Fast Facts: Purple bream otoliths

Black and purple bream!

Image: Juvenile black bream (Acanthopagrus butcheri)

Q: Why would a black bream have purple otoliths?

A: When it was part of a stocking project!

A black bream caught during estuary monitoring in the Blackwood River has given researchers an insight into the long-term outcomes of a fish stocking project from more than 20 years ago.

Researchers from the Department of Primary Industries and Regional Development’s Nearshore, Estuarine and Freshwater team captured the 34 centimetre black bream while undertaking fish stock and ecosystem health surveys in estuaries across the South-West of Western Australia.

In an exciting discovery while examining the fish, the researchers found it had unusual purple otoliths (ear bones), and by counting the annual growth rings of those otoliths, it was aged at 22 years old.

This meant that the black bream was part of a stocking project conducted in the Blackwood River, near Augusta in the early 2000s, where juvenile black bream were released after having their otoliths stained with a purple dye.

The stocking was a collaborative project between Challenger TAFE and Murdoch University to help replenish depleted black bream stocks in the estuary.

Staining otoliths before releasing fish is known as otolith tagging. By being able to identify stocked fish from wild fish (which have plain white otoliths), it allows researchers to examine growth and survival rates of stocked fish and their contribution to the overall population.

The finding demonstrates the value of long-term monitoring and highlights how stocking programs could influence wild population dynamics.

Estuaries are important transitional environments between freshwater and marine ecosystems and support a range of valuable fish species, including black bream, however, they are also vulnerable to the impacts of human activity and climate change.

Image: Whole and sectioned stained otolith from the 34 cm, 22-year-old black bream.