Fishy Situation


Thousands of goldfish have taken over a lake in Colorado. (Boulder Camera)

Get an introduction to invasive species with our activity.

Teachers, scroll down for a quick list of key resources in our Teachers’ Toolkit.

Discussion Ideas
Read through our activity “Introduction to Invasive Species.” Apply its questions to the fish in Teller Lake.

  • What type of fish are “taking over” Teller Lake?
    • Trick question! The lake is being populated by either goldfish or koi, fish that are closely related, but actually different species. Both are types of domesticated carp, and both were bred for decorative (coloration) purposes. (Learn more about domestication here.)
      • Goldfish were bred from Prussian carp, native to freshwater lakes in Siberia. The most common colors of goldfish are orange, white, and yellow.
      • Koi were bred from common carp, native to rivers and lakes throughout Europe and Asia. The most common colors of koi are orange, white, black, yellow, and blue.
      • If you get nose-to-nose with these fish, you can see if they have catfish-like “barbels” near their mouth. If they do, they’re probably koi. If they don’t, they’re probably a variety of goldfish!



  • Are the fish an invasive species?
    • Probably, although their impact on the ecosystem is still being evaluated. Not all introduced species are considered “invaders.” Even in Teller Lake, the non-native population of tiger muskies (fish native to the chilly freshwater of the Great Lakes and St. Lawrence River) are not considered invasive—”because they help ‘manage’ the undesirable population of suckers” in the lake.


  • How might the koi or goldfish impact the ecosystem of Teller Lake?
    • According to the video, “we don’t know what kind of diseases [the introduced species] might carry; they can outcompete the native fish” for food and other resources. (Like all carp, koi and goldfish are omnivores.)
    • The huge number of constantly-moving fish can also disrupt the turbidity of water, which may have a ripple effect (get it?) on the habitat. Koi and goldfish are fairly big, fast-moving fish, and often feed near the bottom of the lake. This behavior can stir up mud and make the water cloudy. Such cloudiness (an increase in turbidity) reduces the amount of sunlight that can penetrate the lake’s upper layers. This makes it more difficult for aquatic plants, producers in Teller Lake’s marshy ecosystem, to photosynthesize and grow. Any reduction in plant life impacts the lake’s entire food web.


  • One of the possible responses to the koi invasion is draining the lake. What does this mean?
    • Just what it sounds like. Hydrologists can use canals, dams, and pumps to permanently or temporarily drain the water from a lake. This is sometimes done to eradicate a species (such as mosquitoes) from a region. Lakes and other bodies of water can also be drained to prevent floods or expose land for farming, housing, or industry. (The Netherlands has famously drained entire regions along its North Sea coastline to create fertile, low-lying tracts of land called polders.)





Boulder Camera: Non-native fish may mean draining one Boulder lake, monitoring at another

Nat Geo: Introduction to Invasive Species

Nat Geo: What is an invasive species?

Nat Geo: What is domestication?

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