COLUMBIA - Nature has found a way to limit the destructive and voluminous Asian carp population in the Missouri River.
Asian carp, an invasive species whose population dramatically expanded after the 1993 floods, have irritated fishermen and threatened native species throughout the river.
Duane Chapman, a research fisheries biologist at the Columbia Environmental Research Center, said this year’s silver carp population is starving, since the Missouri River is unable to sustain such a large number. Columbia Environmental Research Center is a U.S. Geological Survey environmental science research facility.
Chapman said the silver carp might face a die-off sometime this year. The bighead carp, another type of Asian carp, have been decreasing in numbers since 2005.
“Right now we have an extremely stressed population, because they are starving and trying to spawn, which expends energy,” Chapman said. “When that happens, it is very common to have a lot of fish die since the chance of disease is very high.”
Despite their condition, Chapman said some of the Asian carp remain in good shape and will continue to survive. A die-off does not mean the fish would become non-existent in the river, but they will drop in numbers.
“I don’t expect the fish to disappear or become rare,” Chapman said. “We’re going to continue to see problems with bighead and silver carp even if a die-off does take place.”
Although the magnitude of the likely drop in population is unknown, Chapman said the starvation of Asian carp has appeared in the center’s research and observations Their results revealed low fat content, meaning the silver carp are much thinner this year.
Chapman said researchers have also been trying to spawn Asian carp in their laboratory to understand more about the environments they can tolerate but have been unsuccessful so far this year. The gametes, or reproductive cells from the fish, have shown very little promise.
“In my opinion, the reason we aren’t able to get good gametes to spawn is because they are too skinny and can’t produce good eggs,” Chapman said.
Because of the difficulty within the lab, he predicts the Asian carp will have trouble reproducing in the river, hopefully leading to fewer baby silver carp next year and a smaller population.
Considering their poor condition, Chapman advises people to not consume Asian carp from the river because they will not taste good.
“They are usually excellent eating fish, but right now the fish are so skinny that the meat inside of them is translucent and soft in texture,” Chapman said.
In spite of this, Chapman hopes the intrusive species decline enough to help preserve the native species that compete with them for food, which research has shown is already scarce.
The zooplankton supply, the source of food for many fish, is much less abundant now, a fact that was discovered after a two-year study, Chapman said. Since an earlier study done in 1979 by MU graduate student D.K. Jennings, the total rotifer zooplankton, microscopic invertebrate animals, has decreased by 50 percent, and crustacean zooplankton is down to a mere 10 percent of the original number found by Jennings.
Because the 1979 data was collected before the abundance of Asian carp, and the current data after their overpopulation, the dramatic decline of zooplankton may be because of their presence, but Chapman said it is difficult to determine from only two sets of data.
Since the plankton supply is low and silver carp are able to eat smaller plankton particles, they can prevent other plankton eaters from finding any, Chapman said.
Unless they are able to find another food source, filter-feeding fish like paddlefish and gizzard shad could be affected, as well as baby fish of many otherspecies, which almost always eat plankton, Chapman said.
Not a lot of research has been done on this problem, but the influence of Asian carp on native species was analyzed in a study on gizzard shad. Research done by the Columbia National Fish and Wildlife Conservation Office from 2000 to 2006 revealed a 5.4 percent decrease in body weight for gizzard shad fish, a species that directly competes with Asian carp for food.
Although the results were inconclusive, Wyatt Doyle, branch chief for the Missouri River study at the U.S. Fish and Wildlife conservation office in Columbia, said the decrease in health might be a result of competition amongst themselves or with the Asian carp population.
Doyle said the overabundance of Asian carp interferes with the entire food chain within the Missouri River.
“Gizzard shad directly compete with carp for food, and they are a food source for a variety of sport fish,” Doyle said. “If they can’t get enough food, there aren’t enough prey fish, and therefore the sport fish don’t survive as long.”
While fishermen might fear the repercussions on their prize catch, Chapman said there is no evidence that Asian carp affect the catfish.
The influence of Asian carp on native fish species was discovered in other rivers as well. Kevin Irons, large-river ecologist at the Illinois Natural History Survey’s Illinois River Biological Station, said they have found poor health in some of their fish.
“Since 2000, we’ve been looking at bigmouth buffalo and gizzard shad in the Illinois River, and their conditions have declined,” Irons said. “This leads us to believe that there is competition in food sources, since we looked at environmental factors and none were significant to the declining condition of the fish.”
As the Asian carp population increases, Irons said the Asian carp conditions are also declining in the Ohio River and Mississippi River.
The ever-expanding Asian carp river population may have hit its limit, but Chapman emphasized the need to prevent their introduction into lakes and ponds.
“It’s pretty easy to move Asian carp around, so it is important that if you catch your own bait, use it where you caught it,” Chapman said.
Since silver carp jump out of the water, sometimes injuring those they hit, Chapman said it is important to keep them out of lakes where people enjoy recreational boating.
Even though Chapman and other researchers predict that a die-off is likely, they may not know if it happens or not.
“In the Missouri River, the water flows really quickly, so we might not actually see many dead fish,” Chapman said. “There’s a good chance they might just float on down the river.”
In the end, Asian carp can’t remain in this poor condition for much longer, and any decrease in numbers is good for the river, Chapman said.
“If there is a die-off or a recruitment failure, and either or both are likely with such stressed fish, it should give the resources and native competitors a breather,” Chapman said.