It all starts with an aquaticfood web
Every ecosystem has a food chain – a network of species that rely on each other to maintain a delicate natural balance. Lake Michigan is no exception.
A food web is made up of layers of four energy levels: Top Consumers, Primary Consumers, Secondary Consumers, and Primary Producers and Decomposers. Here are some of the cast of characters who fill these roles in Lake Michigan:
Piscivorous fish are large predator fish like salmon, trout, burbot, and walleye that eat other smaller fish.
Planktivorous fish eat plankton and are eaten by predator fish. They are also called prey species or forage fish.
Primary consumers are tiny organisms such as zooplankton and macroinvertebrates that live in the water column or in the sediments at the bottom of the lake and feed on primary producers.
Primary Producers + Decomposers
Primary producers are tiny plants that live in the water column and convert sunlight and nutrients into biomass. Decomposers are tiny animals that feed on detritus and release nutrients for producers to use. Producers and decomposers are the foundation of the food chain.
History of a Changing Ecosystem
Food webs are a delicate balance. One little change can throw the whole system into disarray, Lake Michigan's food web has changed significantly in the last 200 years, all because of a few invasive species with monumental impact.
The little change that started it all was the introduction of two non-native species, the sea lamprey and the alewife into the Great Lakes. By the 1950s, sea lamprey had decimated the native lake trout population. Another non-native species, the alewife, had been kept in check by the lake trout predation. Absent a predator, the alewife population exploded and then crashed, causing massive, stinking die-offs along beaches.
Great Lakes fisheries managers were on the lookout for a new predator to help help keep alewife populations in check. During the 1960s, they started stocking the lakes with Chinook and Coho salmon. This was a spectacular success, for a time. Not only did the salmon help balance the ecosystem in the lower Great Lakes, but it also created a multi-billion dollar sport fishery.
But more changes were in store. During the late 1980s, invasive Zebra and Quagga mussels arrived in the lakes via ballast water discharge from ocean-going lake freighters. The mussels consume much of the primary production in the lake ecosystems, leaving less for higher levels of the food web. Now, prey species are declining and so, too, are the salmon that depend on them.
Native Great Lakes peoples fished the Great Lakes from canoes or ice with spears and hooks made of bone, antler and copper. They fished plentiful stocks of native lake sturgeon, northern pike, and suckers.
The Welland Canal connecting Lake Ontario to Lake Erie allows ship traffic to bypass Niagara Falls and access the upper Great Lakes.
Alewives and sea lamprey enter Lake Erie from Lake Ontario via the Welland Canal. Sea lamprey are parasitic, jawless fish that feeds by attaching themselves to their victims. Alewives are small prey fish.
Commercial and charter fishing expand in the Great Lakes.
Native fisheries decline from overfishing. Ciscoes (lake herring and chubs), lake trout, and whitefish populations plummet.
Sea lamprey discovered in Lake Michigan
By the 1950s, sea lamprey decimated the native lake trout population. This caused the alewife population to explode in the absence of a predator.
During the 1960s, Chinook and Coho salmon were introduced into the lake to control the alewives. The experiment was a success, and the result was a multi-billion dollar sport salmon fishery that lasted into the first decade of the 21st century.
Zebra mussels discovered in Lake St. Clair.
Quagga mussel discovered in Lake Erie.
The Round goby discovered in St Clair River.
Since 2002, salmon and alewife populations in Lake Huron have collapsed, and in Lake Michigan, they are on the decline. The main culprit? Another invasive species; this time a primary consumer. The Quagga mussel.
The Zebra and Quagga mussels are efficient filter feeders and consume much of the primary production in the lakes. This leaves little over for prey species and the predators who depend on them. In 2004, alewife and salmon populations collapsed in Lake Huron.
Native lake trout and whitefish have learned to feed on yet another invasive species, the round goby. Chinook salmon, however, have not made the switch. The result is rising populations of native lake trout, cisco and non-native steelhead as Chinook populations decline.
Chinook salmon population in Lake Michigan begins decline.
Great Lakes states consider reduction of Chinook stocking.
Great Lakes Salmon Sport Fishery
Salmon populations decline
Prey Populations at Historic Lows
The biomass of prey species preferred by salmon is at an historic low in Lake Michigan. Fish have less to eat… and what they eat has less to eat. The end result is less salmon.
Changes in Lake Michigan
As the density of quagga mussels has increased in Lake Michigan, the density of the zooplankton Diporeia has declined. This means less to eat for prey species that some top predators like Chinook salmon depend upon.
Slide down the timeline to see changing mussel and plankton density in Lake Michigan
Great Lakes Salmon Sport Fishery
Total Economic Impacts: 1990-2009
Proposed Stocking Plans
The current proposal calls for a 25 percent reduction in Chinook salmon stocking across the Great Lakes. The goals are to preserve some fishing opportunities without allowing the prey species to collapse. These efforts are coordinated with fisheries agencies in the other Great Lakes states.