Rainbow trout

Oncorhynchus mykiss

Oncorhynchus mykiss (Rainbow trout)
    • Osteichthyes
      • Salmoniformes
        • Salmonidae
          • Oncorhynchus mykiss
Distribution map: Oncorhynchus mykiss (Rainbow trout)


Author: Maria Filipa Castanheira
Version: 2.0 (2021-06-28) - Revision 1 (2022-07-20)


Reviewers: Pablo Arechavala-Lopez, Jenny Volstorf
Editor: Billo Heinzpeter Studer

Cite as: »Castanheira, Maria Filipa. 2022. Oncorhynchus mykiss (Farm: Short Profile). In: FishEthoBase, ed. Fish Ethology and Welfare Group. World Wide Web electronic publication. First published 2016-11-26. Version 2.0 Revision 1. https://fishethobase.net.«


Oncorhynchus mykiss
Home range
Depth range

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

Li = Likelihood that the individuals of the species experience good welfare under minimal farming conditions
Po = Potential of the individuals of the species to experience good welfare under high-standard farming conditions
Ce = Certainty of our findings in Likelihood and Potential

FishEthoScore = Sum of criteria scoring "High" (max. 10)


No findings

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  Home range

Many species traverse in a limited horizontal space (even if just for a certain period of time per year); the home range may be described as a species' understanding of its environment (i.e., its cognitive map) for the most important resources it needs access to. What is the probability of providing the species' whole home range in captivity?

It is low for minimal farming conditions. It is medium for high-standard farming conditions. Our conclusion is based on a high amount of evidence.


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 m5.

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 8FARM PARR and SMOLTS.

SPAWNERS: usually 1-15 km 14FARM tanks: 1m3.

2  Depth range

Given the availability of resources (food, shelter) or the need to avoid predators, species spend their time within a certain depth range. What is the probability of providing the species' whole depth range in captivity?

There are unclear findings for minimal farming conditions. It is medium for high-standard farming conditions. Our conclusion is based on a medium amount of evidence.


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  Migration

Some species undergo seasonal changes of environments for different purposes (feeding, spawning, etc.) and with them, environmental parameters (photoperiod, temperature, salinity) may change, too. What is the probability of providing farming conditions that are compatible with the migrating or habitat-changing behaviour of the species?

There are unclear findings for minimal farming conditions. It is medium for high-standard farming conditions. Our conclusion is based on a medium amount of evidence.


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 24FARMEURYHALINE > 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 23SPAWNERSFARM: 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  Reproduction

A species reproduces at a certain age, season, and sex ratio and possibly involving courtship rituals. What is the probability of the species reproducing naturally in captivity without manipulation?

It is low for minimal farming conditions. It is medium for high-standard farming conditions. Our conclusion is based on a medium amount of evidence.


WILD: spawning occurs from November until May in the Northern hemisphere and from August to November on the Southern hemisphere 27 2Males 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  Aggregation

Species differ in the way they co-exist with conspecifics or other species from being solitary to aggregating unstructured, casually roaming in shoals or closely coordinating in schools of varying densities. What is the probability of providing farming conditions that are compatible with the aggregation behaviour of the species?

It is low for minimal farming conditions. It is medium for high-standard farming conditions. Our conclusion is based on a medium amount of evidence.


ALEVINS and FRY: WILD: territorial, establish social hierarchies 31. LAB: territorial, establish social hierarchies 32 33 34 35FARMFRY: 2,000 to 5,000 fry/m3.

PARR and SMOLTS: WILD: territorial, establish social hierarchies 36. FARM: stocking densities: tanks: 40-265 kg/m37 38; raceways: 8-160 kg/m38; cages: 30-40 kg/m2. 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/m38 10.

6  Aggression

There is a range of adverse reactions in species, spanning from being relatively indifferent towards others to defending valuable resources (e.g., food, territory, mates) to actively attacking opponents. What is the probability of the species being non-aggressive and non-territorial in captivity?

It is low for minimal and high-standard farming conditions. Our conclusion is based on a high amount of evidence.


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 45LAB: 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  Substrate

Depending on where in the water column the species lives, it differs in interacting with or relying on various substrates for feeding or covering purposes (e.g., plants, rocks and stones, sand and mud). What is the probability of providing the species' substrate and shelter needs in captivity?

