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

Effects of photoperiod manipulation on the development of seawater toleranc e were studied in Arctic charr. Three groups of fish, previously reared und er natural photoperiod and ambient water temperature conditions, were subje cted 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 simula ted natural photoperiod (nLD). Temperature of the fresh water was held cons tant at 4 degrees C until mid-May, after which it increased gradually to re ach 8.5 degrees C at the termination of the experiment on 2 July. All group s displayed improved seawater tolerance during the course of the study, ass essed as changes in plasma chloride and osmolality concentrations following 72-h exposure to seawater (33-34 parts per thousand). The tolerance to sea water was positively related to fork length within some sampling dates in a ll groups. Exposure to 24L:0D advanced the development of seawater toleranc e by approximately 6 weeks, compared to the nLD group. Both groups displaye d increases in gill Na+/K+-ATPase activity that coincided with the period o f improved seawater tolerance. Seawater tolerance of the 4L:20D group was d elayed by 6 weeks in comparison with that of the nLD group, but without any concomitant increase in gill Na+/K+-ATPase activity. The results corrobora te previous findings, and suggest that the seasonal changes in seawater tol erance of Arctic charr are controlled by an endogenous, circannual timing m echanism 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. (C ) 2000 Elsevier Science B.V. All rights reserved.