Temperature
Water temperature influences the physiological processes in rainbow trout. As ectothermic animals, rainbow trout rely on the external temperature of water to regulate their own body temperature. Deviations from preferred temperatures (particularly if rapid) can cause physiological stress, including changes in metabolic rates, respiration, blood pH, and osmoregulation. Temperature is a compounding welfare indicator because water’s capacity to hold dissolved oxygen decreases with increasing temperature. Consequently, it can be challenging to separate the direct effects of temperature from those of reduced oxygen availability.
| Level 0 | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
|
> 13 to ≤ 16 °C
|
> 10 to ≤ 13 °C
OR > 16 to ≤ 18 °C |
> 4 to ≤ 10 °C
OR > 18 to ≤ 20 °C |
≤ 4 °C
OR > 20 °C |
Protocol application
Measuring temperature at various points is crucial for a comprehensive understanding of the system’s environment. This involves taking readings at different depths (e.g., surface, mid-column, and the bottom).
It is recommended to monitor temperature continuously, with manual verification at least twice daily, at dawn and dusk. This is to capture the minimum and maximum water temperatures for the day. Where temperature is measured at a single depth only, use the 24-hour average temperature from all readings to determine the score level. Where temperature is measured at multiple depths, take the 24-hour average separately for each depth, and then use the most favorable of these per-depth averages as the final score level.
Extreme readings (e.g., 2°C above or below the daily average) and rapid temperature changes (e.g., a change of 3°C in 24 hours) should be flagged for investigation. The duration of suboptimal conditions should also be noted, with increased monitoring of fish behavior during these periods.
Scoring notes
- Temperature tolerance in fish depends on factors such as species, past acclimation, physiological capabilities, and health status.
- Natural thermal tolerance varies seasonally. Anticipate temperature challenges during extreme weather.
- Gradual temperature changes are better tolerated than rapid fluctuations.
- Poor water movement can create temperature gradients.
- Low temperatures can reduce feeding and slow growth, while high temperatures can increase respiratory rates, reduce oxygen-carrying capacity, cause stress, and increase the risk of mortality.
- Validation: The thresholds are primarily based on evidence from rainbow trout in freshwater systems and have not been validated for rainbow trout in seawater conditions.
Key evidence
| Source | Summary of source | Key result |
|---|---|---|
| Sutterlin and Stevens, 1992 | Experimental study investigating thermal preferences of 6 triploid rainbow trout (1.2–2.3 kg) in commercial sea cages in a stratified Newfoundland fjord (10–24 ppt) across two trials of 4–5 days each, during summer. Fish distribution was measured using acoustic telemetry tags. |
The average selected temperature was 13.3°C, regulating body temperature between 13°C–14°C. Individual trout ranged from 10.8°C (somewhat of an outlier) to 14.5°C mean temperatures, with a total range of 7.2°C to 17.3°C. A diel cycle was observed, with 5 trout showing a 12-hour rhythm, spanning 3–4°C around the mean. Fish tended to stay in the upper 2m of the water (where it was warmer), and 94% of observations were in the top 4m. Salinity also increased with depth, and the authors noted that temperature preference could not be fully separated from salinity avoidance. |
| Jiang et al., 2021 | Experimental study investigating the impact of temperature and oxygen on 360 ongrowing rainbow trout (110 g) in freshwater RAS over 28 days. Three temperatures (13°C, 17°C, and 21°C) and two dissolved oxygen levels (4.2 mg/L (hypoxic) and 9.6 mg/L (hyperoxic)) were tested. There were 6 treatments for each combination of temperature and oxygen, with 3 replicates each, resulting in 18 tanks with 20 fish per tank. |
Although temperature did not have a statistically significant impact on mortality, mortality rates correlated with temperature, particularly at the low dissolved oxygen levels. At high dissolved oxygen levels, survival rates were 93.3% at 13°C, 96.7% at 17°C, and 90.0% at 21°C. At low dissolved oxygen levels, survival rates were 86.7% at 13°C, 86.7% at 17°C, and 70% at 21°C. 17°C also had the highest growth rates and feed consumption, with 21°C having the lowest final weights, growth rates, and feed intake. 13°C and 17°C yielded generally good welfare indicators with minimal stress responses, whereas 21°C showed some welfare concerns. |
