It was great to read in Stargazer last week about fishing guru Alex Julius's recent visit to lakes Julius and Moondarra. I believe the natural beauty impressed him and I am sure he will generate some very good publicity for our fisheries. However I needed to respond to his question why we do not have a barrier net.
For many years the Mount Isa Fish Stocking Group have looked at what we call the barrier net issue. In April 1996 the Director General of Natural Resources wrote to the MIFSG informing us that a favorable decision on a barrier net was unlikely due to “dam safety implications”. Furthermore we have come to our own conclusion that it is unfeasible, too expensive, requires high maintenance and its effect would be detrimental to the fishery we are trying to manage.
Firstly; Lake Moondarra actually has 3 spillways. When the flow on the main one reaches about 20 cm the secondary starts flowing then at about 1 metre the third spillway starts running. We have a proposal to chain mesh fence the secondary and third spillways but that is simply to save our members the big job rescuing barra caught in the rock holes.
The barrier net at lake Tinaroo was installed in April 1997 at a cost of $95,000. It was placed 800 metres up stream of the dam wall. A net cannot go directly at /on a spillway wall, because the water pressure at that point is too great. It could fowl up with debris or dead fish; it could get washed over; and potentially seriously damage the whole structure.
The Tinaroo net is 750m long and 40m deep and weighs 6 tons when dry. Each year it is removed and washed clean. Take a minute to think where we could site ours, who will pay for it and who is going to maintain it? Lake Moondarra fills on average only every 5 years, is it worth the expense and effort?
With out a net Lake Moondarra has proven to be the best stocked impoundment barramundi fishery in the state when you consider “catch per unit effort”. Sure we lost most of our big fish last wet but we want to allow the big barra to follow their instinct and make it to the Gulf to spawn. Barramundi need to find salt water before they can successfully breed. The fish that go down river help stock Lake Julius and sustain the natural gulf fishery when they do reach the sea.
Once a lake has had a couple successful barra stockings you are wasting your money putting in any more fingerlings. The survival rate of even 50mm fingerlings diminishes to almost nil. Barramundi are cannibalistic and a very awesome predator. At Lake Tinaroo they have been forced to trial stocking 300mm “footlings” to try and get smaller size classes of barra in the lake. Note they have to have a barrier net because no big fish would survive going over Barron Falls.
Our most pressing issue with the Lake Moondarra fishery is not a barrier net but how we can increase the survival rate of the big fish that go over the main spillway. That is where the Fish Stocking Group will put its effort.
Mark Van Ryt
MIFSG