IMOS Acoustic Observations Sub-Facility leader, Associate Professor Robert McCauley of Curtin University, has been throwing expensive sea noise recording instruments into the deep ocean in the Perth Canyon west of Fremantle for nearly 20 years. For all of this time they have been listening to a cacophony of noise every evening; however the source of that noise was unknown.
Now, McCauley and an associate, Dr Doug Cato from the University of Sydney and the Defence Science and Technology Group, have published an article in the Journal of the Acoustical Society of America suggesting that the noise they have been detecting is most likely produced by small fishes.
In the Perth Canyon each evening, timed around sunset, lantern fish (family Myctophidae) rise from their daytime resting places at 200 to 500 metre depth, to forage in the top 160 metres of the water column. Once the fish reach their foraging depth there is a massive increase in noise throughout the Perth Canyon which lasts until about 5 hours after sunset but may continue until sunrise at a lower level.
The researchers argue this noise – a chorus – is produced by these small lantern fish. Although small, lantern fishes are one of the most common fish in the oceans. They have all sorts of novel sensing systems, including a system of flashing lights along their body (hence the name lantern fish), sophisticated hearing systems and good vision for the low light world they live in. Some lantern fish also have a bubble of air inside them, their swim bladder, and from work done in the 1960's by a deep sea fish researcher, N. Marshall, they appear to have muscles attached to this swim bladder capable of vibrating it. By driving this bubble they will make a noise and so the researchers argue, are the source of the choruses that they have been listening to for so long.
The work done by McCauley and Cato shows that at a small scale within the Perth Canyon, and at a seasonal scale, the behaviour of the choruses matches where all the small planktonic food are and when they are most likely to be found. The lantern fishes will be preferentially targeting the places where the highest amounts of food are. The researchers argue that perhaps these small fishes are using these sounds during their feeding behaviour, which results in the location and levels of sound produced following trends in when and where all the food is.
The choruses occur at a massive scale, well beyond the researchers' sample range, which was almost 40 km along the 300 m depth contour and 15 km to seaward of this. This means the choruses act as giant beacons in the ocean, advertising places where there are lots of small food. While they have not measured the outside range at which the Perth Canyon choruses can be detected, the researchers estimate that these 'beacons' may be acting in the ocean at the many tens of kilometres scale, which they state are conservatively in the 30 to 60 km range, or most likely more. This makes the ocean a slightly smaller place where help is at hand in finding food, for all the animals which can hear the choruses, such as whales, dolphins, seals, tuna and many other fishes.
Support for this work has been through Environment Australia, Australian Defence, and since 2008, IMOS.
The abstract of Evening choruses in the Perth Canyon and their potential link with Myctophidae fishes, Robert D. McCauley and Douglas H. Cato is available online. Full text of the research paper, published in the Journal of the Acoustical Society of America, is available for a fee and to subscribers.
This IMOS news article has been adapted from a Curtin University media release.