The fry continue to feed and grow throughout the year, if sufficient quantities of warm water are available. If the water temperature falls below about 5°C, feeding may cease and over-wintering procedures will be needed. Though trout can be raised on natural foods, most farms at present use more or less complete artificial feeds.
There is a greater use of concrete or fibreglass tanks for growing yearling or two-year-old trout. The same tanks, particularly the circular tanks used for fry rearing, can also be used for grow-out of yearling trout. Tanks with a good circulation of water carry 25–35kg fish per m3 water. Long raceway type tanks, 30m x 3m and about 1.2m deep are commonly used. Circular tanks of about 5–12m diameter and 0.75–1m depth are also in use. In circular tanks with a flow of 10l/min of water, 1000 fry are reared per m2. The silo type of tanks, briefly described, have been used experimentally for high density culture of rainbow trout.A unit 5m high and 2.29m in diameter with a flow rate of about 28.4l/s is reported to carry 2820kg trout without any problems. This is a stocking density of 136kg/m3 or 27.5kg/m3 per second of water flow. Raceways are widely used for grow-out as well as fry and fingerling production in North and Central America and Europe. Each concrete raceway system may be up to 500m long (10m wide and 1–2m deep),divided into several segments, with arrangements for aeration.
During the grow-out period of about 4.5–5 months, fish are graded and sorted by automatic equipment and transferred by pumping through water pipes to different raceway segments.
In the Danish type of freshwater trout farm, with sufficient fresh water supplies, 25–50 fry are stocked per m2. It is possible to produce up to 30kg/m2 with proper feeding and abundant water supply. In ponds with a high flow-through of water, higher stocking densities are adopted. It is believed that ponds with a high degree of aeration can support a stocking density up to five times greater than unaerated ones. A safe high density of stocking is around 60kg/m3, with a water replacement of at least four to six times per day.
Grow-out of trout in land-based salt-water ponds seems to have originated in Denmark and Japan. One of the main reasons for utilizing sea water is to take advantage of its higher temperatures in colder northern countries in the winter season. Also, sites with abundant supplies of clean sea water can be found more easily in many areas for land-based farming as well as cage and enclosure farming. Young trout and salmon acclimatize to sea water conditions, but it is advisable to acclimatize them over a period of time by gradually increasing the salinity. When the fish is transferred to sea water, water from its body fluid is drawn out by osmosis because the salt concentration in the body is slight. Water loss is countered by drinking sea water, and this results in accumulation of excess salt in the body, which has to be excreted through special cells in the gills. The migratory steelhead acclimatize more readily, because of the increase in the number of salt-excreting cells in the gills during the period when they change into smolt. If other trout races are fed with a high-salt diet while in fresh water, their ability to adapt to sea water can be increased. Rainbow trout are ordinarily transferred to sea water when they are about 70– 100g in weight, but 150–200g fish would acclimatize with less mortality. The growth rate of rainbow trout in sea water is reported to be double that in fresh water.
Shore-based farms consist of tanks of different sizes, with a pumped water supply for rearing fry to market size. The majority of the farms have tanks for rearing fry and ponds of different sizes for fingerlings, market fish and brood fish. If all sizes of fish have to be reared, there should be access to both fresh and sea water. The acclimatization of fingerlings to sea water can best be done gradually over a long period of time, by very gradual dilution with sea water. Pumping will be a major additional cost, but the ease with which the farm can be serviced and the stock monitored has made this system acceptable for growing fingerlings and smolts. Market fish are grown in these when the price is high enough to make it economical.
Instead of shore-based farms, tidal enclosures have been used on a limited scale, where free exchange of water is possible during the course of low and high waters. An enclosure can hold about 8–10kg rainbow trout per m3 water. The fish are grown to sizes of 3–4kg.
The trout culture system that has received considerable attention in recent times is cage culture in fresh- and sea-water environments. The design features of cages have been described in. There is very little difference between the designs of cages used in fresh water and in protected areas of the sea, except that the marine cages are made stronger to withstand rough weather conditions. Floating cage culture has several advantages, among which the more important are the relatively high level of control the fish culturists can have on the stock and the possibility of starting operations on a small scale with a small investment (for example, staring with only two or three cages) and building on in the course of time to the maximum size that the site and local regulations permit. Rainbow trout is the most common species used in cage culture and fingerlings are stocked in cages in spring and are harvested in the autumn after a culture of 1.5 years or stocked in the autumn for harvest after one year. Fingerlings of about 70g weight can attain a size of about 3kg in less than 1.5 years. As male rainbow trout has a tendency to mature under two years of age, many farmers prefer to grow all-female or sterile stock for sea water grow-out. An innovation, started by some production units in Norway, is to hold the brood fish in spring water, with a temperature of about 7°C, through winter and strip them in January or early February. By using heated water at about 10°C, the incubation and fry growth are accelerated, so that by the autumn of the first year itself, smolts are produced which can with-stand transfer to sea water. The duration of cage culture is restricted to one year, as other-wise the fish will start maturing in the cages. Even though the fish are smaller, the whole cycle of production is reduced to 1.5 years and all the cages can be kept under production throughout the year.
The stocking density of fish in the cage is 10–20kg/m3 at harvest, depending on water circulation. Generally, larger quantities of fingerlings than are required to obtain the above density are stocked in each cage and as they grow larger they are graded out to other cages.
The double cropping system of trout and channel catfish, in the southern USA, is an attempt to improve the income from raceway trout culture. Rainbow trout are cultured for about 132 days when the temperature is below 21°C. After the fish are harvested and the temperature goes above this level, channel catfish (Ictalurus punctatus) are reared for over 200 days.