Browns Mountain in the spotlight

BROWNS MOUNTAIN: FACT FILE

Browns Mountain is the remains of an ancient volcano, also known in the geological circles as Mount Woolnough.

A voyage led by Iain Suthers (UNSW) last year deliberately passed directly over the peak (June 17, 2015) on their return to Sydney Harbour. The vessel’s instruments recorded the size, shape and acoustic characteristics of the mountain.

The productive reefs that form Browns Mountain are also home to large deep-sea species such as gemfish, blue-eye trevalla and bass grouper, especially during winter aggregations.

Game fishers catch oodles of yellowfin tuna, southern bluefin tuna, marlin, mahi mahi, wahoo, sharks and other pelagic fish in the upwellings here, too.

Browns Mountain is on the edge of the continental slope, between 375m to 550m deep, and rises approximately 175m above the seafloor.

While only two kilometres of the volcano base are visible on the sonar, other research reveals the full extent to be more than 13km in diameter! So troll around the area and don't just stay glued to the peak.

RV Investigator’s sonar revealed a double peak, with a trough in between. It is the remains of a volcano approximately 260 million years old, composed of distinctively hard rock compared to the surrounding sediments.

The origin of the name “Browns” Mountain may have been named by Jack Paton (a life-long member of Sydney Game Fishing Club) which he named after his old deckhand ‘Browny’.

Paton's boat, the M.V.Signa, a purpose-built game-fishing boat he built himself in 1970 and named after his wife Signa (Sig) is still operating as the
Signa charterboat in Kiama today.

The name Mount Woolnough is due to Hedley (1910, p.20) who wrote: “... I propose with the permission of the Society [i.e. the Linnean Society of NSW] and of Dr. Walter G. Woolnough, to name this submerged cone Mount Woolnough, ...”. 

Walter Woolnough was a professor of geology at the Universities of Sydney and Western Australia, and advisor to the federal government. The mount was originally discovered by the famous around-world oceanographic survey by HMAS Challenger in 1874, and was a source of interest for Woolnough.

This latest revealing voyage led by scientists from the Sydney Institute of Marine Science (SIMS) deliberately passed directly over the peak June 17, 2015, on their return to Sydney Harbour while towing plankton nets. But the news of this research is only now emerging in the public domain.

The vessel’s instruments recorded currents around the mountain and also the size, shape and acoustic characteristics of the volcanic structure. 

SIMS scientists Iain Suthers and Moninya Roughan were investigating how tidal currents and ocean currents flow around obstacles such as Browns Mountain, as they can attract fish as well as contribute to overall fisheries’ production. 

Ocean currents deliver plankton to the small baitfish, which attract large pelagic fish such as marlin, mako shark and tuna.

The productive reefs that form Browns Mountain are also home to large deep-sea species such as gemfish, blue-eye trevalla and bass grouper, especially during winter congregations. 

At the time of the survey, a large warm-core eddy of the East Australian Current was sweeping over Browns Mountain at about 1 knot (0.5 meters per second). The current often runs fast and up to 4 knots here in summer, making bottom fishing impossible at times.

A similar process probably contributes to the recreational harvest from Sydney’s Offshore Artificial Reef, which although only 12m x 15m in area, nevertheless results in a harvest initially estimated by SIMS scientists to be 1-2 tonnes of fish per year but since downgraded to 500kg per year.

Credits: These words were reprinted courtesy of Professor Iain Suthers from the Fisheries and Marine Environmental Research Facility, School of Biological, Earth & Environmental Sciences, at University of NSW. Video footage courtesy of CSIRO.