Gill Na+,K+-ATPase activity, plasma cortisol level, and non-specific immune response in Atlantic salmon (Salmo salar) during parr-smolt transformation

Atlantic salmon ( Salmo salar ) yearling presmolts were reared under two different photoperiod regimes (simulated natural photoperiod, SNP, and continuous light, CL) and identical ambient temperature conditions from 21 November 1990 until 29 May 1991. Sampling was done biweekly from 21 February. Gill Na⁺,K⁺-ATPase activity increased under both photoperiod regimes from early April, concurrently with the increase in ambient temperature. The initial increase and final levels (29 May) in enzyme activity were much lower in the CL regime fish. Seawater challenge tests (24 h, 35 ppt) on 2 and 15 May resulted in lower plasma chloride levels in the SNP regime fish, indicating better hypo-osmoregulatory ability. This difference wasANNOUNCEMENT not present on 29 May. Resting plasma cortisol levels increased from March under both regimes, but the increase was much greater in the SNP regime fish (75 nm on 29 May v 22 nm in the CL regime). The individuals from the SNP regime were of lower weight and condition coefficient on 29 May, whereas length was uniform. Plasma lysozyme activity was unchanged throughout the sampling period, with the CL regime fish showing an overall higher activity. The in vitro leucocyte stimulation test did not indicate a reduced immune response in May in smolting fish exposed to the simulated natural photoperiod regime compared to fish reared under continuous light.