John,
You ask some insightful questions. First, most fishery biologists think about dissolved oxygen levels in milligrams per liter, roughly equivalent to parts per million. Thus, 2 ppm is about 2 mg/l. The dissolved oxygen (D.O.) levels required by most trout species to maintain general good health is around 8 mg/l. That being said, it should not be surprising that trout leave the area when the D.O. levels are 2 mg/l. Certainly, 6 mg/l would be better, but, to the trout, it is analagous to the difference between being outright suffocated, and being somewhat asphyxiated. They probably want to leave either way. So, in answer to your question, yes, 6 mg/l is better, but it is generally cited as a minimul level required for trout suvival.

Also important is the time of day that D.O. levels are measured. For example, is 6 mg/l the average over 24 hours, or measured at a specific point during the day? The relevance of this is that during the night, when photosynthesis is at it's lowest, D.O. levels will also be at their lowest, and during the mid to late afternoon, D.O. levels will be at their highest (coinciding with the highest levels of photosynthesis). If the D.O. levels being cited are moving averages or mid-afternoon levels, then you are probably having periods with substantially lower D.O. levels during the night (referred to as D.O. sags).

There are many habitat features (both physical and chemical) that have the potential to limit trout populations, but if the D.O. levels that you cite (2-6 mg/l) occur with some regularity,then you could be correct in your assertion that this might be the issue of greatest concern to Arkansas trout fisheries.

So, I recommend that you, the citizens of Arkansas, begin to ask some pointed questions. Such as, when and how are D.O. levels being measured? Is 6 mg/l of D.O. really desirable for trout, or it is a minimum level to avoid fish kills? (a D.O. level below about 4 mg/l is generally considered lethal to most trout species) You should also begin inquiries into when the White River system dams are scheduled to undergo Federal Energy Regulatory Regulatory Commission (FERC) relicensing. It is during the relicensing process that you can exert the greatest amount of influence over future operating practices.

If you have further questions, please let me know. - Rick