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Grading machines

155. Automatic grading machines vary in construction and fish can be netted into the grader, pumped in by vacuum pump, or moved in by means of a screw elevator. The grader separates the fish by size usually by allowing them to travel along a slowly widening "V" shaped conveyer. The fish drop through the widening gap into one of several outlets from where they are conveyed, usually by tube, to different tanks for further growth. Many graders will have an automatic counter built in to each of the outlets. The total time taken for a fish to travel from the pump through the grader and back to the tank can be as little as two to three minutes. Depending on the system, the fish are either immersed or sprayed with water throughout the operation. Passive grading is sometimes used whereby smaller fish are able to move through a grid of a specific size into a different tank or container. This method does not ensure that all the small fish swim through nor does it count them.