Reef fish get lost as climate changes

Michael Perry, Reuters 6 Mar 08;

SYDNEY (Reuters) - Climate change might be causing reef fish to get lost, unable to return to breeding grounds from the open ocean, which could have profound implications for the survival of reef ecosystems, Australian scientists say.

Climate change-induced environmental stress, including warmer and more acidic seawater, could be hindering the development of the ear bones in young reef fish, which rely on sound for navigation, the marine experts said on Friday.

The scientists from the James Cook University and the Australian Institute of Marine Science found that fish with asymmetrical ear bones struggle to return to their home reef.

"In our opinion, ear bone asymmetry in the early life stages of reef fish interferes with their capacity to find and settle on coral reefs," fish ecologist Monica Gagliano said in a statement.

Fish at the end of their "ocean stage" after hatching navigate by homing-in on reef-associated sounds, such as the gurgling of fish and the snapping of crustaceans, said the scientists, whose study was published on Friday in the British scientific journal Proceedings of the Royal Society.

Vertebrate animals make sense of sounds by comparing differences in the acoustic signal between their two ears. To do this well, ear structures must be relatively symmetrical. Asymmetrical ear bones do not appear to make the fish deaf, but might interfere with the ability of the fish to hear effectively.

The scientists said ear bone asymmetry could be closely linked to rising sea surface temperature and acidity, caused by high atmospheric carbon dioxide levels, as well as localized stresses. Oceans absorb CO2 from the atmosphere, acting as a giant sink for the greenhouse gas.

Fish ear bones, like fish skeletons and reef-building corals, are made from calcium carbonate. When seawater becomes more acidic, there is less calcium carbonate available for building calcium-based structures, including fish ear bones.

LOST AT SEA

The scientists studied damselfish, which are abundant on Australia's Great Barrier Reef and western Australia's Ningaloo Reef, and found that at hatching, 41 percent of a sample group of fish had symmetrical ear bones and 59 percent asymmetrical.

When the scientists examined the ear bones of fish returning from open ocean to settle on the reef a few weeks later, far fewer asymmetrical fish made their way back to the reef.

The scientists also found that those with asymmetrical ear bones that did make it to the reef took longer to do so than their symmetrical counterparts.

"There is a degree of asymmetry that is acceptable in the population, some is natural," said scientist Martial Depczynski.

"Not all the babies are created equal and not all of them are going to make it, even in pristine environments," said Depczynski.

But Depczynski said the already high mortality rate among reef fish hatchlings was likely to rise even higher if young fish could not navigate by sound.

The scientists said they suspected asymmetrical ear development might be responsible for a drop in the number of damselfish in recent years, but more study was needed.

"Five years ago we used to see them in the thousands, now they are not so plentiful," said Gagliano.

(Editing by David Fogarty)

Reef fish eavesdrop to find home
BBC News 7 Mar 08;

Coral reef fish spend weeks scouting out a new neighbourhood before they move in - just like human house buyers do, Edinburgh scientists have revealed.

The fish use their acute sense of hearing to eavesdrop on locations before deciding where to live.

Edinburgh University researchers studied shoals of fish near the Great Barrier Reef off Australia.

They found damselfish, cardinalfish, emperors and blennies chose communities by the "reef noise" they give off.

They then choose one based on their needs - in much the same way that a human would choose a new neighbourhood based on local schools or work commitments.

Very young fish - those who have just developed as larvae in the open sea - choose locations with invertebrates such as shrimp, which give off a high frequency sound, and hide in holes snatching passing food.

As they grow older, scientists say the fish become "more aware" of the social groups and communities they would like to live in.

They are then drawn towards reefs populated with fish - which produce a lower pitched sound - where they can find shoal mates, forage for food and expand their territories.

The research shows that these natural "cues" are important for determining how communities are established.

Shipping, drilling, mining and active sonar are all adding to levels of noise pollution, but the impacts on community structure are not known.

Dr Steve Simpson, of Edinburgh University's school of biological sciences, said: "These findings show that hearing is crucial for the survival of fish.

"Reef noise includes the sounds of invertebrates and fish feeding, and so provides other fish with direct information about the residents on the reef."

The research, funded by the Natural Environment Research Council (NERC) , the Fisheries Society of the British Isles (FSBI) and the British Ecological Society, has been published in the journals Coral Reefs, Animal Behaviour and Proceedings of the Royal Society.