Recirculation diets clear the water in Tassal hatchery
Tassal, Australia's largest producer of Atlantic salmon, found that Skretting dedicated recirculation diets keep the water in its new hatchery far cleaner, with several incidental benefits, reports http://www.megafishnet.com/ with reference to Tassal.
The freshwater hatchery in southern Tasmania was recently completed at a cost of AUD20 million and is the most advanced smolt rearing facility in Tasmania. It has the capacity to produce four million smolt to support Tassal's production of more than 24,000 tonnes of salmon each year.
The hatchery has three distinct production areas; egg and larval incubation, start-feeding and smolt-rearing tanks. Completing the hatchery is an advanced particle and bio-filtration system. Diets for all stages are supplied by Skretting Australia. Recently the Tassal hatchery began using Skretting's recirculation-specific diet, Nutra RC. Rhys Hauler, Product Manager for Skretting in Australia, says, "RC is the abbreviation for recirculation. We supply Nutra RC in 1.2, 1.5, 2 and 3 mm grades."
A diet with specific benefits for recirculation is critical for Tassal's hatchery, where 98-99% of the water is reused. Water is exchanged hourly in each tank and to accomplish such high rates of recirculation the system includes denitrification and processes to remove phosphorus, and final ‘polishing' of the water with ozonation and UV treatment. Doug Paveley, Hatchery Manager, comments, "Until recently we were not using recirculation specific diets. We found that while we had good growth, the water was turbid making it difficult to view the fish behaviour and the feeding response. As submerged lights are also used to help with the smoltification process, it's possible the dark water was preventing the lights having their full effect."
Paveley indicates that once they moved to the new Nutra RC they noticed profound differences. "The most obvious change has been the colour of the recirculated water. Previously you couldn't see the bottom of the tanks, but now the water is quite clear. You can see smolt swimming at the very bottom of 3-meter deep tanks. Other benefits include a noticeable lessening of the demand on the filtration system. Since using Nutra RC less ozone is needed to maintain water quality and there are fewer organics getting into the biofilters," adds Paveley.
Hauler explains, "Nutra RC contains patented, functional ingredients that help the faeces bind together. It also improves the elasticity and structural stability of the faeces. Basically you end up with a larger faecal ‘pellet' that is quite durable even with water turbulence, allowing it to be easily stripped out by physical filtration."