Factors affecting feed intake, aggression, growth and condition around transfer to sea in Atlantic salmon (Salmo salar)

Atlantic salmon is an anadromous species undergoing smoltification to adapt to the seawater environment. In aquaculture despite being fully smolted when transferred from freshwater hatcheries to sea cages, feed intake is suppressed, sometimes for long periods of time. Farms try to ensure salmon recommence feeding rapidly to reduce loss of condition which may lead to pinheading. The current research aimed to better define the term pinheading and examine factors effecting feed intake, aggression, growth performance, condition and nutritional status of fish around seawater transfer. Pinheads and non-pinheads from a commercial farm were compared. All pinheads had condition factors ‚Äöv¢¬ß0.865. Pinheads had significantly (p<0.05) lower body weight, fork length, condition factor, visceral fat ratios, dry matter, total lipid and gross energy and significantly higher ash than non-pinheads, all indicating decreased condition. There was no significant difference in crude protein, osmolality, or digestive ability (based on trypsin activity). Feeding frequency following seawater transfer was examined in fish fed eight meals per day before transfer. One meal per day following transfer resulted in a greater initial decrease in feed intake than four or eight and significantly increased feeding hierarchy strength (not significant for four or eight meals) before it decreased again to pre-transfer strength within three weeks of transfer. No differences between meal frequency treatments were observed in growth, condition or chemical composition. In a second experiment changing feeding frequency, whether increasing or decreasing, concurrently with seawater transfer had little effect on post-transfer feed intake. Following seawater transfer one meal per day fish had lower initial feed intake than eight meal fish regardless of pre-transfer frequency. Feeding and dominance hierarchies were examined immediately before and following seawater transfer. Feeding ranks were stable (Kendalls coefficient of concordance) in both freshwater and seawater but no correlation existed between mean freshwater and seawater hierarchies. Dominance ranks were also stable in freshwater but not seawater and mean dominance hierarchies in freshwater and seawater did not correlate. Findings suggest seawater hierarchies can not be predicted from freshwater hierarchies. A period of feed-deprivation for the first fourteen days following seawater transfer resulted in feed-deprived fish being significantly out-competed by non-deprived fish in terms of feed intake, growth and final condition. Although not significant, aggression also tended to be lower for feed-deprived fish. Further findings suggest that at higher densities the advantage to non-deprived fish may be diminished. Experiments show advantages of high feeding regimes immediately before and following seawater transfer and the difficulties in predicting performance of individuals in seawater based on their freshwater performance. They also suggests that fish that wait too long to recommence feeding may be at some disadvantage to those recommence sooner.