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David Edwards

Definition of “coldwater” fish

Ichthyologists divide freshwater fishes into three groups according to their temperature requirements:

Tropical fishes can only live in warm water. For example, the tilapias generally die if water temperature falls below about 12°C.

Warmwater fishes often tolerate a wide temperature range, but usually have a minimum temperature requirement for reproduction (often around 20°C). Growth frequently stops or is poor below 10°–15°C. The most important groups of warmwater fishes is the carp family, especially the common carp, Chinese carps and Indian major carps.

Coldwater fishes will not tolerate temperatures above the mid-twenties Celsius for long periods. By far the most important group of cultured coldwater fish is the salmonid family.

The salmonid species

The salmonid species most important to man are the salmon, trout and char.

True salmon are native to the world's two biggest oceans and the rivers draining into them. The Atlantic Ocean has only one native species, the Atlantic salmon (Salmo salar), while in the Pacific Ocean there are several species: pink salmon (Oncorhynchus gorbuscha), chum (O. keta), sockeye (O. nerka), coho (O. kisutch), chinook (O. tschawytscha) and amago (O. rhodurus).

The most important trout species are brown trout (Salmo trutta) and rainbow trout (Salmo gairdneri). The Atlantic Sea trout and the Caspian “salmon” are races of Salmo trutta.

The chars, Salvelinus fontinalis (known in USA as “brook trout”) and Salvelinus alpinus (Arctic char), are important locally in some areas.

In Iran the most important salmonid species for culture are the trouts Salmo gairdneri and S. trutta, but the same farming methods are generally applicable to all the salmonid species.

Geographic distribution

Man has transplanted salmonid fishes to areas well outside their natural range. The rainbow trout is probably the world's most widely transplanted fish. Originally a native of California, it is now found in all the continents except the Antarctic. Even in the tropics, rainbows thrive in the coldwaters of high country lakes and streams. Where natural waters are too hot or too cold for spontaneous reproduction, populations can often be maintained by artificial culture and stocking.

The current boom in commercial salmon-farming has led to transplantations of the valuable Atlantic salmon to areas as far apart as the western coast of North America and Tasmania, Australia.

Natural life cycles

Salmonid fishes can be divided into two groups on the basis of their natural life cycles. Some species spend their whole lives in freshwater. Others reproduce and live during their juvenile stages in freshwater, but then migrate to the sea, where most of their growth occurs. The latter are called anadromous species.

All the true salmons, sea trout and Caspian salmon, and some races of rainbows and Arctic char, are anadromous. However, even races of rainbow trout and Arctic char which are not naturally anadromous can be acclimatized to life in seawater when they reach a size of around 50 g or more.


All salmonids spawn naturally in freshwater Except for non-migratory chars, which lay their eggs in lakes, spawning typically occurs in the head-water and tributary streams of rivers, though it can occur anywhere in a river if the substrate is suitable. In spawning streams the water is shallow, cold and clear, and the stream bed on which the eggs are deposited is of clean stones and gravel free of fine silt which might overlay and smother the eggs. Usually the female fish will excavate a depression, called a “redd”, in the gravel with her tail, and deposit her eggs into this. One or more male fish then discharge sperm over the eggs to effect fertilization. The fertilized eggs are covered with gravel to a depth of several centimetres by the female fish. The parents then leave the eggs, and there is no further parental care.

Since most salmonid species spawn in these head-waters the adults must undergo a “spawning migration” upstream before spawning, regardless of whether their previous home was a river, lake or the sea. Spawning most often occurs in autumn-winter, though sometimes in spring, but some species may begin to move up-river months before this. The spawning migration is frequently triggered by environmental conditions associated with an increase in water flow in the stream.

Pacific salmon normally die after spawning. Mortality is sometimes significant also in Atlantic salmon, especially in males, but most spent fish (or “kelts”) survive to return to the sea. Survival of trout and char after spawning is usually good, and many individuals return to spawn again in subsequent years.

Development of eggs and young fish

Since winter water temperatures in most spawning streams are low, development of eggs spawned in autumn takes a long time. In Scandinavia, for example, eggs spawned in October-November will not be come “eyed” (i.e., the eyes of the embryo can be seen as two black dots) until about the following February, and eggs hatch in April or May (see Fig. 1).

The time eggs take to develop at a given temperature varies for different species, and will be considered in more detail later as it relates to hatchery practice. The newly hatched fry, or “alevins”, live for the first few weeks of life on their yolk sacs, the remains of the food supplied from the egg. When most of the yolk sac has been consumed, the fry become active and leave the protection of the redds to begin searching for food. They must also rise to the surface of the water to take a gulp of air with which they fill their swim bladder, giving them a neutral buoyancy which makes it easier to swim and hold their position in the water column. This period is therefore referred to as “swim-up”, and fry often reach this stage of development in northern Europe around May-June.

During their first few months the young fish feed and grow, at the same time tending to move downstream from the spawning areas to richer feeding grounds. Some species develop vertical stripes on the sides of their bodies and are referred to as “parr”.

From this stage onwards no basic change occurs in the life-style of those salmonid species which spend their whole lives in freshwater. They simply feed and grow until they are ready to mature and spawn. The time taken to reach sexual maturity differs between species, races, climatic zones and other environmental factors such as food abundance.

Migration to and from saltwater

Amongst the species which migrate to sea, some, e.g., Atlantic salmon and sea trout, undergo a clear physiological preadaptation to life in seawater while still in freshwater, by smolting. The osmotic forces encountered by fish in the sea are the opposite of those in freshwater. In the latter, water is drawn into the fish's body by osmosis, and must be eliminated as dilute urine. In seawater, osmosis draws water out of the body. To avoid dehydration, the fish must replace lost water. It does this by drinking seawater, then excreting the salt through special cells located mostly in the gills. These cells develop during smoltification. In addition to the internal changes in the salt-regulating mechanisms of the body, the appearance and behaviour of the fish also change. Most noticeably, a subcutaneous layer of guanine is laid down, concealing the parr markings and giving the sides of the fish a silvery colour. The tail of salmon often develops a black edge, and the fish change from swimming against the current to moving with it. Migratory char do not go through the same clear change of body conformation and colour as Atlantic salmon and sea trout, and therefore cannot be said to truly smolt. Some Pacific salmon species migrate to sea at a very early stage of development, virtually as fry. Downstream migration of young fish to the sea normally occurs in spring, and is thought to be triggered by high water flows and rising temperatures.

After anything from one to five years' growth in the sea, the fish are ready to return to the rivers to spawn. All migratory salmonids show a remarkable “homing instinct”, by which a very large proportion of them are able to find the river in which they were themselves spawned. The fish are thought to be attracted by the chemical “smell” of their river, including “pheromones” (chemical substances released by other fish in the river and present in very low concentration in the water).

Artificial culture

When salmonids are cultured artificially in hatcheries or fish farms the stages in production must broadly follow the natural life cycles However, by manipulating the fishes' environment some modifications can be made to suit the convenience of the culturist. Also, the proportion of the fish's life cycle which is artificially controlled varies according to the purpose for which the fish are being cultured.

Fish for angling and commercial capture

Fish reared for re-stocking enclosed waters or for release to enhance wild runs (so-called “ranching”) are of course grown in captivity for only part of their lives, then released to grow in the wild.

