New research explains why barramundi changes gender



New research from a Charles Darwin University doctoral student has found that the timing of sex change in barramundi is more related to height than age.

Most barramundi are sequential hermaphrodites – starting life as males before becoming female later in life.

Research undertaken by CDU doctoral student Brien Roberts found that a faster growing barramundi will change sex at a younger age.

“Most barra start life as men, then grow into women later in life – that’s why most of the great ‘meterys’ are women,” said Mr Roberts.

“These large females are by far the most valuable breeders in the population.

“There is a limited evolutionary benefit to being a small barramundi female, as a larger female can produce more eggs and will be much more successful in passing on her genes.”

Mr Roberts said his research has shown that the timing of sex change in barramundi is more related to height than age.

“A faster growing barra will change sex at a younger age than a slower growing fish because it will reach ‘threshold’ size faster,” he said.

“Becoming female at a younger age means that a fish spends more of its life being female – and more time producing a baby barra.”

The research looked at barramundi captured from the Fitzroy River in Western Australia, but the results apply to barramundi in all tropical rivers in northern Australia, including the Kakadu Wetlands, listed by Ramsar.

In addition to being the most popular fishing fish in the Top End, barramundi is also very popular with traditional owners for food and cultural reasons.

The Top End has had its best rainy season in nearly five years, with rainfall in the NT between October 2020 and February 2021 around 32% above the long-term average, the highest since 2016-17.

An exceptional wet season will be good news for the barramundi population of the Northern Territory.

Mr Roberts said the healthy soaking kept river flow intact and allowed floodplains to be flooded more often, ensuring healthy barra populations for the future.

“The age at which barramundi mature as females depends on the environmental conditions they face as juvenile fish,” he said.

“The great rainy seasons provide good food and habitat for the barra juveniles, allowing them to grow and mature faster, develop into females earlier and produce more barra babies for the future.

The research is funded by the Australian Government’s National Environmental Science Program through the Northern Australia Environmental Resources Hub. It also involved scientists from the University of Melbourne, the University of Murdoch and NT Fisheries.

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