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MDNR Report

What Happened to the Au Sable North Branch Trout Population?

When last winter finally relented, and anglers were able to get out and enjoy fishing their favorite spots, the DNR Fisheries Division’s Northern Lake Huron Management Unit started getting phone calls from concerned anglers about their lack of success on the North Branch of the Au Sable River.

The unit receives “there are no fish in my lake” calls on a regular basis, and usually they are based on an angler’s couple of days of poor fishing. This year on the North Branch, however, staff received calls from professional trout fishing guides who had never complained about the fishing before. They told local staff they were experiencing extremely low catch rates and weren’t seeing the feeding activity they normally would during insect hatches.

The North Branch was scheduled for Fisheries Division to conduct population estimates in the late summer at three different sites. However, with the number of anglers reporting startlingly low catch rates of trout, the division decided to conduct electrofishing spot checks on May 30.

To build on the strong partnership in the Au Sable River, Fisheries Division invited several of the guides who informed the department to come and help conduct the electrofishing efforts. The first round of electrofishing spot checks were at places that are surveyed regularly. Shortly after the survey started, it was readily apparent the trout population was down from levels normally seen in late summer.

“Most anglers understand that trout can move fairly large distances, usually seasonally, so making direct comparisons of the survey results occurring at different times of the year should be avoided,” said the Northern Lake Huron Management Unit manager, Dave Borgeson. “Regardless, the decline in the number of fish surveyed at the sites corroborated the angler reports so we decided to conduct four more spot checks at other locations on June 7, and got similar results.”

For some unknown reason, it became clear the trout population in the North Branch had declined from the previous year. There was a lot of head-scratching and hypothesis-sharing regarding the cause of the apparent decline, and some of the angler’s ideas centered on the possibility of mortality due to toxic substances introduced into the stream.

While trout populations can vary widely from year to year for a variety of reasons, Fisheries Division decided to notify the Michigan Department of Environmental Quality of the angler’s observations and the electrofishing results. A working group of concerned anglers, DEQ staff, and Fisheries Division employees Neal Godby and Borgeson teamed up to discuss the status of the stream and plan a strategy for additional information gathering in 2018.

The DEQ planned to do some water chemistry work and conduct aquatic invertebrate sampling in the North Branch, Fisheries Division would conduct trout population estimates at three stations on the river, and the angling groups planned to do their annual quantitative aquatic invertebrate sampling as well as cooperate with the U.S. Geological Survey (USGS) to conduct a contaminant survey using lipid-based collection gear deployed in the stream.

The DEQ’s Water Resource Division conducted three P51 Habitat and Macroinvertebrate surveys in mid-June and found “All three sites surveyed in June 2018 scored excellent for both habitat and macroinvertebrates. All sites had excellent macroinvertebrate diversity with 37 taxa found at Dam Four, 32 at Twin Bridge Rd., and 31 at the Ford. Of these, 20 taxa at Dam Four, 18 taxa at Twin Bridges, and 15 taxa at the Ford were ephemeroptera, plecoptera, or tricoptera (EPT) taxa indicating excellent water quality.”

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.

“So, what does all this mean?” asked 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?”

Sometimes trout populations can be impacted by extreme water temperatures. Since the decline occurred after the DNR’s fall survey, and before this summer, warm temperatures do not appear to be the culprit. Last winter did not have too many of the extremely low temperatures that can greatly affect trout. So, temperature may not have been the primary force impacting the population in this case.

Also, the area of the North Branch was the recipient of some tremendous amounts of precipitation in the fall of 2017, and in the spring of 2018. Many long-time river residents and users reported they have never seen the North Branch so high, even out of its banks. It is known that high flows can impact trout populations, especially those occurring in the spring. Fisheries Division also surveyed some other streams that had markedly lower trout abundances. For example, division crews surveyed the West Branch of the Sturgeon River and they said it was very clear the trout population had declined since 2017. The department also had reports of another small, shallow tributary to the Sturgeon River that had a much lower trout population. A tributary to the Muskegon was surveyed and the population was down noticeably.

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 from this situation: 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 (last fall and this spring). 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.

“Overall, productivity in the North Branch has declined from the 1960’s and 1970’s, and the trout population reflects that decline,” explained Borgeson. “Perhaps this has to do with the long-term effects of the Clean Water Act. Those of us old timers remember the good ‘ol days when there were more brook trout in the stream. Maybe the rooted vegetation that used to be more abundant in the North Branch provided those young trout enough cover to survive better to older ages. When a trout population begins with more 1 and 2-year-old fish, then it usually ends up with more 2 and 3-year-old trout. In the past 30 years the stream’s trout population has varied around a new lower average biomass.”

This year on the North Branch of the Au Sable highlights the importance of having a suite of streams where status and trends surveys are conducted. They help put the trout population variability of one stream in a greater context. That is, are there regional trends in all sampled streams, in certain types of streams, or is there a stream that had a unique event occur?

It also points to the importance of strong working relationships with local Fisheries Division staff, concerned anglers, and other agencies or groups that can bring resources to the table to solve complex problems.