Spawning aggregations are one of the ocean's great mysteries. Where the fish are going and what they're doing when they get there are two questions that need answering. And in Western Australia , researchers are unlocking some of the mysteries.

Different species aggregate to reproduce in hundreds of different ways. Some, like the Western Australian dhufish, have quite a complex social structure, where the males and females have their own hierarchies and breeding rules.

Others, like samson fish, gather in their thousands at the same time and place each year.

Anglers often find the temptation to target aggregating fish too hard to resist, unaware of the massive repercussions of taking out a dominant male or female from the group.

Samson fish aggregate west of Rottnest Island annually.

We know from examples around the world that targeted fishing of these species can deplete the stocks to the point where they're in serious strife.

Mike Mackie is heading up a team of scientists from WA Fisheries and Perth universities to learn more about spawning aggregations in their part of the world.

They're studying four different species and each one needs a different technique to monitor their gatherings.

Because samson fish are large and don't move too much when they aggregate, a high-tech sounder is used to map the fish and a computer program can turn it into a 3D format.

Other species, like snapper and dhufish, gather in clean waters where video cameras are sent down to record them.

For mulloway in the river, they can't really use video because they're around at night and it's murky but they grunt and make noises.

Mulloway are known as 'croakers' - they make a noise by using muscles to vibrate their swim bladder.

PhD student Miles Parsons is studying the grunts mulloway make during their spawning aggregations in the Swan River with underwater microphones.

The results have been pretty amazing . For instance, the calls of the male mulloway change as the evening progresses.

Miles is even able to identify individual fish, and is building up a database of grunts to match the mulloway.