Fyfer - When I get air temps at -12C (10F) my pond creates 1" ice per night.
esshup says
If you had to put a time limit on those DO numbers where the trout would survive, how long (days) do you think they could survive in those DO levels? (at the 35°F-42°F water temp range)
So based on what I found on the web (see below), at 35F-39F and DO stays at a steady 2.5 to 3ppm (mg/L), I think the trout would survive for at least 3 to 4 weeks. The main problem is when ponds are ice covered and losing DO due to extended snow cover the dissolved oxygen is in a continual steady decline or decrease due to the lack of sunlight stimulating DO production by plant photosynthesis. Also remember usually the DO loss begins at the bottom sediments and DO loss moves upward toward the ice layer. The fish are forced into shallower and shallower water as the DO is extinguished until the entire pond DO decreases to lethal concentrations depending on species tolerance to low DO. Trout will normally be first fish to die because they can not locate survivable DO concentrations.
DO loss and production under ice-snow cover is highly variable from gradual to sudden dependent on the pond eutrophy and conditions; IMO- biochemical oxygen demand. In Pasinski's pond on Feb 12 DO under the ice fell from 12.3 to 2.ppm on Feb 14; a 5ppm loss per day. whereas on Green Lake under ice on Feb 5 DO increased from 1.8ppm to 8.0ppm on Feb 8 in 3 days a rate of 2.7ppm per day due to photosynthesis when snow was removed from the ice.
A little web searching I found:
The 1985 EPA Water Quality book says " Although the acute lethal limit for salmonids is at or below 3 mg/L, the coldwater minimum has been established at 4 mg/L because a significant proportion of the insect species common to salmonid habitats are less tolerant of acute exposures to low dissolved oxygen than are salmonids.
And I found this - Although O.mykiss (rainbow) have been recorded in a range of dissolved oxygen levels of 2.6 - 8.6 mg/L (Thurston et al. 1981),
A summary of various field study results by WDOE (2002) for trout reports that significant mortality occurs in natural waters when dissolved oxygen concentrations fluctuate the range of 2.5 - 3 mg/L. Long-term (20 - 30 days) constant exposure to mean dissolved oxygen concentrations below 3 - 3.3 mg/L is likely to result in 50% mortality of juvenile salmonids (WDOE 2002). North Coast Regional Water Quality Control Board.. IMO longer survival times will occur at DO of 2.5 to 3mg/L when water temperature is close the the 39F-40F.
Lower temperature limits.
Highest mean temp maximum and lowest mean temp minimum measured over these acclimation temperatures were 40.3 and 2.7 °C (catfish), 38.5 and 3.2 °C (bass) and 29.8
and ∼ 0.0 °C (trout). . I did not have the entire article so I do not know if 0C was the lowest temperature used for trout. Water moving will not freeze at 32F (C).
Currie, R.J., W.A. Bennett, T.L. Beitinger. 1998. Critical thermal minima and maxima of three freshwater game-fish species acclimated to constant temperatures. Env. Bio. of Fishes 51:187-200.
