Rainbow trout are fed by hand, mechanical feeder or demand feeder. In commercial production, mechanical feeders are used to deliver frequent small feedings to fry, as often as every 10–15 minutes. These feeders use vibration to deliver feed. Other mechanical feeders used in trout farming are belt feeders propelled by spring mechanisms (Figures 8 and 9). Feed is placed on a belt that slowly moves, dropping feed into tanks. A final type of mechanical feeder, which is used in marine farming of rainbow trout, uses tubes stretching from a feed bin to each pen. Air pressure blows set amounts of feed to each pen on a regular schedule.
Demand feeders are widely used in freshwater trout farming in the United States of America (Figures 10 and 11). Feed is placed in the feeder, which has a cone-shaped opening at the bottom. A rod extends into the water, and along the rod inside the cone is a plastic disc slightly smaller in diameter than the tapered cone. The disc acts as a plug to prevent feed from falling out the bottom of the feeder. When fish move around the rod in the water, the rod moves, shifting the position of the disc, allowing a small quantity of feed to fall into the water. The amount of feed delivered with each movement of the rod is adjusted by moving the disc up or down the rod, resulting in a larger or smaller clearance between the edges of the disc and the walls of the cone. Rainbow trout learn to move the rod to deliver feed very quickly, and spend virtually all day feeding. The use of demand feeders offers several benefits. First, fish feed themselves, resulting in low labour costs and little wasted feed, assuming the device is adjusted properly. When fish want to eat, they eat. If, for some reason, they are not hungry, they do not feed. Second, since fish feed all day, water quality remains high because there are no low oxygen periods after feeding, as is the case with periodic hand or blower feeding. Rapid activity by trout during feeding times lowers the oxygen content of the water by as much as two–three parts per million. Third, fish take turns feeding, so aggressive fish feed first, then move away, allowing less aggressive fish to have access to feed. Demand feeders thus result in efficient feeding with low labour costs, little wasted feed and low size variability of stock as compared with hand feeding small amounts frequently throughout the day.
