1 2For the development of commercial scale semi-closed sea systems for farming post-smolt 3 Atlantic salmon (Salmo salar), further knowledge is required on the interaction between fish 4 density, farming conditions and fish welfare. In this experiment post-smolts (115.0 g 13.6) 5 were stocked at 5 different densities (25, 50, 75, 100 and 125 kg m -3 ), and kept at these 6 densities for 8 weeks. All treatments received an equal specific flow rate of 0.6 L kg fish -1 7 min -1 of flow-through seawater (fully oxygenated, salinity 34 ‰ and temp. 9.3°C) and water 8 oxygen (O2), pH, carbon dioxide (CO2) and total ammonia nitrogen (TAN) levels were 9 monitored in the outlet and kept within recommended limits. Over the 8 week period, specific 10 growth rate (SGR %) was significantly reduced in stocking densities of 50 kg m -3 and above.
Environmental challenges related to open sea cage production of Atlantic salmon have sparked interest in developing commercial-scale semi-closed sea systems for post-smolt Atlantic salmon (100–1000 g). Determining the mass-specific water flow required by post-smolts will largely influence the design and dimensioning of such systems. In this experiment, post-smolts were exposed to four levels of specific water flow: 0.2, 0.3, 0.4, and 0.5 L kg fish−1 min−1. All treatments involved flow-through seawater with full oxygenation, a salinity of 34‰, and a mean temperature of 9.3 °C. The stocking density was kept stable at 75 kg m−3. Water pH decreased with reduced flow, while partial pressure of carbon dioxide (pCO2) and total ammonia nitrogen (TAN) in the water increased. The increase in water CO2 was reflected in the blood with increased pCO2, HCO3−, and decreased Cl− in the lowest water flow treatment (0.2 L kg fish−1 min−1), indicating a typical regulatory response to increased water CO2 over the eight-week experimental period. No negative effects on osmoregulation, external macroscopic welfare, or performance indicators were observed, suggesting that within the time period of this experiment, post-smolts can compensate for reductions in water flow down to 0.2 L kg fish−1 min−1. However, to avoid activating and exhausting potentially energy-costly physiological regulatory mechanisms, it is suggested to keep specific water flow above 0.3 L kg fish−1 min−1 in large-scale operations with semi-closed sea systems at intermediate temperatures.
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