Descaling did not affect osmoregulation, growth or mortality in Atlantic salmon smolts after transfer to brackish or sea water

Scale loss is often a result of various mechanical impact such as handling and is assumed to increase the risk of impaired osmoregulation and disease. For this reason, scale loss is a commonly used welfare indicator for farmed fish. Knowledge about the consequences scale loss has for fish welfare under different conditions is however limited, especially the long-term effects. We studied the effect of removal of scales on 0 (control) 5, 10 or 20 % of the scale covered body area on individually tagged Atlantic salmon (Salmo salar, Linnaeus) smolts. Descaling was done at the transfer from freshwater to either brackish (25 ppt) or full-strength seawater (34 ppt) at an optimal temperature (10 °C) that was kept stable during the 6-weeks study period. Fish were sampled for blood and examined for wounds after 1, 7 and 42 days. No fish had wounds at any of the sampling occasions, and no fish in any treatment died during the experimental period. Although all plasma parameters measured (concentration of plasma cortisol, glucose, lactate, Cl⁻, Na⁺, K⁺, and Ca²⁺) were affected by sampling day and/or salinity, scale loss did not have a significant effect on any of the plasma parameters. Neither did scale loss affect growth. Thus, no negative effects of scale loss on salmon welfare could be documented in the present study. Scale loss may still have negative effects in more challenging conditions, which should be investigated in more detail.