Michigan Trout Unlimited
MICHIGAN Trout
Au Sable River Trout Population Estimates
24
by Joe Barker
Summer 2021
The Michigan DNR has periodically electrofished 5.3 miles of river with two electrofishing boats on the Au Sable River from the Mio MDOT public launch/powerline downstream to Meadow Springs. An update report issued in March of this year by Tim Cwalinski, Senior Fisheries Biologist for MDNR (Gaylord), summarized the results. “The data was a bit different in 2020 since we didn’t complete a population estimate (marking and recapture 2-day run) due to stocked brown trout not being clipped in 2020. However, we were able to complete a one-day, one-pass electrofishing run of this reach from Mio to Meadow Springs, allowing us to simply compare the ratio of age 2-4 clipped versus non-clipped brown trout,” said Cwalinski. The MDNR has reviewed stocking practices for this reach of river over time. Fishing regulations have remained conservative, emphasizing the reduced harvest of brown trout as assisted by higher size limits. The report believes the public’s objectives have been the catch of large brown trout in the “big water” of the Au Sable River below Mio. According to the report, densities of sizeable brown trout may not be exceedingly high in this reach but are acceptable, and certainly, growth rates are not an issue. The objectives of trout estimates are to determine if the stocking of hatchery yearlings contributes significantly to producing a high-quality fishery for large brown trout. A more recent objective was to determine if stocking contributes enough to the large fish fishery compared to wild brown trout. Trout estimates from Mio to McKinley section from a 2010-2013 statewide study to examine brown trout strain survival showed that stocked fish survival was overall low. The Sturgeon River strain outperformed the Wild Rose strain in the Au Sable River, despite smaller size at stocking. The study also revealed that wild brown trout comprised a significant part of the overall brown trout density and biomass. Wild brown trout were not considered important to the overall trout population below Mio Dam before this study. More recent population estimates covering the years 2017 to 2019 had a two-fold purpose: 1) to examine the size structure of brown and rainbow trout, and 2) to determine the percentage of wild versus stocked brown trout, particularly as they recruited to larger sizes and older ages. The current study was designed in conjunction with a special Mio workgroup consisting of anglers and business owners. Data collected was shared with the workgroup, which
included Michigan Trout Unlimited and Anglers of the Au Sable members, to collaborate in future decision-making. The study is scheduled to continue at least through part or most of the 2020 decade with population estimates and clipped stocked yearling brown trout. Currently, the MDNR stocks 48,000 brown trout yearlings and 24,000 yearling rainbow trout in this entire reach (Mio Dam to 4001 Landing). A study update found that brown trout density and biomass estimates have changed dramatically during the survey period from 1999-2019. The average brown trout density among the 12 sampling periods was 32 fish/acre. This average was not attained in either 2018 or 2019 and has rarely been surpassed during the survey period. This average is likely skewed by the high-density estimate of 2010. Juvenile brown trout numbers were low both in 2018 and 2019, thus reducing overall density estimates. Brown trout biomass estimates have averaged just shy of 15 pounds/acre during the surveys. This was at or near the average for the Mio to Meadow Springs reach both in 2018 and 2019 and well above the average in 2017. It was evident from the 2019 sampling that the density and biomass of large brown trout were high below Mio. However, these variables were low for young brown trout. There appears to be a good number and biomass of larger brown trout in the Mio reaches based on the 2019 survey. This is true for fish 15-inches and larger, but particularly true for fish 20-inches and larger, which is more of what the Mio reach of the Au Sable River is known for. The density of 20-inch and larger brown trout below Mio is about average compared with this variable across Michigan fixed sampling stations. However, many of the stations that scored higher were riverine reaches with access to migrating fish from the Great Lakes. This is not the case for the Mio reach. There has been some speculation that periodic high densities of large brown trout may reduce the numbers of juvenile brown trout through factors such as predation. Based on 1999-2019 survey data, there is a relationship between these two variables, but it is relatively weak. For instance, 26% of the variability in yearling density can be explained by the density of brown trout 15-inches and larger. The update found that brown trout growth in the Au Sable River below Mio is very good compared to the statewide average for this species. This has held across the sampling years. Food resources do not appear limiting. However, survival may be limited by holding cover and space available for all sizes of fish.
