By Asbjørn Bergheim*
Brown trout is the freshwater form with typical habitat in rivers and lakes, while the anadromous form, Sea trout, is found spawning in freshwater, migrating to sea in the spring and returning to its native river in autumn. This trout is native in most European countries and in parts of the River Volga in Western Russia and has been introduced into highland regions of South and North America, Oceania, Japan, India, Pakistan, and some other countries (www.iucnredlist.org/details/19861/0). Compared to other trout species, brown trout is rather tolerant to fluctuating temperature; a main reason why it was possible to transfer it to other parts of the world even 150 years ago.
Sea trout and brown trout are, as indicated, the same species but a combination of genetics and environmental factors (principally lack of food), will mean that some trout in rivers along the coast will go to sea to feed before returning to spawn. The stay at sea commonly lasts for less than three months. It is impossible to distinguish residential and migrating individuals as fry and juveniles in freshwater and they might even originate from the same parents (www.fagrad.com/). Going to sea gives the trout a much larger source of food and sea trout will nearly always be bigger than the resident trout of the same river. In less nutrient-rich rivers, such as most Norwegian coastal rivers, sea trout are mostly females.
Lacustrine brown trout migrate from lakes into rivers or streams to spawn. Older big trout in lakes often feed on fish and may reach large sizes, such as the reported state record in Michigan of 18.8 kg (caught in a tributary of Lake Michigan in 2009).
Salmo trutta is rarely farmed for the table, but there is a considerable production of fry for re-stocking rivers and lakes in many countries. As an example, 74 farm sites in the UK produced around half a million fry and 500 tons of re-stockers annually in rivers to motivate trout anglers (http://www.fao.org/fishery/culturedspecies/Salmo_trutta/en). River Lærdalselva is a famous salmon river which reaches the sea in the inner part of the long fjord system, Sognefjorden, in Western Norway. This river also attracts trout anglers. According to the FAO, brown/sea trout is probably the first species of fish for which artificial reproduction was performed. This probably occurred in Germany around 1739 and the first sea trout hatchery was established in 1841 in the UK.
FAO’s production statistics include all morphs of S. trutta. In 2010, around 80% of the global brown trout production in aquaculture took place in Russia, mainly in freshwater. The other recorded producers are primarily European countries, but even Kenya and Zimbabwe are on the list. Compared to the dominating salmonid species in aquaculture, the global production of brown and sea trout is modest and below 5,000 tons per year. Since 1990, the annual production volume has been at the same level.
Though most of the brown trout farming today takes place in freshwater, the future aquaculture potential seems to be more focused on cage farming in sea. Fjord localities which are protected against wind and waves could be actual sites for sea trout farming. Sea trout has a potential advantage as it can tolerate higher summer temperature than salmon (up to 18 – 20 °C). In the long term, the global warming may favor sea trout farming again (FAO’s report).
At present, farming of sea trout cannot compete with high-priced salmon. Marine trout farming only constituted around 1% of the total global trout production of 5,000 tons in 2010 (https://aquatrace.eu/documents/80305/142567/brown+trout+leaflet.pdf). In saltwater, growth rates of brown trout are reported to be similar to those of Atlantic salmon and Rainbow trout. However, sexual maturation in brown trout strongly reduces growth, increases mortality and reduces the fillet quality. Maturity is considered a serious constraint to commercial production and options to minimize early maturation should be emphasized. The potential of brown trout for mariculture seems to be connected to introduction of female monosex material and/or sterile all-female material in order to reduce the maturation problems.
The mentioned usage of female brown trout for aquaculture has been ‘dwelling in backwater’ for a long period – there seem to be no available reports dealing with these issues since 1992. Undoubtedly, mariculture of brown trout will attract interest sooner or later and hopefully it will become a competitive species.
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.