Salinity
Rainbow trout must constantly regulate their internal salt and water balance through osmoregulation, but this process works differently depending on their environment and smolt status. In freshwater, rainbow trout are hyperosmotic, meaning their body fluids have higher salt content than the surrounding water. In seawater, rainbow trout are hypoosmotic, meaning their body fluids have a lower salt content than the surrounding water. Pre-smolt trout lack osmoregulatory systems for seawater tolerance, meaning they can become severely dehydrated if exposed to high salinity. Rapid salinity transitions can cause osmotic shock.
Post-smolt trout have broader tolerance ranges, but high salinity levels can still be challenging to adjust to, particularly on transfer to seawater or for poorly smolted fish. There is minimal information on which salinity levels are best suited for post-smolt rainbow trout, but based on the evidence, they are likely best suited to brackish waters. Concrete salinity percentage thresholds could not be developed from the evidence, so thresholds instead are based on signs of osmoregulatory stress.
| Level 0 | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
|
Fish have access to a salinity gradient, allowing self-regulation |
No access to a salinity gradient, but fish are healthy, acclimated adults with no osmoregulatory stress |
Some signs of osmoregulatory distress |
Active signs of osmoregulatory distress |
Protocol application
Salinity can be measured using a digital salinity monitor.
It is recommended to measure salinity for rainbow trout in seawater at different cage depths (e.g., 3 m, 5 m, 15 m, and the maximum cage depth) and to monitor for large fluctuations.
It is recommended to measure salinity in high-risk periods (e.g., during the first 1–2 months after transfer to seawater or where there are signs of osmoregulatory distress). Outside of these, the fish are likely to have acclimated to the new salinity levels.
Scoring notes
- If there are many stratified layers of salinity, fish are unlikely to experience osmoregulatory stress because they can swim to their preferred salinity.
- Signs of osmoregulatory stress include:
- Lethargy or abnormal behavior;
- Reduced appetite;
- Skin darkening;
- Small post-smolts; or
- Maturing fish.
- Validation: The threshold is based on the salinity-scoring methodology in the SWIM 1.0 protocol for salmon. It has not been validated for rainbow trout.
Key evidence
| Source | Summary of source | Key result |
|---|---|---|
| Bahabadi and Morshedi, 2015 | Experimental study investigating osmolarity, cortisol, and thyroid hormones in response to different salinity levels in 450 rainbow trout (93 g) over 60 days in laboratory conditions. The fish were initially held in freshwater. Salinities of freshwater (control), 10 ppt, 20 ppt, 30 ppt, and 40 ppt were tested. Upon transfer, the fish were gradually acclimated to the new salinity over 10 days. |
Fish in the 30 ppt and 40 ppt treatment groups all died after the 10-day acclimation period. The freshwater, 10 ppt, and 20 ppt groups had very similar mortality rates of approximately 4% over the entire 70-day period. Significant reductions in final weight, daily growth rate, specific growth rate, and feed conversion ratio were observed up to a salinity of 20 ppt. Osmolality, cortisol, glucose, and thyroid hormones all increased with salinity, indicating stress. |
| Stien et al., 2013 | Semantic model for overall welfare assessment of Atlantic salmon reared in sea cages, based on existing literature. 17 welfare indicators were selected, including salinity. | Salinity is scored as 1 (best) if fish have access to brackish water, scored as 2 if it is only adult fish with no access to brackish water, and scored as 3 (worst) if there are small post-smolts, maturing, or clearly impaired fish with no access to brackish water. On reviewing the literature, the authors concluded there was little evidence that salinity levels have significant effects on the welfare of adult Atlantic salmon in sea cages, while mature or small salmon may struggle to adapt to high salinities, and so may benefit from access to brackish water, citing evidence that small salmon display a preference for haloclines. |
| Gonçalves et al., 2006 | Experimental study investigating the possible ameliorating effects of reduced salinity on 400 adult Atlantic salmon (802 g) over 15 days, initially reared in seawater (35 ppt). A total of 4 tanks were used, with 2 maintaining seawater at 35 ppt and the other 2 at 28 ppt. Before the experiment, fish displayed signs of osmotic stress (skin darkening, lethargy, reduced appetite), and mortality was approximately 3% per day. |
During the first 3 days of the experiment, there was no significant difference in mortality between the groups. By the end of the experiment, the 28 ppt salinity group had a survival rate of 79.8%, while the control had a survival rate of 31.2%. Plasma cortisol was approximately 4 times higher at the end of the experiment in the 35 ppt group compared to the 28 ppt group. The condition factor was also improved in the 28 ppt group, whereas it remained low in the 35 ppt group. |
Supporting evidence
- Altinok, Ilhan, and John M. Grizzle. “Effects of Salinity On Yersinia Ruckeri Infection of Rainbow Trout and Brown Trout.” Journal of Aquatic Animal Health 13, no. 4 (December 2001): 334–39. https://doi.org/10.1577/1548-8667(2001)013%3C0334:eosoyr%3E2.0.co;2.
