Not all of the interactions between the fish on a farm are positive.  Some fish are bullies, bigger fish tend to get the bulk of the feed and larger fish will often eat smaller tank mates!  One of the primary methods a fish farmer can use to avoid or limit these negative behaviours is to keep the fish in each tank, cage or pond as close to each other in size as is possible, and this is achieved by sorting the fish according to their size.
 
Size sorting is performed at various life stages and these vary between species.  Predatory fish, such as catfish or kob, are size sorted very frequently to limit the losses caused by cannibalism and aggressive dominance.  In the case of catfish we sort them weekly from week 2 to 8 inclusive.  Peaceful fish such as carp, which show no cannibalistic tendencies, are still sorted but less frequently, only twice between eggs and the 1kg market size.
 
Sorting is achieved by placing the fish inside a cage or box which has holes or parallel slots of the appropriate size to allow smaller fish through but retain the larger fish on the inside.  Once the fish are in the box, it is gently raised and lowered to encourage the smaller fish to swim through the gaps.  After the entire batch has been sorted in this manner each group is counted by mass, enabling the farmer to know how many fish there are in the group.  Small fish may be culled at the first sorting to prevent wasting feed and other resources on fish that take too long to attain market size.
 
Having too many fish in a culture tank is a major cause of slow growth.  Regular sorting and counting of the fish enables the farmer to avoid crowding and thus enhances rapid growth to market size.  In addition, each time the fish are sorted they are counted, providing the farmer with accurate stock numbers across the farm, warning her of losses and enabling him to plan harvesting and feed ordering.
 
Sorting for size is not a luxury it is an essential farm task.  Let’s keep those fish in same size groupings and reap all the associated benefits.