Brown trout (Salmo trutta)
Marc Vandeputte (INRA, France)
1. General information on production and breeding
History of the domestication
Brown trout is perhaps the first species of fish in which artificial reproduction was performed. This was first done by Jacobi in Germany, probably around 1739, and the earliest brown trout hatchery was established in 1841 by Boccius in Hammersmith, UK . The technique of artificial fertilization was optimized by Coste in the 1850s . Since then, brown trout has been produced extensively in Europe and introduced to other continents; but it has never been specifically domesticated for food fish production, as the principal aim of brown trout culture has always been restocking of natural waters. Only briefly, at the end of the 1980s, was the culture of brown trout in sea cages considered as an alternative to salmon production in the French waters of Brittany . This led to the development of brown trout strains selected for fast growth .
No FEAP or FAO data is available on brown trout production.
Biological features of interest for breeding practices
Main commercial data on weight and age at harvest and slaughtering performances
For restocking, mostly eggs and fingerlings are used. A small amount of production is sold as food fish
, usually at portion size (250-350 g), which can be reached in 12 to 18 months post-fertilisation. At large sizes (2 kg or more) yields exceed those of rainbow trout: gutted yield 92% vs. 85%, trimmed and skinned fillet yield 55% vs. 39% (Bugeon et al, unpublished data).
Generation interval for males and females
Age at puberty is 1-3 years for males, 2-4 years for females.
Brown trout is affected by several bacterial diseases: it is highly sensitive to furonculosis (Aeromonas salmonicida ),and bacterial kidney disease (Renibacterium salmoninarum), but may also suffer moderately from yersiniosis (Yersinia ruckeri ), rainbow trout fry syndrome (Flavobacterium psychrophilum ), and vibriosis (Vibrio spp. ). A vaccine exists for furonculosis and vibriosis.
The main viral diseases affecting brown trout are VHS (Viral Haemorrhagic Septicaemia), IHN (Infectious Haematopoietic Necrosis) and IPN (Infectious Pancreatic Necrosis). The first two are diseases requiring notification of the authorities, according to the EU Council Directive 91/67/EEC defining Approved Zones free of such diseases. Brown trout are moderately susceptible to pancreas disease, caused by a salmonid alphavirus [7,8]
Brown trout may also suffer from fungal infections (Saproleignia spp), especially during the reproductive season when males are particularly sensitive, and parasitic infections by Costia spp. .
2. Genetic variability of the species
Wild genetic resources available (Figure 1)
As brown trout is the native species of trout in Europe, many wild populations of this species exist. Population genetics studies have identified five main lineages: Atlantic, Mediterranean, Adriatic, Danubian and the subspecies Salmo trutta marmoratus [9-12]. Resident and sea migrating ecotypes coexist (except in southern Atlantic and Mediterranean sea ), with little or no genetic differentiation between them [14-16].
Native distribution of brown trout (red) and distribution of sea-trout (green). According to Elliott 1994 .
Differences between wild and/or domesticated populations
The zootechnical comparison of five domesticated French strains with one domesticated Danish population, one wild French population from Brittany, and some crosses between these groups showed that the domesticated populations had better early survival rates and growth rates than the wild ones . Even among the domesticated fish, considerable variation for growth rate existed both in fresh water (≈50% of the mean) and in sea water (≈30% of the mean).
Interaction between wild and domesticated stocks
Interactions between wild and domesticated stocks have been extensively studied in brown trout, especially in the Mediterranean area, where restocking with Atlantic populations has taking place for a long time [17,18]. It appears in many cases that stocked trout, even if restocked at very young stages, are more vulnerable to angling than wild trout [19,20]. The admixture of domesticated trout in heavily restocked rivers may remain remarkably low, but this is highly variable among situations and in some cases high admixture proportions are observed [21,22]. Even when wild parents are used, reduction in effective population size appears to be common .
Fecundity and main reproductive features
Brown trout show an egg production of 1000 to 2000 eggs/kg female. Artificial fertilization is easy.
Cryopreservation protocols for brown trout sperm exist [24-26]. Thawed sperm give variable but sometimes very high fertilization rates (27-95%).
Genetic and environmental sex determination, sexual dimorphism
XX/XY sex determinism has been demonstrated by meiotic gynogenesis . Neomales may be produced by an early methyltestosterone treatment, but have non functional testes that make the use of intra-testicular sperm the only method possible for producing monosex female populations .
Genetic variability per trait
Brown trout was one of the first species in which variation of quantitative traits was studied, a study in 1919 demonstrated heritability of vertebrae number . However, since then, only a few traits have been extensively studied. Skin pigmentation shows good heritability, both for the number of black spots in the dorsal area (h²=0.40 ) and for the number of red spots (h²=0.67-0.73 [12,30]). Among other “exotic” traits, number of pyloric caeca , and swimming performance  have also shown some degree of genetic determinism. The genetics of growth rate has been studied through selective breeding, revealing a realized heritability around 0.3-0.5 , which is still quite variable between strains . Genetic variation for growth at the larval stage is mainly determined by maternal effects linked to egg size .
Genetic correlations and undesirable side effects
Genetic correlations with growth rate have been studied through the observation of correlated responses to selection for growth on processing yields and quality traits (, Vandeputte et al, unpublished data). These studies show little or no effects of selection for growth rate on carcass and fillet yield, flesh fat content or texture. No correlated response is seen on feed efficiency [36,37], but feeding behaviour is clearly altered by selection for growth [38,39].
Genetic responses, progresses and control lines
The “Prosper” program, an optimized mass selection procedure followed for four generations in France, caused a weight increase of 21.5% at 1 year of age in one line, when compared with an unselected control line from the same base population . In a second line however, the response was only 6.3% per generation. This may be a result of differing selection environments: if the one where the response was lower was more competitive, it could have led to the selection of more aggressive fish .
Dominance and intraspecific crossing
Crosses between wild and domesticated strains gave performances that were intermediate between the parental strains . Dominance appears to have a significant impact on larval growth (d²=0.34-0.39) .
5. Polyploidisation and monosexing and hybrids
Triploid induction and performances
Early trials produced either relatively low proportions of triploids  or low survival rates . Using a 28°C heat shock for 10 minutes led both to >90% triploids and hatching rates equivalent to the control . Triploid females exhibit no reduced growth compared with maturing diploid females, and have better carcass yields, 90.6 vs. 81.5% at 4 years in sea water , due to their total absence of maturation and gonad development.
Tetraploid induction and performances
Gynogenesis, androgenesis and mitotic clone performances
Only meiotic gynogenesis has been performed in brown trout, using the same shock as for triploid induction [27,42].
Diploid hybrids have been made with brook trout (Salvelinus fontinalis) and, though these performed well in fresh water , they were not suited to sea water , and were therefore never used in practice. Hybrids with rainbow trout are not viable in the diploid state, but can be produced as allotriploids [40,45]. Their performances are not however sufficient for them to be of practical use .
Tools to evaluate population genetic variability
Allozymes, mtDNA and microsatellites have been used to describe the genetic variability of brown trout (e.g. [9;10;23]) A SNP multiplex allowing the distinction of the five main lineages has also recently been produced .
Genetic markers for genealogical traceability
The number of microsatellites available (at least 288 polymorphic ones ) would clearly allow their usage for parentage assignment. However, no multiplex has been published, and no laboratories perform commercial parentage assignment for this species.
QTL and Marker Assisted Selection
A genetic map has been made with 288 microsatellites and 13 allozymes , but no QTL detection program has yet been set up in brown trout.
No transgenic brown trout have ever been produced.
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