Aquaculture Magazine

April/ May 2017

Post-smolt production in floating enclosures – a new era for the salmon industry?

Production of large post-smolt fish in land based tanks or floating enclosures before transfer to open cages has become a significant alternative in several major Atlantic salmon producing countries.

By Asbjørn Bergheim*

Compared to the traditional farming regime based on stocking of newly smoltified fish of around 100 g size, a later stocking of post-smolt of 400 – 1,000 g in the cages represents some obvious advantages. On the other hand, introducing a ‘third production stage’ represents extra challenges connected to costs and production technology and these aspects are still at an early phase.

Onshore farming of post-smolt takes place in recirculating systems (RAS) supplied with pumped seawater. Such systems are run at high intensity and characterized by increased fish density, maximized growth rate at optimized water temperature (12-15 ºC), and low water consumption, probably even below 500 L/kg of fish produced. Nofima’s research station at Sunndalsøra in Norway performs comprehensive studies on performance of post-smolt salmon in seawater RAS.

Floating enclosures – or semi-closed containment systems (S-CCS) – for production of salmon were initiated almost 30 years ago without convincing results. A decisive reason for the meagre outcome was lack of efficient systems for injection of extra oxygen enabling increased production compared to traditional open cages. The Canadian company AgriMarine Industries then introduced S-CCS with floating, solid-wall cages that incorporate low-pressure pumping of water, oxygen supplementation, separation of solid waste and efficient feed management. 

Such systems are subject to great interest in Norway: at present, 9 farms are already in operation while another 14 farms are under project planning (Terje Fyhn Terjesen, Feb. 2017). The construction presented in Figure 1 consists of 6,000 m3 cages with flexible walls supplied with water from 28 m depth. Thorough optimization of smaller pilot concepts since 2012 has led to the existing design that is an approved R&D facility. Several publicly funded research projects are in progress at the site.

Salmon performance tests so far in Northern Norway demonstrate potentially increased growth rate in semi-closed cages mainly due to favourable temperature levels of pumped deep water during the winter. On a brackish water fjord site, post-smolt grew about 2 times faster in S-CCS cages (concept: Figure 1) compared to the growth in nearby open cages (Arve Nilsen, personnel communication). The temperature at pumping depth of ca. 28 m was 2 – 4 ºC above the surrounding surface water from January - March, i.e. 6 – 8 ºC and 3 – 6 ºC, respectively. Not least, the temperature of deep water was decreasing steadily throughout the winter, unlike the surface temperature where fluctuations of several degrees from one day to another often occurred.

Obviously, the length of the production cycle in the sea based upon post-smolt rearing in S-CCS followed by open-cage farming until harvest strongly depends on the temperature advantage of the S-CCS site in the cold season. Due to this higher winter temperature, the cycle from smoltification (100 g size) until 6 kg may be reduced by up to 6 months (Arve Nilsen). Semi-closed cage sites without the brackish water surface layer do not provide the temperature advantage.

The performance of the fish in closed cages maybe a result of additional influencing factors rather than temperature. Increased occurrence of winter ulcer disease has occasionally caused increased mortality in closed systems. The type and quality of the smolt at stocking seem to play a role, e.g. initial tests indicate better performance of under-yearling compared to 1-year old smolt demonstrating higher growth and feed utilization.

There are no available reports on sustainability analysis of S-CCS vs traditional cage farming yet. However, most reports so far describe strongly reduced sea lice attacks in closed cages, which means improved welfare of the cultured fish and less spreading to wild fish. In general, the walls of closed cages help ensure that disease organisms are not spread among fish groups both within the farm and between farms.

Unlike open cages, S-CCS enable removal of solids from the effluent water by sieving. Thus, the treated outflow only contains smaller particles (< 0.1 mm) with low settling velocity and consequently low risk of any influence on the seabed beneath and around the farm. Collected sludge is considered a valuable resource as manure in agriculture, compost substrate in horticulture or for production of biogas. Other potential fields for utilization of aquaculture sludge are under consideration, e.g. as fuel or an ingredient in concrete.

Dr. AsbjØrn Bergheim is a senior researcher in the Dept. of Marine Environment at the International Research Institute of Stavanger. His fields of interest within aquaculture are primarily water quality vs. technology and management in tanks, cages and ponds, among others.

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