– a gourmet’s cool choice
If you are looking to produce a delicious fish that grows well in cool water then a trout aquaponics system should be at the top of your list. The culture of trout is well established throughout the World, particularly so in the UK. Young fish for stocking are available all year. Trout nutrition is now well understood. There are ample suppliers of excellent and convenient pelleted feed.
Main species and breeds
Trout are members of the salmonid group that includes Atlantic salmon. Two main species of trout are produced in the UK.
a) Rainbow Trout, Oncorhynchus mykiss
This species is native to NW America but has now been widely introduced across the world. This has happened because of its suitability for aquaculture. There are around 350 trout farm in Britain with the main concentration being in Scotland, Yorkshire and southern and south-western England. The UK produces around 12,000 tonnes per annum with 75% going to the table market. The rest are sold live for the restocking angling market.
b) Brown Trout, Salmo trutta
Brown trout are indigenous to the UK and increasing efforts have been taken to protect native stocks from disease and genetic loss. Production levels are far less than those for rainbow trout with just 750 tonnes being produced in 2010.
Female trout grow faster than the males but fortunately, it is now possible to buy all female stock. There is also a move towards triploid trout. These are essentially sterile with the consequence that all food goes towards growth rather than reproduction. They play an important role in helping protect native brown trout.
- Water Temperature – suitable range 10 – 20 °C. with the optimum around 15°C. Find out more about fish tank temperature.
- Dissolved oxygen – minimum around 7mg/l. Find out more about oxygen and fish
- pH – 6.0 – 8.0
- Maximum unionised ammonia – 0.02 mg/l. This is not TAN (Total Ammonia Nitrogen) and shouldn’t be confused with each other. Find out more about fish ammonia.
- Maximum nitrite – 0.2 mg/l.
- Maximum nitrate – 150mg/l. Less important for fish health but critical for plant nutrition.
- Maximum carbon dioxide – 20 mg/l.
Table fish production
A harvest weight of 300 – 400g. can be achieved in 7 – 8 months under good conditions. Trout are classified as an oily fish and have excellent nutritional qualities. They are high in vitamins A and D and have high levels of omega-3 fatty acids that help protect us against heart disease. The flesh is pink with a delicate flavour that marries exceptionally well with natural herbs. It is also ideally suited to smoking with about 10% of the nations output being processed in this way. There are several home smokers available on the market that are well worth considering.
Trout are carnivorous fish and generally require high levels of protein in their diet, typically 45 – 55%. This does inevitably increase feed costs, the largest single cost for most trout farms. Much research is being carried out to reduce the reliance on natural fish meal stocks. Insect protein sources appear to offer much potential for the future.
Waste fresh fish used to be used as a feed. This is totally unsuitable for an aquaponic or recirculating aquaculture system on account of the water pollution that occurs. Dried pellets are definitely the way to go! Trout nutrition is now well understood and there are many excellent commercial feeds available. These lend themselves well to variety of automatic feeding systems.
In addition to high protein trout diets typically have a higher level of oil the omnivorous fish species such as carp and tilapia. The oil is cheaper than protein and is used to supply energy thus saving the protein for growth. Another significant benefit of using a commercial diet is that feed manufacturers produce feeding charts that given recommended ration sizes. This will vary according to water temperature and individual fish weight no matter what species you intend to grow. Below is a typical chart for trout. These are not intended to be strictly adhered to no matter what. It is always best to adapt ration size in the light of fish behaviour.
|35 – 100g||0.82||0.92||1.11||1.34||1.62||1.96||1.59|
|100 – 200g||0.61||0.74||0.89||1.08||1.30||1.57||1.27|
|200 – 300g||0.55||0.66||0.80||0.97||1.17||1.42||1.15|
|300 – 400g||0.51||0.62||0.75||0.91||1.10||1.33||1.08|
|400 – 500g||0.49||0.59||0.71||0.86||1.04||1.26||1.02|
|500 – 750g||0.46||0.56||0.67||0.81||0.99||1.19||0.97|
|750 – 1000g||0.43||0.52||0.63||0.76||0.92||1.11||0.90|
Table 1: Recommended Feed Rations for Trout as %BWD
Note the above table is based on maintaining the best feed conversion possible. Slightly higher growth rates can be obtained if the ration is increased slightly.
Trout tank design
Trout thrive in relatively high water flows. This helps ensure that wastes are readily flushed away and oxygen levels are maintained at an adequate level. They do not take well to warm, static ponds that carp, tilapia and catfish inhabit. Provided high flow rates of good quality water are provided, (minimum 30 – 60 minute retention time) trout can be kept at stocking densities approaching 60 – 80 kg/m3. This however, brings other risks and a requirement for a failsafe power supply. I would recommend that the home trout producer does not exceed 20 kg/m3.
Farm gate prices for table fish tend to be £2.10 – £2.40 /kg. (2012) Supermarket prices for whole trout, on the other hand, are between £5 – 6 /kg (Tesco, 2019)
As previously stated feed costs tend to play a significant role in the economics of a trout farm. For this reason careful stock management and accurate automatic feeders are essential to optimise food conversion rates. These can be as low as 1.2 – 1.4 under ideal conditions.
10g all female rainbow trout are available (2019) for around £100 per thousand. Expect to play more for smaller quantities and delivery. Triploids attract a 30 – 40% premium. Brown trout in the range of 3″ – 5″ are likely to cost £60 per hundred.
The UK government, quite rightly, demand that aquaculture and aquaponic systems be registered. The live movement of fish is also strictly controlled.. It is important that purchasers of live fish familiarise themselves with the legal requirements of fish movements. They are in place to help protect all fish stocks from the spread of disease. The regulations are not difficult to comply with and cost very little, if anything. Reputable fish suppliers will be only too pleased to offer advice and assistance as they recognise just how important this aspect is.