What’s up with that: why do the big fish rise near the NU bridge?

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What’s big, scary, and unattractive that congregates under and on both sides of the bridge connecting the Northwestern University campus to the lake fill area?

The Round Table was surprised by the response.

But really, the Round Table was generally surprised at the behavior of the fish: more large fish than could easily be counted clustered on either side of the bridge, prevented from entering the artificial lake by the rocks forming a dam , but ignoring free access to the great waters of Lake Michigan.

The Roundtable couldn’t convince Northwestern to answer our questions, so the Roundtable took pictures of the fish and sent them to the experts at the Shedd Aquarium in Chicago. According to the fish and research teams at Shedd, the fish act normally for what is likely based on common carp or grass carp.

Shedd scientists sent photographs back to the panel explaining the differences between the two types of carp.

  • Common carp have an elongated dorsal (upper) fin, barbels or whiskers on either side of the mouth, and a thicker body structure.
  • The grass carp is slimmer, has no barbels near the mouth and no scales on the head, and a single dorsal fin. They can grow up to 4-5 feet.

According to the Clean Lakes Alliance website, carp can weigh up to 80 pounds and live an average of 20 years. (Who knew?) Spring to early summer is their breeding season and “female carp can produce hundreds of thousands of eggs during the breeding season”.

Carp are edible, although they have a strong flavor and darker flesh. (Traditionally, carp is one of the main ingredients in gefilte fish. Pro Tip: Ambitious cooks If you are making your own gefilte fish from scratch, we strongly advise you to befriend a local fishmonger and order boneless, ground carp.)

The roundtable also asked the Shedd why carp were acting in ways that seemed counter-intuitive to fish health and welfare.

Johnny Ford, the Shedd’s public relations director, kindly forwarded the scientists’ response. It said: “Common carp prefer shallow waters with soft bottoms and because they root around eating a variety of foods, including aquatic vegetation and bottom insects, they often stir up sediment.

Looks like they’re not stuck in there, but probably enjoying the food and feeding on it and swimming later. They usually hang out near the bottom of the water column, which might explain why you don’t usually see them. Maybe because it’s so clear right now, that’s why you’ve been seeing them lately.

Glad we asked. Now we know.

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