At the simplest level, human interference in the fishes' life can be confined to helping natural populations to overcome some obstacle to their reproduction. For example, where feeding grounds are good, but there is no suitable substrate for spawning, artificial “spawning channels” can be built. These consist of a length of natural stream or a man-made canal, on the bottom of which graded, clean gravel is spread. Screens are erected at the end of the channel to exclude predators and hold salmonid broodfish, but they allow movements of water and fry. Ripe brood-fish can be caught in adjacent or distant waters, and transported to the channel. There they spawn in the normal way. The resulting fry eventually swim up from the gravel and spread to colonize the feeding areas to which the spawning channel is connected.

In areas where good spawning grounds are present, but separated from the feeding grounds by a natural or man-made barrier (e.g., waterfall or a dam) an alternative strategy can be used. Ripe broodfish captured downstream of the obstruction are stripped artificially (see below) and the eggs fertilized. The fertilized eggs are put into special incubation boxes, usually made of plastic. These “Vibert” boxes have slit-shaped perforations in their sides, tops and bottoms, which allow the passage of water and fry, but not of eggs (which are wider). Full boxes are planted out in the gravel of suitable spawning areas and left. The resulting fry spread downstream after swim-up.

In most enhancement or ranching programmes, however, juvenile fish are reared to the fry, fingerling or smolt stage of development in hatcheries and nursing facilities before release. The techniques used are the same as those employed for young fish destined to spend their whole lives in captivity before slaughter for human food, and are described below.

In “ocean ranching” the released juvenile fish go to natural feeding areas, where they rapidly gain weight. When they approach sexual maturity they will “home” to the lease sites, where they can be captured with nets or traps. This technique clearly has the great advantage that the “fish farmer” does not have to feed the fish during the phase of rapid growth. However, there are also significant disadvantages to the system. Briefly they are:

  1. Survival of released fish is often low, the majority falling victim to predators and natural accidents.

  2. When they return to the release/capture sites, the fish are approaching sexual maturation. Their condition is therefore not first-class (see below).

  3. Most of the returning fish will “home” at the same time, resulting in a seasonal supply which is not the ideal basis for marketing purposes.

  4. Ownership of released fish is often impossible to establish, and a large proportion of the returning run may be taken by “free-for-all” fishing in the approaches to the release/capture site.

Fish grown in fish farms for human food

Fish to be grown for human food are normally reared through their entire life cycle in captivity.

Frequently the production process is divided into two parts: production of fry or fingerlings followed by on-growing to market size. In many countries the salmonid farming industry is clearly split into these two parts - some farms specializing in growing fry, fingerlings or smolts for sale, while others buy these fish and grow them up for sale at table size. Often the seed-fish units are few and large, whilst the on-growing farms are smaller and more numerous.

Market size varies between fish species and between countries. For example, rainbow trout are most commonly grown to “portion-size” (i.e., 200–350 g), but in Norway the same species is reared to 2–5 kg before sale. Atlantic salmon are always grown to as large a size as possible, frequently 3–6 kg, but Pacific salmon can be sold at any size from portion (or “pansize”) upwards.

Environmental requirements of salmonids

Water temperature: As mentioned above, salmonids do not tolerate high temperatures. The lower lethal limit for trout and salmon is about -0.5°C, at which temperature ice crystals form in the blood. Optimum temperatures for growth are in the range 14°–18°C, but survival of eggs is best below 12°C.

Oxygen: The main reason why salmonids need low temperature is their high oxygen demand.

Solubility of oxygen in water decreases with increasing temperature (Table 1). For fish farm purposes, the water leaving the farm should always have an oxygen concentration of 6 mg/l (= parts per million, ppm) or more. The water entering the farm should be 100% saturated with oxygen. If it is not, it can be made so by passing it though an aeration device of the type described below for removal of excess nitrogen. The oxygen demand of salmonids is the major factor determining the water requirement of trout and salmon farms, and this will be discussed further below.

Table 1


Temperature (°C)Oxygen concentration at saturation
16  9.76
18  9.35
20  9.00
22  8.67
24  8.36

The values shown are for sea level. The figures must be reduced by approximately 0.5 ppm for every 300 m increase in altitude.

pH: Neutral or slightly alkaline water is preferable for freshwater salmonid farms, but any pH between 6 and 8 is acceptable. Water pH above 9 and below 5.5 can kill fish, especially eggs and early fry. Acid water resulting from the effects of acidic rain and snow is a big problem in areas where soils are poorly buffered and which are subjected to heavy industrial air pollution. Seawater has a much more constant pH, averaging 8.1.

Pollution and heavy metals: Copper, zinc and other metal ions are toxic to fish at very low concentrations, especially where the water is soft and of low conductivity. Iron, which is present in many freshwaters, can be precipitated onto the surface of eggs and cause suffocation. A high silt content in the water can have the same effect, and it may be necessary to filer hatchery water supplies through sand filters. Silt does not usually harm larger fish, but in some areas it causes problems by rapidly filling ponds and raceways.

Many agricultural and industrial pollutants are highly damaging to salmonids. Where more than one pollutant occurs in the same water their effects can be synergistic, so that fish can be killed even when no single pollutant is present in a lethal concentration.

Water requirement

The water requirement of salmonid fishes depends largely on their demand for oxygen. In general, rainbow trout have a higher oxygen demand than salmon at the same temperature, because the trout are more active. Small fish need more water per kilogramme of their bodyweight than larger ones. Approximate water flow requirements for small rainbow trout are shown in Table 2. Atlantic salmon need approximately 30% less. The figures indicate requirements for feeding and growing actively.

Table 2


Size of fish(g)

For larger fish, a general “rule-of-thumb” is to allow 0.7–1.0 1/min/kg.

From these figures, the approximate water requirement of any proposed farm can be calculated. However, as a very rough guide, a 100 t/year rainbow trout unit producing portion-sized fish needs about 1 m3 /sec of water.

In surface waters, the flow rate tends to be lowest at the warmest time of year. Also the amount of oxygen fish need (and hence their water requirement) increases as water temperature rises, whilst the oxygen content of water is also decreasing. The volume of water available at the warmest, driest time of year must therefore be taken as the measure of how much the source can supply.

When planning a new unit, a site should be selected where considerably more water is available than the minimum thought immediately necessary. This allows for later expansion.

Other site requirements

Good road access, an electricity supply and a telephone connection are normally needed at freshwater fish farm sites. The manager or owner should also live on the site, otherwise thefts of fish or equipment may be expected.

Water sources

It is better to have water supplied to a fish farm by gravity flow rather than by pumps. This is partly for economic reasons -electric pumps use power which costs money; and partly for secu rity reasons - a gravity supply is less likely to break down than a pumped one. If electric pumps are relied on, there must always be a back-up system of diesel pumps or generators in case of electricity failure.

Gravity supply is often possible from surface waters, i.e., lakes, rivers or streams, and these intakes to the fish-farm pipeline or canal must be screened to prevent the entry of debris or wild fish. The screens must be placed in such a way that they cannot become blocked by ice or debris.

The disadvantages of using surface water in fish-farms are the wide fluctuations in temperature and water flow, and their liability to flooding with its consequent increase in water turbidity. Springs frequently have a more constant water flow but the temperature, though stable, is usually lower than that of surface water in summer so that fish will grow less well.

