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Rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) have a long history of aquaculture and rainbow trout is one of the few species of fish which may be regarded as truly domesticated. Rainbow trout originate from western North America, while brown trout are native to Europe. In 1995, world aquaculture production of rainbow trout was approximately 360,000 tonnes, valued at approximately US $1.3 billion, while the total global production for all salmonids (including smelts) was approximately 943,000 tonnes and US $3.7 billion species whereas brown trout are generally grown only for recreational fishing.
Brown trout have been in Western Australian waters since 1931 and rainbow trout since 1942. Both species of trout were originally introduced to our waters to provide food and recreational fishing, there being no large freshwater native species in southern areas except for the native catfish (Tandanus bostocki). As trout came from cooler, temperate climates, their distribution in WA is restricted to the south-west corner of the State.
In 1935, a hatchery was developed at Pemberton in the south-west of WA, for breeding both brown and rainbow trout. The hatchery is now operated by the Department of Fisheries as part of the South West Freshwater Research and Aquaculture Centre (SWFRAC) and provides juvenile and yearling rainbow and brown trout for stocking of public rivers and dams for recreational fishing, to farmers to stock their dams, to fish farms for commercial trout grow-out, and for saline inland aquaculture of rainbow trout.
There is a small established trout aquaculture industry in Western Australia. Currently, there are three farms commercially producing trout in WA and this low number of producers prevents the release of specific trout production data (see Cole et al., 1999). The commercial production from the aquaculture of trout in WA had reached 40 tonnes per annum, valued at around $400,000 (McNee et al., 1993). However, due to the reduced number of commercial farms and the diversion of production for fish-out (pay to fish) operations, the local industry has contracted. Australia-wide, over 9,000 tonnes of trout and salmon were produced in 1997/98, worth more than A$75 million (ABARE 1998). Rainbow trout (also called 'ocean trout') are produced in sea cages in Tasmania, but over 2,000 tonnes of rainbow trout were produced from land-based culture with a value of A$12.7 million, mostly from Victoria. Only land-based trout culture is practised in Western Australia. Attempts to farm rainbow trout in sea cages off the south coast have been limited by warm water temperatures, the lack of sheltered sites and poor public reception. However, alternative sites for large-scale production of rainbow trout are currently being examined.
Trout have been cultured since the mid-1800s using ponds, tanks, raceways and, more recently, cages. Consequently, the environmental and nutritional requirements and design of farms for trout are well understood and have been clearly described in a number of books (see recommended reading list at end of this brochure). Overall, trout have a high demand for water of a high quality throughout the year. A major review of the environmental requirements of trout is being undertaken by Molony (in prep).
Trout
are cold-water fish. Although the optimum temperature for rainbow trout
metabolism is 18oC, in practice, the preferred water supply for
trout farms in Europe is between 10oC and 15oC and
should not exceed 22oC (Sedgwick 1985). High mortality rates of
trout generally occur at between 25oC and 27oC (Sedgwick
1985). Rainbow trout show reduced feeding, and therefore lower growth rates,
in water temperatures above 20oC. However, Morrissy (1973)
conducted research in WA which showed that the rainbow trout strain maintained
at the SWFRAC (Pemberton, Western Australia) has an increased ability to
tolerate higher temperatures than stocks in eastern Australia, so data for
interstate and overseas trout populations may not be directly applicable to
trout stocks in WA. At the SWFRAC, rainbow trout feed in water temperatures of
up to 23oC. It is likely that the WA stock of rainbow trout has
been passively selected over many generations to tolerate higher temperatures
than other stocks. Nonetheless, survival of rainbow trout is enhanced by
reducing feeding rates and keeping the fish lean throughout summer. Gibson's
(1998) recommends that farmers stop feeding trout in the Eastern States if
water temperatures exceed 18oC for rainbow trout and 17oC
for Atlantic salmon in seawater.
Trout require high levels of dissolved oxygen and this should remain above 5 to 5.5 parts per million (ppm) (Bromage & Shepherd 1990). As water temperature increases, dissolved oxygen levels and the percent of oxygen saturation decreases, causing trout to become stressed during the summer months. This stress may be reduced by oxygenating the water, especially during the hotter months of summer. Gibson's (1998) emphasises that growth of rainbow trout in other States is reduced (despite maximum food consumption) levels when the dissolved oxygen levels are below 7 ppm (parts per million). Fresh water, saturated with oxygen at 17oC, contains approximately 9.5 ppm oxygen; however, in practice, oxygen demand by fish in culture systems, particularly if heavily fed, will depress oxygen levels to well below saturation.
Rainbow trout are tolerant of a wide range of salinities, ranging from pure freshwater, up to full-strength seawater (35 parts per thousand ppt). Various stocks of wild rainbow and brown trout around the world migrate to the sea to feed and grow until maturity, when they return to fres hwater to spawn. Tolerance to saline water depends upon the age at which acclimatisation to saline water occurs, the rate of acclimatisation and water temperature (Bromage & Shepherd 1990). Trout which have been acclimatised to seawater between the ages of 6 months and 2 years may be reared in marine conditions until maturity. In WA, yearling trout (approximately 8 - 10 months old) are currently being stocked into saline farm dams in the Wheat Belt on a trial basis.
The preferred pH range of trout is between 6.4 and 8.4, with a pH between 7.0 and 7.5 (without rapid fluctuations) being optimal. At higher pH levels, relatively low levels of ammonia are dangerously toxic (Bromage & Shepherd 1990, Sedgwick 1985). Tolerance of trout to variables such as ammonia, chlorine, hydrogen sulphide, suspended solids and metals (such as. aluminium, copper and cadmium) is well documented, while the water-quality criteria for optimum health in salmonids has been summarised by a number of authors, including Bromage & Shepherd (1990) and Gooley (1998).
