The effects of daily ration on growth and smoltification in 0+ and 1+ Atlantic salmon (Salmo salar) parr

The effects of long-term variations in feed ration were studied during two experiments on Atlantic salmon parr. In the first experiment, three duplicate groups of approximately 500 salmon parr were fed at rates of 100%, 66% or 33% of the manufacturer’s recommendation from shortly after first feeding. Each group were exposed to a photoperiod regime which was expected to result in smoltification 9 months after first feeding. In the second experiment, three duplicate groups of 550 fish were fed 100%, 66% or 33% of the manufacturer’s recommendation from first feeding and exposed to a simulated natural photoperiod, which was expected to result in smoltification 13 months after first feeding. In both experiments fish size increased with ration, with recruitment to the upper modal group (UMG) of the population also related to ration (85-96%, 64-88% and 28-42% UMG fish for the full, two-thirds and one-third ration groups respectively, recorded at the conclusion of each experiment). Throughout each experiment the full and two-thirds ration fish maintained similar whole body lipid concentrations, although lipid concentrations in the one-third ration fish were generally lower. At the conclusion of experiment 1, gill Na+, K+-ATPase activity in UMG fish fed full rations reached 9.5 μmol ADP hydrolysed. mg-1. protein-1. h-1, whereas ATPase activities were lower in the other ration groups. In experiment 2, all groups had similar gill Na+, K+,-ATPase activities at the conclusion of the experiment (6.4-9.3 μmol ADP hydrolysed. mg-1. protein-1. h-1). Following 24h seawater challenges, conducted during the parr-smolt transformation, UMG fish from the full and two-thirds groups of experiment 1 displayed high survival rates (100%) and low serum osmolalities (335 mOsm.kg-1), with lower survival rates (75%) and higher serum osmolalities (370 mOsm.kg-1) recorded in the one-third ration fish. In experiment 2 similar survival rates (100%) and serum osmolalities (350 mOsm.kg-1) were found in all ration groups. It is concluded that under accelerated production regimes, feed restriction may result in underyearling Atlantic salmon smolts developing a poor hypo-osmoregulatory ability. Variations in ration significantly influence growth, although it is believed that growth is dependant on the maintenance of a specific lipid level in the body