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Research continues on alewife population

• By Steve Begnoche Special to the Daily News
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  • Mar 28, 2023 Updated 14 hrs ago

 Alewife biomass in Lake Michigan 2022 surveys didn’t show a significant increase in numbers. Nor has a cause been determined for the die-off that saw carcasses of the important prey species rotting on beaches last summer.Dave Warner, a research fisheries biologist with the U.S. Geological Survey based out of the Great Lakes Science Center in Ann Arbor, shared data from trawls and acoustic surveys in 2022 during the recent Lake Michigan Ludington Regional Workshop.In general, the survey results didn’t find significant change in overall population of the silvery forage fish fed upon extensively by sport fish species such as steelhead, lake trout and salmon. Alewife make up nearly the entire diet of Chinook salmon, which in turn is the backbone of the local charter fishing industry.In recent years, some anglers had challenged findings of the annual prey fish surveys stating they were seeing more alewife than the surveys indicated present.The surveys are an important component of fish stocking management decisions. Anglers contended Lake Michigan could support more Chinook salmon than were being stocked due to fishery managers’ concerns over the health of the prey population.Wanting to avoid a collapse of the forage base and sport fishery Lake Huron experienced years ago spurred the cautious approach.With reduced Chinook stocking in recent years, size of Chinook caught increased in Lake Michigan in 2022. The average size of catch, according to Jay Wesley, Michigan Department of Natural Resources Lake Michigan fishery manager, was about 18-19 pounds – up seven pounds from its low mark when stocking reductions began. This year, the DNR intends to plant 1 million more Chinook in Lake Michigan, in part because the larger size of fish caught indicates the lake can support more Chinook.Big Sable River at Ludington State Park will get 100,000 Chinook this spring, the first in several years. All Chinook plants now will be done using net pens such as the Ludington Area Charterboat Association puts out at the Big Sable.Warner and other researchers, meanwhile, continue to conduct the annual assessments using research ships from the DNR, USFWS and USGS to do trawls and acoustic surveys.Warner reported on work assessing the accuracy of the surveys and tests trying to determine bias in reporting numbers due to factors such as the accuracy of assessments due to ship noise.Initial findings of the 2022 assessments trawls out of six Lake Michigan ports, including Ludington, used for years in the survey were reviewed.Deep water sculpin, bloaters, burbot and alewife made up the primary catch. Alewife appear to be remaining in deeper water, farther offshore and longer than in the pre-dreissenid mussel (quagga and zebra) era.In 2021, 85 percent of the alewife sampled were yearlings, Warner said. Alewife aging from the 2022 catch continues so the makeup of it had not been determined.The fall forage survey of 80 deep- and 37 mid-water trawls as well as 25 acoustic transects showed slightly different results compared to 2021, but Warner said it didn’t indicate a pattern.The highest density of alewife were off more nutrient rich areas such as Green Bay and the Garden Peninsula, as well as in southern Lake Michigan where water entering the lake tends to be murkier and contains more nutrients. Quagga mussels, which now cover the bottom of Lake Michigan efficiently filter the water storing phosphorous and other nutrients away from the food chain that previously supported the base of the food chain leading to a sport fishery.Alewife numbers remain low compared to pre-mussel years.Warner described work done in part with Michigan Tech to answer whether ship noise affects the accuracy of the acoustic survey assessments.Each research vessels has its own sound profile. Fish density results from the Baird, the Steelhead, and the Sturgeon which operate on Lake Michigan and the Arcticus and Kiyi and a sail drone used in 2022 were compared.The wind-powered sail drone suspends a measuring device beneath it as it moves through the lake without an engine that causes noise in the water below it – noise that potentially could scare away skittish forage fish researches want to assess.At times, research vessels overtook the sail drone. Warner said that allowed for a comparison of results from each vessel independently, compared to one another and compared to when passing the sail drone to determine if the noise made by the ships changes the results.“This process is complicated,” Warner said. “But we don’t see any evidence it is causing problems with acoustic survey compared to the sail drone. If ships are scaring fish away, we would expect different numbers.Two other possible sources of error in acoustic sampling have been identified, Warner said.A blind zone immediately below the measuring vessel is possible due to the width of the signal, which spreads out as it goes deeper into the water, is so focused and narrow at the surface. The other is at the bottom, where because of the way the sound waves used in measuring travel as they bounce back towards the surface, density of fish in the water column can be under-recorded by as much as, in the case of walleye, 60 percent.“We simply can’t see that part of the water column. In Lake Huron, alewife densities could be off by 50 percent,” he said. They are working on how to predict and react to potential error.An autonomous underwater vehicle (AUV) was used in 2022 to measure fish density in Lake Michigan and Lake Superior in the upper blind zone and bottom dead zone, Warner said.The AUV is a torpedo-shaped device hauled near the middle of the water column measuring fish presence below and above it.The 2022 data is being studied now, so no comparison was offered.Plans for 2023 are similar to work done in 2022, Warner said, with spring bottom trawl sampling to begin in mid-April. Acoustic surveys will be added above bottom trawl depth.“We will be able to see if there are fish there or not,” Warner said.Sail drone and AUV will be added to August and fall surveys, but where they will be used in Lakes Michigan, Huron and this year, Erie, is still being determined.Warner noted improvements to onboard technology to connect the ships sounder, GPS, water thermometer and weather system to create an automated recording of each will be ready for this season.“This 20th century tech should help us a lot,” he said, noting until now, such recordings had to be done by hand.