Re: NANFA-L--question about effect of grass carp on native fishes and plants

Subject: Re: NANFA-L--question about effect of grass carp on native fishes and plants
From: Christopher Scharpf (
Date: Wed Aug 25 2004 - 05:08:23 CDT

Grass carp live up to their name. Although juveniles feed largely on aquatic
invertebrates, adults prefer to munch on plants, actually pulling them out
of the substrate. (Other herbivorous exotics, such as rudd, eat individual
leaves or fragments.) The bigger the grass carp, the more herbivorous it is.
Specimens, which reach up to 1.25 m (49 in) and 45 kg (99 lb), can consume
their body weight in vegetable matter each day. Surprisingly, grass carp
intestines are short, just 2-3 times the length of their body. Herbivorous
fishes usually have much longer digestive tracts in order to break down
plant material. The grass carp's voracious appetite for veggies presumably
makes up for its short digestive system. They'll eat virtually any kind of
plant, including terrestrial vegetation hanging over the water, although
some plants are far more palatable than others. Homeowners who live along
Silver Lake in Cowlitz County, Washington, are often amused to find grass
carp schooling along the shore as they mow their lawns, slurping up grass
clippings! Once grass carp have depleted beds of aquatic plants, they will
switch to a carnivorous diet of crayfish, clams, and other invertebrates.

The effects of grass carp on aquatic ecosystems depend largely on the number
present, the amount of available plant material, and the overall community
structure of the ecosystem. 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. Clearly, the use of grass
carp for aquatic weed control should be tightly monitored and regulated, if
not outlawed altogether.

> From:
> Reply-To:
> Date: Tue, 24 Aug 2004 23:41:40 +0700
> To:
> Subject: NANFA-L--question about effect of grass carp on native fishes and
> plants
> Since there is a move by fisheries department to re-introduce grass
> carp into Borapet pond in Thailand to control undesirable "weed" and
> keep lotus in good shape, do you NANFA people know about grass carp's
> impact on plants and fishes that use plants as living habitat,
> breeding ground or other use in areas with submerged and emersed
> macrophytes in the US? Are minnows effected by grass carp?

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: Wed Sep 29 2004 - 12:24:21 CDT