ACCLIMATION OF ATLANTIC SALMON (SALMO-SALAR) PARR TO SEAWATER IN AUTUMN - STIMULATORY EFFECT OF A LONG PHOTOPERIOD

Starting on 29 September, 1+ year-old Atlantic salmon parr (ca. 70 g) were maintained under either a long (LD18:6) or simulated natural photoperiod (LDN;45-degrees-N) for a 40-day period during which groups were either acclimated to seawater (SW; ca. 32 parts per thousand, ppt) by increasing salinity by 5-10 ppt every 5 days, or remained in fresh water (control). The increase in salinity and photoperiod independently resulted in a significant increase in gill Na+K+ATPase activity and high salinity (37 ppt) tolerance, with a combination of the two treatments being additive. Parr maintained mean plasma osmolality between 291 and 319 mosmol kg-1 during acclimation, indicating a maintenance of iono/osmotic balance. On 5 November SW acclimated parr from LD18:6 and LDN groups were individually tagged and then maintained for 11 months in a single tank supplied with SW under an LDN photoperiod. During winter (SW temperature = 1-2-degrees-C) overall cumulative mortality of parr acclimated under LD18:6 and LDN was 10% and 60% respectively, with the number of sexually mature male mortalities being disproportionately high. Death appeared due to breakdown in hypoosmoregulatory ability. Parr surviving the winter in SW completed smoltification in spring as judged by a decline in condition factor and change in appearance. The results support the hypothesis that an increase in photoperiod during autumn can improve the acclimation of Atlantic salmon parr to SW, and indicate the feasibility of transferring such fish to commercial marine facilities in autumn.