America must kill this fish
Fish poison, electric fences, and quarantined lakes might not be enough to eradicate Asian carp
Ask any US
wildlife official how the invasive Asian carp have managed to stick
around US rivers for four decades, and they'll all say different
versions of the same three things. First, the carp is a voracious eater
that outcompetes other species for plankton, causing native US fish to
die and allowing the carp to thrive. Second, the carp reproduces with
incredible ease, so its populations have grown at tremendous speed. And
third, the US doesn't possess any native species of fish large enough to
prey on adult Asian carp, so they have very few predators. But a closer
look at the four Asian carp species currently plaguing America's waters
points to one glaring fact: we clearly don't know much about these
fish.
The fish best known for its ability to jump out of the water
Fortunately, researchers have
finally started getting a handle on the animal best known for its
ability to jump out of water when startled, says Duane Chapman, a
biologist and leader of the Asian carp research team at the US
Geological Survey (USGS). And thanks to that knowledge, scientists are
now developing technologies — some of them deadly — that could soon make
a dent in populations of this big, ravenous fish.
The Asian carp first
infiltrated the US by way of fisheries. "Back in the 1970s, fish farmers
were using them in aquaculture because they eat plankton and algae,"
says Kevin Irons, aquatic nuisance species program manager at the
Illinois Department of Natural Resources. "We brought them over to the
southern US and put them in with the catfish so they would clean the
ponds, but they escaped almost immediately." The carp have been making
their way up the Mississippi and Illinois River ever since.
Currently, the
northernmost population is located in the Dresden Island Pool, about 50
miles south of Chicago. This is extremely worrisome for biologists,
because just beyond Chicago lie the already fragile Great Lakes. If the
Asian carp make it up there — researchers say this could happen within
the next 20 years — they will devastate the entire ecosystem.
Poison particles so small that only Asian carp pick them out
The first and
deadliest idea is to develop a poison that can kill the Asian carp — but
leave other species unscathed. To come up with that poison, scientists
first examined the carp's biology. They found that these fish eat about
10 percent of their body weight in plankton each day, which means they
need to filter a lot of water through their gills to catch food. And the
four Asian carp species in US waterways all have one thing in common:
incredibly effective gills that can filter much smaller particles than
the average native fish. So scientists at the USGS are trying to produce
poison particles so small that only Asian carp pick them out of the
water.
But small particles alone,
which some other species can ingest, won't cut it for such a large and
dispersed population of fish, so researchers are going one step further
by putting the carp's unique digestive system to good use. "The
particles are designed to be broken down, but only by the enzymes in the
carp's gut," Chapman says. Other fish that might ingest the particles,
such as largemouth bass or catfish, don't produce this enzyme, so they
won't be affected. "And when carp break it down," he says, "they release
the toxin and it causes them to die."
"If the eggs settle to the bottom, it can have a lethal effect on the fish."
Unfortunately, these
particles — called matrices — won't be ready for some time, so
researchers are also looking into another way of disrupting carp
populations by going after their eggs. "Carp eggs require a certain
amount of drift in the water to survive," Chapman explains. "If the eggs
settle to the bottom, it can have a lethal effect on the fish."
If researchers can
build dams that only come up halfway to the water's surface, they might
be able to interrupt the drift in the carp's favorite spawning
locations. "We are still at that point where we are just beginning to
understand how these fish spawn," Chapman says. "But down the line, we
may work with management agencies and state officials to build these
sorts of barriers."
But the most drastic
measure floating around is one that could completely change the city of
Chicago, because officials are thinking about blocking off
the Chicago Sanitary and Ship Canal completely. "The US Army engineers
conducted a major study, and they found that the only end solution is
the full separation of the waterways," says Marc Gaden, communications
director at the Great Lakes Fishery Commission. The canal system links
directly into the Great Lakes, he says, so "it's the most likely pathway
for the carp to take."
The project would take 25 years to complete and could cost up to $18 billion.
It would also completely disrupt Chicago's shipping economy and sewage
system, Gaden says. "The barrier itself is the cheap part, it's the
tunnel and water storage systems that need to be built that will cost
the most money."
Yet the Great Lakes
aren't defenseless. Already, an electrical barrier located about halfway
between Chicago and the northernmost carp population is in place, and
its purpose is to keep the fish at bay. "When fish are near it, they get
increasingly uncomfortable and unable to swim," Gaden says.
Unfortunately, the electrical barrier would have little effect on
juvenile carp and larvae, so it's not a permanent solution.
"They aren't a favored food for Americans"
For now, the only measure that
seems to be putting a dent in the fish population is commercial
fishing. "The USGS and the state of Illinois contracted with commercial
fishermen, so they could fish the Asian carp out of the Illinois
waterway system," says Jesse Trushenski, a nutritionist specializing in
aquaculture at Southern Illinois University. "But given that they aren't
a favored food for Americans, human food use is a very small market."
Trushenski thinks that one way
to encourage commercial fishing is to find alternative uses for the
fish meat. So she's come up with various forms of fish meal made out of
Asian carp, which can then be fed to fish in aquacultures. "Our industry
is always looking for new protein sources," she says, "so you can take
Asian carp, grind them up into fish meal or turn them into oil."
According to her, the Asian carp protein holds up well to current
alternatives, and the fish she has fed it to seem to appreciate its
taste.
Yet none of the
current solutions are perfect, Irons says. And even if the methods in
development turn out to be effective, "blocking the Chicago canal might
still be necessary," he says. The Asian carp certainly aren't the first
invasive species to spread their fins in North America, and they won't
be the last, he explains. And depending on how long it takes to get a
handle on the biology of future aquatic invaders, we might not be able
to stop them. So blocking off whole areas of freshwater from each other
once and for all, Irons says, might actually be our best — and only —
defense.
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