Reduced growth, condition factor and body energy levels in Atlantic salmon Salmo salar L. during their first spring in the sea

We have investigated the feed intake and growth in autumn-transferred Atlantic salmon (S0) during their first spring in the sea, a period of low performance in commercial production. We have compared the results with those obtained from spring-transferred smolt (S1), in order to determine whether this reduction in performance is accompanied by changes in nutrient retention, levels of muscle fat, energy status or condition factor (CF). The practical importance of the results obtained in the small-scale experiments was evaluated by studies performed at two commercial farms, both using S0 salmon. The feeding rate, rate of growth and degree of feed utilization were low during the first spring in sea, for both S0 and S1 smolt. In both commercial farms, the apparent feed intake in S0 was reduced by approximately 50% in the spring. This low-performing period coincided with reduced fat and energy retention, low levels of muscle fat and poor CF. Fat retention was reduced from 44.8% (March-May) to 15.4% (May-June) in S0, whereas protein retention did not change, indicating that the energy demand was high during the first spring in sea.