12.  GRADING AND SORTING FISH

12.0   Introduction

1. Grading fish into groups of similar individual sizes is a common management practice in fish farming.

2. You may need to grade:

3. Grading has several advantages:

4. Fish sorting usually involves separating a mixed group of fish into different species, males and females, immature and mature fish, diseased and clean fish, etc. This is often done at the same time as grading and usually needs more skilled people.

5. The sorting of fish into groups of the same species occurs only when different species of fish are farmed together. At harvest, either for stocking as juveniles or for marketing as food fish, the fish are first sorted by species before being graded, if necessary, by size.

Note: fish may also be sorted according to sex, for example when stocking fattening ponds for monosex culture of tilapia males or when stocking breeding ponds with a specific ratio of males to females.

6. It is always best to grade and to sort live fish some time after harvest to give them a chance to overcome their stressful experience. First hold the live fish in a good storage place for a few days before handling them again. If the fish are not to be sold alive, they can be sorted and graded either during or immediately after harvest.

7. If smaller batches are involved, you may be able to sort and grade immediately, preferably during cool conditions. Fish should be handled as little as possible and taken rapidly to their recovery areas.

Grading or sorting your fish

8. Sorting fish by species is best done on a sorting table (see Section 12.1). For resistant fish such as warm water fish in general, this method can also be used to grade fish by size. But there are also several other simple ways to grade fish by size which you will learn later (see Sections 12.2 to 12.7). Select the most appropriate grading method according to the accompanying chart.

Simple methods for grading fish by size

12.1   How to sort and grade on a sorting table

Building a simple sorting table

1. You can easily build a simple, portable sorting table yourself or with the help of a local carpenter. You will require the following supplies:

2 pieces 140 cm long (A)
2 pieces   87 cm long (B)
3 pieces   60 cm long (C)
2 pieces   30 cm long (D)
2 pieces   20 cm long (E)

4 pieces 105 cm long (F)
2 pieces 130 cm long (G)
4 pieces 110 cm long (H)

2. When you have these materials proceed as follows.

(a) Build the table top:

173a.GIF (15316 byte)
Note: the side boards enclose two angle openings at the upper end of the plywood sheet and an 85 cm-wide opening at the lower end.

(b) Build the folding legs to support the table top:

Trim ends and cut holes in leg pieces

(c) Place removable boards to regulate sorting:

Place removable sorting boards

Note: if necessary, adjust the measurements of the table openings and their blocking boards to accommodate your particular kind of fish.

3. Open the folding legs and place the table top on them, within the underside braces. Your table is now ready to be used for sorting or grading fish into four categories (sizes and/or species).

4. If you have only three categories, remove the triangle and its two lateral boards to open the table end.

Note: this illustration shows all removable boards in the closed position; they can be removed one by one to provide openings for sorting and grading fish from any or all corners or from the full-length lower end opening; place a container with clean, fresh water below each opening to collect the fish.

Using the sorting table

5. When using the sorting table, you should try to keep the fish in good condition. Proceed as follows.

(a) Set up the sorting table close to the harvesting site, it possible in the shade.

(b) Before starting, wet its surface well with clean water.

(e) Under each opening of the table, place a container with clean, fresh water, such as buckets, plastic basins or half metal drums which can be used to transport the fish (see Section 14.1).

(d) Use good handling nets with no rough or torn edges.

(e) Bring only small batches of fish to the table at a time. Put them in suitable temporary holding containers, with a regular change of clean water. This also helps to rinse them clean of mud, algae and plants.

(f) Transfer the fish gently on to the top of the sorting table.

(g) Have three or four qualified persons working around the table. The fish should be handled as little as possible and sorted quickly.

(h) Start sorting:

(i) After having sorted or graded this batch of fish, rinse the table well, with plenty of clean water.

(j) Place another batch of clean fish on the table and continue sorting or grading.

(k) Move the sorted fish as soon as possible to their recovery or destination areas.

6. At the end of the working day, clean the table well, dry it and store it in a protected place.

Note: the surface of the sorting table should always be perfectly smooth to avoid bruising the skin of fish. Regularly check your table surface and, it necessary, smooth it well with sandpaper and repaint it.

12.2   How to grade fish with a calibrated container

1. This method is particularly useful for smaller fish, which are more sensitive and more difficult to handle than larger fish. The basic idea is to place a group of fish in a container held in water, from which the smaller ones can drop through or swim away through holes of a specific size made in the bottom and in the walls of the container. If possible, the container should be held in flowing water.

2. You may use one of the following:

Grading Nile tilapias with woven floating baskets1

Simple containers for grading fish

3. For grading a stock of fish of widely mixed sizes, it may be better to use a series of large meshed boxes or net cages with different mesh sizes which fit into one another. Put the fish stock in the inside cage and let the smaller ones swim through the largest mesh towards the outside cages, as far as they can go according to their size. Select the series of mesh size well, on the basis of the general shape of the fish to be graded (see Section 12.3).

4. If the fish will not all swim through by themselves, raise and gently shake the grading container to move the fish through the meshes or holes.

Three net grading cages suspended in a concrete basin ready for use

Note: the largest fish are kept in the inner net cage while progressively smaller fish are kept in the two outer cages; the smallest fish escape into the concrete basin.

Diagram of three net cages for grading fish

Note: you can also build three light wooden boxes covered with mesh or screen for this kind of fish grader

12.3   How to grade fish with nets

1. Another simple way to grade live fish is to use a series of nets with specific mesh sizes. Such a method is particularly useful to grade large quantities of fry and fingerlings before stocking.

You may use one of the following:


2. In both cases select the mesh size of the netting material carefully, according to the general shape of the fish to be graded (see the accompanying chart). If necessary, make your own tests to determine which are the most appropriate mesh sizes for your particular case.

