LANSING — Denise Purvis’ family began fishing the waters of northern Lake Huron off Manitoulin Island in 1882. Over the years, they came to expect the unpredictability of a livelihood that depended on their ability to capture wild fish.

Purvis came back to the family business in the mid-1990s after college. Her return coincided with the arrival of zebra and quagga mussels into the Great Lakes.

The mussels have become synonymous with the problem of invasive species in the Great Lakes. They’ve colonized the lakes and damaged their ecology.

For Purvis, based in Ontario, and a dwindling number of Great Lakes commercial whitefish fishers, the fishery has fallen on hard times. Whitefish have declined across much of lakes Michigan and Huron, and many scientists and fishers suspect part of the reason is the effects of mussels on the lake’s food web.

“The health of our fishery in northern Lake Huron is not healthy whatsoever,” Purvis said.

Dave Caroffino is a fisheries biologist in Charlevoix, working in the tribal coordination unit in the Fsheries Division of the Department of Natural Resources.

Since 1985, the agency has collected data on whitefish in lakes Michigan, Huron and Superior.

“The vast majority of the monitoring data starting in 1985 comes from agency staff collecting biological samples from the fish caught by commercial fishers,” Caroffino said. “That data wasn’t used for a lot of stuff. It was kind of general patterns, general trends.”

But now the data shows clear declines in whitefish—declines that coincide with the expansion of invasive mussels.

The agency’s estimates of whitefish biomass in northern Lake Huron dropped 45 percent from a peak in 1997, when the mussels began to widely colonize the lakes, through 2017, when quagga mussels had covered much of the lake bottom.

Invasive mussels and whitefish

Whitefish are native to the Great Lakes. They’re bottom feeders, foraging for invertebrates like diporeia, a relative of shrimp that grows to less than 1 centimeter — 4/10ths of an inch — long. The diporeia live in the sediment of the lakes, feeding on material like plankton.

Since the 1990s, diporeia numbers have plummeted in most of the Great Lakes.

Because mussels are filter feeders, pulling plankton out of the water, some experts think the invasives caused the disappearance of diporeia and declines in whitefish.

Steve Pothoven, a fishery biologist at the National Oceanic and Atmospheric Administration’s Lake Michigan Field Station in Muskegon, studies the relationship between diporeia and whitefish.

“Lake Michigan had a spring phytoplankton bloom that would feed the diporeia,” Pothoven said.

Now, mussels feed on the plankton all winter.

In Lake Michigan, “that spring bloom is gone now, and that is thought to be a consequence of the mussels,” he said.

Is that enough evidence to blame the loss of diporeia and drop in whitefish numbers on the mussels?

“It seems like it should be really straight forward, if you look at a food web, but it’s been really complicated,” Pothoven said.

Ashley Elgin is a benthic ecologist, also at the Lake Michigan Field Station, who studies benthos: the life living in and on the lake bottom. Quagga mussels are the focus of her research.

To understand what’s happening on the lake bottom, scientists use a ponar grab sampler: a set of metal jaws lowered to the bottom to snap up sediment and benthos. Researchers sample 150 sites in Lake Michigan and 100 in Lake Huron every five years.

“We survey 46 sites in the southern third of Lake Michigan and we see (diporeia) in only one site,” Elgin said.

That site historically had thousands of diporeia in a grab.

“Now we get excited if we see 20,” she said.

Elgin echoes Pothoven in noting the difficulty in blaming diporeias’ collapse solely on the mussels.

“You had diporeia decline in Lake Huron at the same time as Lake Michigan, when the mussel numbers were very low in Lake Huron,” Elgin said. “Also, Lake Superior has low food levels but healthy diporeia populations.”

Commercial fishers see problems

Jamie Massey has been fishing northern Lake Huron out of St. Ignace for 44 years. He sees a link between the mussels, diporeia and whitefish.

In the past, “we’d lift our trap nets and see diporeia all over the deck of the boat and hanging all over the trap nets,” Massey said. “We watched mussels) come in, filter everything out, and slowly but surely we could see the diporeia disappear and the health of the whitefish deteriorate day by day.”

With diporeia scarce, whitefish began to eat quagga mussels, though they’re less nutritious. Fishers began catching fewer, thinner, less commercially valuable whitefish.

Slime rises in the lakes

As mussels pulled plankton out of the water, lakes Michigan and Huron cleared dramatically. That allowed sunlight deeper into the lakes, opening new habitats for cladophora, an algae that grows in stringy masses on lake bottoms where fishers place their nets.

With cladophora present around Manitoulin Island, Purvis’ crew must carefully consider when to fish. On windy days, cladophora gets picked up and trapped in nets, wrecking fishing gear. It can be so bad they have to pull up the nets, losing fishing days.

Purvis is lucky. The company also sells fish wholesale. But these changes in Lake Huron have altered operations at Purvis Fisheries.

“What’s changed for us to stay in business, now we have to buy a lot of fish that we never bought before,” Purvis said.

“Now I spend my whole time in the spring looking to people to buy fish. I have a harder time keeping employees and keeping those guys employed.

“My company in the end can still make money,” she said, but her employees who do the fishing can’t.