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The aerojet at work. Note the nutrient- stripping duckweed and the body of eels lying in the current for maximum oxygen uptake The charm of tropical Queensland. The fishout pond at Sugarland Barramundi left in them at this stage and it's not cost-effective to hold them once they have reached an acceptable market size. "There's a slight difference in colour and general appearance," says Mark. "It's not all that well defi ned, but with a bit of experience you can pick out the males from the females pretty quickly." This should be the eels' last grading. However, larger or smaller eels are pulled from the female ponds and kept in size cohorts should the size differential threaten to set off a bout of cannibalism. At low stocking densities of up to fi ve or six tonnes per pond (15-20 tonnes/ha) they use the aerojet aerators, but once dawn-dissolved oxygen (DO) levels fall below 4mg per litre they will add a 2.5hp paddlewheel aerator. Aeration during the production cycle is around the clock. Mark says the eels handle these stocking densities quite comfortably as long as they keep the aeration up to them. Turbidity is high. This prevents benthic weeds from developing and shields the eels from direct sunlight. Daily water exchange during the early growth stages when pond loads are low is between one and two percent, but once the stock biomass builds up, daily water exchange is increased to 10 percent. Mark likes to keep early morning DO levels above 6mg/litre whenever he can. In the past they have grown 5-10g elvers out in fl oating cages. Mark says they grew very well, but the young eels were masters at getting out of cages and managing the cages themselves was labour-intensive. However, based on the growth rates, he hasn't totally given up on taking smaller eels and running them through the nursery stage in pond- based cages. Mark tried a few diets and has settled on Skrettings' high- energy fully fl oating Atlantic salmon ration. They use salmon starter for the juveniles and the salmon growout feed for the larger eels. "We were using an eel feed from Skretting's French mill that was 25 percent lipids and 55 percent protein. This really got the young animals going but it's no longer available. Now the salmon starter rations we get from Skrettings are either from France or Chile," Mark says. "They're 15-20 percent lipids and 45 percent protein. But we're getting newsletters from the feed mill all the time relating to ingredient shortages and price hikes. It's a worry." Feed conversions at the Eels Down Under nursery can be better than 1:1. However, the growout diet they are using has been designed for salmonoids, and while they have found it to be adequate it's a long way from a species-specifi c longfi nned eel ration. Food conversion effi ciency for the females in the growout ponds is 1.5:1. Conversion effi ciency in the male ponds, with their much slower growth rate, "can kick up to 2:1". "They eat, but they've stopped growing in some cases so The bio-remedial pond at Sugarland. The native woodlot can be seen in the background we're not getting any gain from it. It's just a management thing. You have to be on top of it all the time. "The Asians get better performance from their growout diets. We've explored importing feed, but logistically it has its drawbacks. Not that getting feed up from Tassy doesn't require half a dozen phone calls to see where it is and to push it along the way." So far, with stock just starting to come through at the premium size, they have been able to sell all they can produce. However, Mark is a realist. "We don't know what the global fi nancial situation is going to bring. Our main interest has come from China. There is a cultural signifi cance to longfi nned eels over there – the bigger the better, as far as they're concerned. There are other large, tropical species in the Indo-Pacifi c basin, but environmental and fi shing pressure has all but wiped them out. "We get a good cash fl ow from the males in a short period. This also gets them out of the way early so we can dedicate production capacity to growing the premium-value females." The core costs of the aquaculture industry are under the same cost pressures as they are for other industries. Feed, transport and energy costs are all fi rming. Talking of aquaculture in general, Mark says. "Unless you have a high- value product to start with it makes it diffi cult. Along with not knowing what you're going to get, or even if the markets are going to hold up, there's no guarantee of a profi table return." He says northern Queensland is a great area for land-based aquaculture. There's plenty of water and the climate, apart from the odd cyclone, is pretty stable. Having said that, there seems to be an increase in the number of larger growers. "The government has tried to streamline the permit application process, but from what I can see it's not getting any easier or any cheaper to get a licence. In fact, it seems to be taking longer and becoming more expensive. "Investors don't like uncertainty and delays. Setting up an aquaculture operation from scratch is becoming out of reach for most average people." JANUARY/FEBRUARY 2012 ■ NZ AQUACULTURE ■ 7