Rainbow smelt, small, silvery fish known for their cucumber-like scent, once held a prominent position in the Lake Michigan ecosystem. Historically abundant, these fish were a familiar sight, playing a role in the aquatic food web and local communities. Their presence supported a popular recreational fishery, particularly during spring spawning runs when anglers gathered along shorelines. Their numbers made them a significant component of the lake’s biomass, influencing other species and fishing practices.
The Smelt’s Introduction and Abundance
Rainbow smelt were not originally found in Lake Michigan but were introduced in the early 20th century. In 1912, landlocked rainbow smelt eggs were stocked in Crystal Lake, Michigan, to provide forage for introduced Atlantic salmon. Smelt escaped into Lake Michigan by 1923, rapidly spreading throughout the Great Lakes.
Conditions in Lake Michigan proved highly favorable for smelt, leading to a population explosion. A lack of natural predators, particularly after the decline of native lake trout due to sea lamprey, allowed smelt numbers to flourish. Smelt, primarily zooplanktivorous, found abundant food sources in the lake’s rich zooplankton communities. This propelled their populations to peak abundance, establishing them as a major commercial and sport fishery throughout the mid-20th century, with catches reaching millions of pounds annually.
Primary Drivers of Smelt Population Decline
The decline of smelt populations in Lake Michigan stemmed from other invasive species and changes in the lake’s food web. A major driver was the proliferation of the invasive alewife, another non-native fish abundant in Lake Michigan by the 1960s. Alewives competed with smelt for zooplankton, a primary food source, and also preyed on the larvae of other fish.
Further pressure came from the widespread stocking of Pacific salmon species, such as Chinook and Coho, beginning in the mid-1960s. These salmon were introduced partly to control alewife populations, but they also heavily preyed on smelt, which became a substantial part of their diet. This increased predation from a recovering salmonine population added top-down pressure on smelt.
Broader ecological changes within Lake Michigan also contributed to the smelt’s decline. The invasion and expansion of dreissenid mussels, including zebra and quagga mussels, altered the lake’s food web. These mussels are efficient filter feeders, removing large quantities of phytoplankton and seston from the water column. This filtering led to increased water clarity and a reduction in primary production, diminishing the zooplankton available for smelt. Coupled with oligotrophication, or reduced nutrient pollution, the food web’s base became less productive, further stressing the smelt population.
Ecological Changes Following Smelt Decline
The reduction in Lake Michigan’s smelt population had effects throughout the aquatic ecosystem. Smelt had served as forage fish for many native and stocked predators, including lake trout and various salmon species. With the decline of smelt, these predators shifted their diets to other available prey.
Changes in the zooplankton community, partly driven by invasive mussels that reduced the food base for smelt, also impacted the lake. While smelt consume zooplankton, the overall structure of zooplankton communities in Lake Michigan has shifted, with some groups decreasing and others increasing. These effects show the interconnectedness of species within an aquatic environment, where the decline of one abundant species can influence many others.
Present Status and Outlook for Smelt
Smelt populations in Lake Michigan remain at lower levels compared to their historical peak. Their numbers declined from the mid-1980s, and commercial harvest fell from over a million pounds annually to less than 2,000 pounds per year since 2013. While not absent, their current biomass represents only a fraction of their long-term average.
Despite indications that smelt productivity has seen a slight increase since 2000, the survival of new recruits to adulthood remains low. Fisheries managers in Lake Michigan do not prioritize efforts to restore smelt populations to their former levels, as smelt are a non-native species. The focus instead lies on managing the overall health and balance of the lake’s ecosystem, which includes supporting native species and managing predator-prey dynamics. Smelt continue to exist in Lake Michigan, but at a much lower equilibrium than their peak, reflecting a changed ecosystem.