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Rainbow Trout Farming: Considerations for Aquaponic systems
We at Greenfish are delighted to bring you an excellently written article by Elena Piana, an Italian aquaculturist working at Goatsbridge trout farm in Kilkenny.
Rainbow trout (latin name Oncorhynchus mykiss) is native of North American rivers draining into the Pacific Ocean and it is genetically closer to the Pacific salmon rather than the most familiar Atlantic salmon. The body is elongated with a characteristic adipose fin between the dorsal and caudal fin. The colour ranges from blue to olive green with a pink band along the lateral line and small black spots that cover most of the body. This species has been farmed in Europe for over 400 years and it is now commercially produced all around the world: Ireland, the UK, Italy, France, China, South Africa, the U.S. and many others.
Rainbow trout has many characteristics that make it suitable for aquaponics: it is easy to farm, it is quite resistant to diseases and to environmental factors. It can be grown in water temperatures ranging from 5-6ºC to 22-24ºC with an optimum pH water of 7, although it can tolerate pH between 5 and 9. This fish is not too sensitive to ammonia level (recommended below 0.012 mg/l of unionised ammonia) and nitrites (better below 0.1 mg/L) either. Oxygen concentration should be no lower than 5.5 mg/l, but considering that fish stocking density (the number of fish put in a certain space) in aquaponic system is never too high, oxygen shouldn’t be a problem. Eggs or fingerlings (young rainbow trout) are relatively easy to find and they shouldn’t be too expensive.
You can watch videos of aquaponic systems holding trout at:
The life cycle of wild rainbow trout (fig.2) begins from little orange eggs laid on the bottom of the river bed on gravel. After fertilization by the male the embryo develops and within a month or two a little fish larvae will hatch. Hatching time depends on water temperature: the warmer the water, the sooner the hatching and viceversa. The little (1-2 cm) larvae has a yolk sac attached just underneath its belly and this will be its only source food for the first few weeks of life. Then active feeding starts. Adult rainbow feed on insects and small invertebrates such as freshwater shrimps, mollusc and other little fish. At around 2- 3 years of age the fish reaches sexual maturity and the cycle start all over again: the female deposits eggs in the river and the male fertilizes them.
Figure 1: downloaded from: http://www.teara.govt.nz/en/trout-and-salmon/2/5
Farmed trout life cycle differs in some parts.
- Sexual maturity of farmed rainbow is reached at around 2 years of age (a bit earlier than wild trout).
- Ready-to-spawn farmed trout won’t release eggs spontaneously like in natural conditions (for reasons still unknown to science). Therefore it needs to be manually stripped by application of a gentle pressure on the abdomen (same principle of milking a cow). Eggs are then collected in bowls, artificially fertilized, and incubated in the dark.
- When it is time, hatching embryos are moved to running water troughs and fed a powdery feed until the jaw is fully developed and ready to swallow bigger feed.
- The next phase is called on-growing. Now the fish are usually moved into bigger spaces and are fed bigger feed pellets for 8 to12 months until it reaches the desired market size (between 350 and 500 gr). Like for the hatching, the warmer the water, the faster the growth.
- It is important to note that trout farming utilizes all-female populations. The reason being is that male trout are aggressive and tend to bite each other causing serious external damage that can eventually lead to death. However, females also have their own drawback: at the onset of sexual maturation most of the fish energy is invested in egg production resulting in a considerable decrease in flesh quality. Therefore the harvest is usually done before this happens.
As the fish develops its diet requirement develops too, and supplying your fish with the appropriate feed formulation is essential for a positive and healthy growth. Every life stage needs a certain balance of protein, fat, carbohydrates and other micronutrients. Nowadays specialised fish feed manufacturers carry out research in this field and are able to provide several types of feeds, tailored for different fish species and their life stages.
A good rule of thumb is to feed your trout more as water temperature rises and less as the fish grows. Generally, the amount of feed needed is calculated as a percentage of the fish body weight. For example, a 50 gr fish in a 5 ºC water temperature will be fed 1% of its body weight (i.e. 0.5 gr per day), whereas the same fish in a 16 ºC waters will need 2% (i.e. 1 gr per day). Detailed feeding charts where you can find this information are usually made available by the feed producer website or will be given at feed purchasing.
Downloaded from http://fywclh.en.ec21.com/Feed_Pellets_Fish_Feed–2372150_3020617.html
Stress and disease:
A further trait which made rainbow trout a good candidate for aquaculture is its resistance to diseases. Generally, fish “get sick” consequently to stressing situations, such as handling, crowding (too many fish in the same space), loud noise or bad quality water (MH3+ or NO3 may have risen too high).
You can understand whether your fish are in a status of stress by observing its behaviour. Common signs of stress are: schooling (the fish swim all together in one direction), rapid movement of the gills (open and close) or damaged dorsal fin. If you notice any of these signs you should try to leave the fish as quiet as possible and check the water chemistry for any abnormalities. Your rainbow trout can recover from stressful situation and damaged fins can fully regrow.
Enjoying your harvest:
Trout has a pink soft but firm flesh texture and can be cooked in many ways: oven baked, grilled, barbecued, fried or even boiled!
More delicious recipes can be found on: