Rainbow Trout

Oncorhynchus mykiss

Sources, Quantities and Cultivation Methods

Sources and Quantities

Rainbow trout are named so because of the iridescent pink band along the flank of the fish, most evident in live animals1. The species was originally classified as Salmo gairdneri within the same family as Atlantic salmon (Salmo salar) and brown trout (Salmo trutta, also known as ‘sea trout’; to be distinguished from sea-grown rainbow trout). However, in 1989 rainbow trout was reclassified as Oncorhynchus mykiss, and grouped with the Pacific salmon species (pink, sockeye, chinook, chum, coho and masu), reflecting its Pacific coast of North America geographic origin2. Like brown trout, rainbow trout may remain in freshwater throughout their life, or migrate to sea before returning to spawn. The migratory strain of rainbow trout is known as ‘steelhead’ in North America3.

Rainbow trout has been cultured since the late 19th century1, making it one of the species with longest presence in modern aquaculture4. As a result, it is often considered domesticated, having been selectively bred for many generations5. Rainbow trout have been spread globally for farming and angling, and are now present in eastern North America, Europe, South America, Africa, Asia and Australasia6. Because of its temperate origins and optimum temperature for farming of below 21°C1, in low latitude countries it is usually farmed in mountainous regions6.

Commercial rainbow trout farming took off in the 1950s in Denmark3, 4 and has heralded the global growth of aquaculture. It has been termed “the farmers’ fish3 due to ease of breeding and feeding, fast growth and tolerance for a wide range of environmental conditions and handling1, 7. Rainbow trout can be farmed in a variety of systems, in both freshwater and seawater. In addition to being grown for consumption, the fish are valued for their sporting quality and farmed trout are commonly stocked into closed waters for angling1.

Rainbow trout are harvested year-round at both portion size (400 g) and large (1.5-5 kg) sizes1, 7. The species is valued as an ‘oily fish’ due to high omega 3 content of the flesh7. In addition to being retailed fresh, fillets of rainbow trout (usually large size) can be smoked, similar to salmon filets. Trout harvested in the UK typically have pink flesh, whereas both white- and pink-flesh fish are produced in other parts of the world1, 7. Salted trout eggs are sold as a form of caviar, especially in northern Europe7.

The global production of farmed rainbow trout has increased from 4,400 tonnes in 1950 to around 800,000 tonnes per year in recent years8, as shown in the graphic. There is, however, an indication that global production peaked in 2012 and has declined subsequently.

Global Rainbow Trout Production: Key Locations and Volumes

Production of farmed rainbow trout in 2016 was reported from 75 countries, totalling 814,091 tonnes. The bulk of that was grown in freshwater systems (75%), with the remainder (25%) in net-pens placed in marine and brackish waters. Most of the growth in the global production of rainbow trout in the last two decades has come from expansions in the trout industries of Iran and Turkey (predominantly freshwater culture). These two countries accounted for 43% of the total amount of trout produced in freshwater in 2016. In turn, Norway and Chile were the leading producers of marine-grown trout, benefiting from sharing production technology with the well-established Atlantic salmon farming industry in those countries. The UK is the 14th largest producer of rainbow trout, with 27% of its harvest coming from marine net-pen systems.

Domestic Market Information9, 10

From 2008 to 2017 trout has declined in value and volume in Great British retail (i.e. in England, Scotland and Wales), by -18.8% and -45.6% respectively from a base of £44.2 million and 4,989 tonnes in 2008.

In 2018, UK retail sales of trout were worth £39.1 million (+9.7% compared to the previous year) with a volume of 2,983 tonnes (-0.4%), average price £13.12 per kg; ranking as the 13th most popular species in the 52 weeks to 16/06/2018 (including discounters).

In 2018, the UK imported 1,327 tonnes of trout.

Production Method

The farming process starts with reproduction. Rainbow trout will mature but not spawn naturally under farm conditions. Broodstock are, therefore, “stripped” of eggs and milt by applying manual pressure to the abdomen of ripe fish. The eggs and milt are then mixed. In order to either delay or prevent sexual maturation of fish destined for consumption, and thus avoid associated health, welfare and flesh quality problems, two techniques are used: ‘all-female’ and ‘triploid’1, 2.

  • ‘All-female’: female trout mature at a larger size than male fish. In order to obtain only female cohort of fish, eggs from normal female broodstock fish are fertilized with the milt from physically male broodstock that are genetically female, having been sex-reversed when fry by addition of male hormones to feed1. All the progeny grown-out to harvest therefore only have female chromosomes and have not been exposed to hormonal treatments1.
  • ‘Triploid’: stripped eggs are exposed to a ‘pressure shock’ which effectively doubles the number of maternal chromosomes; when fertilised by the male’s milt, the progeny have three sets of chromosomes instead of two, which results in sterility.

