Au Sable River Trout Population Estimates

by Joe Barker

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.

The MDNR determined the ratio of stocked to wild brown trout in all years for various ages based on fish marking. The ratio of stocked to wild yearling (age-1) brown trout was roughly 50:50 during the survey years when clipping was done, except for 2013 (more wild) and 2019 (more stocked). For age-2 brown trout, there was generally more wild fish except in 2019. For age-3 and age-4 fish, sample sizes were generally small. Regardless, there were more age-3 and age-4 wild brown trout in the population from Mio to Meadow Springs in most years.

Brown and rainbow trout were again stocked at eight Mio sites in 2020, but stocked yearling brown trout were not clipped in 2020 due to hatchery constraints (COVID-19). Two electrofishing boats were used to sample the Mio to Meadow Springs reach on one day, September 22, 2020. This allowed for examination of stocked (clipped) brown trout versus wild brown trout at ages 2-4 based on straight catches from this one shocking event.

Catches of age 1-3 brown trout were relatively low, while catches were relatively higher for fish aged 4-6. According to the report, no brown trout age-7 or older were found, but this should be interpreted with caution since older brown trout are hard to age, and older fish were likely present. Ratios of stocked to wild brown trout were significantly weighted to wild fish for age 2-6 and highly weighted to wild fish for age 3-6. Growth was considered excellent for all brown trout, particularly for older wild brown trout. As in most years, fair numbers of wild age-0 brown trout were captured, despite not being highly vulnerable to sampling efforts.

Rainbow trout stocking numbers from Mio to Alcona Pond have been highly variable over the decades but steady recently. Rainbow trout were stocked initially to provide a daytime surface fishery for anglers in the lower Au Sable River. In 2016, MDNR and the public collaboratively reduced stocking from 48,000 to 24,000 for the entire reach. More specifically, this included the three stocking sites from Mio to Meadow Springs, which now receive 7,500 annually. Size at stocking has also been variable over the stocking period.

Both density and biomass for rainbow trout have declined over the survey period below Mio. Some of this decline can undoubtedly be linked to reduced stocking numbers, and rainbow trout survival may also be reduced by higher brown trout density and biomass, according to the report. Most rainbow trout found in the survey reach during the fall are yearlings and in the 9-11 inch size range. Survival to age2 and older for rainbow trout was very low and based on inherently low densities.

Despite this, growth rates were extremely good for rainbow trout below Mio. Growth rates for this species examined over the survey years were much higher below Mio when compared to the statewide average. It doesn’t appear that food is as much a limiting factor for rainbow trout as is other mortality sources (predation, water temperature, winter severity). The limited numbers of rainbow trout found in this reach are believed to be primarily dependent on stocking. However, more age-0 (wild) rainbow trout were recorded in 2019 than in any other survey year.

Summer water temperatures and winter severity are believed to be causes of mortality in the Au Sable River below Mio. Temperatures frequently are above 70F in the summer below Mio Dam, and cold water refugia are limited (though present) in the reach of the river between Mio Dam and Alcona Pond. Average July water temperatures have been at or above 70F frequently since 1997. This was particularly true for the survey period of 2010-2013, while a general warming trend has also been observed in recent years. Cooler July temperatures (2014) may help explain the increased survival of some cohorts.

The population estimate of trout below Mio is planned for 2021. DNR was able to hand clip all 48,000 brown trout internally this spring over a period of three cold days in April. These fish were stocked in early May. All 24,000 rainbow trout were also stocked this spring and were much larger than in past years due to photoperiod and growth manipulation at Oden Hatchery. MDNR hopes to continue clipping and surveying similar to pre-2020 to allow MDNR managers and the public to make a better-informed decision on the future of stocking efforts below Mio. Currently, 48,000 brown trout and 24,000 rainbow trout are stocked in this reach annually. This is an approximate annual cost of $61,000 and $41,000, respectively. Past data shows significant variability in the survival of trout, regardless of species, and more information is needed according to the report.

“The pandemic threw our study a hurdle in 2020—fish were stocked, but not clipped—but we believe we are back on track to continue the long term study in 2021,” said Cwalinski.

Introduction to New NRC... continued from page 7

I’ve learned since coming aboard that the role and responsibilities of the NRC has changed over time and is more limited than I believed at the outset. I would like to see the NRC, working in partnership with the DNR staff, be more involved in an advisory capacity with the overall stewardship of our natural resources and outdoor recreation activities. I would also like to re-establish a committee structure within the NRC that allows commissioners the opportunity to take deeper dives into some of the more pressing issues impacting our natural resources.

Cozad: In serving the people of the state of Michigan, I will strive to ensure that natural resource management decisions which come before the Natural Resources Commission are made based on sound scientific principles. Michigan has a deep, rich history with respect to the origins of professional natural resource management. Today’s Natural Resources Commission continues to build upon that conservation legacy so that future generations will be able to both appreciate and enjoy Michigan’s natural resources.

Baird: My goal is to help the DNR manage Michigan’s natural resources for the future, including new and thoughtful hunting and fishing regulations relevant to our future needs. Our natural resources are confronted by a number of new pressures which we have not seen before (or in a long time): climate change and severe weather patterns, invasive species, emerging diseases, user conflicts, etc. In my view, it is imperative that we get ahead of the curve to avoid being caught flat-footed when it is too late to take effective corrective action. The time to begin planning and placing relevant programs into action is now.

Michigan TU thanks Commissioners Carol Rose, Mike Lashbrook, Dave Cozad, and Tom Baird for taking the time to introduce themselves to our members and for agreeing to serve the public’s interest in natural resources management in Michigan. It should be clear that each of these new commissioners shares an active love and appreciation for the types of fishing we all hold dear. As a side note, we can report that all the new commissioners are members of TU!