Sub-surface water usually has to be pumped. It sometimes contains high levels of metals and other undesirable ions and has little oxygen, but it is free of silt. Temperature, usually fairly constant over the year, is higher than that of surface water in winter but lower in summer. When it is necessary to pump surface waters up to the fish-farm, the intake should be placed under a thick layer of stones and gravel. This acts as a filter to prevent debris entering the pump.

Care must be taken to ensure that intakes and pumps cannot suck in air, otherwise supersaturation problems can arise. Similarly whenever water is heated in the hatchery, or when warm water supplies are drawn from a power station or factory, super-saturation with air may be encountered. For practical purposes, supersaturation with oxygen is no problem to a fish-farm. Salmonids can tolerate oxygen levels up to 340% of saturation, and such levels are never reached in practice. Nitrogen super-saturation, on the other hand, is very dangerous, especially to young fish. Levels of nitrogen saturation in water above about 105% can kill salmonid fry and fingerlings. The cause of death is the formation of bubbles of nitrogen gas in the blood. These enter the vital organs, and can often be seen in the gills, skin, eyes and fins. The syndrome is known as “gas bubble disease”.

Nitrogen supersaturation can be removed by any method which brings the incoming water into intimate contact with the air. The most common way of achieving this is by allowing the water to fall in small drops through a series of stacked perforated aluminium plates (see Fig. 2).

Alternative, more compact systems comprise vertical cylinders filled with plastic components giving a very large surface area over which the water falls, or boxes in which air from a compressor is blown through a thin film of flowing water.

Regardless of its initial content of oxygen and nitrogen, water leaves the de-gassing apparatus 100% saturated with both. Thus the same system can be used both to remove excess nitrogen and to increase the oxygen content of poorly oxygenated water.

For salmon smolt units, seawater is also required. This is pumped from deep in the sea, where suspended matter and the planktonic larval stages of sessile marine fouling organisms are absent. Water temperature at depth is also more stable than at the surface.

It can be seen that all water sources have both advantages and disadvantages. Where possible, therefore, fish-farms are often built with access to several different waters. This gives security in the event of a breakdown in one supply, and allows some control of temperature, and sometimes also salinity, by mixing different waters before they enter the fish tanks.

Fish-farm buildings

Where climatic conditions permit, it is cheaper to site the main production tanks outside. However, most hatcheries have their egg incubators and early fry tanks indoors, where they are more easily worked and the light can be controlled. Buildings are also necessary for storage of feeds and equipment, preparation of food, office and canteen space for staff, and living accommodation for the manager.

To accommodate incubators and fish tanks, prefabricated agricultural buildings with no windows are satisfactory. Floors should be of concrete, sloping to drainage channels for easy cleaning. Eggs must be incubated in darkness, but fry can be fed up to 24 hours daily if electric lights are left on. Dimmer switches are an advantage, allowing light levels to be kept low for the sake of the fish but increased when necessary for the workers to do their job.

To avoid the risk of metal poisoning, all pipework and fittings should be made of plastic, not copper, brass or galvanized steel.


Nowadays almost all the fish used as broodstock by commercial fish-farms come from captive stocks. Wild fish are rarely used. This is for two main reasons:

  1. Captive stocks can be improved by application of a planned programme of selective breeding. The traits of most interest in salmoind culture are growth rate, age at sexual maturation, disease resistance and carcass quality. Genetic improvement is a long-term undertaking, but already Norwegian work in this field is producing improvements in salmonid growth rates of around 15% per generation.

  2. The health status of captive populations can be monitored and controlled. This is impossible with wild fish, and consequently the use of eggs stripped from wild populations carries a higher risk of introducing disease to the farm. The authorities of most countries require certification of freedom from major diseases before they will allow import of salmonid eggs from other countries. This certification cannot be given to eggs from wild sources.

Effective programmes for gentic improvement and health control are more easily accomplished when broodstock and egg production are concentrated in a few large units, rather than spread in many small ones. In particular, the investment needed to equip a fish-farm to do meaningful genetic work is very high. Consequently, there is a trend in some countries towards concentrating salmonid breeding into a small number of specialized units capable of supplying the fish seed requirements of the whole nation. Indeed, there are often good financial reasons for this, in addition to the genetic and health advantages outlined above. When the costs of maintaining broodstock are added to the income which could be generated if broodstock ponds were instead devoted to market fish production, it is often found that small farms could buy eggs more cheaply than producing their own.

Appearance and age of sexual maturity

As salmonid fishes approach sexual maturity, their physiology and appearance change. Externally, the skin darkens, often becomes blackish in rainbow trout and brownish in Atlantic salmon. Males usually develop a hooked lower jaw, and the mouth is longer than in females. Internally, the quality of the meat changes in ways which make it less desirable as human food. The flesh becomes more watery. It loses much of its pink or red colour, the pigments being used instead in the production of eggs. The growth rate of meat also slows down or stops, since all the fish's energy is directed to the development of its sexual products. Sometimes the mortality rate also increases in maturing fish, especially in trout held in seawater. For all these reasons it is essential that salmonids intended for human consumption should be harvested before the onset of sexual maturation.

The time taken to reach maturity is partly determined by genetic factors and partly by the environment. In Iran, rainbow trout frequently mature at three years of age for females and two for males, whilst Caspian salmon take an average of one year longer.

Maturing broodfish must be checked about once a week during the spawning season to ascertain when they have ovulated and are ready for stripping. The fertility rate of under-or over-ripe eggs is poor. Males give usable milt (sperm) for a longer period than females give eggs. Ripe females can be recognized by their distended and soft bellies, protruding and reddened genital papillae, and by the way the belly sags towards the head when fish are held up by the tail. Sexual products can be expelled from ripe fish of both sexes by gently pressing the abdomen.

It is possible to use the same broodfish for two or three successive years, but this practice is declining in some countries in favour of slaughtering broodfish for virological examination after stripping.

Stripping and fertilization

Stripping of sexual products is much simplified if broodfish are anaesthetized first; where its use is permitted, MS 222 is often used. This substance dissolves directly in water. Fish placed in an anaesthetic bath must be carefully watched to ensure that respiration does not stop.

Fertilization is now most commonly done by the dry method. It is very important to ensure that neither eggs nor sperm come into contact with water until they are well mixed. This is because eggs quickly absorb water and swell, closing the micropyle (a small hole through which sperm can enter) and rendering the egg incapable of being fertilized. Also sperm become active as soon as they come into contact with water, but their activity is very short-lived. Within less than a minute their motility stops and thereafter they cannot fertilize eggs. For these reasons only dry containers (usually plastic) are used to hold newly-stripped sexual products and many people dry the fish with a towel to remove surface water before beginning stripping.

Eggs from female fish are expelled into plastic bowls by firmly stroking the belly backwards towards the vent. Milt from males can be expelled in the same way either directly onto the eggs or into separate plastic or glass containers (see Fig. 3).

Eggs and sperm are then well mixed together with the fingers. A little water is added, and fertilization takes places almost immediately (see Fig. 4). After standing for a couple of minutes, the eggs are washed with several changes of clean water. They can then either be put directly into incubators or into buckets of water and left for 1 frac12;–2 hours to “harden” before transfer to incubators. During hardening, the fertilized eggs absorb water and increase their volume by about 20%.