- Bahabadi, Mahmoud Nafisi, and Vahid Morshedi. “Changes Osmolarity, Cortisol and Thyroid Hormones (T3, T4) of Rainbow Trout (Oncorhynchus mykiss) in Response to Different Salinity Levels.” Experimental Animal Biology 4, no. 1 (September 2015): 67–78.
- Cortès, Bernat Morro. “The Physiology of Smoltification and Seawater Adaptation in Rainbow Trout.” PhD Thesis, 2019. https://www.storre.stir.ac.uk/bitstream/1893/30076/1/Morro%20-%20Accepted%20Thesis.pdf.
- Gonçalves, J., S. Carraça, A. Damasceno-Oliveira, B. Fernández-Durán, J. Diaz, J. Wilson, and J. Coimbra. “Effect of Reduction in Water Salinity on Osmoregulation and Survival of Large Atlantic Salmon Held at High Water Temperature.” North American Journal of Aquaculture 68, no. 4 (October 1, 2006): 324–29. https://doi.org/10.1577/a05-056.1.
- Jackson, A.J. “Osmotic Regulation in Rainbow Trout (Salmo Gairdneri) Following Transfer to Sea Water.” Aquaculture 24 (1981): 143–51. https://doi.org/10.1016/0044-8486(81)90051-x.
- McKay, L. R., and B. Gjerde. “The Effect of Salinity on Growth of Rainbow Trout.” Aquaculture 49, no. 3-4 (November 1985): 325–31. https://doi.org/10.1016/0044-8486(85)90089-4.
- Morgan, John D., and George K. Iwama. “Effects of Salinity on Growth, Metabolism, and Ion Regulation in Juvenile Rainbow and Steelhead Trout (Oncorhynchus mykiss) and Fall Chinook Salmon (Oncorhynchus Tshawytscha).” Canadian Journal of Fisheries and Aquatic Sciences 48, no. 11 (November 1991): 2083–94. https://doi.org/10.1139/f91-247.
- 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. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
- Pourmozaffar, S., M. Nafisi Bahabadi, A. A. Movahedinia, M. Mohammady, and Kh. Pazir. “Effect of Salinity on Growth Performance, Hematological Variables and Gill Chloride Cells of Rainbow Trout (Oncorhynchus mykiss).” Experimental Animal Biology 2, no. 4 (March 2014): 1–14.
- Ranjbar, Milad, and Majid Mohammad Nejad. “Effect of Water Salinity on Enzymatic and Hormonal Indices of (Oncorhynchus mykiss) Fingerlings.” International Aquatic Research 12, no. 4 (December 2020): 309–14. https://doi.org/10.22034/iar.2020.1906357.1076.
- Ranjbar, Milad, Majid Mohammad Nejad, and Mohammadeeza Ghomi. “Effect of Different Salinity on Growth Factors, Survival and Hematology Indices of Rainbow Trout (Oncorhynchus mykiss).” Journal of Plasma and Biomarkers 14, no. 1 (2021): 87–98.
- Stien, Lars H., Marc B. M. Bracke, Ole Folkedal, Jonatan Nilsson, Frode Oppedal, Thomas Torgersen, Silje Kittilsen, et al. “Salmon Welfare Index Model (SWIM 1.0): A Semantic Model for Overall Welfare Assessment of Caged Atlantic Salmon: Review of the Selected Welfare Indicators and Model Presentation.” Reviews in Aquaculture 5, no. 1 (March 2013): 33–57. https://doi.org/10.1111/j.1753-5131.2012.01083.x.
- 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.
- Xiong, Yinghuai, Xin Wang, Shuanglin Dong, Fang Wang, Jingwen Yang, and Yangen Zhou. “Comparisons of Salinity Adaptation in Terms of Growth, Body Composition, and Energy Budget in Juveniles of Rainbow and Steelhead Trouts (Oncorhynchus mykiss).” Journal of Ocean University of China 18, no. 2 (April 2019): 509–18. https://doi.org/10.1007/s11802-019-3770-4.
- Zeitoun, I. H., J. E. Halver, D. E. Ullrey, and P. I. Tack. “Influence of Salinity on Protein Requirements of Rainbow Trout (Salmo Gairdneri) Fingerlings.” Journal of the Fisheries Research Board of Canada 30, no. 12 (December 1973): 1867–73. https://doi.org/10.1139/f73-298.