Feeding is the most expensive part of trout production.
In the past, trout were fed with trash fish and slaughterhouse by-products, offal and wastes. It is a widespread opinion that using the feeds for fattening listed in Table 5 is rather inconvenient and also very polluting both to the rearing tanks/ponds and to the surrounding environment.
The next period in the development of the trout farming industry was the formulation and use of different types of high protein* feeds. Their feed conversion ratio (FCR*) varied between 2 and 3.
I. Water should not flow faster within 1 second than the actual total length of the reared fish. However, the maximum velocity of water should not exceed 20 cm/sec (12 m/min) even if the fish is longer than 20 cm.
II. The optimal velocity of water is 2–3 cm/sec (1.2–1.8 m/min) for smaller fish and 4–10 cm/sec (2.4–6 m/min.) for larger ones. However, the actual speed of water per second should not be faster than from one-half to three-quarters of the length of the reared fish.
Production work and tasks
In the modern trout farming industry, the traditional feeds have been definitively replaced with very efficient pelleted dry feeds (0.6–1.1 FCR).
There are publications that advocate the use of home-made feeds, which may be feasible only with some reservations. Home-made feeds seem to be a good solution, especially where commercial trout feeds are not readily available. However, the ingredients of home-made feeds should be easily locally available, with continuous supply in the required quantity and quality and at competitive prices. In this case, one of the numerous recipes of formulated trout feeds should be selected and blended. Extensive experience has proved that purchasing commercial feeds is often the only feasible and profitable option.
In evaluating the commercial feeds, the expected FCR and the related price are those characteristics that should be considered at purchase and use. It is a general rule that the price of a feed is inversely related to its FCR – the lower the FCR, the higher the price of a feed will be. However, economic calculations may prove that a feed with a lower price but a higher FCR will be more expensive than an expensive feed with an outstandingly low FCR. For this reason, many farmers choose high-quality expensive feeds for the first stages, where little feed is used but where the fish are most vulnerable and sensitive.
Normally, commercial feed manufacturers determine the recommended daily quantities of their feeds. If not, Figures 44 and 45 provide guidance for adjusting the daily rations.
Daily feed rations should be given in 2–24 equal portions. It is a general rule that the younger fish should be fed more frequently than older ones (Figure 46). The frequency of feeding should also be increased with the temperature of the water. Concerning the size of feed particles, they should be small enough that fish can comfortably grab and swallow them.
Practical aspects of feeding and feeds
Hand and mechanized feedings are the two widely practiced techniques. Of these, hand feeding is the recommended one. Loss of appetite among fish is one of the most obvious symptoms of many different problems. It indicates, among others, insufficient oxygen content of water or a developing disease in fish. Therefore, regular daily feeding is an excellent opportunity to observe fish and detect problems and diagnose diseases.
Demand feeders are those that release feed according to the appetite of fish. Because rainbow trout are very greedy fish, these feeders may allow unnecessary overfeeding of fish unless the portions are controlled.
The advantage of mechanized and automatic feeders is that they save on labour.
The most typical mechanized and automatic feeders are the demand bar feeder, used from fish size 50 g, and the clock-driven feeding belt.
Signs of feeding problems
Obvious signs of feeding problems are the increasing differences in individual sizes, growing aggressiveness and cannibalism. Lack of sufficient feed manifests itself in bitten/damaged fish and dead fish.