Note: always start by grading out the largest fish. If fish get caught in grading meshes, remove them quickly to a safe storage area.

Average size of smallest fish retained by various net meshes1

12.4   How to grade fish in harvesting structures

1. There are simple ways for grading mixed-size fish stocks, either with a harvesting box or in a harvesting basin (see Section 11.3).

(a) Design your harvesting structure so that it can be used for grading fish into two or three size groups, as shown below.

(b) Make a series of grading panels to fit into this harvesting structure.You may use wooden or metal frames covered with meshed material such as netting, plastic or metal mesh or set with vertical steel, wooden or plastic rods, for example 4 to 6 mm in diameter.

(c) Select the spacing or mesh size of each panel according to your particular needs (see Section 12.3).

2. A mobile harvesting box should have two sets of grooves along its length to accommodate grading panels. Add another set of grooves at the end of the box using either a fixed end screening or a final grading panel. For better handling, use a solid section for the lower half of this end panel to help keep your fish in water at all times.

3. A permanent harvesting basin should have two (or more) sets of grooves along its length to accommodate grading panels.

4. Move the fish from the pond in small batches to give them the opportunity to swim (generally upstream, see next section) through the grading panels as far along as possible without excessive crowding. Remove each size group of fish with dip nets as soon as possible. Note the necessary data for proper monitoring (see Section 16.0). Dispatch each group to its next destination, for example storage pond, fattening pond or delivery to market. Grade another batch of fish.

Grading fish in a harvesting box

12.5   How to grade in troughs, basins or tanks

1. Several fish species such as common carp, tilapias and trout react to a strong water current by swimming against it. You may take advantage of this tendency to grade your fish. This method is particularly useful for fry and fingerlings. To apply this method, you require:

(a) The water flow should be sufficient to create a good current within the grading structure. You may use water from a nearby feeding canal or pond, fed by pump, pipe or siphon. The flow should be adjustable (see Section 11.3).

(b) A well-designed grading structure can be:  

(c) A series of grading panels (see Section 12.4) should fit into two or more sets of grooves set along the sides of the grading structure.

2. For grading proceed as follows.

(a) Set the required grading panels in the grading structure, with the largest openings towards the downstream end.

(b) Start the water flowing through the structure.

(c) Put the batch of fish to be graded in the downstream end of the structure.

(d) Regulate the flow, so that the fish move towards the inflow. The smallest ones should be able to swim against the current and reach the upstream end of the structure.

(e) Remove each size group of fish separately:

(f) Grade the next batch.

(g) If necessary repeat this grading process with another series of grading panels.

Note: your fish should be in good health to be strong enough to swim against the current. Usually, it is best to grade them a few days after harvest from the pond.

12.6   Floating grading cages

1. This method is particularly useful for grading fish from partial harvests, to separate those which have reached market size from the smaller ones, which are returned to the pond.

2. A simple floating grading cage can be built with the following materials (as shown):

3. Do not build a floating cage too large. Size it according to the depth of the ponds where it will be used and the quantities of fish to be graded. You will need at least one cubic metre of cage volume per 80 kg of fish. Allow at least 30 cm between the base of the net and the pond bottom.  

Example

You plan to grade at the most 500 kg of fish for market in a cage floating in a pond 1.50 m deep. Build your cage with the following dimensions:

4. To use the grading cage, simply put the harvested batches of fish within the cage. Wait for a few hours, giving the fish time to rest. Check closely on their condition, in particular if they have to be marketed live. Then drive the small fish from the cage by splashing the water and progressively lifting up the cage bag to crop the market fish out. Be careful to keep the cage meshes open to allow the small fish to escape.

Building a simple floating grading cage

Diagram of completed floating grading cage with floats and netting in place

Note: the top of the frame should float about 30 cm above the water level; allow at least 30 cm between the base of the net and the bottom of the pond.

Construction details

12.7   Box and mechanical graders

1. You can buy more sophisticated graders of various designs from specialist suppliers. You can also make simpler versions yourself. These graders normally consist of a wood, metal or fibreglass box, with one or more components with grading bars or slots set in its base. Small grading boxes are used directly inside the pond or harvesting basin. Larger boxes are usually transported around the farm and are set up on the pond bank.

2. There are three main types:

Making a simple grader

3. You can make a simple fixed-width grader as follows.

(a) Make a metal, wood or fibreglass box with typical dimensions:

(b) Finish the inside of the box as smoothly as possible.

(c) Fit simple handles and, at the base of the box, add the inner ledge on which the grader screen is placed.

(d) Make several grading screens, using netting, or preferably metal, plastic or fibreglass rod, set under a wood, metal or plastic frame, sized so that it will fit neatly inside the box on the inner ledge.

(f) If necessary use simple wooden pegs or- rubber bands as simple clips to hold the grader screen in place.

Using the grader

4. The grader is used by placing it in the water, in the area where the smaller fish are to go, and netting batches of fish into it. The box is lifted up and down, to let the smaller fish drop through, leaving the larger sizes, which are then tipped out into a receiving cage or transport box, for example.

5. Fully mechanical graders are usually transported to the side of the pond and are frequently trailer-mounted. They are usually variable-width graders, with a sloping upper trough in which the fish are placed (or pumped), which is separated from two or more lower receiving boxes by the grader screen. These receiving boxes are usually connected by pipe, through which the fish return by gravity to their final locations. A continuous flow of water and fish enters the upper end of the trough where the screen width is narrow. Smallest size fish drop through in the first receiving box. The fish move progressively down the trough until the widening screen allows them to drop through into their appropriate size group.