By Asbjørn Bergheim*
Rainbow trout (Oncorhynchus mykiss) farming is going on in many parts of the world outside the species’ native region in western North America, such as in South America, many European countries, Japan, Oceania, the Far East and Africa. Altogether, more than 60 countries are represented in the trout production statistics. Though the natural habitat of rainbow trout is freshwater with around 12ºC in summer, it tolerates from zero to 25ºC and the anadromous strain – steelhead trout – makes runs to the sea.
There are two principal ways to raise trout in aquaculture: the freshwater based production in ponds/raceways with on-growing of 6 months to one year and harvest at 400 g – 1 kg (“small trout”), and on-growing in seawater cages to a harvest size of 3 – 5 kg (“large trout”). Trout transferred to seawater cages have faster growth rates and can reach more than 3 kg weight after 18 months from stocking as 70 g fingerlings.
Steelhead trout has no smoltification process such as salmon and arctic char, but they are so-called euryhaline, i.e. they can adapt to seawater by osmoregulation which controls the salt concentration in the blood. In a reported study, rainbow trout demonstrated higher growth performance in the high-saline Aegean Sea (36 ppt salinity) compared to in freshwater during on-growing from 200 to 600 g in winter.
The high growth potential of this species is clearly demonstrated in a recent test performed at the Freshwater Institute in West Virginia, U.S., where an average weight of 5 kg was reached 22 months after hatching in freshwater tanks at 13ºC (Fig. 1) Thus, cohorts of trout may grow faster in freshwater under controlled conditions than steelhead trout with grow-out in seawater.
The annual global production of O. mykiss amounts to around 600 thousand tons which is equally distributed between small and large trout. Chile and Norway are the biggest producers of large trout, while the freshwater production volume of smaller fish is dominated by Iran and several European countries such as Italy, France, Spain, Germany and Denmark. Idaho is the predominating producer in the U.S. In Canadian lakes, 7,000 tons are being produced every year in net pens.
The fastest growing trout producer over the last 20 years is actually Iran which has increased its annual volume from less than 1,000 tons in 1993 to 91,500 tons in 2010. Not least, the impressive increase in productivity from below 10 kg to 40 kg/m2 pond area during this period has contributed to this successful growth.
Danish trout in sea cages is an all-female production method based on two products, roe for export and meat from the stripped fish (carcass with reduced quality). Such combined production is more profitable than traditional harvest before maturation. In order to intensify the production and to reduce environmental effects, such as reduced water usage and effluent, the traditional freshwater systems (dambrug) are increasingly being converted to RAS farms with recirculation of water and various water treatment technologies.
According to Global Writes (www.global-writes.com), the world’s largest trout farm is located in Snake River Canyon in Buhl, Idaho. This farm produces some 11,000 tons of trout every year and at any given time there are around 10 million fish in the numerous raceways! This facility also runs a nearby processing plant. The water source is the enormous underground lake in this canyon supplying spring water at a constant temperature of 58 degrees F (14.5ºC).
Some 20 million trout are stocked in Norwegian cages every year. Trout farms are mainly located in fjords with brackish water towards the surface. The number of escaped trout fluctuates a lot from one year to another; only 200 escapees were totally reported in 2013, while as much as 133,000 trout escaped the year before. Damage of cages and nets in stormy weather and other harmful episodes are decisive factors. However, the behavior of the trout makes it possible to recapture a high portion as they normally dwell around the farm for several months. The main negative effects of escaped trout are the risk of spreading diseases and parasites, and these rainbow trout rarely breed in the wild.
This species tolerates high density in aquaculture without reduced growth and lower welfare standards. Probably due to numerous generations of domestication and the species’ unique adaptability, trout density tests with up to 100 kg of juveniles/m3 tank water – or around 1,000 individuals of 100 g size! – seem to be acceptable assuming optimal water quality. However, the commonly applied production rate in commercial farming is considerably lower.
No doubt, farming of rainbow trout is facing a prosperous future with increasing production volume in several parts of the world.
Dr. AsbjØrn Bergheim is a senior researcher in the Dept. of Marine Environment at the International Research Institute of Stavanger. His fields of interest within aquaculture are primarily water quality vs. technology and management in tanks, cages and ponds, among others.