The yield of eggs to be expected varies with fish age, size, species and environmental factors such as the feeding regime used for broodstock. High fat diets are best avoided during broodstock preparation, because the presence of a large amount of fat in the body cavity makes stripping more difficult. However, on average rainbow trout females can be expected to give around 1 500–2 000 eggs/kg of body weight.

The sex ratio used for fertilization varies according to the farmer's habit. The sperm produced by a male is actually sufficient to fertilize the eggs of many females. However, in practice sex rations of anything between one male to one female and one male to ten females are used, with an average of about 1:4. Milt from at least two males should be used on each batch of eggs in case a sterile male is encountered. Where milt is stripped into a container before use, it is possible to check its motility by examining a drop in water under a microscope.

Incubation of eggs

Several different types of incubators for salmonid eggs are available. The most common is the so-called “California” system. This consists of a longitudinal trough containing trays which hold the eggs. Troughs and trays can be home-made from metal or wood, but commercially-produced models are usually made of fibreglass (see Fig. 5). The bottom of each tray is of perforated aluminium sheet on which the eggs are spread, and this lies a few centimetres above the bottom of the trough. The downstream vertical side of each tray extends down to the bottom of the trough, whilst near its top is a wide slit covered with perforated aluminium through which water passes out of the tray. Water flowing in at the up-stream end of the trough passes under the first tray, up through the aluminium sheet at its bottom, past the eggs, and then out through the aluminium screen in the downstream side of the tray. The same pattern is repeated at the next tray, and so on down the trough, so that the water passes through every box in the trough and then out through a standpipe or over a weir at the downstream end of the trough (see Fig. 6).

“Battery” systems are also available in which the egg trays are arranged vertically one on top of the other in a cabinet like in a chest-of-drawers.

Companies producing millions of eggs for sale sometimes use large incubating cylinders or “silos” (see Fig. 7). These can be home-made of plastic pipe or metal, and commercially-produced ones are available in polythene. Incubation cylinders hold many litres of eggs (often 25–40 1), but need only small amounts of water and occupy little space. However, they are only suitable for incubating eggs to the eyed stage of development, when they are sold. If eggs were allowed to hatch in them, the fry would die of oxygen deficiency.

Incubation cylinders holding 25 1 of eggs need only about 8 1/min of water. California-type systems containing, for example, seven 40 × 40 cm trays can hold 70 000–80 000 eggs with a water flow of around 12 1/min. Salmonid eggs are damaged in bright light. Incubators should therefore either be kept in a dark room or be individually covered to exclude light.

Dead eggs usually turn white due to coagulation of the yolk. They are best removed, otherwise fungus will grow on them and spread to infect live eggs. Dead eggs can be removed from California systems with a siphon tube (see Fig. 8) or by a rubber suction bulb attached to a glass tube. For units producing many millions of eggs, automatic machines are available which separate dead eggs from live ones and at the same time count the eggs. A photo-cell detects the difference. After eyeing, eggs are usually “shocked” by pouring them into a bucket before returning them to incubators. Dead eggs which had not previously turned white then do so and can be removed.

To keep down the growth of fungus, most units use the chemical malachite green. This is essential in incubating cylinders because it is impossible to remove dead eggs from them. Normally, the malachite green is made up into a stock solution of about 0.25% concentration. One hundred ml of this is added to the upstream end of each incubator trough every two or three days.

Where an automatic counting machine is not available, eggs can be counted by hand using special perspex plates, in which a known number (often 250) of countersunk holes are drilled. Each time the plate is dipped into a bowl of eggs, this precise number of eggs is picked up. Alternatively, the diameter of eggs can be measured by lining eggs up in a special V-scale. The number of eggs filling the 25 cm scale is counted, and the number of eggs per litre can be determined by consulting the following table:

Eggs/25 cm3638404244464850
Eggs/13 6004 2004 8005 6006 4007 3008 3009 400

Subsequently eggs can be measured by volume.

Development of eggs

The development time of eggs and early fry depends on species and water temperature. It is usual to express development time in day-degrees (i.e., the number of days after fertilization multiplied by the water temperature during the period). Approximate development times for several salmonid species at about 8°C are shown in Table 3.

Table 3

SpeciesAverage diameter of eggs
Development time in day-degrees
To eyeingTo hatchFrom hatch to first feeding
Atlantic salmon6.2245510290
Rainbow trout5.1175370150
Sea trout5.2240500280
Migratory char4.3220445225
Pink salmon7.6280640290

Rear fry and fingerlings

Newly-hatched fry are left in their hatching troughs for a few weeks while they live on the nutrients stored in their yolksacs. During this time they must be kept in the dark and protected from disturbance. However, the water flow to the troughs must be increased because the oxygen demand of fry is higher than that of eggs. In the California system described above an increase in flow from about 12 to about 20 1/min is sufficient.

When yolk sacs are about two-thirds used up and the young fish start swimming about, they are ready to begin feeding. At this stage they require light.

Fry can be first-fed in their hatching troughs, or be transferred to special fry tanks for first-feeding. In many places small concrete raceways are used for this purpose, but circular or square tanks with a circulating flow of water are considered better (see Fig. 9). Typically, tanks are between 1 and 4 m2 in surface area and hold about 25 cm depth of water. However, in some modern hatcheries deeper tanks holding about 1 m of water are used instead.

Regardless of size, all circular (or square with rounded corners) tanks work on the same principle. The water enters tangentially, setting-up a circulation around the tank. In the centre of the base of each tank is a rectangular aluminium or steel screen through which water leaves. Water level is regulated by an external stand-pipe (see Fig. 10). The stand-pipe can be removed or turned down to one side to empty the tank. Water generally enters the tank at the top and leaves at the bottom. This arrangement, combined with the circulation of water, ensures that no “dead areas” of water are left in the corners or at the bottom. Tank bottoms slope slightly towards the central drain, and the water circulation washes waste food, faeces and dead fish onto the outlet screen. Thus these tanks are said to be partially self-cleaning. The water circulation also serves to spread food around the tank, and fish orient themselves against the current as they would in a natural stream.

In concrete raceways, on the other hand, water frequently both enters and leaves at the top of the water column, and dead areas in the corners accumulate wastes. Raceways are therefore less easy to keep clean than circulation-type tanks, and conditions of hygiene in them are therefore generally poorer.

First-feeding fry can be kept fairly densely crowded. Frequently they are initially stocked in small tanks at about 10 000 individuals/m2 of surface area. The water current in the tanks should not be greater than the fish can comfortably swim against. If they crowd over the central outlet screen the current is too fast.

Dry, pelleted food is now used in almost all commercial hatcheries. Experiments have shown that growth of fry is better if the fish are fed their daily ration divided into many small meals, rather than a few large ones. Also feeding can continue 24 h/day if light is available. To assist in feeding small fish optimally, therefore, automatic feeders are often used. (Details on foods and feeders are given in the section of this report devoted to the subject.)

As fish grow, they must be periodically graded so that all the fish held in any one tank are of approximately the same size. This is necessary for three reasons:

  1. It prevents the establishment of a strong feeding hierarchy, in which the larger fish take all the food at the expense of the smaller individuals.

  2. It prevents cannibalism.

  3. It allows the manager to choose a food pellet size suitable for all the fish in the tank.

The optimal stocking densities for various sizes of fish are not precisely known. However, approximate densities used in commercial practice in Norway are shown as a guideline in Table 4.

