Long-term performance of Arctic charr following early and late juvenile stage entry to brackish water

The performance of Arctic charr ( Salvelinus alpinus ) was monitored following transfer from low-salinity water (LSW, 3.6 ppt) to brackish water (BW, 20 ppt) at two juvenile stages. The early-entry group (EE) was transferred at ~8 g, whereas the late-entry group (LE) entered BW 151 days later at 176 g. Mortality did not differ between groups. Growth was initially slower in BW than in LSW, but after the LE group was transferred, the EE group gradually caught up, and from day 328 to harvest on day 502 no differences in body length were detected, although the LE group had significantly higher final weight. Length growth was relatively stable, whereas weight growth followed cyclical patterns of reduced and accelerated growth. At harvest, the LE group showed a significantly higher incidence of skeletal deformities, suggesting that the timing of transfer to BW may influence bone health. Fish with elevated gonadosomatic index (GSI) at harvest showed contrasting growth trajectories between sexes. Males with slightly elevated GSI (Mean GSI ≈ 0.13%) were heavier than males with lower GSI (< 0.1%), whereas females with slightly elevated GSI (mean GSI ≈ 0.47%) exhibited reduced somatic growth during the latter part of the production cycle. Overall, the study suggests that Arctic charr juveniles of the Hólar strain can be transferred to BW earlier than is currently practiced, without compromising welfare.