Trout Decline Investigated On Iconic Au Sable

A lack of fish in one of the nation’s most iconic trout fisheries.

The Michigan Department of Natural Resources has been investigating a concerning lack of fish in one of the nation’s most iconic trout fisheries. Spurred by angler complaints of extremely low catch rates this past spring, DNR took a closer look at the situation on the North Branch of the Au Sable River.

Through electrofishing spot checks, it was apparent trout populations were down from normal levels, but the reason remained unknown. The Michigan Department of Environmental Quality conducted water chemistry work and aquatic invertebrate sampling in the North Branch, while Fisheries Division continued trout population estimates at three stations on the river. Meanwhile, angling groups conducted annual quantitative aquatic invertebrate sampling and also cooperated with the U.S. Geological Survey (USGS) to conduct a contaminant survey using lipid-based collection gear deployed in the stream.

Some data from the Department  of Environmental Quality:

Department of Environmental Quality sampling showed excellent water quality as well as habitat and concentrations of microinvertebrates. The USGS organic chemical sampling results are pending. Fisheries Division electrofishing surveys revealed:

• At Twin Bridges the brook trout density and biomass were at the lowest recorded level in the past 30 years, and brown trout density and biomass levels were on par with the past two years.

• At Eamon’s Landing the brook trout density was around the long-term average and biomass was on par with the past two years (but low compared with the long-term average).

• At Dam 4 the brook trout density was well below average and biomass was at its lowest recorded level in the last 30 years. Brown trout density was about average, but the biomass was well below its average.

What does the data mean?

“So, what does all this mean?” asked Northern Lake Huron Management Unit manager, Dave Borgeson. “Do we know why the trout abundance in the North Branch declined substantially? It appears the aquatic invertebrate populations appear to be in good shape, and that non-trout species are in decent numbers. Because of that, contamination or an acute toxin event is not likely the cause. Additionally, trout species are still present albeit in relatively low numbers. So, what else could it be?”

The area of the North Branch was the recipient of tremendous precipitation in the fall of 2017 and in the spring of 2018. It is known that high flows can impact trout populations, especially those occurring in the spring.

Maybe there was a regional phenomenon that affected certain types of streams disproportionately more than others? Could the high flows have been the primary culprit? We probably won’t know with an ironclad degree of certainty, but we can make some conclusions. It occurred between early fall 2017 and May 2018. It doesn’t appear to be a toxic event. The relatively large one-year reduction in trout abundance coincided with two extremely high flow events. Also, there appears to be enough numbers of young trout in the system that with decent overwinter survival the numbers of catchable fish should improve in the coming years.

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