Table 4


Average weight of fish (g)Stocking density (kg of fish/m3 of water)
Under  24–6

As fish grow, they are frequently transferred to larger ponds or tanks. These are normally sited out-of-doors. Earth ponds are popular in some countries (see Fig. 11), e.g., Denmark, but in most places modern farms use either circular tanks (or square with rounded corners) (see Fig. 12) or long raceways. Raceways are almost always made of concrete, though the bottoms are sometimes of earth or stones. Tanks are commercially produced in fibreglass but can be made on-site of concrete or of steel on a concrete base. Small fish should not be put into earth ponds before they reach a size of about 5 cm, because small fry can become infected by the spores of the protozoan parasite Myxosoma cerebralis, the causative agent of whirling disease. The spores are not present in concrete, fibreglass or other hard substrates, and fish are immune from infection once the bones of their cranium are fully developed.

Routine measurements

Some farmers routinely monitor oxygen concentration, temperature of water and pH. All farms should measure temperature each day, otherwise they cannot properly calculate the required food ration. Dead fish could be removed quickly, and a record kept of their number. A separate scoop or net should be provided for each tank to avoid transferring pathogens from one tank to another. For the same reason, each tank should have its own cleaning brush. Some larger farms employ a fish pathologist. Those which cannot afford this should call in a competent specialist if mortalities increase. In addition, the amount of food given to each tank each day should be recorded. At intervals samples of fish must be weighed to estimate growth, so that the food ration can be adjusted accordingly. The number and weight of fish put into each tank, and transferred between tanks, must be recorded. These basic records help the farm manager to plan his production.

On-growing fish to market size

On-growing of fingerlings or smolts to market size can either be done in land-based facilities or in floating cages. Most land-based units use freshwater, whilst cages can be sited either in lakes or in the sea.

Land-based units

These grow their fish in earth ponds, concrete raceways (see Figs. 13 and 14) or, less commonly, in large circular ponds (see Fig. 15). The construction of these facilities is similar to that of their smaller counterparts used for fingerling production described above. All have advantages and disadvantages.

Earth ponds are cheap to construct where the soil holds water well and is flat or slightly sloping. Where seepage is a problem, they can be lined with rubber sheeting. However, hygienic conditions in earth ponds are generally poorer than in enclosures built of non-porous materials which can be more easily disinfected. Earth ponds also require considerable maintenance to repair banks, cut vegetation, etc. Concrete raceways need less maintenance and have a longer working life, but are more expensive to construct. Water circulation is not as good in raceways as in circular ponds. This can clearly be seen if a dye is introduced into the water inflow. The dye will spread to fill the whole of a circular pond, showing that water exchange is occurring throughout the enclosure. In a raceway, on the other hand, the dye will form the pattern shown in Fig. 16.

In the corners of such enclosures water exchange is poor and water quality consequently suffers.

The water supply normally reaches circular ponds through a closed pipeline, but for earth ponds and concrete raceways open channels are common. Channels can be of earth (see Fig. 17) or lined with concrete. Similar channels collect the water from pond outlets and discharge it, usually back into the river from which it came.

Earth ponds and concrete raceways are typically rectangular, and arranged in parallel (see Fig. 18). Frequently raceways are built utilizing the slope of the land in a series of “steps” (see Fig. 19). The water absorbs more oxygen as it falls between successive enclosures. On large farms, many such series of raceways may be built in parallel. The size of earth ponds and concrete raceways is variable, but ponds are commonly about 30 × 10 m, whilst raceways are narrower, e.g., 30 × 2.5 m. Depth of water in both types of facility commonly averages 0.7 –1.2 m. Pond bottoms should be sloped slightly down towards the outlet to facilitate emptying and fish harvest. Circular tanks are often of similar depth, though for fish culture in seawater tanks up to 5 m deep are used. Diameter can be up to 20 m.

The inflow water can enter ponds and raceways from the supply canal through pipes set into the bank or via screened sluices. Raceways supplied by a closed pipe recieve their water through valves in the same way as circular tanks. The outlet structures of large circular tanks operate in the same way as described above for small tanks of similar design. Earth ponds and some raceways are equipped with outlet “monks” (see Fig. 20) which draw waste water from the bottom of the enclosure. However, in some raceways water leaves via a simple weir. This arrangement is not as good, because water both enters and leaves the enclosure at the surface, allowing build-up of “dead” water and waste on the bottom. In such systems the movements of the fish are of major importance in maintaining water circulation and exchange.

In some, generally older, trout farms, raceways are arranged as a single winding channel divided into sections by screens (see Fig. 21). This arrangement has the disadvantage that the same water is used by all the fish in the farm, with consequent problems of water quality and potential spread of disease.

Raceways and circular tanks can be constructed either above or below ground level. Sometimes concrete tanks are painted with non-toxic waterproof paint. Where seawater is pumped into tanks, antifouling paint can be applied to reduce the build-up of marine algal and animal growth.

Stocking densities and water flow

The water flow required for portion or larger fish production are similar to those described above, i.e., around 0.7–1 1/min/kg. Generally, water exchange is faster in concrete raceways or circular tanks than in earth ponds, and consequently stocking rates can be higher. Without aeration, maximum stocking density in raceways or tanks is best kept at around 25 kg/m3, though some farmers claim to use much higher densities successfully. If necessary, extra aeration can be provided by pushing air through diffusers on the pond bottoms from blowers (low pressure compressors delivering a large volume of air) on the pond bank or housed in a nearby building. To be effective, the diffusers must release the air in very small bubbles. Paddle or propeller-type aerators are also produced commercially. They float on the water surface in the pond.


Fish in production ponds are graded for the same reasons as described above for small fish, but less often. In addition, production fish must be graded to select individuals of the right size for the market and sometimes to separate fish which will mature in the near future from those which can safely be grown-on for another year.

Grading by size can be done by hand using special boxes (see Fig. 22). The bottom of a grading box contains a removable grid of parallel bars. Fish small enough to pass between the bars fall out of the box, whilst larger individuals are retained. Grid size can be changed to suit the size of fish being sorted. For fish averaging more than about 100 g, or when large quantities of fish must be sorted, bigger grading machines are available. Basically these consist of a sloping "table composed of longitudinal bars. The bars are not quite parallel, the gap between them being smaller at the top of the table and widening towards the bottom. Fish are poured into a receiving hopper at the top end of the table, and slide by gravity down the table on top of the bars. When a fish reaches a section of the table where the bars are widely enough spaced for it to pass through, it falls between the bars. Underneath the table are either tanks of water to catch the fish, or pipes which carry them directly away to ponds. Some large grading machines have motor-driven rotating bars to speed the operation, and most are provided with water jets or sprays to keep the fish wet.

For grading by eye, e.g., to separate maturing from immature fish, a Y-shaped table is often used. Fish are netted and placed in the bottom arm of the Y. One man stands to each side of the table to push fish into separate containers located underneath the ends of the top two arms. Vertical sides about 10 cm high stop fish falling off the table except from its ends.


Fish can be harvested from ponds, tanks or raceways after lowering the water level. Frequently fish are harvested from earth ponds by a seine net, but in raceways and circular ponds it is more usual to use a seine only to “crowd” the fish into a small area (see Fig. 23) from which they are removed by dip net. As an alternative to a seine net, mobile rigid mesh-covered screens can be used to crowd fish before harvest or for sorting or grading.

