Brackish water and low specific water flow during smolt production affect growth and susceptibility to Moritella viscosa after seawater transfer.

Norwegian Atlantic salmon (Salmo salar L.) smolt production strategies are multifaceted. One strategy involves rearing in brackish water during the smoltification process, often in combination with relatively low specific water flow (SWF). Even though such practices are common, there is still a lack of scientific documentation of how this affects fish welfare and performance. In the present study, we examined the performance and susceptibility to winter ulcer in Atlantic salmon smolts (52 g) reared in flow-through systems, either in fresh water (FW, 0 ppt) or brackish water (BW, 13-14 ppt) at three different SWFs ranging from 0.27 to 0.12 L kg min for 6 weeks during smoltification (Phase 1). As SWF decreased, carbon dioxide (CO) levels increased and pH decreased in the rearing water. The fish were individually tagged and introduced to the different treatment regimens 120 degree-days after start of light induced smoltification. Subsequently, all treatment groups were transferred to seawater and subjected to an infection trial with Moritella viscosa or mock-infected (Phase 2). During the experimental period all groups developed typical smolt characteristics, including loss of parr marks and silvering, development of hypoosmoregulatory capacity and a decrease in condition factor. The different SWFs affected growth in both FW and BW groups. In Phase 1, growth was lower in the groups with the lowest SWF (p < 0.0001) and within these, the growth of fish in BW was significantly lower than of fish in FW. However, at moderate SWF levels, fish in BW grew better than fish in FW. No differences were found between the two groups with the highest SWF. In Phase 2, growth increased in all groups compared to Phase 1. Fish in the FW that had previously been subjected to the lowest SWF had significantly higher growth than those subjected to higher SWF; this was not the case in the BW groups. Blood chemistry analyses revealed increased partial pressure of CO (pCO) in the groups with lower SWF, irrespective of salinity (p < 0.0001). Both blood pCO and bicarbonate (HCO ) levels were positively correlated with increasing CO concentrations measured in the rearing water. Lower survival rates in the disease challenge test in the BW groups than in the FW groups indicate that premature exposure to higher salinities during smoltification may increase the susceptibility to M. viscosa. These results highlight that the use of a salinity around 14 ppt during smoltification does not alter growth when used in combination with a sufficient SWF, but make the fish less robust against a common bacterial pathogen.