Trout farms require a reliable water supply to provide sufficient levels of dissolved oxygen, which is affected by a number of factors, particularly temperature. For example, assuming that the dissolved oxygen of incoming water is at 100 per cent oxygen saturation levels, if incoming water is at or below 14oC a farm requires approximately 14.3 litres of water per second per tonne of 200 gm rainbow trout (14.3 L.sec-1.tonne-1). Should the water temperature increase to 18oC, a flow-rate of 20.9 L.sec-1.tonne-1 is required in the culture of 200 g rainbow trout (Bromage & Shepherd 1990). In WA during summer, water demands of trout culture are very high.
Trout have been farmed in a wide variety of situations. Originally, trout were grown in drainable earthen ponds, generally 30 m long, 10 m wide and 1.5 m deep, with flow rates of 10 to 15 L.sec-1. Ponds will only permit low stocking densities, generally from 2 kg per cubic meter (kg.m-3), depending on water exchange rates and other variables (Bromage et al., 1990).
Raceway culture is also commonly practiced. Raceways are usually concrete, generally 30 m long, 3 to 10 m wide and 1 m deep and typically have higher flow rates than ponds, ranging from 75 to 250 L.sec-1. Consequently, increased stocking densities are possible and densities of trout greater than 32 kg.m-3 have been reported from intensive raceway systems (Bromage et al., 1990).
Circular tanks, typically 4 to 6 m in diameter and 0.75 m deep, have also been used for trout culture. A water supply of 4 L.sec-1 permits stocking densities of trout of up to 21 kg.m-3. However, by increasing the water flow, stocking densities of 35 kg.m-3 and over may be achieved. Circular ponds are popular in WA and are used in the SWFRAC. However, circular ponds are not well suited to automated handling, grading or harvesting techniques that have been adopted overseas to improve productivity of trout culture (Bromage et al., 1990).
Trout production from still-water farm dams is generally restricted to approximately 100 kg.ha-1.year-1 (Morrissy, 1985). However, growing trout in still-water dams is being widely practiced in WA for private use, recreational fishing and extensive production trials. Similarly, the rearing of trout within floating cages in dams, which experience poor or little flushing, generally results in eutrophication and pollution. However, cages in freshwater ponds are used successfully in other states for more tolerant species such as barramundi (Fowler, 1999). In Tasmania and overseas, trout have been reared in cages within semi-enclosed water bodies such as Macquarie Harbour, but this is generally done in lochs or fjords which are much larger and deeper than farm dams (Morrissy 1985).
Floating sea cages have also been used for raising trout with stocking densities as high as 30 to 40 kg.m-2 being reported (Sedgwick 1985). However, due to environmental constraints and site selection problems, raising trout in sea cages has not yet been successful in Western Australia.
Trout have also been cultured in man-made lakes for recreational fishing and the design and requirements of trout lakes can be found in Barrington (1983).
The reproduction of trout, including control of spawning, induction of spawning, stripping, fertilisation of eggs, egg incubation and larval rearing, is well understood. A number of books provide further information on all aspects of breeding and rearing of trout (See Bromage et al., 1990, Sedgwick 1985, Stevenson 1987).
Spawning of trout occurs during winter months in WA (June and July). Female rainbow trout can produce up to 2000 eggs.kg-1 of body weight (Morrissy 1985), and similar egg production is recorded from brown trout (Novotny and Nash, 1985). Trout produce relatively large eggs of 3-7 mm in diameter (Novotny and Nash, 1985)) and the larvae are well developed at hatching. In general, trout will not spawn naturally in culture systems and juveniles must be obtained either by artificial spawning from a hatchery or by collecting eggs from wild stocks.
The Department of Fisheries operates a trout hatchery in Pemberton at the SWFRAC. The hatchery provides juvenile trout for stocking and is able to reliably provide fingerlings, yearlings and older fish to farmers and members of the public.
The special nutritional requirements of trout are well known and have been described in a number of books including Sedgwick (1985) and Shepherd & Bromage (1990). Trout are carnivorous fish, although feeding and growth rates decline and cease when water temperatures exceed 20oC (23oC in WA). Juvenile trout are well developed at hatching and, after absorbing their yolk sac, will readily feed upon commercially available artificial diets. Pelletised diets are produced locally and are available from stock feed manufacturers in WA. Feeding charts (available from most suppliers), describing recommended pellet size and daily ration (for example, Gibson's (1998)), are generally based on trout cultured in colder climates and are not readily applicable to WA stock or conditions, although reduced feeding during hot weather is critically important. Most large trout farms use mechanical demand feeders to distribute the trout feed, which can also be distributed by hand to allow observation of the stock for demand feeding. From experience, leaner fish fed at a lower ration level cope far better with high summer temperatures than generously fed fish.
Food Conversion Ratios (FCR) are ratios calculated from of the amount of feed required to produce one kilogram of trout. For example, a FCR of 1.5:1 means that it requires 1.5 kg of feed to produce 1 kg of trout. FCR ratios are useful to predict the costs of feeds required during trout culture. Most trout farms achieve FCRs of at least 1.5:1 and, under ideal conditions, FCR may be as low as 1:1. However, with low protein or poorly digestible diets, FCR may be as poor as 2:1 or more (Bromage et al., 1990, Gibson's, 1998).
© Copyright 1999-2001 Department of Fisheries.
