A fishy tale of how invasive species shaped an ecosystem and a fishery
It all starts with an aquatic food 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.
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. Here's how Lake Michigan's food web has changed
Lake Michigan has been transformed since humans first began exploiting it for shipping, industry and recreational and commercial fishing.
The changes began in the 1830s with the first shipping channels connecting the Great Lakes with the St. Lawrence Seaway and the Atlantic Ocean. These pathways allowed exotic species to enter the Great Lakes. Humans introduced Chinook salmon in the 1960s as a way to rebalance the local food web while creating a sport fishery.
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.
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.
Logging of most of Michigan's virgin forest resulted in loss of fish habitat and dammed tributaries.
Commercial and charter fishing expand in the Great Lakes.
Native fisheries decline: Ciscoes (lake herring and chubs), lake trout, and whitefish populations plummet.
Sea lamprey discovered in Lake Michigan
In the 1950s, the sea lamprey, a parasitic, jawless fish that feeds by attaching itself to its victims, decimated the native Lake Trout population. This caused the alewife (another invasive species) 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.
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? Yet another invasive species; this time a primary consumer,the Quagga mussel, has altered the lake ecosystem once again.
The Zebra and Quagga mussels are super-efficient filter feeders and consume much of the primary production biomass in the lakes. This leaves little over for secondary consumers, or prey species, and hence the predators who depend on them. IN 2004, Alewife and salmon populations collapse in Lake Huron.
Chinook salmon population peaks in Lake Michigan and begins decline.
Native Lake Trout and Whitefish have learned to feed on yet another invasive species, the Round Goby, a bottom feeder that depends less on the primary consumers in the water column for its diet. Chinook salmon, however, have not made the switch. The result is rising populations of native Lake Trout, Cisco and non-native steelhead, even as Chinook populations decline.
Great Lakes states weigh reduction of Chinook stocking.
Abundance of Species Vs. Time
Great Lakes Salmon Sport Fishery
The salmon sport fishery in the Great Lakes is significant, generating millions of dollars in local economies. Anglers spend money on food, gas, hotels and entertainment, which builds local economies and provides jobs in coastal Michigan towns.
Total Economic Impacts: 1990-2009
Historic Low Prey Biomass
However, that fishery is on the decline. The biomass of prey species preferred by salmon is at an historic low in Lake Michigan.
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 denisity in Lake Michigan
Proposed Stocking Plans
The current proposal calls for a 50 percent reduction in Chinook salmon stocking across Michigan. 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.