Jumping several feet into the air, a single silver carp can knock a boat driver unconscious before a passenger even has time to say “Duck!” For that reason, the Illinois Natural History Survey station in Havana installed netting around the steering wheel and dashboard of its electro-fishing boats.
“It’s simply one of the most dangerous things that we’re doing, so we have to protect ourselves. We can’t have a fish jumping on the throttle [or] a fish knocking somebody out,” says Kevin Irons, INHS fisheries specialist.
The invasive species has become a common sight on the Illinois River and its tributaries, including parts of the Sangamon River, but leaping silver carp are only the most visible representation of a much deeper problem – one that scientists fear will soon spread to the Great Lakes, where Asian carp threaten a $7 billion fishing and tourism industry.
“Economic damage is the fish hitting people, people not wanting to spend time on the water,” says Irons. “But the ecological damage is much worse. … People can see this and say ‘Oh my gosh, this is horrible,’ but they don’t understand the effects of having them in the water year after year after year.”
While biologists and fishermen now see Asian carp as an environmental detriment, the fish were originally brought to the country as an environmentally safe alternative to chemical treatment, Irons says. In the 1960s and 1970s, southern fish farmers imported Asian carp to help keep catfish ponds clean.
“It seemed like a good idea at the time,” Irons says. “We can use a biological control to control nutrients in our catfish ponds, our catfish is healthier, tastes better, and then you have a large fish that you can use for either cleaning up the next pond or you can use for animal feed or fertilizers.”
But once flooding washed Asian carp out of the controlled ponds and into the Mississippi River, the fish quickly became an environmental problem and have been threatening to change the ecology of major waterways ever since.
Fish food INHS’ Havana group saw its first bighead carp – the Asian carp known for weighing up to 100 pounds – in 1995, and its first silver carp in 1998, Irons says. But it wasn’t until 2000 that the fish finally started spawning on the Illinois River in great numbers. That year, through its annual sampling of 400 random Illinois River sites, INHS caught about 1,200 bighead carp and several hundred silver carp, mostly small, young specimens, Irons says.
By 2007, sampling brought in more than 10,000 silver carp. “The next year it was over 100,000 juvenile [silver carp]. … Most of those are small, but if only 1 percent of those survive you can see how those numbers [rise],” Irons says.
If a substantial number of silver carp survive to adulthood, they will continue to compete with other young fish for food. The adult silver carp’s gills are so spongy that they can consume plankton as small as 3 microns (a human hair has a minimum diameter of about 40 microns), food usually consumed only by juvenile fish, says Matt O’Hara, INHS biologist.
“All that food is impacting every other fish species because all fish need plankton at some point in their life,” Irons says. While he hasn’t seen any extreme long-term population decreases in native fish species, those fish are looking much thinner than in the past.
“The populations are generally fairly robust until 2007-2008, when we thought we were seeing a tremendous crash in our bluegills and sunfish,” Irons says, adding that they hit 17-year lows during the drought-like conditions. “After that, we had good floods and those numbers have rebounded.”
While the relationship between Asian carp and the populations of other fish species is complex, it appears that when water levels are low, the impact of Asian carp is greater. At the same time, Asian carp, like other fish, have more successful spawns under flood conditions.
“It’s really dynamic,” Irons says, explaining
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