SRAC Publication No. 391

SouthernRegionalAquacultureCenter

June, 1990

Sorting And Grading Warmwater FishGary L. Jensen*

Commercial fish farming involvesmany steps between producers andend users of aquaculture products.One important step in this process isthe sorting or grading of live fish.Grading allows the producer to maxi-mize profits when prices vary withfish size. Large fish mixed with

nailer ones can affect the ap- earance of a shipment of fish.

Smaller fish may appear less attrac-tive, and the shipment may be con-sidered of poor quality. Fish shouldbe graded for economic reasonseither to increase the value of thecrop, to increase fish yields or both.In some markets and production sys-tems, however, there are no ad-vantages to grading fish.

Many methods and types of equip-ment can be used successfully tograde fish. Sorting can be ac-complished in ponds, holding vats,tanks and raceways. As with anylive fish handling practice, there aresafe ways and proper methods thatwill influence the success in sortingand maintaining live fish in goodcondition. When grading is required,proper equipment and techniquesreduce scale loss, stress and mor-talities.

Grading involves holding. It is essen-tial that fish be in good health and

condition prior to grading, and that

● Louisiana Cooperative Extension Service,Louisiana State University AgriculturalCenter

the water and environment in theholding facility are of good quality toreduce stress and associatedproblems. Whenever possible, grad-ing should be done in ponds toreduce the number of fish to betransported to holding facilitiesbefore returning them to ponds asunusable.

Advantages of gradingMany fish are sold by size or grade.Grading increases the crop’s marketvalue by supplying the sizes orgrades of product desired by cus-tomers. For example, smaller min-nows are worth more per poundthan larger fish. Feeder fish of thesame species may be worth twice theprice of larger fish used for bait. Cat-fish are sold by the inch andcategorized as small to large finger-lings just like redfish, striped bassand their hybrids. State laws mayprohibit marketing certain fishspecies, especially game fish, thatare longer than a specified length.

When fish of a similar size and ageare stocked in a pond, differentialgrowth among individuals producesfish of various sizes at the end of thegrow-out cycle. This is morepronounced in ponds where fish aregraded only at the time of harvest-ing. Some markets do not acceptpond-run, ungraded fish of mixedsizes.

Some species are cannibalistic andperiodic size grading is important toobtain good survival and yields andto eliminate runts. Largemouth bassand striped bass and their hybridsare examples of cannibalistic fish.These and other species may requirefrequent grading when maintained athigh densities in tanks or troughs.Grading is also required during thevarious stages from fry to food-sizedfish in outdoor pond culture. Fre-quency of grading depends on thegrowth rate of fish and their unifor-mity in size. The first 2 months areusually the most critical to minimizelosses from cannibalism.

Grading also allows more accuratesample counting to estimate fishnumbers from a weight-numberrelationship. This is important tobaitfish retailers who sell fish of auniform desired size by the number.Fish of mixed sizes make this count-ing method more difficult because ofa wider size variation. The processof grading provides an opportunityto remove unwanted fish species, un-desirable organisms, and nuisanceaquatic plants that could become es-tablished in, or contaminate stockedwaters. Species separation is re-quired in polyculture systems wheremore than one species of fish aregrown together in a pond. Eachspecies may require sorting becauseof different market outlets orprocessing requirements.

Often it is desirable to separatemales from females, or to determinethe numbers of each sex for breedingpurposes. Sexes of fathead minnowscan be effectively separated using a15/64- to 16/64-inch bar spacing.Other species like channel catfishand tilapia exhibit growth differen-ces between sexes. This fact can beimportant, depending on how fishare used after grading. Sometimes aspecial variety of fish species withunique breeding or market charac-teristics is desired, or fish with de-formities or poor breeding traits mayneed to be culled. Accurate length,weight or size information may alsobe required for research purposes.

Food fish growers may want to gradefish to avoid harvesting fish too smallfor marketing or processing. Proces-sors may want to have fish graded be-cause of differences in the value ordemand of fish by size. Grading re-quires additional labor, time andequipment but in many markets it isessential and may improve profits.

Grading and sortingequipmentA variety of commercial graders isavailable. Many producers constructgrading devices specially designedfor use in their facilities. Based onthe market situation, know the sizeor sizes of fish that require sortingbefore purchasing or constructing agrader. Grader panels or grills canbe purchased and inserted intohomemade frames to save money.PVC pipe is a popular material usedin making graders. Plastic netting isalso available in various meshes andcan be used for grading small fishwhen the material is strong and wellsupported. Fish are injured less ifthe grading material is smooth andnon-abrasive, without any sharpedges or roughness.

