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New Diets Counter High Temperatures for Atlantic Salmon, Sea Bass and Rainbow Trout

13 August 2012

It is widely understood that each fish species has an optimum temperature range and will lose appetite and grow more slowly when water temperatures go much above that range. Until recently the physical effects on the fish leading to these symptoms were not fully understood. Scientists at Skretting Aquaculture Research Centre (ARC) identified the changes caused by high temperatures and developed a feed solution to the problem that can be applied across species.

Following validation trials showing benefits in growth and feed conversion ratios, high temperature (HT) grower diets have been developed for various species, including Atlantic salmon, sea bass and rainbow trout. New Optiline HT salmon grower feeds were introduced by Skretting Australia in October 2011, in time for the southern hemisphere summer and are becoming available in Europe.

The research journey began in 2010, in a joint research project funded by the Research Council of Norway. The project was in anticipation of potential problems from climate change bringing warmer waters to the coast, especially in the south of Norway, but was also developed keeping in mind the annual challenges faced by Tasmanian salmonid farmers during summer. Skretting joined Marine Harvest, the Norwegian Institute of Nutrition and Seafood Research (NIFES) and the Institute of Marine Research (NOFIMA) to investigate.

A focus of the research was to investigate the effects of high temperature on gut integrity. Previous investigations from the EU-funded AquaMax project demonstrated that the gut wall is more permeable at higher temperatures, making the fish more vulnerable to invasion by pathogens. The Norwegian project showed that higher temperatures lead to faster metabolism of anti-oxidants and that disruption of the gut wall leads to loss of appetite. That prompted the ARC team to consider ways of counteracting these effects through a modification of the diet.

“We could counteract the loss of anti-oxidants by adding an appropriate anti-oxidant to the feed,” reports Ingunn Stubhaug, who led the research project at ARC. “The real challenge was to identify micro-ingredients that would stabilise the gut. We ran high temperature trials with several blends at our Lerang Fish Research Station. By January 2011 one combination really stood out. The fish were healthier, ate more, grew better and had a lower feed conversion ratio. Then, using new digitised histology equipment, Charles McGurk from the ARC Fish Health team, revealed that this blend reduced disturbance of the gut wall, which explained the positive effects seen in the trial. That blend is the basis for the HT feeds.”

Sissel Susort, Senior Product Manager Skretting Norway and a member of the Skretting global Standard Feed Team, translated the results into commercial grower feeds for salmon. Fellow team member, Rhys Hauler, Marketing Manager in Skretting Australia, saw an opportunity. “We began selling Optiline HT in October 2011,”reports Hauler. “We also took the opportunity with HT competence to challenge the high protein formulations conventionally used over summer in the Australian salmon industry. Unfortunately these lead to higher excretion of nitrogen. Optiline HT, however, has less protein, subsequently nitrogen outputs are less and therefore the diet delivers sustainability benefits as well.”

“Having the knowledge from the ARC and the practical experience with Atlantic salmon, we began formulating HT feeds for sea bass and rainbow trout,” says Umberto Luzzana, Product Manager, Skretting Southern Europe. Optibass HT is recommended for use when water temperatures exceed 26 °C. Optiline HT for rainbow trout is for when water temperatures reach 18 °C. “In addition to anti-oxidant and micro-ingredient changes, we changed the balance of protein and energy and adjusted the amino acid ratios. At the higher temperatures we encounter in this region, oxygen levels in the water are lower. It is wasteful to include too much energy as the fish cannot get sufficient oxygen to use it. Instead, the proportion of protein is higher. Also, the basal metabolism of the fish is altered at higher temperatures, which means they use amino acids differently. We adjusted the balance of amino acids to match these changes.” The HT formulations became available to Skretting operating companies in Spain, France, Italy and Turkey in June 2012 and they are introducing them to customers as appropriate in their markets.

Trout trials in Italy

Trials in Italy with rainbow trout demonstrated the advantages of HT feeds. Rainbow trout weighing 180 g were fed either Optiline HT or a conventional grower diet, both given at 0.8% of body weight per day. The water temperature during the trial was in the range 18–21 °C. After 54 days the Optiline HT trout showed a specific growth rate 18% higher and an economic feed conversion ratio 14% lower than with the conventional diet. An additional benefit was observed with outbreaks of RTGE (rainbow trout gastro-enteritis). The interval between the outbreaks in the Optiline HT batch was 40 days but 25 days with the conventional diet. Also, mortality for Optiline HT was 1% compared with 2% with the conventional feed.

Optiline HT at Tassal

David Kiemele, Head of farming at Tassal, was encouraged to use Optiline HT for salmon production over the Australian summer based on the ARC knowledge of improved fish performance and the potential benefits of reduced nitrogen outputs. “We had a very challenging summer this year, water temperatures reached as high as 19-20oC in most of our production areas”, Kiemele says. “Optiline HT has helped us maintain performance during this difficult period,” he reports. The HT diet was fed in commercial cages as water temperatures climbed above 15oC and from then on all the way through the summer months. “Compared to the conventional summer diet, fish fed Optiline HT showed 8% improvement in growth as well as reduced FCR and increased feed intake. We are very pleased with the results of the commercial trials this past summer, and plan to incorporate the HT option into how we manage fish performance through warm summers in the future”.

August 2012

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