A closer look at closed cages: Growth and mortality rates during production of post-smolt Atlantic salmon in marine closed confinement systems

Abstract The most controversial environmental problems in commercial salmon farming are the negative effects of sea lice (Lepeoptheirus salmonis, Caligus spp.), the genetic introgression of farmed salmon in wild populations, nutrient waste load and the emission of potentially toxic waste to coastal waters. Moving production from sea cages to land-based facilities, offshore farming or marine closed containment systems (CCS) are suggested as possible ways to solve these problems. However, there are few published studies on production capacity and fish welfare in such systems. The main aim of this study was to describe growth rates, mortality rates and mortality causes in the commercial-scale production of Atlantic salmon (Salmo salar) post-smolts in CCS from sea transfer until the size of 1000 g. From October 2014 to May 2017, we recorded growth rates, feed use, mortality and mortality causes during 23 CCS production cycles, including 18 CCS periods with off-season smolt (S0) and 5 CCS periods with one-year smolt (S1). The mean (SD) growth rate, thermal growth coefficient (TGC), for all 23 CCS was 3.03 (0.34), with no difference between cages with S1 (n = 5) and cages with S0 (n = 1 8). Cumulative mortality three months after sea transfer (CM3mo) was 2.6%, while cumulated mortality throughout the total trial period (CMtotal, mean number of days = 159) was 3.6%. Both CM3mo and CMtotal were higher in S1 groups than in S0 groups. Mean (SD) feed conversion ratio in CCS with S0 (n = 18) was 1.11 (0.07). The two main mortality causes were ‘Ulcers and fin rot’ (S1 and S0) and ‘Failed smolt’ (S1), accounting for 36.1% and 19.3% of the total mortality, respectively. Water flow, oxygen saturation and other water quality parameters were within safe limits for fish health and welfare.