A variety of commercial graders is available. Many producers construct grading devices specially designed for use in their facilities.
Based on the market situation, know the size or sizes of fish that require sorting before purchasing or constructing a grader. Grader panels or grills can be purchased and inserted into homemade frames to save money.
PVC pipe is a popular material used in making graders. Plastic netting is also available in various meshes and can be used for grading small fish when the material is strong and well supported.
Fish are injured less if the grading material is smooth and non-abrasive, without any sharp edges or roughness.
Mechanical bar graders often have 3/16-inch parallel aluminum or bronze rods that are separated by measured distances in a frame. The spacing between the bars determines the size of fish retained or allowed to pass through (Figure 1).
These graders may be vertical panel graders like those often used in holding vats and raceways, or grader panels may be mounted on rollers or wheels for easy movement across the tank bottom. The manually pulled panel graders should have handles for easier use (Figure 2).
To compensate for any slight difference in the widths of holding vats, some type of flexible sheeting, like rubber stripping, can be placed around the frame to provide a snug fit. To sort fish into a variety of sizes, multiple panel bar graders of various spacings work well.
Grading boxes are used to sort large numbers of minnows or fingerling fish. These boxes vary in size from small units for use in tanks to larger ones designed to handle more fish in ponds. The boxes usually float.
Both pond and tank grader boxes can be constructed with frames that accommodate standard sized grader panels with various bar spacings. Some commercial boxes have simple adjustable or interchangeable bar graders.
Polyethylene fish baskets have a square mesh of 5/16 inches and can grade out small fish. A series of grading baskets with various bar spacings can be used to sort mixed sizes of fish. The grading boxes can be enclosed on the sides and bottom with galvanized hardware cloth, but plastic coated wire cloth is preferred because of greater durability and less injury to fish.
The series can be built as nest boxes and should have long handles that permit easy use and support by the walls of the holding vat (Figure 3). Sorting or grading tables work well when hardy species require accurate processing for research or breeding purposes.
Tables can be portable or permanent. The tables are of various dimensions but all have smooth surfaces and slots in at least the corners and sometimes sides. Slots should be large enough to allow passage of fish sizes anticipated.
Fish can be collected in containers filled with fresh, oxygenated water. A chute from the slots properly directs fish to containers. If fish are sorted by length, reference length measurements can be inscribed into the table surface, or rulers can be located on the sides.
Every time fish are crowded with a harvesting seine, some rough grading occurs. Net graders of various types can be used to sort fish. Fish of a minimum desired size can be harvested by selecting a harvesting seine of the proper mesh size.
Nylon nets should be treated with a net coating material. Polyethylene nets require no special treatment. Seines can be ordered or made with end panels of a desired mesh for grading. This means fewer seines for storage and use.
Live cars or socks can be used to further grade fish after they are seined. These grading or holding nets, usually 10 to 20 feet wide, 10 to 60 feet long and4to41/2 feet deep, are commonly used by catfish producers to sort and hold both fingerlings and food-sized fish.
They usually have an 18-inch wide skirt inside the net with floats to prevent fish from jumping out of the enclosure. These devices are equipped with a 3-foot by 5-foot tunnel and metal frame attached to the harvesting seine by a drawstring.
Fish pumps equipped with grader boxes can be used both to load or transfer and grade fish simultaneously. Fish pumps are used in the salmonid industry where fish are commonly grown in concrete raceways.
They are also used in some other countries. Until now, fish pumps have not been widely used with warm water fish. Specially designed dewatering towers are required to separate water from fish.
Fish weights are also determined by water displacement rather than by weighing scales. They may move fish well, but simultaneous mechanical grading of catfish has proved difficult. When handled, catfish often lock their pectoral spiny fins in an extended position.
New innovations in fish pump and grading technology may remedy this problem in the future. Other species should grade well.
Recommended bar and mesh spacings:
Bar spacings or mesh sizes should conform with the desired sizes of fish to grade. Commercial grader basket spacings range from 12/64 to 74/64 inches. Net or seine square mesh sizes normally range from 1/4 to 3 inches.
Whether or not a fish is retained by a grading device depends on its body shape and dimensions. Another factor is the number of days off feed before grading. The condition factor of fish will also determine lengths of fish retained; thin or fatter than normal fish will affect lengths graded.
The following tables are recommended dimensions or guidelines to grade various fish species to different desired sizes. There is always a range between the largest and smallest fish in a length group; the fish sizes reported are approximate values.
The length-weight relationship of catfish and other species varies depending on the condition of individual fish. Differences are greater with smaller fish because a l-inch length increment can result in a big difference in weight between different populations or lots of fish.
Repeated trials are required to determine bar spacings or mesh sizes required to grade other species to desirable sizes.
Author:
Gary L. Jensen