| Cocherell et al., 2014 | Experimental study investigating temperature preferences of 40 ongrowing rainbow trout (568 g) using a freshwater chamber which had a temperature gradient from 12°C to 24°C. 3 groups of fish were acclimated to different temperatures (12°C, 15°C, and 18°C) for 15–22 days, then individually tested in the gradient chamber for 1 hour. |
12°C- and 15°C-acclimated fish preferred temperatures of 15.8–16.2°C. 18°C acclimated trout preferred a significantly warmer temperature of 18.4°C. The 12°C and 15°C groups actively avoided temperatures higher than 19°C. The 18°C group avoided temperatures below 16°C and above 20°C. |
| Bear, 2005 | Experimental study investigating the survival and growth at different temperatures in juvenile rainbow trout (mean weight ranging from 13.9 to 38.3 g) in freshwater laboratory conditions over 60 days. Survival and growth were assessed at 2°C increments between 8 and 30°C. 36 tanks were used, with 50 fish per tank and 3 replicates per temperature. |
The study found that the best growth and condition occurred between 10–18°C, with 12–16°C being the optimal performing range. Feed conversion efficiency declined significantly above 20°C, and growth ceased entirely above 24°C. Survival remained high between 8–18°C, with these temperature groups showing survival rates of 96.3%–100%. However, at 20°C, survival dropped to 92.9%, but increased again to 98.6% at 22°C. Above 24°C, conditions became rapidly lethal for rainbow trout. At 24°C, the survival rate fell dramatically to 72.8%, but at 26°C, there was a massive die-off with only 1.3% survival, and at 28°C, there was complete mortality. |
| North et al., 2006 | Experimental study investigating the impact of stocking density on 100 female rainbow trout (starting at 180 g) in freshwater laboratory conditions over 9 months. Three stocking densities were tested: 10, 40, and 80 kg/m³. Water temperature ranged from 0.1°C to 13.2°C. |
There was a correlation between increased plasma cortisol and water temperatures below 4°C between January and April, particularly in the 10 kg/m³ group. The authors noted evidence that cortisol levels are elevated during the winter months in juvenile salmon. |
Supporting evidence
- Bear, Elizabeth Ann. “Effects of Temperature on Survival and Growth of Westslope Cutthroat Trout and Rainbow Trout: Implications for Conservation and Restoration.” MSc Thesis, 2005. https://scholarworks.montana.edu/server/api/core/bitstreams/c05655be-e5fb-46a3-bbf1-0365127baea2/content.
- Cocherell, Dennis E., Nann A. Fangue, Peter A. Klimley, and Joseph J. Cech. “Temperature Preferences of Hardhead Mylopharodon Conocephalus and Rainbow Trout Oncorhynchus Mykiss in an Annular Chamber.” Environmental Biology of Fishes 97, no. 8 (August 2014). https://doi.org/10.1007/s10641-013-0185-8.
- Jiang, Xuyang, Shuanglin Dong, Rongxin Liu, Ming Huang, Kang Dong, Jian Ge, Qinfeng Gao, and Yangen Zhou. “Effects of Temperature, Dissolved Oxygen, and Their Interaction on the Growth Performance and Condition of Rainbow Trout (Oncorhynchus Mykiss).” Journal of Thermal Biology 98 (May 2021): 102928. https://doi.org/10.1016/j.jtherbio.2021.102928.
- MacIntyre, Craig. “Water Quality and Welfare Assessment on United Kingdom Trout Farms.” PhD Thesis, 2008. https://dspace.stir.ac.uk/bitstream/1893/434/1/FINAL%20THESIS.pdf.
- Mikkelsen, Heidi, Thomas Lindenstrøm, and Michael Engelbrecht Nielsen. “Effects of Temperature on Production and Specificity of Antibodies in Rainbow Trout (Oncorhynchus Mykiss).” Journal of the World Aquaculture Society 37, no. 4 (November 27, 2006): 518–22. https://doi.org/10.1111/j.1749-7345.2006.00065.x.
- 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.
- North, B. P., J. F. Turnbull, T. Ellis, M. J. Porter, H. Migaud, J. Bron, and N. R. Bromage. “The Impact of Stocking Density on the Welfare of Rainbow Trout (Oncorhynchus Mykiss).” Aquaculture 255, no. 1-4 (May 2006): 466–79. https://doi.org/10.1016/j.aquaculture.2006.01.004.
- Sutterlin, A. M., and E. D. Stevens. “Thermal Behaviour of Rainbow Trout and Arctic Char in Cages Moored in Stratified Water.” Aquaculture 102, no. 1-2 (March 1, 1992): 65–75. https://doi.org/10.1016/0044-8486(92)90289-w.
- Taylor, S., S. Egginton, and E. Taylor. “Seasonal Temperature Acclimatisation of Rainbow Trout: Cardiovascular and Morphometric Influences on Maximal Sustainable Exercise Level.” Journal of Experimental Biology 199, no. 4 (1996): 835–45. https://doi.org/10.1242/jeb.199.4.835.
- Wagner, Eric J., Thomas Bosakowski, and Steven Intelmann. “Combined Effects of Temperature and High pH on Mortality and the Stress Response of Rainbow Trout after Stocking.” Transactions of the American Fisheries Society 126, no. 6 (November 1997): 985–98. https://doi.org/10.1577/1548-8659(1997)126%3C0985:ceotah%3E2.3.co;2.