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Research: Sex Change Makes Female Fish 'King of the Pack' News Service
April 15, 2007 17:42 EST

NEWCASTLE, Australia -- Local research has revealed a remarkable tale of fish changing sex and fish harems in waters off the New South Wales Central Coast.

PhD student at the University of Newcastle, Ourimbah campus, Jason Morton, has spent the past four years researching the secret life of two local species of fish; the Crimson-banded wrasse and the Maori wrasse.

"My research has uncovered the fascinating life of local wrasses, including the ability to change sex," Jason Morton said.

"Individual wrasses first function as females. At around two years of age, and approximately 18cm in length, they mature and begin producing eggs.

"They continue to function as females until approximately four to five years of age, when they are about 28cm long, then change sex and become males in a process known as sequential hermaphroditism.

"Within each territory a single male has a harem of up to 10 socially ranked females with the largest, highest ranking female likely to be the one to change sex after the male dies.

"But my research has found that the females are not always committed to one male. Some make excursion trips into other harems to determine whether joining a new group will improve their social rank and therefore reduce the waiting time to become a male.

"My research provides insight into the likely social disruptions to the wrasse population caused by fishing, particularly because the larger, male fish is more likely to be caught.

"As a result, the highest-ranked female fish changes sex and the remaining females within a harem are likely to re-contest their social rank."

Mr Morton said local enthusiasts of fishing, snorkelling and scuba diving would be familiar with the wrasse, particularly because they readily take bait ahead of other species, but very little was known about them.

"Over the years I have observed patterns of distribution, social organisation and behaviour, age and growth, reproduction, and feeding ecology," he said.

"I have found that unless captured by fishermen or eaten by predators, these fish can live for more than 10 years and reach 40cm in length, a fact that will surprise many people."s Division of Biology, who worked on the study, explained: "Intensive fishing makes populations vulnerable because if they rely on recruits to replenish their numbers, there is always the danger that some kind of environmental factor will devastate the recruits in one season. This would leave the population close to collapse, with very few young fish coming into the group to replace those being caught."

Professor Beddington adds that the increased variability has serious implications for the way in which fish stocks are managed: "Typically fish populations are managed by governments setting total allowable catch limits (TACs), but a fixed TAC which doesn't take into account the variability of abundance over time, may mean that in some years it is completely incompatible with the population size. This means that fishing vessels could unwittingly overexploit the population, even though they are abiding by set limits."

Researchers hope that their findings will mean future decisions about fishery management take into account the variations caused by fishing, to safeguard the future of key fish populations.