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Hearing the way home – how fish find reefs

Hearing the way home – how fish find reefs

The symphony of underwater clicks, pops and clacks is a powerful navigation tool for juvenile reef fish to find their way home, according to new research conducted by New Zealand scientists from the National Institute of Water and Atmospheric Research (NIWA) and The University of Auckland working as part of an international team.

The research, published in “Science” magazine today (April 8), has shown for the first time that reef fish not only locate settlement sites using reef sounds but also discriminate between sounds.

Dr Andrew Jeffs of NIWA and Professor John Montgomery, director of the Leigh Marine Laboratory in the University of Auckland’s Faculty of Science, have been funded on a joint Marsden grant to study larval fish and crustacea.

They say the picture of reef fish larvae as passive passengers on ocean currents has collapsed in recent years under evidence of their enviable swimming capabilities.

“Most reef fish spawn on the reef, and the larvae are swept out to sea where they live in the plankton for weeks to months. The problem is – how do they return to the reefs when it is time to settle?

“Although only millimetres long, we have shown that reef fish larvae can consistently swim faster than the currents they float in – and in some cases we recorded them swimming for as far as 200 kilometres. So they are exceptional swimmers for their size, but the problem then becomes - how do they know which way to go?”

The research team built 24 patch reefs near Lizard Island on the Great Barrier reef. They used an innovative submersible speaker system developed in New Zealand for broadcasting imitation reef noises, which included the sounds of snapping shrimps and fish calls, on half of the patch reefs. The other patch reefs did not have pre-recorded sound. Larvae settled in greater numbers on noisy patch reefs than on silent reefs.

The study also found that one group of fishes, the damsel fish, were preferentially attracted

to patch reefs emitting high frequency sound, predominantly of shrimps, than the low frequency sound, predominantly of fish.

The “Science” paper says the study provides direct field evidence that settling reef fish use sounds to orientate towards and select reefs, and indicates that some fish groups may also be selectively using specific components on the reef sound to guide their settlement behaviour.

“This important use of sound at this critical life stage does raise the possibility of potential adverse effects of increasing marine noise pollution from for example, shipping and drilling, but may lead to development of new tools for fisheries’ managers for restocking fisheries or newly established marine reserves.”

Professor Montgomery says his interest now is to better understand what sounds attract the fish to reefs and how the fish are receiving the sound.

“Fish don’t hear in the same way as humans. Their ears are more like our otolith bone in the inner ear, which lets us know when we are upright. That means fish simply detect the accelerations of sound waves, and have no obvious way of telling the direction the sound is coming from.

“Some fish have small air bubbles in their bladder that help them process sounds, but some of the fish under investigation by our team did not. To make things more difficult, sound travels so fast underwater that even a human ear has difficulty pinpointing particular sound sources.”

Despite this, Professor Montgomery says fish can determine the direction of sound. Put in a T-shaped chamber that allowed them to choose to swim in one direction or another, most fish swam towards the replayed reef sound.

Dr Jeffs has particular interests in charting the nature of underwater sound around reefs and helped to show that larval crabs and rock lobster are probably using the same acoustic cues to find the coast from out at sea.

A better understanding of the nature and use of sound in navigation by marine animals could have far-reaching implications.

“It could be a particular component of the sound, such as breaking waves or the biological noises, or the overall ambient noise of the reef,” he says.

“If fish are attracted by the crooning of their peers, then areas emptied of fish through over-fishing may be slow to re-colonise. Or, if fish only sense less specific noise, there is a danger that other noises could draw them into unsuitable areas. A better understanding of the underlying sensory and behavioural mechanisms will help the active management of reef fish populations in the future.”

The research is a collaboration with the National Institute of Water and Atmospheric Research (NIWA), the Leigh Marine Laboratory and School of Biological Sciences at The University of Auckland, Dr Stephen Simpson, Edinburgh University, Dr Mark Meekan, the Australian Institute of Marine Science, and Dr Rob McCauley, the Centre for Marine Science and Technology, Curtin University.

- Ends -

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