Effects of pellet contrast and light intensity on salmonid feeding behaviours

Abstract

Two of the outstanding questions related to fish feeding in enclosures are why all fish do not feed at once and why daily feeding rates vary. Our results will indicate that few fish feeding at once may be caused by a desire to feed under low light if pellets are detectable there, while variable daily feed discharge rates (kg min −1) under similar ration level are correlated with underwater light levels. We hypothesized that non-uniform feeding and variable feeding rates were linked and related to poor food detectability at large cage depths. We tested this hypothesis by feeding fish in cages on salmon farms different food pellet types (silver and regular brown) while simultaneously measuring feed discharge rate, lighting conditions and feeding activities near the surface and cage bottom. Images of pellet and fish obtained from underwater cameras that were positioned near the cage bottom were used as a basis for judging satiation and setting the feed discharge rate. In morning feedings, the discharge rate of fish given silver pellets was lower than the rate for conventional brown pellets ( P-value=0.002). In both treatments, feed discharge rates decreased with increasing levels of background light intensity (0.44< r 2<0.84). Contrary to our expectations, there was no significant difference in time spent feeding in different spatial locations between treatments ( P-value>0.6). Generally fish in both treatments did not feed throughout the cage at the same time, and fish never appeared to be eating all at the same time. A general feeding pattern emerged; at some time during the feeding event, feeding fish would start to follow food from the surface down into the cage (instead of letting it pass by them). To feed these fish and prevent pellet wastage, discharge rate would be lowered by researchers when the fish reached the cage bottom. Fish given silver pellets and fed under bright conditions would reach the cage bottom sooner then fish given regular pellets and fed under darker conditions (hence the discharge rates under the former conditions were lower than the rates under the latter conditions). The results imply a preference by salmonids for eating at a low light level if pellets are easily available and detectable there. This tendency for fish to eat under restricted conditions (that may not be available for all fish in a cage at any given time) could cause non-uniform feeding patterns.