In Norway the majority of post-smolt Atlantic salmon (Salmo salar) are grown in sea cages, which is where the highest losses occur. According to Sara Calabrese of the University of Bergen, close to 10-15% of the fish die before reaching market size.
Extending the time fish spend in controlled environments, such as land-based recirculation systems, or semi-closed rearing systems in the sea, produces larger, more robust post-smolts, reduces mortality, shortens overall production time, and increases sustainability.
However, for semi-closed sea-systems to be economically profitable, production intensity needs to be increased from the 25kg/m3 legislated in Norway for traditional sea cages. Increased stocking density and lower pumping costs per fish will lower investment- and production costs, but could increase concentrations of metabolites like CO2 that compromise fish health and welfare.
Calabrese addressed this issue at the Aquaculture Europe conference held last year in San Sebastián, Spain by describing recent experiments on the specific effects of CO2 concentrations on post-smolts in semi-closed, sea-based systems, and how sub-optimal conditions like increased CO2 affect fish challenged with additional stressors like crowding and transfer to sea cages.
Experimental design: Samples were taken from all treatments before starting the experiment and after four and eight weeks of CO2 exposure (n=12). Stress responsiveness was studied after four and eight weeks in the control, medium and high CO2 treatment (n=12 fish).
It is already known that even mild stress owing to poor water quality can reduce normal stress responses, and that is critical for rapid adaptation to a new environment, so Calabrese and her colleagues from the University of Bergen; Marine Harvest Norway; Bergen University College; Nofima; and Uni Research AS examined the effects of increasing CO2 on post-smolt growth, physiology and stress responsiveness, with a view to determining optimum conditions for growing post-smolts in semi-closed systems.
Atlantic salmon smolts weighing 91.5±13.9g from Lerøy SeaFood were distributed among twelve, square fiberglass tanks (500 l) at the Industrial Laboratory (ILAB), Bergen Norway. Each tank was initially supplied with full-strength seawater at 10-11°C under a simulated natural photoperiod. All tanks received a specific water flow of 1.0 l/kg/ min, and oxygen levels were automatically controlled to maintain saturation above 80% throughout the experiment. Water quality (temperature, salinity, pH, O2, TAN) was monitored.
After a two-week acclimation period pure CO2 (99%) was administered to obtain the concentrations of 2mg/l (control), 7, 15, 20, 25-30 and 30-35mg/l in duplicate tanks. CO2 levels were monitored daily through pH measurements and adjusted in the header tanks. The experiment lasted for a total of eight weeks. Twelve fish per treatment were sampled every second week for blood chemistry (ions and pH, I-STAT analyzer, EC8+ cartridges) and primary stress responses (glucose and cortisol, I-STAT and ELISA).
Stress challenge tests were done after four and eight weeks of CO2 treatment (at the 2, 15 and 25-30mg/l concentrations). This entailed netting six fish from each tank and constraining them for 10 minutes in the net (10 l) in a 300-l holding tank that received water from the original treatment. After a 30 minute recovery period in the 300-l tank, the fish were euthanized with a lethal dose of MS222 and blood and organs were sampled immediately.
Mean plasma cortisol levels in Atlantic salmon postsmolts after 4 weeks of exposure to either 2mg/L (low), 16 mg/L (medium) or 29 mg/L (high) of CO2 in flow-through sea water. Blue bars are effects of treatment only orange bars are after fish from each treatment have been challenged with an additional stressor (10 min net confinement).
High levels of CO2 not good
Their results indicted that SGR is negatively affected when PCO2 is increased from 29mg/l to 34mg/l for an eight-week period. After eight-weeks of CO2 treatment there was a linear relationship between increased CO2 in the water and regulatory responses in the blood. Concentrations of blood bicarbonate and pH increased with elevated ambient CO2, and blood chloride ions decreased.
After four weeks of CO2 treatment, concentration of the stress hormone cortisol were low and within reported resting values for salmonids (0-10 ng/ml) in the control group. At this time, fish from the high CO2 treatment (29mg/l) had significantly elevated levels indicating that CO2 might be a chronic stressor in post-smolt Atlantic salmon. A similar trend was observed after eight weeks, but no significant differences were detected. After the acute stress challenge test at both four and eight weeks, fish from all treatments had elevated plasma cortisol levels but no differences among treatments were observed.
A similar trend was observed for another primary stress indicator, blood glucose. After eight weeks of exposure to CO2 an acute stress challenge increased blood glucose in the control and medium CO2 groups, but not in the high CO2 group, suggesting that a high CO2 level (29mg/l) reduces the capacity of post-smolts in sea water to mobilize energy resources and elicit a stress response when faced with an additional stressor.
These results suggest that growth rates of post-smolts in semi-closed sea systems are reduced by high CO2 levels and that even slight increases in concentration initiate a cascade of physiological regulatory responses that are energy-costly. Furthermore, the study clearly shows that CO2 affects the ability of post-smolts to respond to additional stressors.
The group plans to further explore the ability of post-smolts to adapt to sub-optimal water quality in semi-closed sea systems by looking at molecular stress markers in the brain.
Hatchery international acknowledges the assistance of Sara Calabrese in providing additional details of these experiments and for furnishing the illustrations. For more information contact her by email at: Sara.Calabrese@marineharvest.com.