The effect of different periods of constant short days on smoltification in juvenile Atlantic salmon (Salmo salar).

The present study investigated the differential effects of a range of constant photoperiods on the development of biomodality and hypoosmoregulatory ability and the changes in coloration and morphology which occur during the process of smoltification in the Atlantic salmon. The aim being to determine the photoperiod conditions required for the successful production of out-of-season (0 +) smelts. In 2 separate experiments, groups of parr reared from hatch under a long day (LL or LD 23:1) and in heated water (10 degrees C or 13 degrees C) were subjected to a short day photoperiod (LD 8:16 or LD 10:14) for periods of 4, 5, 6, 7, 8 or 12 weeks starting at varying times from July through to September after which the groups were then returned to a long day (LD 19.5:4.5 or LD 20:4). Control groups of fish were held under a natural photoperiod and also constant long days (LD 19.5:4.5). The timing of the changes from long to short and short to long daylengths in different photoperiod regimes induced differences in seasonal profile of the various parr-smelt changes in morphology and hypo-osmoregulatory ability. The development of bimodality was related to the timing of the decrease in photoperiod, whereas hypoosmoregulatory ability and smelt coloration appeared related to the increase. The regimes which included periods of 6 or more weeks of short days advanced smoltification compared to the group under ambient photoperiod. By contrast, smoltification was not completed in the groups exposed to 4 or 5 week periods of short days or constant long days. Under all the artificial photoperiod regimes there was some degree of desynchronisation of the various constituent processes of smoltification. Collectively, these data provide information on the specific components of photoperiod regimes which are necessary for the successful production of out-of-season smelts; they also suggest that smoltification is controlled by a series of interrelated endogenous rhythms or clocks. (C) 1998 Elsevier Science B.V. All rights reserved.