Effects of photoperiod manipulation on development of seawater tolerance in Arctic charr

Effects of photoperiod manipulation on the development of seawater tolerance were studied in Arctic charr. Three groups of fish, previously reared under natural photoperiod and ambient water temperature conditions, were subjected to a constant short daylength, 4L:20D, from 21 December to 30 January, followed by exposure to either 4L:20D, continuous light (24L:0D) or simulated natural photoperiod (nLD). Temperature of the fresh water was held constant at 4°C until mid-May, after which it increased gradually to reach 8.5°C at the termination of the experiment on 2 July. All groups displayed improved seawater tolerance during the course of the study, assessed as changes in plasma chloride and osmolality concentrations following 72-h exposure to seawater (33–34‰). The tolerance to seawater was positively related to fork length within some sampling dates in all groups. Exposure to 24L:0D advanced the development of seawater tolerance by approximately 6 weeks, compared to the nLD group. Both groups displayed increases in gill Na +/K +-ATPase activity that coincided with the period of improved seawater tolerance. Seawater tolerance of the 4L:20D group was delayed by 6 weeks in comparison with that of the nLD group, but without any concomitant increase in gill Na +/K +-ATPase activity. The results corroborate previous findings, and suggest that the seasonal changes in seawater tolerance of Arctic charr are controlled by an endogenous, circannual timing mechanism that is entrainable by artificially extended daylengths in spring. Our data further suggest that development of seawater tolerance in Arctic charr may occur independently of changes in gill Na +/K +-ATPase activity.