Every wondered how and where whales, dolphins and porpoises sleep?
New work by Gateway Antarctica’s Andrew Wright released this week reveals, for the first time, sleeping during diving in harbour porpoises. Part of his PhD work in Denmark before coming to the University of Canterbury, Andrew attached behavioural loggers to porpoises and found a new type of dive in the data obtained. The dives are slow, low energy and low in echolocation clicks – the biosonar that porpoises use to find food.
Cetaceans (whales, dolphins and porpoises), sleep with only half their brain at a time because they spend their lives underwater and must return to the surface to breathe. This unusual behaviour is also seen in many migrating birds that sleep on the wing. Yet life underwater means that we know little about sleeping in wild cetaceans. Applying behavioural criteria for sleep developed in terrestrial mammals to behavioural data from tags, Andrew identified a roughly semi-circular dive form that measured up. Stereotypical in not only dive shape, but also the swimming movements throughout the dive, the dives are typically quiet. This discovery means raises the possibility that animals sleeping at depth might be more susceptible to becoming entangled in fishing nets set at those depths because they are not echolocating.
The work raises some interesting possibilities for resolving the conflict between fishermen and cetaceans around the world, including New Zealand’s own Maui dolphin. For example, it may be possible to reduce entanglement rates if fishermen can avoid setting nets at the depths that the porpoises and dolphins sleep at.
“Although the dives make up less than 10% of all the activities for each animal, even small reductions in fisheries bycatch can make a big difference to the long-term survival of many endangered cetacean species,” notes Andrew.
However, the finding also has implications for scientists themselves. The use of passive acoustic monitoring technologies are becoming commonplace. Detecting marine mammal sounds as the whales and dolphins swim past, such devices were thought to be able to detect all porpoises as they were believed to produce clicks at all times. However, the existence of quiet dives means than not all animals will necessarily be detected. This means the finding also has implications for industries relying upon passive acoustic monitoring to protect marine mammals from harmful effects, such as the oil and gas industry.