There are unclear findings for minimal farming conditions. It is medium for high-standard farming conditions. Our conclusion is based on a medium amount of evidence.


Eggs, ALEVINS and FRY: WILD: successful incubation and emergence dependent on gravel characteristics 52 30FARM: use artificial hatching substrate 3 5LAB: 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 55FARM: rearing in earthen-bottom ponds enhances physiology 56 and increase survival when transferred to seawater 57LAB: 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 30FARM: no data found yet. 


8  Stress

Farming involves subjecting the species to diverse procedures (e.g., handling, air exposure, short-term confinement, short-term crowding, transport), sudden parameter changes or repeated disturbances (e.g., husbandry, size-grading). What is the probability of the species not being stressed?

It is low for minimal farming conditions. It is medium for high-standard farming conditions. Our conclusion is based on a high amount of evidence.


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  Malformations

Deformities that – in contrast to diseases – are commonly irreversible may indicate sub-optimal rearing conditions (e.g., mechanical stress during hatching and rearing, environmental factors unless mentioned in crit. 3, aquatic pollutants, nutritional deficiencies) or a general incompatibility of the species with being farmed. What is the probability of the species being malformed rarely?

It is low for minimal farming conditions. It is medium for high-standard farming conditions. Our conclusion is based on a medium amount of evidence.


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  Slaughter

The cornerstone for a humane treatment is that slaughter a) immediately follows stunning (i.e., while the individual is unconscious), b) happens according to a clear and reproducible set of instructions verified under farming conditions, and c) avoids pain, suffering, and distress. What is the probability of the species being slaughtered according to a humane slaughter protocol?

It is low for minimal farming conditions. It is high for high-standard farming conditions. Our conclusion is based on a high amount of evidence.


Common slaughter method: for the related O. kisutch, anaesthesia with high CO2 or iced water 80, then bled by cutting gill arches and immersing in iced water 80 81. High-standard slaughter method: indications that electrical stunning before killing by chilling or bleeding is most effective 82 83 84 85. Percussive stunning before killing by chilling or bleeding is most effective in larger trout sizes 84.

11  Side note: Domestication

Teletchea and Fontaine introduced 5 domestication levels illustrating how far species are from having their life cycle closed in captivity without wild input, how long they have been reared in captivity, and whether breeding programmes are in place. What is the species’ domestication level?

DOMESTICATION LEVEL 5 86 87, fully domesticated. Cultured since late 19th century 2.

12  Side note: Forage fish in the feed

450-1,000 milliard wild-caught fishes end up being processed into fish meal and fish oil each year which contributes to overfishing and represents enormous suffering. There is a broad range of feeding types within species reared in captivity. To what degree may fish meal and fish oil based on forage fish be replaced by non-forage fishery components (e.g., poultry blood meal) or sustainable sources (e.g., soybean cake)?

ALEVINS and FRY: WILD: carnivorous 2 1. FARM: fish meal and fish oil may be completely* replaced by plant protein 88 89.

PARR and SMOLTS: WILD: carnivorous 2 1. FARM: fish meal and fish oil may be mostly* replaced by plant protein 90 91 92 93. 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 89 94.

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


WILD = setting in the wild
FARM = setting in farming environment or under conditions simulating farming environment in terms of size of facility or number of individuals
ALEVINS = larvae until the end of yolk sac absorption, for details Findings 10.1 Ontogenetic development
FRY = larvae from external feeding on, for details Findings 10.1 Ontogenetic development
PARR = juvenile stage in rivers, for details Findings 10.1 Ontogenetic development
SMOLTS = juvenile stage migrating to the sea, for details Findings 10.1 Ontogenetic development
ADULTS = mature individuals, for details Findings 10.1 Ontogenetic development
SPAWNERS = adults that are kept as broodstock
ANADROMOUS = migrating from the sea into fresh water to spawn
POTAMODROMOUS = migrating within fresh water
EURYHALINE = tolerant of a wide range of salinities
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
DOMESTICATION LEVEL 5 = selective breeding programmes are used focusing on specific goals 86


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