Such chromosome manipulations are not classed as genetic modification (or genetic alteration) as there is no deletion or insertion of DNA, and the processes could happen naturally. Chromosome manipulation has the additional benefit of preventing reproduction, should fish escape into the wild1. Almost no mixed sex rainbow trout are farmed in Scotland nowadays, with ‘all-female’ and ‘triploid’ fish constituting 84% and 10% of production respectively11.

The almost universal application of chromosome manipulation techniques, coupled with selective breeding for desired traits such as growth rate, disease resistance and flesh quality1, means that most rainbow eggs are now produced at centralised specialist broodstock sites. Eggs are then transported to be “laid down” to hatch in hatcheries. For example, in Scotland, 92% of rainbow trout eggs in 2017 were imported - from Northern Ireland, Denmark, USA, Norway and the Isle of Man11.

Freshwater Trout Production Schematic (excludes seawater production)

Eggs are incubated in wire baskets in the dark at a stable temperature, typically in flow-through troughs1. When they hatch the alevin (yolk-sac fry) drop through the mesh and lie on the trough bottom whilst absorbing their yolk-sac. Once the yolk-sac food reserve is exhausted they become “swim-up fry”, actively swimming into the water column to feed1. The fry are fed formulated feeds, increasing in particle size from an initial crumb to larger pellets as they grow. The size of rearing systems also increases with fish typically moving from the indoor troughs and tanks of the hatchery to larger outdoor tanks and raceways of the nursery. Further growing-out to harvest continues outdoors in larger tanks, raceways and Danish (earthen) ponds. Fish can also be grown-out in floating net-pens in freshwater lakes/lochs. Farmed fish are graded to reduce size variation which aids stock management, enables appropriately sized pellets to be fed and reduces size-hierarchies12. Grading is usually done 4 times at 2-5g, 10-20g, 50-60g, and >100g1, 7. The grow-out phase to portion size (400g) takes about 9 months1.

Like other salmonids, rainbow trout require a high-quality water supply. Freshwater supplies for hatcheries and nurseries are often sourced from springs or groundwater as these are clean, pathogen free and stable in temperature. Grow-out units typically divert water from rivers or lakes as a freshwater supply, using the slope of the land to ensure adequate water flow. The flow-through of water provides the fish with a current to swim against, supplies oxygen and removes wastes. Wastes include solid particles of faeces and uneaten food, and dissolved products of metabolism such as ammonia and carbon dioxide. Most farms will have systems in place to reduce the discharge of wastes, e.g. settlement ponds.

When grown-out in freshwater, rainbow trout are commonly produced in intensive flow-through monoculture systems1. Freshwater resources are often limited, and flow-through farms typically reuse abstracted water by passing it through successive faming units, before it enters the settlement ponds, with larger fish receiving the most reused water12. Where water is reused, water treatment facilities (e.g. aeration, oxygenation, filtration) are often placed between farming units.

However, water can also be recirculated on the farm, which requires more treatment but has the advantage of minimising abstraction and waste discharge. For example, in Denmark there has been an investment in “model farms” converting flow-through rainbow trout farms to recirculation13.

Rainbow trout are also grown-out in seawater in floating net-pens akin those used for salmon. Rainbow trout do not go through the “smolt” development stage of salmon, and the physiological ability to cope with a transfer to seawater is based upon size. Farms growing rainbow trout in seawater benefit from higher growth rate, with fish transferred at around 70 g reaching 3 kg in less than 1.5 years1.

Rainbow trout are considered an efficient species to farm. The high-quality formulated feeds available and good feeding practices enable efficient use of feed resources1, with food conversion ratios (food weight: fish weight increase) as low as 0.8:1. They are farmed at higher densities than brown trout and Atlantic salmon, up to 80 kg/m3, reflecting a high system productivity14.

References

  1. FAO
  2. Smith GR, Sterly RF. 1989. The classification and scientific names of rainbow and cutthroat trouts. Fisheries. 14, 4-10.
  3. Sedgwick, DS. 1995. Trout farming handbook, 6th edition. Fishing News Books, Oxford, 164p.
  4. Gall, G.A.E, Crandell, P.A. 1992). The rainbow trout. Aquaculture. 100, 1-10.
  5. Donaldson, L.R., Olson, P.R. 1957. Development of rainbow trout brood stock by selective breeding. Transactions of the American Fisheries Society 85, 93-101.
  6. MacCrimmon, HR. 1971. World distribution of rainbow trout (Salmo gairdneri). Journal of the Fisheries Research Board of Canada. 28, 663-704.
  7. EC
  8. FAO FishstatJ
  9. AC Nielson
  10. HMRC
  11. Marine Scotland Science 2018.
  12. North, B.P., Ellis, T., Turnbull, J.F., Davis, J., Bromage, N.R. 2006. Stocking density practices of commercial UK rainbow trout farms. Aquaculture. 259, 260–267
  13. CEFAS
  14. Ellis, T., North, B., Scott, A.P., Bromage, N.R., Porter, M., Gadd, D. 2002. The relationships between stocking density and the welfare of farmed rainbow trout. Journal of Fish Biology. 61, 493-531.