At the end of July 2021, Ludington, Michigan served as the last stop for a 101-foot research vessel monitoring Lake Michigan’s population of alewife, an important prey fish that has struggled in recent years.
The ship, called the R/V Sturgeon, collected over 300 samples of alewife larvae and also sampled the microscopic plankton they eat, said David Bunnell, the biologist leading the research for the U.S. Geological Survey.
The Sturgeon arrived in Ludington on Saturday and left Tuesday morning for its home port in Cheboygan, Bunnell said.
USGS scientists aim to compare the samples with others taken in 2015, which indicated alewives were “not thriving,” Bunnell said.
The 2015 samples showed that larvae were growing 40-percent slower and had less food in their stomachs than in the early 2000s, Bunnell said.
A weak alewife population can mean less salmon stocked in the lake, as the salmon “rely heavily” on them for prey, said Jay Wesley, Lake Michigan basin coordinator for the Michigan Department of Natural Resources.
Reductions in salmon, a fish not native to Lake Michigan, can be “very unpopular” when anglers have come to expect the fish on their hooks, Wesley said.
“Our fear, or concern, is that we get too low levels and (alewives) totally crash,” Wesley said.
This happened in Lake Huron in 2004, and that lake’s alewife and salmon populations have not yet recovered, Wesley said.
Because Lake Michigan alewives tend to be leaner than they were about 20 years ago, salmon may have to eat more of them to get the same nutrients, Wesley said.
Alewives are also younger overall than they used to be, likely due to increased predation, he said.
“We used to have alewife up to nine years old,” Wesley said. “Now they only get up to about 6 years old, mostly because they’re preyed upon before they get to those older ages.”
While the alewives’ predators are sometimes blamed for overeating, alewives could also be having trouble finding prey of their own, Bunnell said.
The Sturgeon sampled the water for zooplankton — a microscopic animal eaten by alewives — as well as for an algae eaten in turn by the zooplankton. Using these samples, scientists will assess the food web’s weaknesses, Bunnell said.
“There’s not as much plankton as there used to be,” Bunnell said. “The lake is more clear.”
The shortage of plankton is credited in part to invasive zebra and quagga mussels, Bunnell said. Earlier this month, an Environmental Protection Agency ship called Lake Guardian was sampling the lake’s mussel populations, as it does every five years, he said.
Though Lake Michigan’s alewives are weaker than they were in their pre-2000s heyday, they’re stronger than they were about five years ago, Wesley said.
“I would say they’re slightly on the rebound,” he said.
Michigan stocked about 880,000 salmon in the lake this year, up from an “all-time low” of about 330,000 in 2017, Wesley said — but there was a time when the state would contribute around 3 million, he added.
Nine sites along the coast were sampled, with seven samples taken at each site at increasing distances from the shore. Water was sampled as close as three miles to shore and as far out as 24 miles, Bunnell said.
The Lake Explorer II, an EPA ship, sailed from its home port in Duluth, Minnesota to assist with the collection.
The alewife study is being conducted through the EPA’s Cooperative Science and Monitoring Initiative, which coordinates research in one of the five Great Lakes each year.
Bunnell said he expects the study’s preliminary findings to be presented next spring and formally reported through the CSMI in 2023.
Ludington Daily News Staff Writer
Justin Cooper can be reached at firstname.lastname@example.org.