Discharge of waste water

In some countries, the water leaving a land-based fish-farm must by law be passed through a sedimentation pond to remove suspended solids before discharge into a natural water course. To remain effective, sedimentation ponds must be cleaned frequently by pumping out the accumulated solids from their bottoms. The pumped sludge can be used as a fertilizer on agricultural land. Generally, farms sited in locations where waste water can be discharged directly to the sea are not required to install sedimentation ponds.

Cleaning and disinfection of tanks

Ponds, tanks and raceways should be thoroughly cleaned and disinfected after their crop of fish has been harvested and before re-stocking. For earth ponds, lime is often used as a disinfectant, but for raceways and tanks formalin or another liquid product is normally employed.

Floating cages

Cage culture is a relatively new technique in most countries. However, it is quickly being adopted all over the world for the following main reasons:

Cage design

Most floating cages are of simple design. Basically they consist of a fish-net bag which is open at the top, where it is suspended from a floating framework (see Fig. 24).

The bag hangs loose in the water, but is kept roughly in shape by a lead-line around its bottom or by weights (usually stones) tied in small net pockets in the bottom corners. The net is often either extended, or an extra net is attached, out of the water for about 1 m all around the top of the bag to prevent fish jumping out. When small fish are present in the cage, a further net (usually of larger mesh) covers the open top of the bag to exclude predatory birds.

Most fish-farmers buy ready-made net bags, but it is possible to make them on site from bought-in fish netting. There are plenty of commercially-made floating frameworks on the market, but many farmers construct their own from local materials. The volume of net bag can vary from about 50 m3 to several thousand m3, but 100–1 000 m3 is the most popular range for salmonid culture.

Cage frameworks can be made in a variety of shapes from many different materials. The simplest to construct are 4-sided structures (see Figs. 25 and 26), but 6 (see Fig. 27), 8 and even 10-sided designs, as well as circular models (see Fig. 28) are also used. The most common material for building is timber, but plastic tubes, galvanized steel, aluminium, fibreglass and rubber are all used. Some cage designs incorporate a working platform or “walkway” around the cage, whilst others lack this and must always be worked from a boat.

Flotation or buoyancy is frequently provided by blocks of expanded polystyrene, which is sometimes encased in fibreglass or plastic tanks. Alternatively air tanks (e.g., empty oil drums) can be employed. In fact anything which floats can be used. The buoyancy is fixed underneath the cage frame, attached to it by bolts, ropes, wires or other convenient method.

Net bags, which actually contain the fish, are almost always made of nylon fish-netting, but steel or plastic meshes are occasionally used. Either knotted or knotless netting can be used; knotless is more gentle to the fish, but knotted is stronger and less easily torn. The mesh size must be small enough to hold the smallest fish in the cage, but it is common to change to a larger mesh size as fish grow. The maximum mesh size recommended for salmonids of various sizes is shown in Table 5. Large meshes are cheaper to buy (per m2), lighter to handle, and allow better water exchange, but they are not as strong as smaller meshes. The depth of the net bag can range from 3 to 20 m, but 4–6 m is average.

Moorings: Cages can be moored singly or in groups around a central working platform. Sometimes the working platform can be moored joining the shore (see Fig. 29), but more usually water depth is too shallow to permit this. Most platforms must therefore be reached by boat. Cages and cage platforms can be moored directly to the shore by chains lying on the sea bed, or to anchors or concrete blocks on the bottom. The weight of moorings required, and of chain and rope connecting anchors to cages, varies greatly according to conditions of wind, waves and currents.

Table 5


Mesh size
Weight of smallest fish to be held

Site requirements and site selection for floating cages

  1. Shelter: Most cage designs are not strong enough to withstand open-sea conditions. They are, therefore, sited in bays or channels between islands where there is shelter from the wind and where the maximum wave height does not exceed about 1 m. Commercially-made cages withstand much bigger waves, but they are expensive and recommended only for areas where more sheltered sites cannot be found.

  2. Water exchange: A flow of water is required through the cages to bring in new oxygenated water and to remove the fishes waste products. In freshwaters, currents are often provided by through-flow of rivers, whilst in the sea tidal movements are often most important. Where currents are poor, fish stocking density must be kept low. Currents between 1 and 30 cm/sec are optimal. Very fast currents should be avoided because they impose excessive stress on cage structures and nets, and force fish to expend much energy swimming against them.

  3. Depth: To allow faeces and waste food to be carried clear of the cage, there should be at least as much depth of open water underneath a cage as the depth of the cage itself. Much greater depths than this are usable.

  4. Amenities: Normally a land base is required to service cages, though a few farms build facilities on floating rafts. Staff accomodation, storage for equipment and food materials, net drying and cleaning facilities, a quay and a crane are useful, but not all essential for very small farms. Road access and services such as electricity supply and telephone are normally required for all but the smallest units.

Working floating cages

Stocking densities can be as high as those used for ponds or raceways on land, provided that water exchange is good, i.e., up to a maximum of about 25 kg of fish/m3 of cage. However, in lakes or other areas with poor water exchange densities should be kept lower - to a maximum of about 10 g/m3. Cages can be stocked with the number of fish which will give this density when fully grown, and in this case they need not necessarily be graded. More usually, however, a larger number of fingerlings is stocked, and fish are thinned out and graded into more cages when they are partly grown. Grading techniques are the same as described above for land-based units. In freshwater, fish can be put into cages when they are as small as 2–5 g, but for stocking in the sea rainbow trout should be at least 50 g, and salmon must be at the smolt stage of development.

Especially in seawater, nets become fouled with marine organisms which settle and grow on them. Fouling blocks the meshes and therefore reduces water exchange. Nets must therefore be removed and cleaned before they become badly fouled. Net changing can be done without removing or handling the fish. The dirty net is pulled part way up and a clean net attached to the cage framework all around it. The old net can then be gradually removed from one side of the cage, and the fish will swim over into their new net. Cleaning of fouled nets is by drying and shaking, with a high pressure water jet, or in a special “washing machine” which cleans the nets by rolling them in a tumbling motion. To extend the interval between net changes, some farms impregnate their nets with a special antifouling preparation which inhibits settlement and growth of marine fouling organisms.

Dead fish can be removed from nets by partly pulling up the bag, then using a long-handled dip net. External parasites are treated with a chemical bath using a plastic bag attached to a small flotation frame. A net inside the bag allows fish to be removed from the treatment bath without loss of the chemical solution, which can be re-used. Because no water flow is possible through the treatment bag, the solution must be oxygenated in use via diffusers from oxygen cylinders carried on the flotation collar. Bacterial diseases can be controlled with antibiotics administered with food in the same way as in tanks or ponds.

In most cage farms, feeding is done by hand, but automatic feeders are available. Dry, wet and moist foods can all be used. Feeding is often done according to the fishes appetite, but care should be taken to avoid overfeeding especially in places where the water is shallow and water exchange is poor. Excess food and faeces can form an organic mud on the bottom underneath the cages. This uses oxygen and, in extreme cases, can kill the fish.

