SHEBOYGAN - As Lake Michigan fisheries managers and stakeholders eye potential changes to trout and salmon stocking levels, two key indicators turned positive last year.
First, the Lake Michigan prey fish biomass, including alewife, increased in 2018, continuing a slight upward trend observed over the last three years, according to researchers with the U.S. Geological Survey.
And in another snapshot of the lake's fishery, the average weight of 3-year-old female chinook salmon in 2018 was 20.6 pounds, heaviest since 1986, according to data provided by the Wisconsin Department of Natural Resources.
Both changes are likely linked to reduced Lake Michigan trout and salmon stocking since 2013.
"There's still a lot of predation pressure on the prey fish, but we're seeing signs it may have reduced," said Chuck Madenjian, USGS fisheries biologist.
Madenjian presented the 2018 Lake Michigan prey fish status report Saturday at a meeting of the Wisconsin Federation of Great Lakes Sport Fishing Clubs in Sheboygan.
Prey fish are sampled in the lake by bottom trawl (annually since 1973) and acoustics (most years since 1992).
Among key findings, the alewife biomass was 0.54 kilograms/hectare in 2018, up from the all-time low of 0.02 the previous year. And the total prey fish biomass was 6.22 kg/ha in 2018 compared to 3.77 in 2017.
In addition to alewife, bloater chub, smelt, deepwater sculpin and slimy sculpin also showed year-over-year increases.
However, Madenjian noted the alewife biomass estimate was fifth-lowest and the overall prey fish biomass was eighth-lowest since the bottom trawl was initiated in 1973.
"Historically, the forage base is still at a low level," Madenjian said.
Prey fish in the lake are subjected to bottom-up and top-down effects, Madenjian said.
Less food is generally available to prey fish because of reduced phosphorous inputs to the lake and a massive invasion of zebra and quagga mussels, filter-feeding organisms which remove plankton from the water column.
And predator fish form the other effect by feeding on alewife and the other forage species.
Last year, aging of about 400 alewives caught in the surveys found none older than 5 years. About 80% were age 1 and 2, Madenjian said, referring to the alewife population as "truncated."
"That's a young population, much younger than we used to see in the lake, and a sign of substantial predation," Madenjian said.
The prey fish data is considered by fisheries managers as they attempt to balance the number of predator and forage fish in the lake.
Several stocking reductions have been enacted over the decades to avoid a crash of the fishery.
The most recent period of reduced plants was initiated in 2013, when chinook numbers were cut by 50%. Another round of reductions followed in 2017, when brown trout and lake trout were cut by 50% or more.
The Lake Michigan efforts have prevented a collapse of the chinook fishery similar to what was observed after alewife declined precipitously in Lake Huron in the early 2000s.
In fact, in 2018 the weight of 3-year-old female chinook in Lake Michigan was heaviest since 1986. The weight is determined from fish lake-wide, said Nick Legler, Wisconsin DNR fisheries biologist.
Legler said while relatively big chinooks the last three years is encouraging, the weight has varied substantially over the last decade which is "concerning and suggestive of ecosystem instability."
Recent improvements in Lake Michigan fish size can likely be attributed in part to improved predator/prey ratios following recent stocking adjustments to protect and sustain the ecosystem and fishery, Legler said.
The alewife population in Lake Michigan, assessed by bottom trawl (top) and midwater trawl (bottom), is "truncated," or has relatively few older year classes, according to USGS scientists.
The alewife population in Lake Michigan, assessed by bottom trawl (top) and midwater trawl (bottom), is "truncated," or has relatively few older year classes, according to USGS scientists. (Photo: U.S. Geological Survey)
A key measure of the fishery, called the predator prey ratio, has not been calculated for 2018 but is expected to be completed in the coming weeks.
For 2018, the DNR planned to stock 810,000 chinook, 450,000 coho, 390,000 steelhead and 362,000 browns (actual stocking numbers are not yet available from the agency).
The 2019 plans are similar, but this year DNR fisheries staff is considering stocking options for 2020 and beyond.
Pressure is already mounting on the agency to increase stocking.
"Let's get to the bottom line," John Hanson of Racine said at Saturday's meeting. "When can we start putting more chinook in?
Madenjian, a research scientist, diplomatically demurred, saying his role was to collect data, not make management decisions.
The DNR will likely hear from many in the Wisconsin sport fishing community over the coming months about their preferences, perhaps especially brown trout enthusiasts who most recently sustained a 50% stocking cut.
The DNR said it plans to engage stakeholders this year at public meetings to "begin discussions about Lake Michigan management and collaboration options for 2020 and beyond."
Information on the meeting content, dates and locations will be discussed at the 7 p.m. Monday gathering at the Lake Michigan Fisheries forum at Lakeshore Technical College in Cleveland, Wisconsin.
A list of the public meetings, likely to be held this summer, will then be developed and released in the coming weeks or months.
