Montana Fish, Wildlife & Parks (FWP) monitors the fisheries in the Madison River drainage to determine potential effects from operations at Hebgen and Madison dams which is funded through an agreement with NorthWestern Energy (NWE), who owns the dams. The agreement is meant to help NWE meet Federal Energy Regulatory Commission (FERC) license requirements issued to operate hydropower systems on the Madison-Missouri system, collectively referred to by FERC as the 2188 Project. It's nice to know if you're gonna block a river with a dam, there's some strings attached. NWE and FWP have a Memorandum of Understanding (MOU) to provide annual funding for protection, mitigation, and enhancement (PM&E) of fisheries, recreation, and wildlife resources.
Every year, FWP produces a report called the Madison River Drainage 2188 Project Monitoring Report which summarizes work completed by FWP with funding provided within the MOU to address requirements that relate to the Madison River fishery. Reports summarize 1) fish abundance estimates in the Madison River, 2) assessment of fish populations in Hebgen and Ennis reservoirs, 3) evaluation of the effects of the 2021 Hebgen gate failure to upper Madison River fisheries 3) conservation and restoration of Arctic Grayling populations, 4) conservation and restoration of Westslope Cutthroat Trout populations, 5) evaluation of opportunities for the enhancement of mainstem and tributary habitats, and 6) evaluation of the effects of high-water on riparian regeneration.
Here I’ll summarize those survey efforts completed in 2023 for Hebgen and Ennis reservoirs as well as the Madison River long-term study reaches. Those sections are: Pine Butte (just above the West Fork confluence), Varney, and Norris (Warm Springs and dowstream).
As for the reservoirs which are popular fisheries themselves, FWP annually monitors those waters for fish abundance and population structure. For Hebgen, FWP uses annual spring gill netting surveys. For Ennis Lake, FWP has historically monitored fish assemblages with biannual fall gill netting surveys on odd years. New gill net locations were established in 2021 to provide better coverage of the reservoir while eliminating gill net sets in shallow habitats that had poor capture efficiencies. Sampling is planned to occur annually for at least three consecutive years to provide data that can be used to establish management goals for Rainbow and Brown trout. Although FWP will assess long-term trends using data collected with the new sampling approach, we sort of need to take it with a grain of salt at Ennis reservoir because we’re comparing apples to oranges as far as sampling techniques go. It’ll take some time to develop long term trends using the new methods.
But in general, the metric used for reservoir sampling is what’s called catch-per-unit-effort (CPUE), and it’s most frequently displayed as mean trout per net night using both floating and sinking nets. Management goals for Hebgen are about 19 trout per net night (14 for Brown Trout and 5 for Rainbow Trout). Management goals for Ennis are about 13 trout per net night (6 for Brown Trout and 7 for Rainbow Trout).
On the river, FWP estimates Rainbow and Brown Trout abundances using mark-recapture surveys in the three study sections. Trout are collected by electrofishing from a drift boat. Fish captured in the initial marking run are weighed and measured, marked with a fin clip, and released. FWP then conduct a second recapture run about a week later to examine trout for fin clips. They then record lengths of marked fish, and record lengths and weights of unmarked fish. Abundance estimates are then calculated to stream miles for Rainbow and Brown Trout.
As with the lakes, FWP have developed management goals for total trout abundances. Specifically, those goals pertain to total trout greater than or equal to 10 inches and for size structure. The size structure goals are defined as the percentage of trout 10 inches or bigger that are ALSO greater than or equal to 16 inches. Basically, they have goals in each section for fish over 16 inches. How are the goals established? It’s not as if some biologist somewhere plucks a number out of the sky that sounds like a good goal to have. They use the approximate 66th percentiles of data collected over the past 20 years as their goal marks. Pretty clever. The abundance goals for the Pine Butte, Varney, and Norris sections are 2,300, 1,200, and 2,500 trout/mile, respectively. The proportional size structure goals for Pine Butte, Varney and Norris are 25%, 35%, and 15%, respectively. So clearly, FWP are managing trout abundances in the Madison River for higher numbers at Pine Butte and Norris and for larger fish in the Varney section.
Hebgen Reservoir
The mean CPUE of total trout in Hebgen Reservoir is stable to slightly increasing from 20 trout/net in 2022 to 23 trout/net in 2023. This is above the long term average of 19 trout. For Brown Trout, the CPUE increased from 14.8 trout/net in 2022 to 17.3 trout/net in 2023, exceeding the management goal of 15.5 Brown Trout/net. Rainbow trout CPUE increased from 5.2 trout/net in 2022 to 6 trout/net in 2023 which remains below the management goal of 7.5 Rainbow trout/net. The mean length of Brown Trout decreased from 18.06 inches in 2022 to 17.95 inches in 2023, remaining above the long-term average. The mean length of Rainbow Trout decreased from 17.05 inches in 2022 to 15.91 inches in 2023, which is slightly above the long-term average. What’s kind of surprising to me is 90% of the Brown Trout captured in gill nets
were ≥ 16 inches, with 61 % of the Rainbow Trout captured over that mark.
