SEASONAL-CHANGES IN FOOD-CONSUMPTION AND GROWTH OF ARCTIC CHARR EXPOSED TO EITHER SIMULATED NATURAL OR A 12 12-LD PHOTOPERIOD AT CONSTANT WATER TEMPERATURE/

Immature male and female and maturing male Arctic charr Salvelinus alp inus were held at constant temperature (4 degrees C) under either simu lated natural photoperiod (nLD) or constant photoperiod (12: 12 LD) co nditions, and feed intake and growth were monitored at monthly interva ls over 13 months. Food was supplied in excess during a 5-h period eac h day, and daily consumption was determined using X-radiography. Food consumption and growth of both immature and maturing fish showed disti nct seasonal cycles irrespective of photoperiod treatment. Feed intake and growth rates were highest in mid-summer and lowest in autumn. The fact that seasonal cycles persisted under constant photoperiod (12: 1 2 LD) and temperature conditions suggests that circannual rhythms of f ood consumption and growth exist in the absence of seasonal changes in these environmental cues. The data support the view that seasonal cha nges in food consumption and growth in the Arctic charr are driven by endogenous rhythms. Reductions in feeding and growth in autumn occurre d about 1 month earlier in the maturing males than in the immature fis h. Males that matured under the 12: 12 LD regime displayed a growth cy cle that was delayed in comparison with that shown by the maturing mal es held under nLD. (C) 1996 The Fisheries Society of the British Isles