Mechanical bar graders often have3/16-inch parallel aluminum orbronze rods that are separated bymeasured distances in a frame. Thespacing between the bars determinesthe size of fish retained or allowed topass through (Figure 1). Thesegraders may be vertical panelgraders like those often used in hold-

on the sides aids a tight fit.

ing vats and raceways, or graderpanels may be mounted on rollers orwheels for easy movement across thetank bottom. The manually pulledpanel graders should have handlesfor easier use (Figure 2).

To compensate for any slight dif-ference in the widths of holding vats,some type of flexible sheeting, likerubber stripping, can be placedaround the frame to provide a snugfit. To sort fish into a variety of sizes,multiple panel bar graders of variousspacings work well.

Grading boxes are used to sort largenumbers of minnows or fingerlingfish. These boxes vary in size fromsmall units for use in tanks to largerones designed to handle more fish inponds. The boxes usually float. Bothpond and tank grader boxes can beconstructed with frames that accom-modate standard sized grader panels

with various bar spacings. Some com-mercial boxes have simple adjustableor interchangeable bar graders.Polyethylene fish baskets have asquare mesh of 5/16 inches and cangrade out small fish.

A series of grading baskets withvarious bar spacings can be used tosort mixed sizes of fish. The gradingboxes can be enclosed on the sidesand bottom with galvanizedhardware cloth, but plastic coatedwire cloth is preferred because ofgreater durability and less injury tofish. The series can be built as nestboxes and should have long handlesthat permit easy use and support bythe walls of the holding vat (Figure3).

Sorting or grading tables work well when hardy species require accurateprocessing for research or breedingpurposes. Tables can be portable or

food-sized fish.They usually have an18-inch wide skirt inside the net withfloats to prevent fish from jumpingout of the enclosure. These devicesare equipped with a 3-foot by 5-foottunnel and metal frame attached tothe harvesting seine by a drawstring.

Fish pumps equipped with graderboxes can be used both to load ortransfer and grade fish simultaneous-ly. Fish pumps are used in the sal-monid industry where fish arecommonly grown in concreteraceways. They are also used insome other countries. Until now, fishpumps have not been widely usedwith warmwater fish. Speciallydesigned dewatering towers are re-quired to separate water from fish.Fish weights are also determined by

Figure 2. Large panel with wheels on top and bottom to move device in holding water displacement rather than by -

tank.

permanent. The tables are of variousdimensions but all have smooth sur-faces and slots in at least the cornersand sometimes sides. Slots should belarge enough to allow passage of fish

sizes anticipated. Fish can be col-lected in containers filled with fresh,oxygenated water. A chute from theslots properly directs fish to con-tainers. If fish are sorted by length,reference length measurements canbe inscribed into the table surface,or rulers can be located on the sides.

Every time fish are crowded with aharvesting seine, some rough grad-ing occurs. Net graders of varioustypes can be used to sort fish. Fish ofa minimum desired size can be har-vested by selecting a harvesting seineof the proper mesh size. Nylon netsshould be treated with a net coatingmaterial. Polyethylene nets requireno special treatment. Seines can beordered or made with end panels ofa desired mesh for grading. Thismeans fewer seines for storage anduse.

Live cars or socks can be used to fur-ther grade fish after they are seined.These grading or holding nets, usual-ly 10 to 20 feet wide, 10 to 60 feet

long and4to41/2 feet deep, arecommonly used by catfish producersto sort and hold both fingerlings and

weighing scales. They may move fishwell, but simultaneous mechanical

Figure 3. A variety of grading baskets and boxes used for sortingandgrading.

Table 1. Boxes, Baskets or Vertical Panels with Bar Graders

Approximate ApproximateBar Space Total Length Weight(64th inch) Held (inches) lbs/1,000 fish

13-15 1 3/4 3

15-17 2 1/4 4

More than 17 2 3/4 6

Approximate ApproximateBar Space Total Length Weight(64th inch) Held (inches) lbs/1,000 fish

12-14 1 3/4 3

14-16 2 1/2 5

16-18 3 8

18-21 3 1/2 13.5

21-23 4 20-25

23-25 4 1/2 30

25-27 5 40

More than 29 5 1/2 60

Approximate ApproximateBar Space Total Length Weight(64th inch) Held (inches) lbs/1,000 fish

27 3 8-10

32 4 18-20

40 5 32-35

48 6 60

56 7 93

62 8 112-140

Note: A bar grader space of 11/2 inches retains channelcatfish between 3/4 and 1 pound or 11 inches long.

