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Predicting Smelt Runs

smelt-photo-jeff-gundersonEach spring in the tributaries of Lake Superior, sportsmen gather to fish for smelt. How do they know where and when to find the smelt? How do they scoop them up? Bring your dip-net, bucket and waders and look at environmental patterns for clues. The U.S. Geological Survey (USGS) provides water data that can help you plan a successful dip-netting outing like those along Lake Superior. And don’t forget to bite the head off the first smelt you catch — to ensure good luck during the run.

Grade Levels:

  • National Science Education Standards, 5-8 grade
  • Michigan Grade Level Content Expectations, 5-7 grade

Performance Expectations:

  • MS-LS1-4 From Molecules to Organisms: Structures and Processes. Use argument based on empirical evidence and scientific reasoning to support an explanation for how characteristic animal behaviors affect the probability of successful reproduction of animals.
  • HS-LS4-5 Biological Evolution: Unity and Diversity. Evaluate the evidence supporting claims that changes in environmental conditions may result in: (1) increases in the number of individuals of some species; (2) the emergence of new species over time; and (3) the extinction of other species.

For alignment, see: NGSS Summary

Objectives

  • Describe rainbow smelt populations living in Lake Superior.
  • Identify environmental factors that influence smelt spawning.
  • Graph water data.
  • Draw conclusions about the timing of smelt spawning using water temperature and water discharge data as evidence.

Background

Smelt were brought into the Great Lakes in the early 1900s to help stocked Atlantic salmon survive. Smelt were initially stocked in Crystal Lake, in northwest Michigan, and a population soon spread to Lake Michigan and other Great Lakes. They were first found in Lake Superior in 1930.

In the water, rainbow smelt are shimmering and colorful, which inspired their name. Removed from the water, they quickly fade to a silvery white. Out of the water, rainbow smelt are said to emit an odor like freshly cut cucumbers.

The smelt move from lakes into tributaries in early spring to spawn. Spawning is related to spring thaw and often begins after consistent warming rain showers when water temperatures reach 5-7 degrees Celsius (C) or 42-44 degrees Fahrenheit (F). Spawning runs last about three weeks, or as long as temperatures stay in the preferred range. Photoperiod — or the changing length of the day — is an additional environmental cue for spawning.

Rainbow Smelt

Name: Atlantic rainbow smelt (Osmerus mordax).

Average size: 7-8 inches long (17-20 centimeters).

Lifespan: Most live 2-3 years.

Predators: Coho salmon, burbot, trout, walleye, yellow perch and other smelt.

Competitors: Alewife, possibly lake herring.

Factors influencing populations: 80 percent of variation in population size is related to spring rainfall and abundance of other prey species like smelt. Prey species are fish that are generally considered prey for larger fish.

Excessive rainfall can cause rivers and streams to fill up or overflow in the spring and may reduce smelt population size. While the abundance of other prey species may increase smelt population size because there are other fish for predators to eat.

Spawning: Smelt spawn in tributaries over gravel beds at night and return to the lake by morning. Spawning is related to spring thaw, water temperature and warming spring rains and usually lasts about three weeks. (Note: Photoperiod is an additional environmental cue, but in this activity, students will use only water temperature and water discharge as indicators.)

Habitat: Cool, dark waters. Optimal water temperature is 6-13 degrees C (43-56 degrees F).

Diet: Opossum shrimp, a small shrimp-like crustacean, insects and insect larvae, other aquatic invertebrates, fish such as smaller smelt, emerald shiners, sculpins, juvenile burbot and whitefish.

Fishing strategy: The best dip-net fishing is in tributaries at night between 10 p.m. – 2 a.m. during the spawning run.

What you’ll need: Dip-net and bucket, warm clothes, waders, life jacket, flashlight and Michigan fishing license if you are 17 or older.

Where to get more information: Department of Natural Resources Fishing Report Hot Line (517) 373-0908. Or if you feel confident after the activity, test your science detective skills.

Activity

  • Predicting Smelt Runs
    Summary: Students use U.S. Geological Survey water data to predict smelt runs.
    Time: One 50-minute class

Lesson Sources

Michigan Department of Natural Resources and Environment – Fisheries Division. Smelt Dipping Opportunities: Streams and Great Lakes Shoreline (PDF). Document accessed July 24, 2019.

Fish of the Great Lakes, Rainbow Smelt, Wisconsin Sea Grant online. Accessed: April 2013.

Data Sources

U.S. Department of Interior, US Geological Survey – Real-Time Data for Michigan. (Data accessed January 29, 2010; link updated July 24, 2019). US Department of Interior, US Geological Survey. Michigan Water-Data Support Team.

U.S. Naval Oceanography – Sun or moon rise/set for one year. Website accessed January 29, 2010. U.S. Naval Oceanography, Astronomical Applications Department, US Naval Observatory, Washington, DC.