Aquaculture Europe 2015

October 20-23, 2015

Rotterdam, Netherlands

SEMI-CLOSED SEA SYSTEMS FOR POSTSMOLT ATLANTIC SALMON  

S. Calabrese1,3*, S.O. Handeland2 , B. Sævareid3, O. Breck3, R. Joensen3, T.O. Nilsen2, J. Kolarevic5, S. Fivelstad4, H. Takle5, C. Hosfeld4, L.O.E. Ebbesson2,  S. Stefansson1, B.F. Terjesen5
1Department of Biology, High Technology Centre, University of Bergen, N-5020, Norway, 2Uni Research AS, Bergen, Norway ,3 Marine Harvest Norway, Bergen, Norway; 4Bergen University College, Bergen, Norway, 5Nofima, Ås, Norway;;  
Email: Sara.Calabrese@marineharvest.com

Introduction

Currently, the majority of postsmolt Atlantic salmon production in Norway occurs in open sea cages. It is during this phase that the highest losses occur, approximately 10-15% of the fish that enter seawater do not make it to market size (Gullestad et al., 2011). Extending the time fish spend in controlled environments, such as land-based recirculation systems or semi-closed rearing systems in sea, prior to being stocked in open sea cages, is expected to produce larger and more robust postsmolts, decrease losses, shorten production time, and increase overall sustainability. For semi-closed sea systems to be economically profitable, rearing densities need to be increased from the legislated 25kg/m3 for traditional sea cages. Increased density and reduced water pumping costs per fish will lower investments and overall production costs, but may lead to a buildup of metabolites that negatively affect fish health and welfare. Several studies on stocking density and water requirements have been done on smolts in land-based systems, however relatively little is known regarding the biological requirements of postsmolts in semi-closed sea systems (Thorarensen and Farrell, 2011; Terjesen et al., 2013). In this study the effects of fish density and water flow on postsmolt Atlantic salmon performance, physiology and welfare were established in a small-scale laboratory experiment. Knowledge gained was used to develop and test an industrial scale semi-closed prototype.

Methods

Fish density and specific water flow for postsmolts in semi-closed sea systems: Postsmolts 115.0 g ±13.6 g were stocked in 5 different densities (25, 50, 75, 100 and 125 kg/m3) with a specific water flow of 0.6 l kg-1min-1 or in 4 different specific water flows (0.5, 0.4, 0.3, 0.2 l kg-1min-1) with a set density of 75kg/m3. Each treatment was run in replicate tanks in which the biomass was adjusted back to original treatment density every second week. The experiment lasted for 8 weeks in 10° C seawater and water quality parameters such as oxygen, pH, CO2 and TAN were monitored and kept within recommended values. In the density trial waste feed was collected and fish were bulk weighed in order to assess FCE, feed intake and biomass growth. Every two weeks, 12 fish per treatment were sampled to determine effects on physiology, external welfare and skin health.

Industrial scale semi-closed sea system pilot test:

The large-scale experiment took place from October 2013 to June 2014 at a Marine Harvest site in Molnessund in the south-western part of Norway. The semi-closed prototype was produced by AquaFarm Equipment AS and has a volume of 21 000 m3 and a pumping capacity of 400 m3/min. The intake depth was fixed at 25 meters to avoid areas in which sea lice are the most abundant and to optimize temperature. Atlantic salmon smolts (n=200 000, ~118 g) were transferred to the prototype November 17, one month after 200 000 smolts from the same group were transferred to a traditional sea cage as a reference. The delay in transfer to the prototype was due to technical issues. Water quality (temp, O2, TAN, CO2), growth and survival were monitored regularly by personnel at the site. More in-depth studies on water quality, fish performance health and welfare (n=100 fish) were performed before transfer and after 1, 3 and 6 months in the prototype.

Results and discussion

The small scale study revealed that fish densities above 75 kg/m3 had a negative effect on postsmolt growth. The highest stocking density (125 kg/m3) also had significantly reduced FCE and elevated plasma cortisol, Na+, PCO2 and decreased blood pH after 8 weeks of treatment. Stocking densities at 100 kg/m3 and above had a negative effect on external welfare parameters such as fin condition and cataract prevalence. Reducing specific waterflow below 0.4 l kg-1min-1 induced a typical physiological regulatory response seen with increased water CO2 (i.e. increased blood PCO2 and HCO3+ and reduced Cl-) and had negative effects on skin cell morphology. However, no negative effects were observed on macroscopic external welfare or growth. Results from this study suggest that stocking density should be below 100 kg/m3 and, without any in-tank water treatment, specific water flow should not be reduced below 0.4 l kg-1min-1 in order to maintain optimal postsmolt performance and welfare in semi-closed sea systems. Further studies will be needed to isolate more narrow ranges for these reccomendations, as well as effects of physiological and health status, fish size and water temperature.

The industrial scale test of the semi-closed prototype ended in June 2014 when fish had reached approximately 1 kg. Good growth and FCR (1.0) were observed throughout the period, especially shortly after transfer and throughout the winter when temperatures were higher in the prototype compared to the reference site. There were no observed effects of the prototype on external welfare indicators (cataract, operculum, fin and skin damage). Water quality parameters stayed within recommended values and a homogenous water quality environment throughout the tank was observed at the 3 and 6 month sampling point. Sea lice were detected in the prototype during winter and spring, although well below levels observed at other sites in the area, and was effectively regulated below treatment limits with cleaner fish. The survival was over 99 % the first 8 months in the prototype.

These studies show that if fish density and specific water flow are maintained at optimal levels, there is potential for good growth, welfare and high survival in semi-closed sea systems. Further tests are needed to determine sea lice entrance and dynamics in the system and possible accumulation effects if a disease outbreak occurs. The knowledge gained in these studies will further be used for optimizing biological conditions for postsmolts in semi-closed rearing systems.

Acknowledgements

This study was funded by The Research Council of Norway (project 217502/E40   Optimized Postsmolt Production "OPP"), The Norwegian Seafood Research Fund - FHF (project 900816), and Marine Harvest Norway, Lerøy SeaFood, Smøla Klekkeri og Settefisk, Grieg Seafood, Lingalaks, and Erko Settefisk.

References

Gullestad, P., Bjørgo, S., Eithun, I., Ervik, A., Gudding, R., Hansen, H., Johansen, R., Osland, A., Rødseth, M., Røsvik, I., Sandersen, H., Skarra, H., Bakke, G., 2011 (In Norwegian, "Efficient and sustainable use of areas in Norwegian mariculture"), Oslo, pp. 190.
   Terjesen, B.F., Rosten, T.W., Ulgenes, Y., Henriksen, K., Aarhus, I.J., Winther, U., 2013. Water quality requirements for efficient farming of Atlantic salmon in closed systems. In Norwegian, English abstract . VANN. 48, 14-27.
  Thorarensen, H. and Farrell, A.P., 2011. The biological requirements for post-smolt Atlantic  salmon in closed-containment systems. Aquaculture. 312, 1-14.