Recirculation technology improves smolt welfare
New knowledge is making land-based smolt production more efficient and improving fish welfare in the process. Recirculation technology is solving the problem of access to an adequate supply of fresh water for additional fry production.
The idea that Norway, where fresh water is so abundant, would soon lack adequate freshwater resources to produce smolt for its aquaculture industry may have sounded farfetched, but this is precisely what a group of researchers accurately warned against back in 2006.
The research group behind the report "Tilgjengelige ferskvannsressurser til fremtidig produksjon av settefisk av laks og ørret" ("Available freshwater resources for future smolt production of salmon and trout," in Norwegian only) comprised members from the Norwegian Institute for Water Research (NIVA), the SINTEF Group, and the Institute of Aquaculture Research (AKVAFORSK, since merged into the Norwegian Institute of Food, Fisheries and Aquaculture Research (Nofima)).
The research team predicted that conflicting interests and the EU Water Framework Directive would make it difficult to satisfy the estimated need for enough fresh water to produce 350 million smolt per year in 2020. This level of production is already near at hand: in 2011 some 309 million smolt were produced in Norway.
Nofima received funding from the Research Council of Norway for a five-year Research Institution-based Strategic Project to learn more about environmental conditions and smolt production in recirculating aquaculture systems (RAS). These systems recycle water, in contrast to conventional flow-through systems that require a constant supply of fresh water passing through a fish tank just once.
"Even though recirculating aquaculture systems are quickly gaining favour in Norway, there has been little documented knowledge about water quality and operating conditions for ensuring high production and good fish welfare," points out Bendik Fyhn Terjesen, Senior Scientist at Nofima's Sunndalsøra Research Station.
"Our findings show that recirculation can provide a more stable environment and improve fish welfare while at the same time meeting the industry's efficiency need." He headed the research project, a collaboration between Nofima, SINTEF Technology and Society, and the Aquaculture Protein Centre (APC) in Ås, Norway. The US-based Freshwater Institute also collaborated.
High fish density is important for ensuring efficient operations and profitability using land-based smolt production facilities. Dr Fyhn Terjesen and his colleagues have concluded that parr (salmon weighing 15-100 grams) have no problem tolerating fish densities up to 100 kg per cubic metre with the right water quality and feed supply.
"Larger fish can probably tolerate even higher densities, but it is important to consider that density increases quickly from day to day," says the project manager. "The system has to be set up so the fish are not subjected to such high density over extended periods – particularly if the water temperature exceeds 14 degrees."
Although roughly 25 recirculating aquaculture systems are currently operating in Norway and some producers have acquired a great deal of experience with them, most of the technical know-how has been located outside Norway. Operating an RAS facility requires specialised training.
Over the past five years, the Nofima researchers have been experimenting with recirculating aquaculture systems (RAS) for Atlantic salmon and have worked out a formula for optimal production of salmon smolt in such systems. The five most salient points are:
- Salmon adapt well to increased concentrations of ammonia.
- The salmon environment should have at least a 100:1 ratio of chloride to nitrite-nitrogen, far higher than previous international recommendations based on other fish species.
- Alkalinity should be maintained at 70 mg/L CaCO3 or higher.
- Tank-water current should be 1?1.5 body lengths per second. If it is slower the fish will lack exercise, faster and their resistance to disease may be reduced in this freshwater phase.
- When dimensioning RAS facilities, use documented values for feed utilisation in salmon of relevant size and genetics, fed on modern feed types.
Additionally, friction through the recirculation pump and pipes warms the water, accelerating fry growth. In flow-through systems, this effect can be achieved only by directly heating the water. Compared to flow-through systems at the same water temperature, recirculation systems result in less fin wear and operculum/gill damage to the fish and lower levels of CO2 compounds in the blood.
Full control of bio-conditions
According to Dr Fyhn Terjesen, simply monitoring the biological needs of the young salmon is not enough. In many ways, the recirculation system itself is an organism whose biological processes must function properly. An RAS facility is equipped with biofilters through which large amounts of air pass.
The biofilters contain plastic chips that greatly increase the surface area – up to hundreds of square metres per cubic metre of medium. Living on these chips are bacteria and other organisms that convert harmful ammonia to less-toxic nitrate.
"It is vital to know which microbial environments work best in the biofilter," explains Dr Fyhn Terjesen.
"We have evaluated different molecular-biological methods to find the one that best describes the microbial community. Together with SINTEF Fisheries and Aquaculture we have also developed a method using biofilter technology for quickly measuring the amount of biodegradable organic carbon. Compared to methods currently in use, these new methods provide more relevant monitoring of conditions for more stable smolt production in recirculating aquaculture systems."
The researchers also tested use of sensors on fish and found that real-time readouts of fish behaviour give ultra-fast indications of changes in the fish environment within a recirculation system, such as changes in feeding and water flow.