically dive for 4 to 5 minutes, after which they surface for a minute before diving again. The time they spend at the surface is often simply to breathe and prepare for the next dive, so the interpretation of whale behaviour based on surface observations is unreliable at best. What is needed is a means of working with these animals underwater. Cameras are of limited use due to poor visibility - which even in clear tropical seas is limited to a few hundred feet (a few whale lengths). To get around this problem, we used a Reson SeaBat multibeam sonar to study the swimming biomechanics and underwater behaviour of grey whales in San Ignacio lagoon, Mexico, and Cape Caution, Canada. Biomechanical results include fluke motion and stroke frequency relative to acceleration and turning performance of the animals. Ethologically, we were able to describe three types of underwater behaviour from the lagoon: bottom-sleeping, mothers teaching calves to feed, and multiple whale behaviour which may be a reproductive display or dance. On northern feeding grounds, we used the sonar to describe the movement of whales relative to the bottom, each other and swarms of mysids, their primary prey in the area. We conclude that the multibeam sonar is a useful tool for the study of underwater whale behaviour. We note some issues with the current technology, particularly with regard to the precision of the instrument and its two-dimensional representation of the underwater environment.