Quick Dips

Earbones Reveal Fishy Secrets

The earbones of young kahawai are providing MAF Fisheries scientists with a more accurate picture of this valuable fish stock. Fish earbones, or otoliths, contain lines or rings which, like those found in trees, are caused by seasonal changes and stressful events.

"Unlike tree rings, however, otoliths can also have daily rings," says researcher Dr John Kalish. He has spent the past three months analysing the rings on the earbones of 75 young kahawai. Despite the popularity of the fish, little is understood of its early life and the relationship between the parent stock that is fished and the juveniles that replenish that stock, according to Kalish.

"We need to know how exploitation of adults will affect the progeny," he says, "so that we can better advise on safe catch levels."

The results of the study will provide a picture of the age and growth rates of kahawai, and may help to pinpoint where and when kahawai spawn. Kalish believes that it could also provide information on the effects of climate change on the growth and survival of the young fish, which has implications for future stocks.

In the study, the fish were immersed for 24 hours in solution of tetracycline, a harmless antibiotic, which was taken up by the bones. The solution's presence, in the form of a yellow ring, can be spotted by an ultraviolet light microscope. Twenty-one days later, the otoliths were removed from the fish and ground into thin sections. Each otolith was about three millimetres long and had to be magnified 200 times for the rings to show. Some 20 rings were found to occur following the yellow tetracycline ring, indicating that kahawai produce one ring a day.

"These findings will be used with length frequency data to complete the picture of the aging and growth rates of kahawai," says Kalish.

Researchers know that kahawai live for 20-25 years, and that much of the commercial catch is 10-15 years old. At present, it is not known for certain where or when kahawai spawn, and at what stage the young fish move out to deeper waters. By studying the otoliths of fish caught in a particular area, researchers will be able to tell how long ago they were spawned. Comparing that with local currents, there is the possibility of working out where the fish could have come from in that time, Kalish hopes.