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Macroinvertebrates On the Back of an Envelope

If you like aquatic macroinvertebrates (river bugs), here are some “back of the envelope” calculations that may amuse you.
The UMOWA 2015 Macroinvertebrate Monitoring Report reported that the greatest density of macroinvertebrates occurred between the Dam and Craig. The density of macroinvertebrates averaged about 12,000 individuals per square meter.

The distance from the Dam to Craig is about 7.8 miles (12,553 meters). The average wetted channel width is roughly 100 m on this stretch of river. If we multiply these two numbers, we get an idea of the area of the riverbed above town:  12,553 m x 100 m = 1,255,300 square meters of river bottom between the Dam and Craig.

We can then multiply this number by the average number of macroinvertebrates per square meter of river bottom (12,000 individuals per square meter) to get a rough guess of the total number of bugs between the Dam and Craig:  1,255,300 square meters x 12,000 individuals per square meter = 15,063,600,000 individual bugs between the Dam and Craig. 

So between the Dam and Craig there are about 15 billion individual bugs.

The most recent MT FWP trout surveys reported that on the Craig stretch of the Missouri River there were 4,073 Rainbows and 433 Browns greater than 10 inches per mile. If we combine these two numbers, then there were 4,506 trout per mile of river around Craig. We can multiply this number by the 7.8 miles of river between the Dam and Craig to get a guess of the total number of trout between the Dam and Craig:  4,506 trout per mile x 7.8 miles = 35,147 total trout greater than 10″ between the Dam and Craig.

If there are 15,063,600,000 individual bugs and 35,147 trout, then there are approximately 428,588 bugs per trout. Now of course, these numbers are very rough and not all of the bugs are available to the trout at all times.

So if you want to impress some trout bum chicks…don’t use this information as a catalyst to the second drift boat date!

What’s the point of all this math? I don’t really have one, other than it’s kind of amazing that the flies on the ends of our lines get noticed at all with so many natural bugs around. But they do. And that’s pretty cool.

fly fishing technique, missouri river
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