Scientists have been amazed to find unusually large groups of Port Jackson sharks lounging amongst colourful sponge gardens and rocky reefs in a previously unexplored marine reserve in Bass Strait.
The Beagle Commonwealth Marine Reserve (CMR) covers almost 3,000 square kilometres of seafloor across Bass Strait from south-east of Wilson's Promontory to north-west of Flinders Island.
But the seafloor reaches to depths of 50 to 70 metres which makes it difficult to explore, said research leader Neville Barrett of the University of Tasmania Institute of Marine and Antarctic Studies (IMAS).
"This reserve was declared nearly 10 years ago, on the basis of extensive sponge gardens but no-one's been out there and collected any imagery or any real understanding of what's going on there, so we really have no idea, we've not really explored this part of our coast at all," he said.
"If we think of a reef in this part of the world, they're not-coral forming reefs like you find in the tropics, they're rocky reefs, so a rocky, hard bottom that a range of things grow on."
During 10 days in July, the Australian Maritime College's research vessel, Bluefin, sent an autonomous underwater vehicle (AUV) to take three-dimensional photographic maps of the seafloor.
The images surprised researchers.
Hundreds of Port Jackson sharks were massed on the sandy seafloor among rocky reefs covered with brightly coloured sponges.
"This is the largest aggregation of Port Jackson sharks I've ever seen," Professor Barrett said.
"While these sharks often gather in places like ledges and appear to sleep, they are very rarely seen in numbers more than half a dozen, so this aggregation appears unique.
"We know they lay their eggs further north —typically mid to southern NSW— in August to September, so they were unlikely to be spawning, but it may have been a mating aggregation where the eggs are first fertilised."
Professor Barrett said the images provided a valuable insight into the sharks' biology.
"Given that this CMR is protected from potentially disruptive activities, such as trawling, it shows that a species such as this may benefit from such closures, particularly at vulnerable life history stages such as mating," he said.
The research will provide a basis for ongoing monitoring of the reefs and shark conservation.