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We provide evidence that in male Atlantic salmon (Salmo salar) parr, maturation is suppressed when mesenteric fat fails to exceed an undefined level by May. In nonmaturing parr the postwinter increase in total lipids began in May, while the mesenteric store started filling in June. However, in male parr which would have matured, total lipids started increasing a month earlier, in April, and the mesenteric store started filling in May. Consequently, maturing male parr had significantly more mesenteric fat than nonmaturing fish by June. Mesenteric fat is needed for maturation. Levels continued to increase in nonmaturing parr during autumn months, but had declined in maturing parr by September. This depletion of mesenteric fat in maturing males coincided with increases in the gonadosomatic index from 0.05 to 10% and with reductions in both feeding and growth. Fasting during spring months delayed increases in total lipids and fat accumulation into the mesenteric store until June and suppressed maturation rates of male parr. The internal decision to suppress maturation is therefore dependent on mesenteric fat levels increasing in May. However, this requires the prior replenishment of other stores in April. A model is proposed to explain the physiological link between fat accumulation during spring and the initiation of maturation.

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The diel and seasonal activity patterns of salmonids are predominantly governed by the annual changes in photoperiod and temperature. In winter salmonids become increasingly nocturnal, hiding in refuges by day but emerging to feed at night. This behaviour may be linked to either one of the controlling influences mentioned above or to an inherent annual rhythm. Here we show that the previously described switch by Altantic salmon (Salmo salar L.) from predominantly diurnal to nocturnal activity in winter also occurs at other times of the year in response to low, "winter" temperatures; this is demonstrated both in laboratory experiments and by field observations in glacial rivers. This indicates that there is no underlying inherent annual rhythm to this behaviour nor any photoperiodic influence. Furthermore, in the laboratory experiment this temperature-dependent shift to nocturnalism was explained by a suppression of daytime activity rather than an increase in activity at night.