Approximate ApproximateBar Space Total Length Weight(64th inch) Held (inches) lbs/1,000 fish

12 1.0 --

19 2.75 --

21-27 3 10

29 3.5 15

31 3.9 20

32 4 23

33 4.2 25

36 4.5 35

38 4.8 40

40 5 45

42 5.2 50

44 5.4 55

46 5.6 60

52 6 78

58 6.3 95

62 6.5 100

Approximate ApproximateBar Space Total Length Weight(64th inch) Held (inches) lbs/1,000 fish

13 1 0.63

15 2 4.8

19 3 16.3

32 5 75

72 9 500

96 11 1,000

104 13 1,333

Table2. Fish Nets, Live Cars or Grading Seines

Approximate ApproximateSquare Mesh Size Fish Length Square Mesh Size Fish Length

(inches) Held (inches) (inches) Held (inches)

1/4 3/4 1/4 1 - 2

1/3 1 1/2 3/8 3 - 4

1/2 2 1/2 4 - 5

1 4 1/2 3/4 7 - 8

11 8 -10

Fish Weight Held (pounds)

1 3/8 3/4

1 5/8 1

1 3/4 1 1/2

Note: Square mesh size equals one-half stretched mesh size.

grading of catfish has proved dif-ficult. When handled, catfish oftenlock their pectoral spiny fins in an ex-tended position. New innovations infish pump and grading technologymay remedy this problem in the fu-ture. Other species should gradewell.

Recommended bar andmesh spacingsBar spacings or mesh sizes shouldconform with the desired sizes offish to grade. Commercial graderbasket spacings range from 12/64 to74/64 inches. Net or seine squaremesh sizes normally range from 1/4to 3 inches. Whether or not a fish isretained by a grading devicedepends on its body shape anddimensions. Another factor is thenumber of days off feed before grad-ing. The condition factor of fish willalso determine lengths of fishretained; thin or fatter than normalfish will affect lengths graded.

The following tables are recom-mended dimensions or guidelines tograde various fish species to dif-

Figure 4. Floating grader box with inter-changeable bar panels used to sorthybrid striped bass.

ferent desired sizes. There is alwaysa range between the largest andsmallest fish in a length group; thefish sizes reported are approximatevalues. The length-weight relation-ship of catfish and other speciesvaries depending on the condition of

individual fish. Differences aregreater with smaller fish because al-inch length increment can result ina big difference in weight betweendifferent populations or lots of fish.Repeated trials are required todetermine bar spacings or meshsizes required to grade other speciesto desirable sizes.

Grading and sortingtechniquesThere are various methods for usingdifferent types of grading devices.The technique should be compatiblewith the species of fish and gradingsituation, and should consider the be-havior and water quality require-ments of the fish. For example,baitfish grade well through the sidesof a grader box, while catfish grademore efficiently through the bottom.Golden shiners are very excitableand more difficult to handle whenwater temperatures are above 60° to65° F. Fish are especially delicate tohandle for sorting purposes duringJuly through September when pondtemperatures are highest. Stripedbass and its hybrids should preferab-

ly be handled and graded at watertemperatures less than 60° to 70° F.The risk of mortality is greater if fishare graded in soft water with lowchlorides at higher water tempera-tures. Fish should be allowed at least2 hours to recover after seining ortransport before they are graded.

Fish stomachs should be empty andany unwanted debris that hamperssorting should be removed eitherbefore or during grading. Fish arecrowded during grading and oxygencan be depleted quickly in a local-ized area. It is essential to maintaindissolved oxygen levels at least 5ppm to 6 ppm in the grading or hold-ing area at all times. If large fish areheld overnight, the top of the vatshould be covered to prevent loss offish from jumping. To minimizestress, all sorting should be con-ducted in a shaded area to preventbrightness from direct sunlight orwarming effects from radiation.

Factors that influence the gradingprocess are the degree of fish crowd-ing, fish condition (normal, thin orplump), water temperature, level offish activity and grading time. Fishrequire crowding for efficient grad-ing, and most are more active andgrade faster at higher temperatures.During colder winter temperaturescatfish, especially, require more timefor pond grading to prevent manysmall, off-sized fish being harvested.Use of larger than normal mesh sizesalso helps grading when tempera-tures are cold. Some fish can be at-tracted through a grader by aflowing water current. This is mostapplicable when a pond is drainedinto an outside harvest basin.

