Attracting fish to reefs

DR CRAIG RADFORD
The main part of my research is working on how larval fish and crabs mainly find their way back to a reef. Once they are hatched as larvae in the water column, they are at the mercy of the currents, and they usually wash quite a way out to sea. For example, the New Zealand rock lobster is found up to 180 kilometres offshore, and we are looking between 20 and 40 kilometres for some fish offshore.

So once these fish have developed, they have to swim all the way back from those distances, and there must be some sort of orientation cue, and we are pretty sure that they are listening to sound. All our work suggests that fish and crabs can respond to sound in that they can detect it, but what we haven’t shown and what we have always assumed was that they can detect it from these large distances based on first principles or physics of underwater sound and knowing that it can travel these large distances with little attenuation.

And a recent paper came out challenging us, really, to say that we have made all these assumptions. So we set up an experiment up at the Hen and Chick Islands, and we had a hydrophone that was basically stationary and was a reference to what the actual sound on the reef was doing. And then we took another hydrophone, and we measured the sound at a series of distances off from that reef, so 500 metres, 1k, 2k, 3k, 4k, 5k, and then we could calculate how the sound dropped off from the reef as we moved away from the reef.

And we found that it didn’t conform to traditional marine models. For the Hen and Chick example, the reef is about 600 metres long, and as you go away from that reef, we didn’t see the sound start to drop off until we were out to 1 kilometre away from the reef. We found that what we called the reef effect – it extends the range at which you can hear reef sound away from the source.

Acknowledgement:
Jenni Stanley
Canadian Broadcasting Corporation
Tony & Jenny Enderby