When giant poplars shed their leaves for winter over Thredbo River, hooked-jawed trout swimming below drop their guard.
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Noses facing upstream, the hefty male browns glide gracefully behind sleek females above a bed of smooth rocks, intent on multiplying in their thousands in their annual spawning dance..
Rapids burble in the background as the ritual is repeated each time a new female in the pool allows the river to sweep her past a male.
As quiet as falling leaves, the mating is at odds with the business-like milking of female fish in Gaden Hatchery on the river bank above Jindabyne Dam, and adds to the mystery of why rainbow trout, unlike the browns, have become so scarce in the Snowy Mountains.
A forum at Cooma on May 29 will bring anglers up to date on trout stocking and introduce New Zealand scientist Dr Michel Dedual from the Department of Conservation, in Turangi, who will provide insights into the iconic Taupo trout fishery, which has experienced changes in catch rates of rainbow trout.
NSW Department of Primary Industry managers will be keen to hear feedback from anglers as well.
Gaden Hatchery's breeding and stocking underpins a $100 million recreational fishing industry and, while the hatchery produces abundant trout and Atlantic salmon, the rainbows' decline is raising concerns.
Monaro Acclimatisation president Steve Samuels says the hatchery is critical because the lakes are too cold for other species of fish. Trout and salmon thrive, but need replenishing. If not for Gaden, people in the mountains would have to find another way of generating tourism revenue over warmer months.
"We have to stock rainbows. Rainbows only live for three years and they are easily targeted by anglers. So if we don't supplement their stocks, their numbers can collapse, as they did in the 1990s,'' Mr Samuels said.
Gaden manager Garry Green said having disease free status enabled the hatchery to provide base stock for Tasmania's salmon industry in the early 1980s. Gaden also provides salmon for virus research at Port Stephens.
Gaden breeds Atlantic salmon and brook trout and traps 2000 browns, and fluctuating numbers of rainbows from Thredbo River each winter, milking the fish and releasing them. Rainbows are taken out about August and September.
Staff attribute the higher fertilisation strike rate of wild trout in the hatchery over captivity-bred fish to their exercise and carrying less fat.
In May female five-kilo, three-year-old salmon are milked. They are partially anaesthetised before being towelled off, given a few slaps to the lower body and squeezed. They eject about 1000 eggs per kilo.
Then males, called bucks are hoisted from tanks and held above the bright orange eggs. Their milky sperm squirts into a dish full of the eggs. Fresh water is mixed in with a small paint brush and, in about eight weeks, a set of pin-sized eyes appears in successfully fertilised eggs.
By October the hatchery has a full quota of fertilised eggs and fry that will grow out and be delivered as fry and fingerlings across the Snowy Mountains,
Gaden also stocks the Blue Mountains, Oberon, Southern Tablelands and provides brown trout ova for Dutton trout hatchery in Ebor, New England.
Mr Samuels said brown trout were not stocked in Macquarie perch waters, and trout were not stocked above an altitude of 1500 metres, because it was the last refuge of native Galaxias fish.