Select graders by visually estimatingthe sizes of fish. Use a test sample offish in the selected grader orgraders, and examine fish for unifor-mity in size. An additional grader isneeded if there is an excessive or un-acceptable size difference amongfish graded to one size. If no fish orvery few fish remain in a grader thenthat grader should not be used. Oc-casional large fish can be sortedmanually.

Figure 5. A live car or sock commonly used in the catfish industry to grade fish. Notemetal loading frame that attaches the sock to the harvestingseine.

For grading in vats or tanks, floatinggrader boxes or baskets work well.Fish are dipped into the graders andare retained or pass into othergraders or the tank. A simplemethod to grade fingerlings uses aseries of two, three or four gradingboxes that nest inside each other.There should be enough space be-tween boxes to accumulate 10 to 30pounds of fish of a similar size. Theinner box may have l-inch mesh withother boxes having l/2-inch, l/3-inch and l/4-inch meshes. Thenested boxes are put in the tank, andungraded fish are dipped or pouredinto the inner box. Fish pass throughthe different meshes until they canno longer pass. The smallest sizedfish may be collected outside theouter box. When enough fish of agiven size accumulate, the boxes areseparated and fish can be processedby counting, weighing or passing toanother tank or compartment(Figure 4).

Splashing inside the grader boxspeeds grading time. Minnows gradefaster when thumping sounds aremade on the water surface by snap-

ping the fingers. The loading rate offish in a grader box should not ex-ceed 5 pounds per cubic foot ofwater.

When using a sorting table, a thinsheet of water should be kept on thesurface for easy movement of fish. Ashort block of wood or othermaterial can be placed in front ofthe slot opening to avoid accidentalentry of unwanted fish if the table isheavily loaded. Fish should beworked up quickly. Sorting duringcooler months of the year greatlyreduces handling stress. Catfishwithstand this type of handling andare less active during coldertemperatures. Other more delicatespecies can suffer severe losses afterprocessing on a sorting table.Whether or not the fish require re-stocking or will be killed should beconsidered before this method isused.

In ponds, large floating grader boxesor a net device can be used. Largerfish are easiest to remove when agrading or cutting seine of the ap-propriate mesh is pulled inside a har-

vesting seine of smaller mesh. Much labor and time are required to sort

fish manually. In other cases, a har-vesting seine may have end panels ofvarious meshes and the seine endscan be used like grading seines oncethe fish are crowded. One problemwith any net grader is the possibilityof gilling fish or trapping fish in themesh net. Bass and their hybridssnag easily in nets by their fins andoperculum, or gill flap, if a seinewith an improperly sized mesh isused to grade widely mixed sizes.

water well. The live car can also beused to hold two lots of sorted fishby putting the center of the sockover the harvesting boat to form twoseparate compartments. This workswell to select and sort males andfemales. Fish loading rates in livecars should not exceed 5,000 poundsper 10-foot length; lower fish loadsare recommended at temperaturesabove 60° F (Figure 5).

Handling of fish in tanks is reducedby using vertical panel graders thatare built to fit the holding tank. Touse this type of grader simply moveit from one end of the tank toanother. This forces the fish to eitherpass through the bar space or betrapped. at the other end of the tank.Disturb the water in the confinedarea to move as many fish as pos-sible through the grader. Sortingmore than one size is accomplishedby repeating the process with panels

of desired bar spacings. The verticalpanel graders require less handlingof fish and cause less stress com-pared to forcing fish throughhardware cloth grading boxes.

Different lots of fish in tanks can beseparated by transferring them toanother tank or by using blockingscreens to form several compart-ments in the same tank. Blockingscreens can be placed in recessedgrooves in the tank wall or wedgedtightly between the side walls andbottom. Suspended nets insidewooden frames (hapas) can also beused to segregate fish.

Be patient when grading and checkto be sure that your efforts producethe desired results. Remember thatfish are crowded and excited andmaintaining good water quality isessential.

Live cars or socks work well for in-pond crowding and grading of cat-fish. Provide adequate aerationduring grading or holding, especiallyduring summer. It is important tosecure the net enclosure firmly intothe pond bottom. Support it withharvesting stakes to prevent rollingup the sock and killing fish fromcurrents created by an aerator or

AcknowledgmentsSpecial thanks to Gerald Ludwig, Tracy Hellerman, Bruce Friedmann and Larry Dorman for providing useful information.

This publication was supported in part by a grant from the United States Department of Agriculture, Number 87-CRSR-2-3218, sponsored jointly by theCooperative State Research Service and the Extension Service.