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Correct Feed Particle Size

BY Leslie Ter Morshuizen

Commercial fish farming depends on providing the fish with sufficient feed of the correctly nutritional quality to promote maximum, healthy growth. Although live feeds are important and serve numerous specific needs, it is seldom workable to feed fish exclusively on live feeds. We therefore generally rely on artificial feeds for the feeding of our fish to enjoy the benefits of bulk availability, storage and operational simplicity offered by this option. As artificial feeds are supplied in different pellet sizes, starting from a powder via crumble to a large pellet, we naturally need to know how to select the feed size that is best suited the cohort of fish being fed.

 

Obviously the gape of the fish’s mouth is the first factor to consider; if the pellet cannot fit into the mouth it will not be ingested immediately. As the pellet soaks water and softens, the fish will nibble at it, causing the pellet to fall apart and enabling the fish to eat pieces of the pellet. This causes large amounts of feed to be wasted as bits fall to the tank floor and certain elements dissolve into the water. However, it is equally important that the pellet should not be too small as the fish will usually not feed to satiation on pellets that are too small, especially in predatory fish where the size of the pellet is important to elicit an attack response.

 

If the pellet can fit into the gape of the fish, but only just, the fish tend not to digest the feed completely. This is evidenced by undigested pellets seen whilst inspecting the faeces. If the feed is not being digested it is clearly not contributing to growth!

 

Small fish require more protein in their diet than do larger fish of the same species, so the feed manufacturers include more protein in the diet of the smaller fish, thus in the smaller feed particle sizes. Feeding large fish on small pellets will therefore also result in unnecessary protein in the diet, which translates directly to financial waste as protein is expensive.

 

All this imprecision begs the question: `how do you know what the correct pellet size is for a group of fish?’

 

I do not believe that there is a simple one-size-fits-all type of solution due to the divergent dietary strategies employed by different species of fish. Predators tend to subdue and swallow large, infrequent meals whereas detritivores and herbivores are more inclined to nibble continuously. It is therefore necessary to consider the behaviour of the species you are farming in determining the most appropriate pellet size. However, the principle is that the feed particle must comfortably fit into the mouth of the fish. Practically, the pellet should be between ¼ to ½ of the area of the gape, with predators tending to receive larger pellets than other species. Ensure that the fish can comfortably eat the feed being fed, but err on the side of larger pellets rather than smaller pellets as this shortens feeding time and the fish tend to reach satiation more easily.

 March 27, 2015
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