Survey of Habitat on Au Sable “Trophy Water” Suggests

Little Refuge for Trout in Heat of the Summer


   There ain’t no cure for the summertime blues, or at least not much if you’re a trout living in the Au Sable from Mio Dam down.

   A recent study prepared for MIDNR by Affiliated Researchers, an Oscoda-based environmental consulting firm, suggests that there is very little quality coldwater refuge between reservoirs on the Au Sable for trout to retreat to in the heat of summertime.  It only gets worse as one goes further downstream.

   The purpose of the study was to determine the extent of quality coldwater habitat available during the warmest period of the year, usually from mid July through mid-August.  Quality habitat was defined as any area with temperatures at or below 70 (F) degrees and deeper than three feet.  Previous research supports both measures as markers for significant increases in trout survival during the warmest periods of the year.

   The study was conducted in 2004, a year that was atypical of recent ones in that it was cooler and wetter on the whole.

   The process for gathering data was very sophisticated and employed state-of-the-art instruments such as Trimble ProXRS for GPS, YSI 6000 for aquatic temperature measures, and Horizon DS150 sonar to log water depth readings as well as the latest software for analyzing the data.

   The data set was, in a word, extensive.

   How extensive?  The YSI 6000 collected over 35,800 aquatic temperature data points while the Horizon DS150 sonar registered over 29,800 depth data points, both instruments did so while integrated with the GPS system. 

   Data collection took place from a rowboat which drifted the river twice in order to improve accuracy in measurement.  A fourth instrument, called a Tidbit, measured daily fluctuations in water temperature at one hour intervals.  Several Tidbits were placed throughout the river.

   The result was a map of the both the river depth and its temperatures layers.  These thermographic and bathymetric contours were coupled with aerial photographs to complete the package.  Resources Managers have been given a template rich with information on the nature of the river during summer’s warmest few weeks.

   So what’s the conclusion?

   Alarmingly, data indicates that for the 22.1 miles of river between MDOT Access in Mio to 4001 Bridge there exists only 3.1 total acres, less than 1%, containing holding water that meets the strictest definition of quality refuge, with temperatures less than 70 (F) degrees and water depths greater than 3 feet.  If the standards are relaxed to 72 (F) degrees – some research does support this figure as acceptable for trout survival – then the amount of quality habitat increases to 12.3 acres, still less than 3% of the river!

   It’s even worse below Alcona Dam, a 25 mile stretch of river that has received trout plantings.  The quality holding area is 440 square feet, about the size of a guest cabin at many Northern Michigan cottages.  If the temperature standard is relaxed to 72 (F) degrees then the area “increases” to 1967 square feet, a nice house on the mainstream.

   Summertime coldwater habitat below Loud Dam – F’gettabodit!

   What’s the Cause?  What’s the Solution?

   “The dams are the cause,” suggested Rollin Reineck, Principle Owner of Affiliated Researchers.  “They need to stop releasing solar heated water into the river below each dam which is intolerable to the trout species during the warmest time of the year.”

   This conclusion was also found by the MI DNR Au Sable River Assessment (Zorn & Sendek, 2001).

   Reineck explained that in the summer months the water in the reservoirs is heated by the sun.  The surface waters on these ponds usually approach the upper 70s to nearly 80 (F) degrees by the end of July.  Cool down does not begin until the end of August.  Slowly, these surface waters move toward the dam and eventually flow over the top into that corresponding section of river.  This water, already warmer since being in the reservoir, comes into the next pond where it is heated to an even greater degree before tumbling into the next section.

   “The water just keeps getting warmer and warmer with each dam,” Renieck said.

   Most of the quality refuge Renieck and his team found is near tributaries and feeder creeks, many of which never get much warmer than 50 (F) degrees.

   “These tribs provide little slivers of sanctuary for trout,” He added.

   Renieck believes if the water flowing over the dams can be cooled, even just a little, it could greatly improved the survival of trout through summer’s “Dog Days.”

   “Even as little as a three degree drop could really change things,” he explained.  “We need empirical evidence concerning the nature of the water depth and temperatures in these ponds.  We will become a lot smarter about all of this if we have that data.  Then we can decide what to do.”

   Reineck proposes doing a similar study on the reservoirs on the Au Sable.  He is currently looking for funding sources.

   “There is no downside to this project.  We could come up with an answer none of us has right now,” he said.

   The potential of changing the water flow through dams is not something that will be welcomed by all.  Consumers Power which owns the dams would likely end up spending a fair amount of money in any type of project.  Renieck also says that the data acquired through his proposed study will confirm whether or not Consumers is complying with FERC regulations regarding water temperature ranges.

   The current study also offers some information on how concerned parties could help out in trout survival right now on this stretch of river. 

   Reineck suggests building habitat or placing woody debris in the cool water areas.  Habitat upgrades in warm spots in the river may be a waste of time and resources especially during warm months. 

   In addition, any trout caught during this warm period may likely die no matter how careful the release.  Either keep the trout caught, if they are legal size, during this time or don’t fish the section.

   Because of the strains of water temperature, it might make sense to plant only brown trout below Mio and Alcona since they are the most temperature-tolerant species of salmonid.  This may explain why there are so few rainbow trout caught at the end of the season in comparison to the start, too many died in the summer swelter.

   Fishers below Mio have long wondered where all those planter trout go?  Reineck has given us an answer.  With additional work he may give us a solution to stop it.


-  Learn more about this study and Affiliated Researchers by going to   













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