Farming of Brown Trout - The basics

Introduction to aquaculture
Ever since the over fishing of the oceans was fully realised, fish culture has increased dramatically all over the world. Different variations of farming methods can be employed and these depend wholly on the species of fish being produced. Systems range from open or non-intensive, involving little investment or labour to high intensity re-circulation systems, needing increased labour and high capital costs. Each system has their place in aquaculture either supplying food or stockfish to the community.
The Brown Trout
The brown trout (Salmo trutta) is part of the salmonidae family with its closest relatives being the brook trout (Salvelinus fontinalis) and the sea trout (Trutta trutta). Trout are carnivorous and feed actively on insects and small fish, which include their own young. They are found naturally in streams, rivers and lakes usually in the shallow areas where feeding is easiest.

 


Figure 1: The brown trout (Salmo trutta) Source: Phillips et al.(1985), p.29

Their commercial importance is far from that of the rainbow trout (Onchorhynchus mykiss) which is farmed heavily throughout Britain and Europe for food. Although the brown trout is rarely farmed for the table there are other important reasons for producing this species. With pleasure angling being one of the largest participant sports in the United Kingdom, the process of restocking will create a demand for brown trout throughout the country. Recent figures show 74 sites in Great Britain produced approximately 478 tonnes of re-stockers and around 479000 fry per year (Weeks 1998). There are subtle differences between the production of brown and rainbow trout. The brown trout is used for restocking therefore the emphasis is on quality not quantity and damage drastically reduces the value of the product. Different rearing systems are used to try and combat this problem commonly involving lower stocking densities.
Site Selection
Many parameters need to be considered before the production of trout can begin. The most important factor is the quality of the water available to the site. Oxygen demands, carbon dioxide, ammonia, suspended solids, pH and temperature levels need close continuous assessment on any aquaculture facility (Meaden, 1987).

Oxygen levels - The trout thrives on high dissolved oxygen levels therefore running water is essential in its production. The brown trout extracts 80% of the dissolved oxygen that passes across its gills and a minimal requirement of a 6 ppm saturation level is needed (Stevenson 1987).

Temperature - There is a direct relationship between dissolved oxygen and temperature with large fluctuations creating serious problems. The ideal temperature for growing trout is around 15°c, which holds a 9.96-ppm oxygen solubility level, but temperatures exceeding 24°c are lethal.

P.H. Levels - Around neutral or just above are ideal but low levels must be avoided as problems with reproduction can occur.

Nitrogen - High levels can lead to gas bubble disease, a common problem in hatcheries with bore hole water supplies. Other forms such as ammonia (NH3) can be early indicators of industrial pollution.

Suspended solids - These can create the minor problem of gill clogging and slow growth rates caused by visibility problems when feeding.

Flow rates - Fast flow rates will aid aeration, but larger energy requirements are needed if the flows are too fast, leading to poor growth.

Water source - Upstream sites are favourable as fish diseases and pollutants are easily carried downstream. Bore hole or spring supplies are commonly used in hatchery systems due to the constant temperatures provided.

Size - This has no major bearing as many new farming techniques utilise the space provided.

The market - Road access and distance to customers need great consideration when choosing a site.

Rearing Techniques

Hatchery systems
Eggs can be purchased from specialist suppliers or alternatively they can be produced on site. Many small farms will produce their own with selected broodstock and the art of successfully spawning and hatching eggs is very skilled. As light is detrimental to ova and small fry the majority of hatchery units are undercover, preferably in a brick building.

Egg production usually takes place inside plastic troughs with an average size of 3.0m x 0.5m x0.2m (Stevenson 1987). The water will flow directly through the trough and out of the hatchery. Situated in these troughs are egg baskets, where the ova are set (Fig 1). The water levels are adjusted by pipes on the outflow to ensure safe depths, so as not to crush the eggs. The supply of water should be as clean as possible because excess suspended solids will cover and suffocate the eggs, 100% oxygen content is advised. Bore hole or spring water is ideal as it commonly consists of both parameters. Flow rates of 1litre/minute per 3000 eggs to 1litre/minute per 1500 eggs will usually be needed with low oxygen levels resulting in the deformities of the trout later on in life (Sedgwick 1990).

 


Figure 2: Horizontal plastic trough
Source: Stevenson (1987), p.88
It is important to remove dead eggs, as they attract disease, with fungus being the most prominent. Dead eggs are white in colour and can be removed with the help of pipettes.

Space can be difficult to find in hatcheries so incubation systems are employed to combat this problem. Instead of horizontal troughs, vertical stacks of trays can be used commonly refereed to as the Heath system. The trays are stacked on top of each other with the water being allowed to filter through them all. These systems can be used in conjunction with re-circulation units as long as the oxygen levels are monitored. It is possible to use this technique, as there are no excretory products from the eggs. Flow rates of 45m3/day is needed for a 16 tray Heath system (Sedgwick, 1990).

  The third method applied in hatcheries involves the use of glass silos. The water is pushed upwards and out of the top of the jar. The continued movement of eggs in silos is essential to avoid the growth of fungus; therefore upwelling techniques have to be used. At 10°c the brown trout eggs will take approximately forty-one days to hatch and a further 20 days before the swim up stage (Stevenson 1987).

Figure 3: Incubation jars Source: Photograph taken whilst visiting Alenbrook trout farm, Verwood, Dorset (1990).
Fry Production
Once the alvins have absorbed their yolk sac they become known as fry and are still present in the egg trays. When swim up occurs it is important to encourage feeding immediately so moving fry at this stage could be disastrous. The water level is increased so the egg tray can be removed leaving the fry in the horizontal troughs. Figure 4 shows a pair of troughs in the hatchery unit at Bibary trout farm. The water can be seen to be black with fry at a size of approximately 150 to the pound. The water entering the troughs needs to be checked on a regular basis, the screens need cleaning and the dead fish and unused food removing. The fish are still fairly weak at this stage and any fluctuations in conditions could cause heavy losses.

