Oxygen content in the water is critical to fish in the same way oxygen in the air is critical for us. The way we measure oxygen content for fish is milligrams per liter (mg/L) of dissolved oxygen, or DO. Habitat requirements, such as average DO requirements, are useful in our research and monitoring programs for interpreting our data and designing management actions.
What is meant by average dissolved oxygen requirements?
The average dissolved oxygen (DO) requirements shown here are generalized habitat requirements for members of the family Salmonidae (which includes Rainbow Trout and Brown Trout) and are not specific to the Henry’s Fork, but are based on field and laboratory studies that average across variation between individuals and variation between streams. Furthermore, it is known that DO requirements vary with species, age, temperatures at which prior acclimation occurred, water velocity, and activity level. Additionally, DO levels and fish metabolic rates depend on temperature: as temperature increases, the saturation level of DO in the water decreases, but the DO requirement for fish increases. As a result, an increase in temperature resulting in a decrease in DO levels below biological demand can be detrimental to trout. Therefore, the figure below summarizes a large amount of information on average DO requirements by life history (eggs and sac-fry vs. juveniles and adults) and by warm vs. cold temperatures (above or below 15˚C, which is approximately 59 ˚F).
So, what does this figure say?
Levels of DO greater than 11mg/L (in the water column) were found to be optimal (bright green) for developing embryos and levels above 9mg/L are sub-optimal (light green), but not stressful. DO levels below 9mg/L were found to be stressful (yellow) for developing trout with slower growth and more abnormalities occurring at levels below 7mg/L. Sustained DO levels below 5mg/L slowed egg development to the point of mortality.
Juveniles and adults can sustain lower DO levels than eggs and sac-fry. Optimal and sub-optimal levels occur at or above 4mg/L in cooler temperatures (below 15˚C) when they aren’t as active. Physiological stress sets in at levels below 4mg/L and if fish cannot find habitat with DO above 3mg/L mortality can occur. DO requirements for salmonids increase as their metabolisms become more active in warmer temperatures. Therefore, optimal and sub-optimal DO levels for adults and juveniles increase to above 6mg/L in warmer temperatures (above 15˚C), lethal levels of DO occur below 5mg/L, and fish undergo stress at DO levels between 5–6 mg/L.
What impacts do stressfully low DO levels have on salmonids?
Salmonids incubated at low DO levels were found to be small with slower development, were weak, and had more occurrences of abnormal structural growth. For juveniles and adults, low DO can result in reduced fecundity and even prevent spawning.
What are the limitations of this information?
The requirements shown are for salmonids and were created by compiling field and lab results for genera Oncorhynchus (including Rainbow Trout) and Salmo (including Brown Trout) where common thresholds that were most conservative for both genera were used. For instance, optimal DO levels for Rainbow Trout at temps above 15 ˚C were found to be greater than or equal to 7 mg/L, but optimal DO levels for Browns at these same temperatures are greater than or equal to 9 mg/L, therefore DO levels at or above 9 mg/L during warm summer temperatures would be optimal for both Rainbow and Brown Trout populations.
What is HFF doing about it?
HFF’s network of water quality sondes measure DO in each major stretch of the river. As we transition from winter’s low temperatures and low biological activity into the spring, we expect to see DO levels in the river decrease from winter highs with increasing temperatures and increasing oxygen demand. HFF staff will continue to monitor DO and will be able to detect if DO levels become stressfully low.
Stay tuned for a seasonal summary of temperature and DO trends over winter and into early spring!
Note: A brief reference list in included on the figure.