Pellet wastage, and subsurface and surface feeding behaviours associated with different feeding systems in sea cage farming of salmonids

Observations of subsurface and surface feeding activities were recorded on salmon farms that utilised a variety of feeding techniques. Those activities and pellet wastage were recorded as indicators of uniform feeding and efficiency, respectively. Fish apparently could not readily detect pellets under conditions of low water visibility (<3 m) or low ambient light levels. At higher visibility: when fish were being continuously fed, they formed a ring-like structure, characterised by organised foraging. Fish initially formed a spiral-like structure when fed in batches. Thereafter and while feed was still available, they foraged in a disorganised style. The foraging pattern by individual fish was an 'S'-shaped searching manoeuvre, and the amplitude of a swerve was greater in batch feeding than in continuous feeding. Feed wastage varied with feed delivery rate and feeding endpoint method. Wastage and underfeeding were associated with ration feeding. With the exception of one farming site, uniform feeding was uncommon. One of the hypotheses, formulated from the data, is uniform feeding may be achieved if feeding rate is optimal and pellets are detectable throughout the water column. (C) 1998 Elsevier Science B.V. All rights reserved.