Average temperature requirements for Rainbow and Brown trout

Average temperature requirements for Brown trout
Average temperature requirements for Rainbow trout

I recently created two figures to accompany the quick reference poster of water quality on the Henry’s Fork that was featured in our blog post on October 30th. These figures illustrate the optimal, sub-optimal, stressful, and lethal temperature ranges for Brown Trout (Salmo trutta) and Rainbow Trout (Oncorhynchus mykiss). Temperature ranges differ according to the life stage of each species and individual temperature columns within each figure represent this. The figures are meant to aid in interpreting the water quality data portrayed on the quick reference poster. Information about where each species is found in the Henry’s Fork, as well as the timing of each species’ spawning events is also included in each figure. Our water temperature data is in degrees Celsius and I have also included a degrees Fahrenheit scale on each figure for reference. Each figure also includes the temperature standards set by Idaho DEQ for cold-water aquatic species. The regulations are written such that spawning salmonids have their own temperature standards as a subset of the cold-water aquatic species standards.


Brown Trout spawn in the fall, whereas Rainbows spawn in the spring and it is this difference in spawning time that underlies the difference in temperature tolerances of eggs and larvae of the two trout species. Brown Trout spawn in October to November, triggered by cooling temperatures, and the eggs incubate during the cold winter months to emerge in early spring. Alternatively, Rainbows spawn in April to June, triggered by warming temperatures, so their eggs cannot tolerate the cold winter water temperatures that Browns endure. There are also differences between the temperature tolerances of older Browns and Rainbows; mainly that Brown Trout adults and juveniles can tolerate higher temperatures than Rainbow Trout adults and juveniles.


Please feel free to come into the lobby, where you will find our quick reference poster and these two figures hanging, and learn the current temperature and dissolved oxygen levels found in each reach of the river! Next week I’ll capture water quality information that our sondes have collected over the last month and compile this information onto our quick reference poster. In the next couple of weeks I will post a reference scale for average dissolved oxygen requirements and I’ll summarize the previous month’s sonde data.