The number of fish feed pellets that an individual fish can catch is related to its swimming activity, which is, in turn, partially temperature dependent.
Rainbow trout could catch larger numbers of pellets in a single feed portion as temperatures increased from 5 to 15°C under laboratory conditions; this change is likely mediated by changes in the fish’s activity at different temperatures. The feeding rate of brown trout increased rapidly between 3.8 and 6.8°C, remaining relatively stable between 6.8 and 19.3°C, before decreasing at temperatures above 19.3°C. Changes in the feeding and swimming activity of salmonids at different temperatures have been presented by others with the greatest change in activity occurring between 5 and 10°C. In aquaculture, the feed delivery rate should therefore be taken into account when calculating a feeding regime.

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The size of feed pellets and the rate at which they are delivered may affect the amount of feed an individual fish can ingest over a period of time. Pellets of sub-optimal size or pellets that are delivered at a high rate may cause wastage, as fish may be unable to catch large numbers of pellets before they sink through the net pen. Salmon farmers use many different sizes of feed pellets during the grow out period, and each time the size is changed, new calculations for the optimal number of pellets per fish and delivery must be done. It would therefore save both time and money if fewer sizes of pelleted feed can be used for larger parts of the grow out period.

At some extreme, pellet size will obviously have an effect on fish performance, and there are indications of this effect presented here. For example, a pellet that is larger than the gape width of the fish or is so large that handling time becomes a limiting factor in the fish’s ability to ingest enough pellets to maintain good growth will clearly have adverse effects. Adult salmon showed a more immediate response to larger pellets but that these were more likely to be rejected than pellets of a shorter length.

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Pellets slightly smaller than the ‘‘normal’’ commercial size were eaten at the fastest rate, thereby indicating that the salmon are perhaps adjusting their feeding behavior to compensate for a smaller feed pellet size. For Arctic charr reared under hatchery conditions, the optimal pellet size has been suggested to be around 2% of the fish’s length. Others have suggested that for a range of fish species, the optimal feed size appears to be 25–50% of the mouth width. It appears that fish can adjust to reasonable deviations from the optimal pellet size with no significantly negative effects on growth.

Extension Reading:
How to Choose Appropriate Feed Pellets for Your Fish
How to identify the best fish feed pellets