Commercial fish farming involves many steps between producers and end users of aquaculture products. One important step in this process is the sorting or grading of live fish.
Grading allows the producer to maximize profits when prices vary with fish size. Large fish mixed with nailer ones can affect the appearance of a shipment of fish.
Smaller fish may appear less attractive, and the shipment may be considered of poor quality. Fish should be graded for economic reasons either to increase the value of the crop, to increase fish yields or both.
In some markets and production systems, however, there are no advantages to grading fish. Many methods and types of equipment can be used successfully to grade fish. Sorting can be accomplished in ponds, holding vats, tanks and raceways.
As with any live fish handling practice, there are safe ways and proper methods that will influence the success in sorting and maintaining live fish in good condition. When grading is required, proper equipment and techniques reduce scale loss, stress and mortalities.
Grading involves holding. It is essential that fish be in good health and condition prior to grading, and that the water and environment in the holding facility are of good quality to reduce stress and associated problems.
Whenever possible, grading should be done in ponds to reduce the number of fish to be transported to holding facilities before returning them to ponds as unusable.
Advantages of grading:
Many fish are sold by size or grade. Grading increases the crop’s market value by supplying the sizes or grades of product desired by customers. For example, smaller minnows are worth more per pound than larger fish. Feeder fish of the same species may be worth twice the price of larger fish used for bait.
Catfish are sold by the inch and categorized as small to large fingerlings just like redfish, striped bass and their hybrids. State laws may prohibit marketing certain fish species, especially game fish, that are longer than a specified length.
When fish of a similar size and age are stocked in a pond, differential growth among individuals produces fish of various sizes at the end of the grow-out cycle. This is more pronounced in ponds where fish are graded only at the time of harvesting.
Some markets do not accept pond-run, ungraded fish of mixed sizes. Some species are cannibalistic and periodic size grading is important to obtain good survival and yields and to eliminate runts. Largemouth bass and striped bass and their hybrids are examples of cannibalistic fish.
These and other species may require frequent grading when maintained at high densities in tanks or troughs. Grading is also required during the various stages from fry to food-sized fish in outdoor pond culture.
Frequency of grading depends on the growth rate of fish and their uniformity in size. The first 2 months are usually the most critical to minimize losses from cannibalism.
Grading also allows more accurate sample counting to estimate fish numbers from a weight-number relationship. This is important to baitfish retailers who sell fish of a uniform desired size by the number.
Fish of mixed sizes make this counting method more difficult because of a wider size variation. The process of grading provides an opportunity to remove unwanted fish species, undesirable organisms, and nuisance aquatic plants that could become established in, or contaminate stocked waters.
Species separation is required in polyculture systems where more than one species of fish are grown together in a pond. Each species may require sorting because of different market outlets or processing requirements.
Often it is desirable to separate males from females, or to determine the numbers of each sex for breeding purposes. Sexes of fathead minnows can be effectively separated using a 15/64- to 16/64-inch bar spacing.
Other species like channel catfish and tilapia exhibit growth differences between sexes. This fact can be important, depending on how fish are used after grading. Sometimes a special variety of fish species with unique breeding or market characteristics is desired, or fish with deformities or poor breeding traits may need to be culled. Accurate length, weight or size information may also be required for research purposes.
Food fish growers may want to grade fish to avoid harvesting fish too small for marketing or processing. Processors may want to have fish graded because of differences in the value or demand of fish by size. Grading requires additional labor, time and equipment but in many markets it is essential and may improve profits.
Author:
Gary L. Jensen
