Photo credit: Ismael Radwan (Egypt)
Description: Abdel Rahman El Gamal
The two photos show a simple 2-unit fish grader made of a metal frame and netting materials which are locally available in Benin. As expected, the inner unit which is smaller and shallower should be made of larger-mesh net to enable larger size fry to be retained while allowing smaller size fish to pass through and got retained by the lower unit. The mesh sizes will determine the targeted fish sizes. For example, if the grading is a part of sex reversal of tilapia, the mesh size of the inner unit should retain tilapia fry larger than 11-mm total length and hence the passing through fry are should be sexually undifferentiated and would be expected to respond successfully to the sex-reversal hormonal treatment (This is conditioned to national regulations in regard to hormonal sex reversal).
If this type of graders are used for management purposes to produce batches of fish of homogeneous sizes whether for feeding, marketing or to address a predation phenomenon such as in African catfish, the mesh sizes for both unites could vary according the size of fish being graded.
This two-unit grader can grade three size groups when the bottom unit allows the smaller size to pass through into the holding facility in which the grading takes place.