As in land-based culture systems, the time taken to produce fish in cages depends greatly on the species, water temperature, the size to which it is desired to grow the fish (i.e., the size the market wants), and the food quality and feeding regime. Where water temperature is either too high (above the low twenties Celsius) or too low (when a thick layer of ice forms) for salmonids for part of the year, the production of fish is not necessarily rendered impossible. Often it is possible to rear market-sized fish within that part of the year when temperatures are suitable, and simply leave the site empty while conditions are adverse. For example, summer brackish and seawater temperatures on the Adriatic coast of Yugoslavia are 25°–28°C for about three months. However, by stocking floating cages with coho salmon smolts or rainbow trout fingerlings in autumn when temperatures begin to fall, it is possible to produce fish of 1–1.5 kg average weight before spring temperatures rise to dangerous levels the following year.

Transport of fish and fish eggs

Dry eggs and sperm

When broodfish are stripped away from the hatchery, eggs and sperm can be stored and transported separately. Provided that they are not allowed to come into contact with water and are kept cool, they will retain their potential for fertilization for a considerable time. According to the literature, milt can be stored for up to 1–2 weeks, and eggs for several days. The following storage methods are recommended.

Milt: Store in glass or plastic containers (plastic bags are suitable), filled with air or preferably with pure oxygen. The milt should not be in layers more than 6 mm deep to allow oxygen to diffuse into it. A milt to oxygen ratio of between 1:50 and 1 : 120 is recommended. Milt containing urine, blood or faeces should be discarded.

Containers can be stored in a refrigerator at 0°–4°C or kept on ice. Transport is on flake ice for a duration of up to 48 hours. It is advisable to check the motility of an example of stored sperm under a microscope before use.

Sperm can be stored long-term by freezing in liquid nitrogen.

Eggs: Dry eggs can be stored in plastic containers packed as above for a theoretical maximum time of 10 days at 0°C. At 3°C they are reported to retain normal fertilization rates for 3 days, reducing to 90% by day 6 at 3°C.

Nevertheless, in practice the results of fertilization after transport of dry eggs and sperm are sometimes poor. This may be attributable to mechanical damage to the delicate eggs. It is therefore recommended that fertilization be done as soon as is practically possible after stripping.

Newly-fertilized or “green” eggs

After swelling (i.e., about two hours after fertilization), eggs can be transported in water for short distances during the next 36 hours. Thereafter they become very vulnerable to mechanical damage and must be left undisturbed until they reach the eyed stage of development.

Eyed eggs

The usual stage of development during which salmonid eggs are transported is at the eyed phase. Eyed eggs are fairly resistant to handling and, properly packed, they can withstand transport without significant loss for at least 48 hours. By air freight, it is thus possible to ship them anywhere in the world.

Eyed eggs are packed in special polystyrene boxes (see Fig. 30). The boxes contain a number of trays with holes in their bottoms. The lowest tray in the box is left empty, the top one filled with flake ice, and the intermediate trays filled with eyed eggs. During transport the ice gradually melts, maintaining a cool, moist atmosphere inside the box. Melt water collects at the bottom of the box.

Fry, fingerlings and larger fish

Small numbers of fry or fingerlings can be transported long distances in plastic bags filled approximately 25% with water and then inflated with oxygen. For safety, often two layers of plastic are used, and the full bags are packed in rigid outer cardboard or polystyrene boxes. To keep water temperatures low, ice can be packed around the bags or even inside them. Bags can be sent by road, rail or air. Generally, the stocking density of small fish in plastic bags should not exceed about 0.1 kg/1 (= 100 kg/m3) of water.

For transport of larger numbers of fish, special tanks are often employed. These can be carried on trucks, trailers or boats (see Fig. 31). Commercially-made fibreglass tanks are available, each often holding 1 or 2 m3 of water. Sometimes larger tanks of 10 m3 or more are built as an intrinsic part of a truck (see Fig. 32). Tanks are provided with oxygen from cylinders delivered via diffusers, or sometimes with air from a pump. Tanks should be filled full to avoid damage to fish by water slopping around during transport. Stocking densities for fingerlings are normally a maximum of 0.15 kg/1 (150 kg/m3) in freshwater or 0.1 kg/1 (100 kg/m3) in seawater. Portion-sized and bigger fish can be carried at up to 0.25 kg/1 (250 kg/m3).

The temperature of water in transport tanks should be kept low to reduce the fish's oxygen demand, preferably below 5°C and not above 10°C. For this reason, commercially-made tanks are usually double-skinned, with a layer of insulating material sandwiched between the two skins. For long journeys, the water inside transport tanks can be changed en route.

For transport of salmon smolts or trout fingerlings acclimatized to seawater and sometimes for transport of market-sized fish, special live-hauling or well-boats can be used. Instead of a normal hold below decks, these boats are equipped with wells full of water. Flow of clean water through the well is maintained during transport by opening screened valves in the sides of the vessel below the water line.

Fish to be transported in plastic bags, tanks or other closed vessels for extended periods should be starved for a few days before loading to empty their digestive tracts. This reduces the risk of fouling the water with faeces or vomit, and also reduces the fish's oxygen demand.

Handling of fish during loading and unloading must always be done as gently as possible. Indeed, considerate treatment is needed wherever live fish are handled, to avoid excessive stress, bruising and scale loss, which can pre-dispose the fish to attack by pathogens later. When netting is unavoidable, dip nets made with soft, knotless netting should be used. Sometimes dip nets are lined with polythene sheeting, so that fish are actually transferred in water. It is important not to take too many fish into a dip net at one time, to avoid crushing them. Sometimes it is possible to run fish directly from their tanks into transport containers with water through a flexible plastic pipe, or to use a special fish pump or hoist (see Fig. 33) to lift them without removing them from water.

By the above methods, fish can be transported long distances for periods of up to several days. For local transport and for moving fish around the farm simple buckets, plastic dustbins, or large open tanks mounted on trailers or trucks can be used. Some farms are designed to allow fish to flow with water by gravity through plastic pipes into larger tanks or out into floating cages.

Quality control

Markets in most countries are now highly competitive. For economic success, the fish-farmer must pay careful attention to the quality of his product. An important part of quality control is the handling of fish immediately before harvest, the method of slaughter, and subsequent packing and transport. Careless handling at this time can spoil in a few hours a product which has taken years to culture.

Starving: Fish should be starved for several days before harvest. A period of one week without food is recommended. This has several functions:

Harvesting: Fish must be taken from the water as gently as possible, so as not to stress them more than absolutely necessary. When stressed, the fishes' muscles use up glycogen and the keeping qualities of the flesh are reduced. Generally, the later the onset of rigor mortis after death, and the longer it lasts, the better the fish will keep. The onset of rigor is brought forward and its duration shortened by stress during slaughter, and the sooner fish are put into ice after death, the better the meat will keep.

Care should be taken to avoid mechanical damage to the muscle or skin by bruising or knocking off scales. This damages both the appearance and the quality of the product and therefore reduces the market price. As with transfer of small fish, animals to be slaughtered should be removed from the water with hand nets made of smooth knotless netting or netting lined with polythene sheeting. Crushing must be avoided by lifting only a few fish at a time in the net. After removal from the water, fish should be killed and packed as quickly as possible. If there is any delay, they must be put into ice and kept out of the sun. Fish which have died in the water should never be offered for sale.

Slaughter: The quality of meat is poorer if fish are simply allowed to die of suffocation than if they are killed quickly. Several methods of slaughter are commonly used:

Hygiene, grading and packing

Regulations governing premises used for slaughtering and packing fish exist in many countries, and public health officers routinely carry out inspections to help enforce them.

