Short profile

FishEthoScore of the species

Abbreviated assessment of the species' likelihood and potential for fish welfare in aquaculture, based on ethological findings for 10 crucial criteria.

Criteria Li Po Ce
1 Home range
2 Depth range ?
3 Migration ?
4 Reproduction
5 Aggregation
6 Aggression
7 Substrate ?
8 Stress
9 Malformation
10 Slaughter
FishEthoScore 0 1 4
Li = Likelihood that the individuals of the species experience welfare under minimal farming conditions
Po = Potential overall potential of the individuals of the species to experience welfare under improved farming conditions
Ce = Certainty of our findings in Likelihood and Potential
                    ?     /  
  High    Medium     Low     Unclear  No findings
FishEthoScore = Sum of criteria scoring "High" (max. 10)

General remarks

Oncorhynchus mykiss is one of the dominant freshwater salmonids farmed in Europe and North America. In addition, it is one of the most widely studied model fish species in the wild and in captivity. Yet, the living conditions and the husbandry systems that maximise the welfare of this species are still to be defined, developed and improved. This lack is quite incomprehensible, given the background and the availability of research performed on this species. The low FishEthoScore is mainly due to the dependence of fish in the diet, need of space, high levels of aggression, needs of substrate, stress under farming conditions and high levels of deformations. In addition, anadromous fish experience changes in morphology, behaviour and environmental requirements through their life history. Thus, husbandry systems and practices need to take such differences into account in order to achieve and maintain higher welfare standards throughout the life cycle of the cultured fishes. The development of new rearing strategies to optimise the husbandry practices, handling with special care and the establishment of a slaughter protocol would be a step forward to solve some specific welfare concerns. Finally, providing feed which does not contain any fish components from wild catch has proven feasible for this species in lab studies, so a protocol for application in farming conditions has to be developed.

1. Are minimal farming conditions likely to provide the home range of the species? What overall welfare potential can be achieved? How certain are these findings?


Eggs: WILD: deposited in redds [1]. FARM: trays, tanks: 40-50 cm x 4 m [2], 10,000 eggs/0.2 m2 [3].

ALEVINS and FRYWILD: salmonids move short distances from the redd [4]. Further research needed to determine whether this applies to Oncorhynchus mykiss as well. FARM: round tanks: 2 m in diameter [2]; tanks: 2 x 2 m [2], 0.5-1.4 m[5].

PARR and SMOLTS: WILD: usually 0-3 km [6] [7] [8] [9]. FARM: raceways and ponds: 2-3 m x 12-30 m [2]; 4-25 m3 [5]; cages: 6 x 6 m [2], 16,000-130,000 m3 [10].

ADULTS: usually 1-15 km [6] [11] [12] [13] [8]FARM PARR and SMOLTS.

SPAWNERS: usually 1-15 km [14]FARM tanks: 1m[3].

2. Are minimal farming conditions likely to provide the depth range of the species? What overall welfare potential can be achieved? How certain are these findings?


Eggs: WILD: deposited in redds [1]. FARM: trays, tanks: 20 cm [2], 50 cm [3].

ALEVINS and FRY: WILD: salmonids move short distances [15] [16]. Further research needed to determine whether this applies to Oncorhynchus mykiss as well. FARM: tanks: 50-60 cm [2], 0.8-1m [3] [5].

PARR and SMOLTS: WILD: usually swim 0-5 m deep [17] [18] [19]. FARM: raceways and ponds: 1-1.2 m [3] [2]; cages: 4-5 m [2], 40 m [20],10-50 m [10].

ADULTS: WILD: usually swim 0-20 m deep [17] [11] [18]. FARM PARR and SMOLTS.

SPAWNERSWILD and FARM: no data found yet.

3. Are minimal farming conditions compatible with the migrating or habitat-changing behaviour of the species? What overall welfare potential can be achieved? How certain are these findings?


Two populations: steelhead trout ANADROMOUS [13] [21] [14] move from fresh water to seawater. Rainbow trout POTAMODROMOUS [13] stationary, stay permanently in fresh water, 

Steelhead trout population:

ALEVINSFRY and PARRWILD: stationary [2] [1], remain in steams and rivers [22] [23]

SMOLTSWILD: migration to sea [24]FARMEURYHALINE > 50g, 70-100g good survival rate at the sea [25]. Brackish and saltwater cages 20-34 ppm [10].

ADULTSWILD: return as GRILSE to natal streams to spawn [24] [23]SPAWNERSFARM: 10-17 ‰ [26].

KELTWILD: return to sea and spawn again in streams [24] [23]

Rainbow trout population:

WILD: All age classes stationary, except adults move upstream to spawn [11].

All age classes: further research needed on welfare parameters to determine whether presenting species with conditions of different migratory phases indeed satisfies their urge to migrate or whether they need to experience the transition.

4. Is the species likely to reproduce in captivity without manipulation? What overall welfare potential can be achieved? How certain are these findings?


WILD: spawning occurs from November until May in the Northern hemisphere and from August to November on the Southern hemisphere [27] [2]Males perform aggressive courtship displays [28] [3]. Female builds redd [29] [17] [30]. FARM: do not spawn naturally, eggs and milt are stripped [2] [3].


5. Is the aggregation imposed by minimal farming conditions likely to be compatible with the natural behaviour of the species? What overall welfare potential can be achieved? How certain are these findings?


ALEVINS and FRY: WILD: territorial, establish social hierarchies [31]. LAB: territorial, establish social hierarchies [32] [33] [34] [35]FARMFRY: 2,000 to 5,000 fry/m[3].

