Re: NANFA-- "river rabbits"

Christopher Scharpf (
Wed, 07 Mar 2001 08:00:51 -0400

>I can't see how you could have a problem with various species of Asian
>carps. Since the introduction of Black Bass and various sunfish into
>Japanese waters, the carps in affected rivers and lakes have been decimated.
>Currently, introduced species of livebearers such as platies, mollies,
>guppies, girardinus have adversely affected the populations of medaka,
>gobies and minnows. Bass, sunfish, gar pike and pickeral species are slowly
>working on the carp populations with the Chinese Grass Carp being the worst

Perhaps I should let Jan answer this, since he's the professional fish
ecologist. However, I've been reading on this subject recently, and these
thoughts are still fresh on my mind.

The thing to realize is, is that exotic fishes can harm ecosystems and native
fish assemblages in ways other that outright predation, which appears to be the
case in exotic fishes wreaking havoc on Japanese natives and introduced
populations of grass carp. (Grass carp are not native to Japan.)

In the United States, grass carp are used to consume aquatic vegetation that
clogs lakes, ponds, and irrigation canals. They're very good at their job. One
specimen can consume its body weight in vegetable matter each day. They'll eat
virtually any kind of plant, including terrestrial vegetation hanging over the
water, although some plants are far more palatable than others. By reducing
plant cover, grass carp can eliminate shelter, spawning habitat, and food
sources of native fishes. Since grass carp digest only about half of the plants
they eat, the undigested plants they expel back into water can cause algae
blooms that reduce water clarity and decrease oxygen levels. Grass carp also
harbor exotic parasites that spread to other species. For example, grass carp in
Arkansas released an exotic parasite that ultimately spread to and affected
populations of a federally endangered minnow as far away as Utah!

The bighead carp is a filter feeder, using extremely close-set gill rakers to
remove plankton, algae, and detritus from the water. It's this feeding strategy
that attracted commercial Arkansas fish farmers, who first imported the carp to
America in the early 1970s to help remove the cloudy build-up of nutrients and
waste products from commercial catfish ponds. Although subsequent studies have
not confirmed whether bighead carp can actually perform this task, the species
was cultured in various hatcheries anyway and distributed to private fish farms
and municipal sewage lagoons in Arkansas and throughout the Midwest. Not
surprisingly, it has escaped into open waters and is now established in two
states, probably more. The impact of the bighead carp (and its cousin the silver
carp) is in the way it upsets the bottom of the food chain. These fish have the
potential to deplete zooplankton populations and thereby adversely affect native
filter feeders such as paddlefish, bigmouth buffalo, gizzard shad, the larval
young of just about all fish species, and freshwater mussels.

Black carp, which are molluscivores, are being introduced into fish farms and
aquaculture facilities to eat a mussel that harbors a particularly nasty fish
parasite. Scientists worry that the fish will escape to open waters -- which, if
history is any indication, it will -- where it will then probably start
feeding on North America's largely endangered native mussel fauna.

I hope this answers your question as to why Asian carps are the "bad guys" in
North America, but not necessarily so in Japan.

Chris Scharpf

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