17 May 2021

New research explains why barramundi switch sex

New research from a Charles Darwin University PhD candidate has found the timing of the sex-switching in barramundi is more related to size than age.

Most barramundi are sequential hermaphrodites – starting their lives as males before switching to being female later in life.

Research undertaken by CDU PhD candidate Brien Roberts has discovered that a faster-growing barramundi will change sex at a younger age.

Most barra start their lives as males, then switch to being female later in life – that’s why most of the big ‘meterys’ are female. These big females are by far the most valuable breeders in the population. There’s limited evolutionary benefit to being a small female barramundi, because a larger female can produce a greater number of eggs, and will be much more successful at passing on her genes.

– Mr Brien Roberts, PhD Candidate, Charles Darwin University

 

Mr Robin Leppitt and Mr Brien Roberts look at some of CDU Aquaculture’s barramundi. Photo NESP Northern Hub.

Mr Roberts said his research showed that the timing of the sex-switching in barramundi is more related to size than age.

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

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

 

Mr Roberts analysed the growth rates of barra by using growth rings in fish ear bones (otoliths).

The research studied barramundi caught in Western Australia’s Fitzroy River, but the results apply to barramundi across all of Australia’s tropical northern rivers, including in the Ramsar-listed Kakadu wetlands.

Besides being the Top End’s most prized angling fish, barramundi are also highly valued by Traditional Owners for food and cultural reasons.

The Top End is experiencing its best wet season in almost five years, with rainfall across the NT between October 2020 and February 2021 around 32 per cent above the long-term average, the highest since 2016–17.

A bumper wet season will spell good news for the Northern Territory’s barramundi population.

A good wet season means rivers keep flowing and floodplains flood more often, ensuring well-fed and healthy barramundi in the future. Photo NESP Northern Hub.

Mr Roberts said the healthy drenching was keeping river flows intact and allowing floodplains to flood more often, ensuring healthy barra populations for the future.

The age at which barramundi mature as females depends on the environmental conditions they experience as a juvenile fish. Big wet seasons ensure good food and good habitat for juvenile barra, letting them grow and mature faster, turn into females sooner, and produce more baby barra for the future.

– Mr Brien Roberts

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, Murdoch University and NT Fisheries.

Prior to his PhD research with CDU, Mr Roberts worked as an aquaculture technician at Southern Ocean Mariculture cultivating abalone in southwest Victoria.

He holds an Honours degree in Environmental Science, with a major in freshwater biology and management from Deakin University.

This research is part of a larger project investigating the environmental water needs of the Daly River led by Associate Professor Alison King.