PARR and SMOLTS: WILD: territorial, establish social hierarchies [36]. FARM: stocking densities: tanks: 40-265 kg/m[37] [38]; raceways: 8-160 kg/m[38]; cages: 30-40 kg/m[2]. Low and high stocking densities affect welfare [37] [38] LAB: stocking densities 10-80 kg/m3, low and high stocking densities affect welfare [39].

ADULTS: WILD: salmonids live in schools during migration [40] [41] [42]. Further research needed to determine whether this applies to Oncorhynchus mykiss as well. FARM PARR and SMOLTS.

SPAWNERS: WILD:  ADULTS. FARM: rearing at low densities with unknown extension [2]

All age classes: FARM: wide variation in recommended stocking densities 2-80 kg/m3, in Europe and North America commercial farmers use normally 15-40 kg/m3, maximum observed at 60 kg/m[38] [10].

6. Is the species likely to be non-aggressive and non-territorial? What overall welfare potential can be achieved? How certain are these findings?


ALEVINS and FRY: higher levels of aggression in wild than under farming conditions [31] [33]. FARM: no food competition at stocking density 9.9-37.6 kg/m3 [32]. LAB: individuals with larger yolk sacs [34] and quick emergence from spawning gravel are more aggressive [35] [43].

PARR and SMOLTS: FARM: more aggressive with feeding schedule compared to free access regime [44] [45]LAB: aggressive when establishing dominant-subordinate relationships [46] [47] [48] [49] [50] [51].

ADULTS: LAB: aggressive when establishing dominant-subordinate relationships [48].

SPAWNERS: FARM: males performs aggressive courtship [3].

For all age classes, no data found yet on aggression behaviour in the wild.

7. Are minimal farming conditions likely to match the natural substrate and shelter needs of the species? What overall welfare potential can be achieved? How certain are these findings?


Eggs, ALEVINS and FRY: WILD: successful incubation and emergence dependent on gravel characteristics [52] [30]FARM: use artificial hatching substrate [3] [5]LAB: emergence from spawning gravel used to distinguish distinct stress coping styles and growth performances [35] [43]. The use of gravel reduce fin erosions [53]

PARR and SMOLTS: WILD: use gravel, stones and boulders as shelters [17] [22] [54] [55]FARM: rearing in earthen-bottom ponds enhances physiology [56] and increase survival when transferred to seawater [57]LAB: cobble substrate reduce fins erosions [58].

ADULTS: ➝ PARR and SMOLTSFARM: rearing in earthen-bottom ponds increase survival when transferred to seawater [57]

SPAWNERS: WILD: use substrate to build redds [17] [30]FARM: no data found yet. 


8. Are minimal farming conditions (handling, confinement etc.) likely not to stress the individuals of the species? What overall welfare potential can be achieved? How certain are these findings?


ALEVINS and FRY: stressed by acute handling [59] and confinment [60].

PARR and SMOLTS: stressed by repeated handling [61] [62] [63] [64], confinement, crowding [65] [66], group hierarchies [67] [48] [49] and transport [68].                                     

ADULTS: stressed by confinement [69] [65], repeated handling [70] [48] [71], group hierarchies [69] and transport [68].

SPAWNERS: stressed by handling [72]; identified quantitative trait loci (QTL) when stressed by crowding [73].

9. Are malformations of this species likely to be rare under farming conditions? What overall welfare potential can be achieved? How certain are these findings?


ALEVINS and FRY: malformations of spine [74] in >15% of individuals.

PARR and SMOLTS: malformations of spine [75] [76] [77] [78] [79] in >10% of individuals.

ADULTS: malformations of spine [76] [77] [79] in >10% of individuals.

For all age classes, no data found yet on frequency of malformations in the wild.


10. Is a humane slaughter protocol likely to be applied under minimal farming conditions? What overall welfare potential can be achieved? How certain are these findings?


Common slaughter method: no data found yet. High-standard slaughter method: indications that electrical stunning before killing by chilling or bleeding is most effective [80] [81] [82] [83]. Percussive stunning before killing by chilling or bleeding is most effective in larger trout sizes [82].

Side note: Domestication

DOMESTICATION LEVEL 5 [84] [85], fully domesticated. Cultured since late 19th century [2].

Side note: Feeding without components of forage fishery

ALEVINS and FRY: WILD: carnivorous [2] [1]. FARM: fish meal and fish oil may be completely* replaced by plant protein [86] [87].

PARR and SMOLTS: WILD: carnivorous [2] [1]. FARM: fish meal and fish oil may be mostly* replaced by plant protein [88] [89] [90] [91]. Further research needed to clarify the feasability of complete* replacement.


SPAWNERSWILD: carnivorous [2] [1]. FARM: fish meal and fish oil may be completely* replaced by plant protein [87] [92].

*partly = <51% – mostly = 51-99% – completely = 100%


ADULTS = mature individuals, for details Findings 10.1 Ontogenetic development
ALEVINS = larvae until the end of yolk sac absorption, for details Findings 10.1 Ontogenetic development
ANADROMOUS = migrating from the sea into fresh water to spawn
DOMESTICATION LEVEL 5 = selective breeding programmes are used focusing on specific goals [84]
EURYHALINE = tolerant of a wide range of salinities
FARM = setting in farm environment
FRY = larvae from external feeding on, for details Findings 10.1 Ontogenetic development
GRILSE = adults returning from sea to home river to spawn, for details Findings 10.1 Ontogenetic development
KELT = adults surviving spawning, for details Findings 10.1 Ontogenetic development
LAB = setting in laboratory environment
PARR = juvenile stage in rivers, for details Findings 10.1 Ontogenetic development
POTAMODROMOUS = migrating within fresh water
SMOLTS = juvenile stage migrating to the sea, for details Findings 10.1 Ontogenetic development
SPAWNERS = adults that are kept as broodstock
WILD = setting in the wild


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