The latest Lake Michigan assessment shows alewife remain near a record low

The biomass of alewife and other forage fish in Lake Michigan continues to sag near record lows, according to recently-released data from the U.S. Geological Survey.

Bottom trawls conducted in 2019 by USGS found the third-lowest level of alewife and third-lowest level of total prey fish since 1973.

The information was included in a report titled "Status and Trends of Prey Fish in Lake Michigan, 2019" presented Feb. 15 by USGS fisheries research biologist Chuck Madenjian at a meeting of the Wisconsin Federation of Great Lakes Sport Fishing Clubs in Sheboygan.

The forage fish play key roles in the ecosystem, including as food sources for salmon and trout.

The low levels continue a trend observed in the USGS assessments since the late 1990s.

Most scientists believe the drop has been due to a combination of factors, including invasive zebra and quagga mussels which remove plankton from the lake and reduce the amount of food available for other organisms, as well as predation on forage fish by salmon and trout.

The bottom trawls are performed annually at seven index sites in Lake Michigan, including Port Washington.

The 2019 trawling data showed a total lake-wide prey fish biomass of 1.77 kilograms/hectare, third-lowest on record. The five lowest totals have come in the last six years.

Bloater chub had the highest biomass, 0.78 kg/ha, followed by deepwater sculpin (0.47) and round goby (0.37).

Significant to sport anglers, alewife biomass was 0.07 kg/ha, third-lowest recorded in the bottom trawl.

Preliminary acoustic data also showed a drop of alewife in 2019 from 2018.

However, the final report on the acoustic work won't be available until later this year, Madenjian said.

The USGS findings are called misleading by many sport anglers and charter captains who report clouds of bait fish on their electronics.

And in 2018 the average weight of 3-year-old female chinook salmon captured lake-wide was 20.6 pounds, highest on record dating to 1986, indicative of an ample food supply.

Fisheries managers have long attempted to balance the populations of predator fish stocked in the lake with the available prey base.

In 2019, the Wisconsin Department of Natural Resources sided with charter captains and others in the business community and announced a 48% increase in chinook salmon stocking as well as hikes in coho salmon, steelhead and brown trout.

The DNR also announced it will attempt to conduct some of its own forage fish assessments. Beginning this year, the agency will work to develop a spring alewife survey to sample spawning aggregations of the fish at two Lake Michigan nearshore areas, said Todd Kalish, DNR fisheries manager.

Madenjian said the lake's forage fish were being squeezed by top-down and bottom-up effects.

Not only are predator fish eating the smaller fish, but other organisms, principally quagga mussels, have limited the amount of food available for forage fish.

Madenjian said the alewife age structure continues to be "truncated."

Out of 600 alewife analyzed for age in 2019, most were 1 or 2 years old and very few were older than 4, he said.

"The most logical explanation is the prey fish are getting gobbled up at a rapid rate by predator fish," Madenjian said.

In addition to the salmon and trout stocked in the lake, a substantial amount of natural reproduction occurs in tributaries, mostly in Michigan. About 70% of chinook salmon caught in Lake Michigan in recent years are wild fish, according to DNR fisheries biologists.

The final report of the 2019 USGS data, which will include acoustic testing, is expected to be released in the coming months.