Water velocity

The water velocity that fish are exposed to impacts how they swim and also plays a critical role in providing sufficient dissolved oxygen and carrying away waste. Water velocity is often measured in body lengths per second (BL/s), as current speed has a relative impact depending on the fish’s body size. While some flow is beneficial as exercise, excessively high water velocity can quickly exhaust rainbow trout, making them unable to maintain position. This can result in the current forcing the trout into uncomfortable areas of the cage for prolonged periods (such as against net walls or other equipment), which can cause mechanical damage.

Water velocity

Level 0 Level 1 Level 2 Level 3
> 1 to ≤ 1.4 BL/s
> 0.8 to ≤ 1 BL/s
OR
> 1.4 to ≤ 1.8 BL/s
> 0.5 to ≤ 0.8 BL/s
OR
> 1.8 to ≤ 2 BL/s
≤ 0.5 BL/s
OR
> 2 BL/s

Behavioral response to flow

Level 0 Level 1 Level 2 Level 3

Fish are swimming actively through the cage, with no forcing against structures

Fish are swimming actively throughout the cage, with occasional use of structures for rest

Fish aggregated in a small area of the enclosure
OR
Fish are occasionally forced against structures
OR
Fish displaying lethargy due to inadequate flow

Fish are frequently forced against structures
OR
Severe spatial restriction

Overall: Overall score is determined by the most severe subindicator

Protocol application

Velocity can be measured using a propeller flow meter or a Doppler velocimeter. Then body lengths per second (BL/s) can be calculated by dividing velocity (in cm/s) by fish body length (in cm). The estimated average body length in the population can be used to calculate this.

It is recommended to measure water velocity during high-risk periods (e.g., during storms or when water velocity is visibly very high). The water velocity should be assessed based on the measurements in the densest area.

Behavioral response assessment is mandatory for all evaluations, as water velocity is never uniform throughout the cage. Fish clustering in specific areas, forced positioning against structures, or lethargy indicates inadequate flow in those zones, even if measured velocity elsewhere is level 0 or level 1.

If calculating velocity in BL/s is not practical, behavioral assessment alone is sufficient for scoring using the qualitative methodology provided.

Scoring notes
  • Water velocity will vary in different areas of the cage.
  • Specialized current/velocity sensors are required to measure velocity, and many producers may not have access to this equipment.
  • Examples of level 0 velocities depending on fish size:
    • 20 cm: 0.2–0.28 m/s.
    • 30 cm: 0.3–0.42 m/s.
    • 50 cm: 0.5–0.7 m/s.
  • Given that there will undoubtedly be size variation in an enclosure, different proportions of the population will be affected differently by the water flow.
  • Smaller individuals may tolerate a higher BL/s speed, but a lower m/s speed, meaning that the welfare impacts of swimming speed can be quite dependent on the size of the individual fish.
  • Validation: Multiple studies investigating how ongrowing rainbow trout respond to different water velocities were relied upon. The qualitative thresholds were developed based on expert opinion.
Key evidence
Source Summary of source Key result
Fløtum Mittún et al., 2025 Experimental study investigating the preferred and optimal swimming speeds at three temperatures of 31 ongrowing (175 g) rainbow trout in seawater laboratory conditions (circular raceway) over 6 hours. The average body length was 25cm.

The three temperatures tested were 10°C, 15°C, and 20°C.
 The preferred speed was between 1.18 and 1.24 BL/s for all temperatures. There was no significant effect of temperature on preferred swimming speeds.

The critical swimming speed (maximum speed a fish can sustain before reaching complete exhaustion) was between 2.1 and 2.6 BL/s for all temperatures.
Waldrop et al., 2020 Experimental study investigating the effects of swimming exercise and dissolved oxygen on growth, fin condition, and survival in juvenile rainbow trout (18 g) to harvest-size rainbow trout (~1 kg) in laboratory conditions over 341 days.

Two water velocities were tested: high (1.5–2 BL/s) and low (~0.5 BL/s) at high (100%) and low (70%) oxygen saturations.		 Swimming exercise at a higher velocity significantly increased the cardio-somatic index. The higher velocity group also had lower levels of aggression.

Low-swimming exercise groups showed more damage to the right pectoral fin.

There was no statistically significant impact of water flow on growth.
Hafs et al., 2012 Experimental study investigating the impact of carbon dioxide, water velocity, and other parameters on growth and fillet attributes in 600 ongrowing rainbow trout (300 g) in laboratory conditions over 90 days.

The low-velocity group had a water velocity of 0.5 BL/s, and the high-velocity group had a velocity of 2 BL/s.		 Overall, there were limited differences between the groups tested. For example, there was no significant difference in hematocrit or wet weight between the groups.

Specific growth rate and feed conversion ratio were not adversely affected by the high velocity, and no fin damage was reported.
Supporting evidence
  • Fløtum Mittún, Ólavur av, Morten Bo Søndergaard Svendsen, Lars Emil Juel Andersen, Heiðrikur Bergsson, and John Fleng Steffensen. “Preferred and Optimal Swimming Speeds in Rainbow Trout (Oncorhynchus mykiss) at Three Temperatures.” Fishes 10, no. 2 (2025): 64–64. https://doi.org/10.3390/fishes10020064.
  • Hafs, A. W., P. M. Mazik, P. B. Kenney, and J. T. Silverstein. “Impact of Carbon Dioxide Level, Water Velocity, Strain, and Feeding Regimen on Growth and Fillet Attributes of Cultured Rainbow Trout (Oncorhynchus mykiss).” Aquaculture 350-353 (June 20, 2012): 46–53. https://doi.org/10.1016/j.aquaculture.2012.04.020.
  • Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. “Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare.” Nofima. Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
  • Parker, Timothy M., and Michael E. Barnes. “Effects of Different Water Velocities on the Hatchery Rearing Performance and Recovery from Transportation of Rainbow Trout Fed Two Different Rations.” Transactions of the American Fisheries Society 144, no. 5 (July 30, 2015): 882–90. https://doi.org/10.1080/00028487.2015.1047533.
  • Skov, Peter Vilhelm, Bodil Katrine Larsen, Michael G. Frisk, and Alfred Jokumsen. “Effects of Rearing Density and Water Current on the Respiratory Physiology and Haematology in Rainbow Trout, Oncorhynchus mykiss at High Temperature.” Aquaculture 319, no. 3-4 (October 1, 2011): 446–52. https://doi.org/10.1016/j.aquaculture.2011.07.008.
  • Timmerhaus, Gerrit, Carlo C. Lazado, Nikko Alvin R. Cabillon, Britt Kristin Megård Reiten, and Lill-Heidi Johansen. “The Optimum Velocity for Atlantic Salmon Post-Smolts in RAS Is a Compromise between Muscle Growth and Fish Welfare.” Aquaculture 532 (February 15, 2021). https://doi.org/10.1016/j.aquaculture.2020.736076.
  • Voorhees, Jill M., Nathan Huysman, Eric Krebs, and Michael E. Barnes. “Influence of Water Velocity and Vertically-Suspended Structures on Rainbow Trout Rearing Performance.” Open Journal of Animal Sciences 10, no. 1 (January 2020): 152–61. https://doi.org/10.4236/ojas.2020.101008.
  • Waldrop, Thomas, Steven Summerfelt, Patricia Mazik, P. Brett Kenney, and Christopher Good. “The Effects of Swimming Exercise and Dissolved Oxygen on Growth Performance, Fin Condition and Survival of Rainbow Trout Oncorhynchus mykiss.” Aquaculture Research 51, no. 6 (March 15, 2020): 2582–89. https://doi.org/10.1111/are.14600.