A New Invasive Hits the Iowa Great Lakes

By Steve Weisman

I am sure by now that you have heard about the new “invasive” that has hit the Iowa Great Lakes. We’ve certainly had our share, and we have been warned by experts that there are a lot of other exotics knocking on the door. However, this is one that totally blindsided me: gizzard shad!

The Iowa Department of Natural Resources (DNR) discovered 26, 3-inch to 5-inch gizzard shad, in East Okoboji Lake on August 11 as part of routine sampling. They definitely are not native, and until this point have never been found in any of the Iowa Great Lakes chain. Right away the DNR published a release about the find.
“We don’t know how gizzard shad got into the lake,” said Mike Hawkins, fisheries biologist for the Iowa DNR at Spirit Lake. The discovery could negatively impact the Iowa Great Lakes.

It is pretty obvious that there is some reproduction going on, or there would be no 3-5 inchers. How many are in the lake? Who knows?

What are they?
Here is a little background on the gizzard shad (courtesy of Iowa DNR). This species is an omnivorous filter feeder taking both phytoplankton and zooplankton.

It is one of the most widespread and abundant of Iowa’s fishes. Except for the far northern central part of the state, it has been found in all the state’s major rivers systems, large reservoirs and man-made lakes. It is found throughout the Mississippi River as well as in parts of the Missouri River.

Gizzard shad have little value as a food-fish and are seldom taken by hook-and-line. Its flesh, and particularly the gizzard-like stomach, are occasionally fermented for use as catfish bait. Dense gizzard shad populations provide considerable forage as young for other predatory fishes, and their schooling behavior as young-of-the-year fish make them easy prey for larger predators.
Massive die-offs of young and yearling shad are commonly reported after spring ice-out because of their susceptibility to fluctuating water temperatures.

The issue
The problem is they don’t stay small enough to be baitfish for long, and they quickly outgrow the vulnerable forage size and rapidly assume pest levels in some closed watersheds or when predator populations are insufficient to control their numbers. Maximum size for adults is 9-14 inches.
Evidence has been collected showing that gizzard shad compete with young bluegill, crappie and largemouth bass for food, and when populations reach very dense levels, their survival is lowered. When the gizzard shad populations explode as they often do, the entire eco system can potentially be destroyed. This has been documented over the years in Iowa’s southern impoundments and smaller southern lakes, and it has been an issue and a thorn in the side of the Iowa DNR where populations have exploded. In some areas, fishing is decimated as is the local economy that relies on people coming to use their lakes.

Although it is illegal for anglers to possess gizzard shad as bait, there have been several documented occasions where gizzard shad have illegally been released in public waters. As a matter of fact, some bodies of water have been renovated, and then people dump gizzard shad back in the lake. The question is whether people think it will improve fishing or whether they are trying to sabotage the DNR.

At one time, eradication of the entire fish population and game fish species restocking, particularly in small lakes was the only way to restore acceptable fishing. More recently, biologists have found that using a low dose of the commonly used fish toxicant, Rotenone can kill the more susceptible gizzard shad, but most gamefish are not affected as severely. However, those treatments were on small lakes and not larger natural lakes like we have around here that contain dozens of other native fish species. These types of “targeted” renovations are just not feasible here.

How did gizzard shad get here?
I answer that with a couple of other questions? How did the zebra mussel get here? How about the yellow bass? Usually exotics get here with the help of human beings, whether knowingly or unknowingly.

It is very important to never transport and release any fish species into any public water body. Introducing species like yellow bass, common carp and gizzard shad can reduce native fish populations, decrease water quality, and limit fishing. Iowa law makes it illegal to possess live gizzard shad. It is also illegal to stock any fish in any public water of the state, including game fish. The public is asked to report any of this illegal activity to their local conservation officer or by calling the Turn-in-Poachers (TIP) hotline 1-800-532-2020. Callers can remain anonymous.
In addition, anglers should always dispose of unused bait in the garbage, not in the lake.

What it means for the Iowa Great Lakes chain
Who knows!?! That is the problem…we really don’t have a clue. They are a significant baitfish in Storm Lake, but this is the first time they have been found this far north in Iowa. We know how important the bluegill, crappie and bass populations are to anglers that fish the Iowa Great Lakes. In addition, Hawkins notes that the gizzard shad can also impact the yellow perch in a lake, and the perch fishery is extremely important around here both for anglers and as a baitfish.

Hawkins said, “We can’t predict their impact in the Iowa Great Lakes at this point. Because we are at the northern edge of their range our winters should hold their numbers in check. Gizzard shad can reproduce in large numbers and our continued mild seasons could favor this species.”

What it does do, however, is add one more species of fish to the fishery that does not belong. Hopefully, we are too far north, and they won’t be able to get a foothold. Young-of-the-year shad can be a quick meal. However, if the population does explode over the next few years, I believe they will have a profoundly negative impact on our lakes. Let’s hope it never gets to that point!

At this point, all that the DNR can do is monitor the gizzard shad and see what happens. As Hawkins said, “There is no way to eliminate gizzard shad from a lake without completely renovating and restocking the lake, and that simply isn’t an option here!”