Ennis Reservoir
FWP historically monitors Ennis Reservoir with biannual fall gill netting surveys on odd years. New gill net locations were established in 2021 to provide better coverage of the reservoir while eliminating gill net sets in shallow habitats that had poor capture efficiencies. 2023 was the third year of consecutive surveys however, and FWP is currently analyzing data to establish management goals for Brown and Rainbow Trout. In 2023, the mean CPUE of total trout (roughly 14 trout/net night) were near the long-term average (15 trout/net night) and down slightly from 2022 (nearly 16 trout/net night). The mean total length of Brown Trout increased from 15.83 inches in 2022 to 16.93 inches in 2023, exceeding the long-term average. The mean total length of Rainbow Trout increased from 14.02 inches in 2022 to 15.35 inches in 2023, also exceeding the long-term average. So, both Hebgen and Ennis seem to be pretty healthy fisheries.
Upper Madison River
The top line here is that abundance management goals were not met for any of the three sections in 2023. Abundances of trout per mile greater than or equal to 10 inches increased slightly at Varney, were stable in Norris, and decreased in Pine Butte. But before you raise your eyebrows and hit the red panic button, let’s dig deeper.
Pine Butte
The estimated abundance of all trout 10 inches or bigger in 2023 (roughly 1,200/mile) dropped for the second consecutive year, is nearly half of what the management goal is (2,300/mile), and is on a downward trend since 2018, the last year it was at or above the management goal for this bracket. Looking closer at the data we see the biggest contribution to this decline in 2023 was the dramatic drop from the cohort of Rainbow Trout between 6 and 10 inches at Pine Butte. We saw a similar drop from 2018 to 2019, which eventually rebounded a few years later. Additionally, Rainbow Trout less than 6 inches were near 20-year lows at this site for 2023. So, smaller Rainbows not doing so hot as they get older. In contrast, young Brown Trout fared better with the abundance of Brown Trout greater than or equal to 6 inches in Pine Butte increasing from 1,159 trout/mile in 2022 to 1,257 trout/mile in 2023. But, this is still near the historic 20-year lows. The good news at Pine Butte was that the high frequencies of trout greater than or equal to 10 inches in 2023 indicate a strong age class was recruited. This may result in large trout contributing even more to the fishery at Pine Butte in the near future. In fact the cohort of large trout at Pine Butte is already strong. Here, 30% (goal is 25%) of 10 inch trout were bigger than 16 inches. It was the only study section to meet or achieve the size structure management goals.
Varney
Abundances of trout greater than or equal to 10 inches have been relatively low at Varney since 2015 (with the exception of 2021). In 2023, the estimate for all trout at or above 10 inches was roughly 1,000 trout/mile with a goal of 1,200. However, when you look at abundances including small fish, estimated abundances of Rainbow Trout greater than or equal to 6 inches remain above the long-term average at 1,574 trout/mile in the Varney Section, despite a decrease from 1,950 trout/mile in 2022. So, lots of little guys in the system here, but we just need more recruitment from them in subsequent years. However, the slight overall increase in abundance of total trout greater than or equal to 10 inches from 2022 to 2023 indicates good survival of previous year classes last year. More positives to glean at Varney was that Brown Trout abundances increased to the historic, 20-year average of 1,610 trout/mile in 2023.
Norris
Abundances of trout greater than or equal to 10 inches in 2023 (roughly 1,600 trout/mile) remain at historic lows in Norris and well below the management goal of 2,500 trout/mile. The estimated abundances of Rainbow Trout greater than or equal to 6 inches also remained below the long-term averages in the Norris section with 1,248 trout/mile in 2023. Brown Trout abundance increased from 523 trout/mile in 2022 to 680 trout/mile in 2023 but remains below the 20-year historical average. The age class frequencies indicate reduced survival of adult Rainbow Trout to the population from that observed in the 2000s and 2010s. The high frequency of Brown Trout greater than or equal to 10 inches indicates strong recruitment in recent years, but abundance estimates indicate an overall decline in the population compared to the 20-year historic averages. Overall reduced abundances of adult trout in Norris may explain why recent recruitment was strong. Less competition for younger age classes.
Here's a closer look at the size structure management goals for the three study reaches.
So Pine Butte looking great for size structure goals. Varney not so bad, seemingly cyclical around the long-term average. Norris is trending down, but there does seem to be a cyclical trend in that data as well.
Here's a closer look at each species at each site.
To read the whole report, which is full of other monitoring efforts related to Grayling and Westslope Cutts, bang it HERE.