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 20 o C (23 o C 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).
The growth rate of trout relies upon water temperature, nutrition and genetic factors. Consequently production of 200 g fish from eyed eggs may take between 10 and 20 months (Bromage et al., 1990).
However, by purchasing hatchery produced fingerlings or yearlings, the grow-out period is decreased to as little as 6-8 months in some cases. Trout raised in freshwater ponds or tanks are generally reared to pan size (approximately 250 g), while fish in sea cages may be grown to 2 kg. In WA, trout can be reared to 300 gm in approximately nine months.
The strain of rainbow trout developed at the SWFRAC (Pemberton) exhibits higher and much more uniform growth rates than fish considered more like the original strain introduced into WA.
The disease status of trout in Western Australia is relatively good as a number of diseases which affect trout in other parts of the world and in the Eastern States are not known to exist in WA.
Trout can be affected by a large number of diseases including the viral disease EHN (epizootic haematopoietic necrosis) and aquabirnavirus infections such as IPN (infectious pancreatic necrosis virus), which cause concern and extra expense to trout farmers overseas and in the Eastern States.
Based on testing of WA trout over four years, the State is considered to be free of EHN. A common problem in WA, due to the relatively warm growing waters, is infection by Mycobacterium marinum .
This causes mortalities during warmer summers. Diseases experienced in the cultivation of trout and their treatment are very well known (see Roberts 1989 and Sheperd & Bromage,1990).