Fish should be chilled in ice to lower their core temperature to a maximum of 4°C immediately after killing. Generally, fish should be gutted, cleaned and packed as soon as is practically possible after death, and certainly before the onset of rigor mortis. Gutting of large fish is normally done by hand, but machines are available for handling large quantities of portion fish. Washing of carcasses must be done with water of drinking quality, and ice used for packing must also be of this standard. Sometimes the gills are also removed.

Fish are graded by weight and packed in polystyrene (insulating) boxes in plenty of ice. For large fish, gutted individuals are packed belly-down, round fish belly-up. Boxes are sealed with a banding machine or with tape, and should be clearly marked with the number and weight of fish they contain. In some countries, boxes must also be marked with the packing station licence number, allowing substandard fish to be easily traced back to their source. Boxes are often loaded onto pallets for ease of transport.

Freezing and processing

In most countries, frozen fish command a lower price than fresh. Nevertheless, it is often necessary to freeze some fish, especially when harvest is to be carried out during a particular, short season. Freezing must be done in special blast or plate freezers at around -40°C or less. Frozen fish keep better if they are “glazed” (i.e., dipped in water when frozen to seal them from the air) or sealed in plastic foil to prevent drying of the meat and oxidation of fats. Salmonids are fatty fish, and will not keep well in deep freeze for very long periods. The maximum recommended storage times at various temperatures are shown in Table 6.

Table 6


Storage temperature (°C)Lean fishFatty (oily) fish
-20  42
-25  64

Fish should always be gutted before freezing. Frozen fish for human consumption must never be kept in the same cold-store as ingredients for making fish feeds.

Much of the profit in the fish business in most countries is made by middlemen who buy and re-sell, often after some processing which “adds value” to the product. Fish-farmers can increase their profitability by doing the processing themselves, or by grouping into cooperatives or associations for marketing purposes. “Processing” can mean simply packing the whole fish in a better or more long-lasting way (e.g., vacuum-packing or controlled atmosphere packing for supermarket sale). Alternatively, it can mean filleting, smoking, or preparation of “speciality” fish products such as marinated fish, fermented fish, or fillets packed in a sauce.

Smoking: In many countries smoked salmonid fishes are popular. The smoking process is done in two phases:

  1. Salting and drying - fish is kept for several hours (time depending on size and custom) in salt or brine.

  2. Smoking - the fish are hung in a container through which wood smoke from smouldering sawdust is passed, often by a fan. A variety of different hard woods can be used for smoking, each giving its own characteristic flavour to the fish. Smoking can be “hot” so that the fish is also cooked, or “cold” resulting in an uncooked product. Generally, small (portion) trout are smoked hot, either as whole gutted fish or in fillets, whilst large fish are filleted and smoked cold. Cold-smoked salmon is an international delicacy, retailing at up to $US 30/kg.

After smoking, the fish or fillets are often vacuum-packed to extend shelf life. They can then be frozen if required.


Effective marketing depends on first-class quality control. To achieve this, the handling of fish before, during and after slaughter should be carefully carried out as described above. However, additional very important factors are as follows:

Fish must be harvested before the onset of sexual maturation.

Based on a consistently first-class product, sales of salmon and trout can be promoted in the market by advertising, dissemination of recipe leaflets, articles in newspapers or magazines, etc. In most countries, the money to finance this comes from a levy on sales of fish paid by the farmers to a cooperative or a producers' association. Such organizations also serve the farmers by lobbying the Government over such issues as licensing, availability of land for fish-farms, rents, environmental and water quality control, waste disposal, etc. Without such organizations, only very large companies can afford a worthwhile marketing campaign or pursue a test case through the courts. Sometimes associations or cooperatives also qualify for Government assistance for building, processing and packing plants, purchasing transport, etc.

References and further reading

Edwards, D.J. (1978) Salmon and trout farming in Norway. Fishing News Books Ltd. Farnham, Surrey, England

Gjedrem, T. (ed.) (1986) Fiskeoppdrett med framtid. (Norwegian). Landbruksforlaget, Oslo

Ingebrigtsen, O. (ed.) (1982) Akvakultur: Oppdrett av laksefisk. (Norwegian) NKS-Forlaget, Oslo

Stevenson, J.P. (1980) Trout farming manual. Fishing News Books Ltd. Farnham, Surrey, England

Fig. 1

Fig. 1 “Eyed” trout eggs (photograph by Vidar Vassvik)

Fig. 2 Diagram of the
“stacked plate”
system for degassing
Fig. 2
Fig. 3

Fig. 3 Stripping eggs from a mature salmon
(photograph by Vidar Vassvik)

Fig. 4

Fig. 4 Mixing trout eggs and sperm together in a plastic bowl (photograph by Vidar Vassvik)

Fig. 5

Fig. 5 “California-type” incubators for salmonid eggs (photograph by Vidar Vassvik)

Fig. 6

Fig. 6 Diagrammatic illustration of the “Californiatype” incubator system

Fig. 7
Fig. 7 Plastic incubation cylinders or “silos” (photograph by Vidar Vassvik)

Fig. 8 Picking out dead trout eggs with a siphon tube (photograph by Vidar Vassvik)
Fig. 8
Fig. 9

Fig. 9 Circular fibreglass tanks for salmonid fry. Note the automatic feeders hanging above the tanks (photograph by Vidar Vassvik)

Fig. 10

Fig. 10 Diagrammatic illustration of a circular or square tank for culture of salmonid fishes

Fig. 11

Fig. 11 Earth ponds for rainbow trout production

Fig. 12

Fig. 12 Outdoor circulating-type fibreglass tanks

Fig. 13

Fig. 13 Small concrete raceways

Fig. 14

Fig. 14 Large concrete raceways with an inspection/ working walkway

Fig. 15

Fig. 15 10 m-diameter circular concrete ponds

Fig. 16

Fig. 16 Distribution of dye introduced into the inlet of a raceway or earth pond

Fig. 17

Fig. 17 The inflow canal supplying earth ponds with water

Fig. 18

Fig. 18 Stylized arrangement of earth ponds or raceways

Fig. 19

Fig. 19 Schematic arrangement of raceways on sloping land

Fig. 20

Fig. 20 The principle of a “monk” outlet from a pond or raceway

Fig. 21

Fig. 21 Raceways arranged as a single, winding channel

Fig. 22

Fig. 22 Sorting trout with a hand grader

Fig. 23

Fig. 23 Selecting salmon broodfish, using a seine-net to “crowd” the fish

Fig. 24

Fig. 24 The basic structure of a floating cage

Fig. 25

Fig. 25 Square timber-framed floating cages

Fig. 26

Fig. 26 Large square steel-framed cages

Fig. 27

Fig. 27 Six-sided floating cages

Fig. 28

Fig. 28 A circular floating cage made from flexible polythene pipe

Fig. 29

Fig. 29 Cages moored alongside floating walkways attached to the shore

Fig. 30

Fig. 30 Polystyrene boxes for transport of live salmonid eggs

Fig. 31

Fig. 31 Live fish transport tanks mounted on a trailer

Fig. 32

Fig. 32 A large truck-mounted tank for live fish transport

Fig. 33

Fig. 33 A mechanical fish hoist

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