Plant Earth Online - Sounds under the Sea
Sarah - Documentaries filmed underwater tend to give the impression that it's a quiet, even serene environment beneath the waves, but stick a microphone in the sea and you'd be amazed what you can hear. One man who's been doing just that is Bristol University Fish Ecologist Steve Simpson and he's found that many marine species use these underwater sounds to find their way around. Richard Holligham went to meet him...
The sounds of a coral reef
Steve - That's a really healthy coral reef in the Philippines. It's a marine protected area, it's very well protected from fishing. First of all, there's a background crackle. It sounds almost like the sound of heavy rain on the pavement.
Richard - Or bacon in a pan or something.
Steve - Or frying bacon. That was how it's first described by mariners. That's the sound of snapping shrimp. They produce a microbubble in their claw that they fire forwards. The bubble implodes when it hits the water and that creates a very loud snap.
Richard - Now the other sound in there was almost a croaking sound.
Steve - That's right. So, there's then a whole suite of diverse sounds that fish have learned to make. So they can be croaking sounds, chirping noises that sound almost frog-like, and they do that to communicate with each other, perhaps to assess whether they're a suitable mate or for territorial behaviour.
The sounds of a coral reef
Steve - The logical next step for our work was to take our recordings of coral reefs and start to look at what information that recording contained. So the first study that we did then was to split the recording into the higher frequency noise, that is the crackling snapping shrimp, and the lower frequency noise that was the fish popping and chirping.
What we found was that larval fish were actually attracted to the higher frequency crackling noises. So we think that might be a cue that brings them into shallow water environments. When we play the sound on artificial reefs, the lower frequency noises are then used by the juvenile and adult fish that move around at night, trying to find habitat.
We've actually just got a paper out this week which shows that when you take recordings from different types of habitat, so we take recordings from an Outer Barrier, we take recordings in a lagoon, and we play that next to artificial reefs, the fish that you would find on the lagoon arrive on that artificial reef. The fish that you would find on the outer reef, you then find settling onto reefs playing those noises.
Richard - Why is this important? I mean, it's interesting, but why is it important?
Steve - The reason that I got interested in it was actually from a much more applied perspective in that I was interested in coral reef fisheries, particularly in fisheries in developing world situations where there is very limited information on what is being caught; it's a multi-species fishery, it's normally artisanal, so the fish are landed on the beach. So to try and actually model or manage populations of fish is very difficult if you've got no idea how the actual whole life cycle works.
Richard - Now your latest research comes back to those crustaceans, the snapping sound that we were hearing at the beginning.
Steve - So there are some crustaceans that do have a pelagic larval phase and then settle on to a coral reef environment. Crabs, lobsters are good examples of those, and we find that late stage larvae of crabs are attracted to coral reef noise, so that's great. But there are lots of crustaceans that live around coral reef environment in an otherwise fairly low nutrient environment; coral reefs are quite high in nutrients. So, to be able to forage on the outskirts of coral reefs, without encountering the millions of mouths that come out at night in every coral polyp, in every planktivorous fish, to find some way that you could live near to the coral reef without landing on it would be very useful.
We find that lots of groups of crustaceans that either sit in the seabed during the day time and come up into the water column at night to feed, or that are constantly in the planktonic realm, are able to detect coral reef noise, but stay away from it. So it becomes a cue that's not just used as an attractive cue but it relates to the ecology of the animals.