Blue whales (Balaenoptera musculus) truly are the giants of the ocean. Actually, they are the giants of the whole planet. Reaching around 30 metres in length and more than 190 tonnes, they are the largest animal currently existing and, to the best of our knowledge, the heaviest animal that has ever existed. Being so huge, risk of predation is low so they seem not to have really developed much of a threat-response system. Of course that all changed with humans who became extremely capable predators, but on an evolutionary timeframe, that is an extremely recent event. Hunting has largely ceased, but many endangered whale populations still face threats to their recovery and long-term persistence, threats like collisions with ships. Research by Megan McKenna from the Marine Mammal Commission, alongside colleagues from Cascadia Research Collection, NOAA, and Sreveals just why the blue whales are so vulnerable to ship strikes.
What did the researchers do? The research team focused their efforts on one of busiest shipping ports in the world - Port of Los Angeles-Long Beach. The area also happens to be a hotspot for blue whales, and known to be an area of high ship-strike risk. To get movement data of blue whales in the area, they used archival GPS and dive logging data, which was obtained by attaching monitoring units with suction cups to the whales, alongside some visual tracking from a small boat. They used ship AIS (Automatic Identification System) data to obtain vessel movements in the area. Combining these datasets together, the team were able to analyse the whale’s response to vessels passing them by up to 3.6km away.
What did the research tell us? Well the research didn’t conclude that the whales are just ‘stupid’ (they are regarded as highly ‘intelligent’ animals*). The whales did react to approaching vessels, but their reaction is… not that great. Instead of swimming out of the way of a vessel, the whales made “a slow dive”. We aren't talking the whole, tail up in the air and diving vertically down dive, more of a sink… horizontal sinking. Sinking can reach speeds of around half a meter per second, but whales need to get below 30 meters depth to escape both the vessel itself and risk of being sucked up into the vessel by the propeller. This, the researchers note, isn’t really fast enough to get out of the way of many vessels. Interestingly, the researchers note that the whales exhibited a similar behaviour when attaching the suction cups. They suggest that the “slow dive” is more of a startle response to unpleasant activity on the ocean surface, as opposed to an active ‘get out of the way of something moving quickly towards me’ response.
How is this research useful? The current research suggests that whales haven’t evolved the ability to get out of the way of vessels, but there are steps we can take to reduce ship strikes. The whale’s “slow dive” response may be useless for fast-moving vessels, but for slow-moving vessels it may be enough. Slow-speed reductions could help, though exactly what speed is slow enough is an area of research.
What’s next The researchers plan to do more tracking work on the blue whales to continue to direct effective management action to avoid collisions. They note that different whale species may react differently, so they are planning to expand their research to look at humpback whales too. They also suggest that reaction to oncoming vessels may be more a visual than an auditory response. Work on visual underwater visual cues may be useful, as could work on reactions to different ship sizes, bow designs, and ship speeds.
The paper, which the authors have paid to be made open access, is published in Endangered Species Research. You can have a read of it here http://dx.doi.org/10.3354/esr00666
*How best to measure intelligence in humans as well as other animals is an area of ongoing research
Image: A blue whale tale taken in Half Moon Bay, California. Credit Gregory "Slobirdr" Smith/Flickr (CC BY-SA 2.0)