There are various methods for using different types of grading devices. The technique should be compatible with the species of fish and grading situation, and should consider the behavior and water quality requirements of the fish.
For example, baitfish grade well through the sides of a grader box, while catfish grade more efficiently through the bottom.
Golden shiners are very excitable and more difficult to handle when water temperatures are above 60° to 65° F. Fish are especially delicate to handle for sorting purposes during July through September when pond temperatures are highest.
Striped bass and its hybrids should preferably be handled and graded at water temperatures less than 60° to 70° F.
The risk of mortality is greater if fish are graded in soft water with low chlorides at higher water temperatures. Fish should be allowed at least 2 hours to recover after seining or transport before they are graded.
Fish stomachs should be empty and any unwanted debris that hampers sorting should be removed either before or during grading. Fish are crowded during grading and oxygen can be depleted quickly in a localized area.
It is essential to maintain dissolved oxygen levels at least 5 ppm to 6 ppm in the grading or holding area at all times. If large fish are held overnight, the top of the vat should be covered to prevent loss of fish from jumping.
To minimize stress, all sorting should be conducted in a shaded area to prevent brightness from direct sunlight or warming effects from radiation.
Factors that influence the grading process are the degree of fish crowding, fish condition (normal, thin or plump), water temperature, level of fish activity and grading time. Fish require crowding for efficient grading, and most are more active and grade faster at higher temperatures.
During colder winter temperatures catfish, especially, require more time for pond grading to prevent many small, off-sized fish being harvested. Use of larger than normal mesh sizes also helps grading when temperatures are cold.
Some fish can be attracted through a grader by a flowing water current. This is most applicable when a pond is drained into an outside harvest basin. Select graders by visually estimating the sizes of fish.
Use a test sample of fish in the selected grader or graders, and examine fish for uniformity in size. An additional grader is needed if there is an excessive or unacceptable size difference among fish graded to one size. If no fish or very few fish remain in a grader then that grader should not be used. Occasional large fish can be sorted manually.
For grading in vats or tanks, floating grader boxes or baskets work well. Fish are dipped into the graders and are retained or pass into other graders or the tank. A simple method to grade fingerlings uses a series of two, three or four grading boxes that nest inside each other.
There should be enough space between boxes to accumulate 10 to 30 pounds of fish of a similar size. The inner box may have l-inch mesh with other boxes having l/2-inch, l/3- inch and l/4-inch meshes.
The nested boxes are put in the tank, and ungraded fish are dipped or poured into the inner box. Fish pass through the different meshes until they can no longer pass. The smallest sized fish may be collected outside the outer box.
When enough fish of a given size accumulate, the boxes are separated and fish can be processed by counting, weighing or passing to another tank or compartment.
Splashing inside the grader box speeds grading time. Minnows grade faster when thumping sounds are made on the water surface by snapping the fingers.
The loading rate of fish in a grader box should not exceed 5 pounds per cubic foot of water. When using a sorting table, a thin sheet of water should be kept on the surface for easy movement of fish.
A short block of wood or other material can be placed in front of the slot opening to avoid accidental entry of unwanted fish if the table is heavily loaded. Fish should be worked up quickly. Sorting during cooler months of the year greatly reduces handling stress.
Catfish withstand this type of handling and are less active during colder temperatures. Other more delicate species can suffer severe losses after processing on a sorting table. Whether or not the fish require restocking or will be killed should be considered before this method is used.
In ponds, large floating grader boxes or a net device can be used. Larger fish are easiest to remove when a grading or cutting seine of the appropriate mesh is pulled inside a harvesting seine of smaller mesh. Much labor and time are required to sort fish manually.
In other cases, a harvesting seine may have end panels of various meshes and the seine ends can be used like grading seines once the fish are crowded. One problem with any net grader is the possibility of gilling fish or trapping fish in the mesh net.
Bass and their hybrids snag easily in nets by their fins and operculum, or gill flap, if a seine with an improperly sized mesh is used to grade widely mixed sizes. Live cars or socks work well for inpond crowding and grading of catfish.
Provide adequate aeration during grading or holding, especially during summer. It is important to secure the net enclosure firmly into the pond bottom. Support it with harvesting stakes to prevent rolling up the sock and killing fish from currents created by an aerator or water well.
The live car can also be used to hold two lots of sorted fish by putting the center of the sock over the harvesting boat to form two separate compartments. This works well to select and sort males and females. Fish loading rates in live cars should not exceed 5,000 pounds per 10-foot length; lower fish loads are recommended at temperatures above 60° F .
Handling of fish in tanks is reduced by using vertical panel graders that are built to fit the holding tank. To use this type of grader simply move it from one end of the tank to another. This forces the fish to either pass through the bar space or be trapped. at the other end of the tank.
Disturb the water in the confined area to move as many fish as possible through the grader. Sorting more than one size is accomplished by repeating the process with panels of desired bar spacings. The vertical panel graders require less handling of fish and cause less stress compared to forcing fish through hardware cloth grading boxes.
Different lots of fish in tanks can be separated by transferring them to another tank or by using blocking screens to form several compartments in the same tank. Blocking screens can be placed in recessed grooves in the tank wall or wedged tightly between the side walls and bottom. Suspended nets inside wooden frames (hapas) can also be used to segregate fish.
Be patient when grading and check to be sure that your efforts produce the desired results. Remember that fish are crowded and excited and maintaining good water quality is essential.
Author:
Gary L. Jensen
