Effect of photoperiod, ration, sea water and sexual maturation on food consumption and growth of Atlantic salmon, Salmo salar L
The effects of photoperiod, sea water transfer, ration level and sexual maturation on food consumption and growth performance were examined in individual Atlantic salmon, Salmo salar L., held in groups and reared under culture at various stages of the life cycle. X-radiography was employed to measure food consumption rates of individual fish. Artificial lighting was used to manipulate day length in freshwater parr and results showed that growth in autumn may be enhanced or reduced when fish are reared for 9 weeks under extended or shortened day lengths of 16 and 8 hours of light and that the effects were reversible. Fish reared under extended day lengths had significantly greater weights, lengths, specific growth rates and food consumption rates than those fish on shorter day lengths (P<0.05). Food conversion ratios were similar for fish irrespective of the number of hours of day light (P>0.05). Consumption-growth relationships were examined in individuals fed at ration levels of 0.5, 1.0 and 3.0% body weight day-1, pre and post sea water transfer. Growth performance in freshwater was not a significant determinant of subsequent sea water growth (P>0.05). Specific growth rates increased with increasing ration level in both freshwater and sea water (P>0.05). Salinity appeared to effect the consumption-growth relationship more at higher rations than lower rations. A second experiment was performed to determine the influence of temperature on the latter observation, however the results were equivocal. Levels of steroid hormones, 11-ketotestosterone and estradiol-17β were measured by radioimmunoassay to detect evidence of sexual maturation in males and females. This study identified 2 phases associated with sexual maturation, an early phase (October 1992-April 1993) and a late phase (May-October 1993). The early phase was characterised by slowly rising steroid hormone levels concomitant with high rates of feeding and growth. In the late phase, steroid hormone levels increased more rapidly and growth rates decreased in association with inappetence.