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Illinois hunting and fishing

Asian carp are very efficient feeders when it comes to filtering plankton out of the water. Their diet overlaps with several native fish including gizzard shad, bigmouth buffalo and paddlefish. Low numbers of gizzard shad have caused some to speculate that Asian carp are out-competing shad for food. Chris Young/The State Journal-Register.

Are Asian carp hurting eagles?

January 30, 2010 at 08:39 PM

SPRINGFIELD STATE JOURNAL-REGISTER

It’s a concept that’s as hard to grasp as a slippery fish.

Even as the battle rages to keep invasive Asian carp out of Lake Michigan, the prolific fish already could be causing problems far beyond the Illinois River.

This winter, bald eagles have been seen in lower numbers than usual, leading some to wonder out loud whether or not the carp are hurting the eagles’ food supply.

At the Starved Rock Lock and Dam, throngs of visitors showed up as usual for Bald Eagle Watch Weekend, but only a handful of eagles were around.

Part of that could have been due to the weather. The river was free of ice and the eagles could go wherever they liked.

But something else was missing.

No line at the buffet

Starved Rock is a hardcore birder’s paradise. At the dam, gulls by the hundreds pick up fish that come through the dam. Bird watchers scan the airborne crowd of gulls looking for the rare needle in a moving haystack. Iceland, glaucous, lesser black-backed,
Thayer’s gulls and more can be spotted by the patient and tenacious.

This year, the gulls were all but absent. Something must have been amiss for both the eagles and gulls to have a reason to give up on the dam that serves up their food chopped or at least stunned.

According to Greg Sass, director of the Illinois River Biological Station in Havana, populations of gizzard shad, the plentiful forage fish that is food to eagles, gulls and largemouth bass, have crashed.

However, gizzard shad are a “boom and bust” fish and their numbers rise and fall cyclically with spawning conditions and other factors.

“Gizzard shad are way down for this year,” he says.

There are a lot of possible reasons why.

Sass says there are more “piscivores” – fish-eating birds and fish.

“There were more largemouth bass and more white bass this year than we’ve seen in a while,” he says. “So some of those predators could have knocked the population back.

“We caught more largemouth bass this year than we have in the past 15 years – in defiance of 15-year trend.”

Strong spring floods made for good spawning conditions for bass.

Asian carp could play a role, too.

Sass and his colleagues at the field station have authored a paper documenting that gizzard shad body condition has declined since Asian carp came on the scene in about 2000. In short, the shad are skinnier.

“That kind of foreshadows a little bit of competition,” he says.

Still, the Illinois River is a tremendously productive environment, with lots of plankton to go around – for now.

“As a group, some of the things we are focusing on now are zooplankton,” he says. “We are finding that the larger zooplankton have disappeared leaving smaller bodied plankton that only the Asian carp can process.”

The Asian carp have gill rakers that filter the food out of the water column. They have the ability to take very small, microscopic animals like rotifers, whereas gizzard shad and bigmouth buffalo can’t filter out organisms that small.

“Are the Asian carp ecosystem engineers?” he asks. “Beavers are the classic example. A beaver will dam a stream to create a pond to benefit itself.

“By the way the carp feed, there is pretty good evidence that there are some big changes going on.”

Out of balance

The natural ebb and flow of nature, like the ups and downs of shad populations, potentially can be pushed out of balance if factors like increased competition for food, poor spawning conditions, additional predation and others are piled on.

The difficulty for scientists is that it can be almost impossible to say for sure what is causing the problem at a given moment.

“It’s too early to tell if this is something to be concerned about,” Sass says. “They are a real boom or bust kind of fish, and we don’t know what is going to happen this year.

“Conditions could be just right and numbers could just explode.”

John Chick, director of the Great Rivers Field Station near Brighton has been studying Asian carp for a decade. He says he was just about ready to publish a paper noting a 15-year trend in a decline in gizzard shad populations when numbers shot back up unexpectedly.

“Zoo and phytoplankton are extremely abundant,” he says. “Asian carp populations would seem to have to increase even more - or it may take even more time (for changes to become noticeable).
“Maybe we’re just starting to see the first effects,” Chick says. “It may take several years or decades before you will see consequences.”

“That is the blessing and the curse of long-term data,” Sass says. “We start to see some trends and then something surprises us.

No easy answers

With up at least 50 species of fish in the Illinois River all interacting with one another and other organisms - both in and out of the water – it can be frustrating for scientists to point to a single cause for any ecosystem problem.

“That’s what get people very frustrated with ecology,” says Chick. “You start to think about how many variables interact in a system, and before you even think about how many fish species are at Starved Rock – it all gets very complex.”

Chick says scientists used to draw very simple food chains, but nature doesn’t go in a straight line. It’s not a chain, but a food web with lots of connecting points.

“Sometimes we forget that there are lots of other factors that affect population dynamics,” he says. “Yes, the availability of prey is an important factor in any ecosystem. But temperature, available oxygen, physical and chemical factors and disease also are variables.

“I think we downplay the role of disease,” he says.

Unintended consequences

As for those who theorize the carp may be hurting the shad and in turn depriving the eagles, Chick says the idea has merit, but is difficult to prove.

“It’s awful hard to point to it right now, but they may be right,” he says. “That’s the frustrating thing.
“If we’re rally seeing a decline in gizzard shad and then bald eagles, where are the bald eagles going from here?

“We might be affecting ecosystems up north,” he says. “They might see a reduction in eagle numbers up there.”

The take home message is tinkering with ecosystems at home could cause unintended consequences elsewhere.

“Who would have ever predicted that a silver carp coming into the Illinois River might affect eagles in Canada?

“That shows how complex this world is.”

 

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I’m just wondering, How much longer until the HOG population takes off up here.

Posted by HawgNSonsTV on 01/31 at 05:06 AM

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