 


Figure 4: Fry in troughs once the egg trays have been removed. Source: Photograph taken whilst visiting Bibary trout farm, Gloucestershire (1991)
The fry are fed with the smallest available food known as crumb, this will take place twice a day. At this stage of production regular batch weighing should take place and average weights gathered. This is important so accurate feeding rates can be calculated as overfeeding is costly and creates an unhealthy environment for the fry.
Fingerling Growing on Systems
When the fry are removed from the hatchery they are allocated tanks as their new home. Although tanks can be constructed of steel, corrugated iron or concrete brown trout grown for the purpose of restocking will be put into fibreglass tanks. These tanks are the kindest of the growing systems, as sharp or rough edges do not occur. The fingerlings will usually go through two moves whilst they are young. Figure 5 shows the first stage of movement encountered by the young. These are tanks in the shape of large hatchery troughs, which can be managed easily and effectively. Many other species involved with aquaculture are released straight into ponds or small lakes for their next stage of growth. Brown trout are susceptible to a protozoan infection known as whirling disease, which is found in earth ponds and has a tendency to effect young fry before their bones have hardened. The use of clean fibreglass tanks lowers the risk of infection from this disease.

Once the fish are a sufficient size they can be moved into circular fibreglass tanks where the growing process is continued.

Earth Ponds, Design and Use
Through out the world many methods are used for the on growing of large fish. Popular systems include corrugated tanks and concrete raceways, but these methods are more commonly used in the table farming of rainbow trout. Dues to the quality expected when producing fish for restocking these methods are found to be unsatisfactory as scale loss and fin damage are common occurrences.

The earth pond system is ideal in the production of fish and has been used for centuries; the original users being the monks when growing carp (Cyprinus carpio) for food. The preference of the farmer or the space that is available at the site dictates the size of the earth ponds, but the larger they are the difficulty of management will increase. The soil must have good water holding qualities, as alternatives such as butyl liners are costly. The standard size is approximately 30m x 10m with a depth of 1m at the inlet tapering to 1.7m at the outlet, which creates a holding capacity of 1.5 tonnes of trout (Sedgwick 1990).

The site should be level and have a high water table, enough room should be left between the ponds for vehicle access. Deed areas with low dissolved oxygen can be created in the corners of the pond therefore aeration at the inlet preferably by natural means should reduce these problems. If this method is not feasible the dissolved oxygen levels can be supplemented by the pumping of air or liquid oxygen directly into the water. Figure 6 shows a small earth pond being used to grow on brown trout. Note the unusual square ends of the pond. The ideal pond will have tapering rounded walls, which aid the use of seine nets as a fish removal technique.


Figure 5: Fibreglass fry tanks Source: Photograph taken whilst visiting Bibary trout farm, Gloucestershire (1991).


Figure 6: An earth pond being used to grow brown trout for restocking. Source: Photograph taken whilst visiting Allenbrook trout farm, Verwood, Dorset (1990)

Feeding
Many factors have been taken into consideration, but the main aim of trout farming is the growing of fish and this can only be achieved by feeding. The fastest transformation from fry to sailable size fish is done with the aid of artificial high protein fish diets. These can be bought in various sizes from fine crumb to 12 mm pellets with the option of slow sinking or floating varieties.

Accurate feeding levels are important because incorrect amounts will effect the food conversion ratio and create unneeded costs for the farm. The size of the fish, water temperatures and dissolved oxygen levels will influence the food intake. This is the reason why food producers will supply their own recommended feeding charts.

Clock work feeders, hoppers or demand feeders can be used to reduce the man hours involved, but hand feeding is still the most commonly used method. If the food is weighed correctly each morning hand feeding becomes the most reliable technique and includes the added bonus of regular stock assessment as early detection of a health problem can be observed if fish suddenly stop feeding.

Other Fundamental Equipment
Nets - Various nets are required during the process of growing trout. Fine meshed plastic fry nets are commonly found in the hatchery and larger wooden or glass fibre handled models are used for upwards of fingerling size. Seine nets may be required to extracted fish from large earth ponds, these must have knot less mesh otherwise damage could be caused in the form of scale loss.

Scales - All farms will use scales for sample weighing exercises and the pre-loading of delivery tanks.

Graders - Fish will be continually graded throughout production to keep consistent sizes together. Mechanical or bar graders can be used but hand grading on wet trays is an effective method and is kinder to the fish

Disinfectant - The use of disinfectants are essential in hatchery and fry units with the most common varieties being iodophors used in conjunction with eggs and benzalkonium chloride as a general site cleanser (Sedgwick,1990)

Disease treatments - Other common chemicals will include chlorime t, copper sulphide, formalin, malachite green and oxyolinic acid (Sedgwick, 1990). All of these are general treatments used to combat the various diseases encountered.

References
Meaden, G. (1987). Where should trout farms be in Britain. Fish farmer, 10, 2, 33-35.
Phillips, R., and Rix, M. (1985). Freshwater fish of Britain, Ireland and Europe. Pan books, Farnham.
Sedgwick, S.D. (1990). Trout farming handbook. Fishing news books, Oxford.
Stevenson, J.P. (1987). Trout farming manual. Fishing news books, Oxford
Uk Trout Farming. (1999). Http//www.homeusers.prestel.co.uk/shegog/trout.htm
Weeks, J. (1998). The 1997 survey of trout production in England and Wales. Trout News, 26, 7-16.

 

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