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When the morning temperature at Mio Dam is 70 degrees or more then I will not fish for trout on that stretch of the Au Sable River, Mio to Alcona Pond. If I choose to fish that day, it will be on other streams with cooler water, further downstream for warm water fish such as smallmouth bass, or on the many lakes in the area. If I catch a trout I will let it go immediately. When the temperatures fall below 70 degrees at Mio Dam I may consider fishing the stretch again, but will do it early in the morning, at twilight, and at night. I will use a thermometer to fish runs with cooler water so the trout have a better chance at surviving catch and release. I recognize that this is a necessary practice during the hottest part of the summer, late June through mid August, and I am helping the fishery when I do it.

Why Should I Take It?

Because trout are coldwater fish their metabolisms are designed to function best at temperatures around 60 degrees. Furthermore, oxygen levels are greater at cooler water temperatures. Thus, it is not surprising how well salmonids fight when the water temps are in this range. Their bodies are at peak performance and there is plenty of oxygen to feed their muscles. When fought fairly and released properly they should have a very high survival rate. That means somebody else gets to enjoy the magic. It also helps to build a trophy fishery. It's very unlikely that any trophy trout, let's say 20 inches and up, has never been caught before. Proper release, which includes understanding the effects of water temperature, is the key to great sport fishing.

As the water temperature rises a trout's metabolism begins to put more demands on the fish than its system can easily handle. Essentially, their body is now out of tune and there is much less oxygen to help them maintain proper function.

This becomes more of a problem at 65 degrees and by 68 degrees some studies indicate significant hook mortality. By 70 degrees the chances of surviving catch and release are very low. Many trout will die at temps of 75 degrees or more whether they are hooked or not. The effect is like running on a very hot day with a tight shirt or vest hindering one's breathing. The body exerts more effort due to the heat, but can't get enough oxygen to the muscles, including the most important one, the heart. The body cannot regulate the effects of exertion without oxygen and death occurs. Here are two charts on temperature ranges during the summer of 2001. Notice how long the temperature stayed above 68 degrees.

ome would say that it appears that the McKinley gauge, actually located at 4001, shows a cooling river. Actually that is an artifact of where the gauge is placed. In the summer months it is fairly shallow around its location so there is a greater temperature fluctuation. This is good but does not signal all clear. Notice that daytime water temperatures tend to be higher at 4001 than Mio. Also, notice that it still stays above 65 for most of the summer. Finally, DNR SFR page 16 makes it clear - tributaries and upwellings DO NOT OFFER relief.