On 7 March, Hilton Pond Center was honored to host a guided field trip for members of the Environmental Education Association of South Carolina (EEASC), which was holding its annual spring meeting in York County. Many of the state's best, brightest, and most enthusiastic environmental educators are EEASC members, and folks on the field trip exhibited all these qualities--plus a special appreciation for nature in the Piedmont. We were able to teach these teachers lots of interesting things about local phenomena, but thanks to the curiosity of the group we also observed something we'd never seen before at Hilton Pond.

As we started out on the trails around Hilton Pond itself, one vigilant participant noticed a light-colored object glistening in the grass at water's edge. This artifact (above) was near the remains of a Grass Carp that died some weeks before and caught the attention of a couple of fish-eating Black Vultures after it washed ashore. The vultures picked the carcass clean (right), leaving only skeleton, scaly skin, and the white object that caught the EEASC member's attention. It was apparent the curious item once had been part of the fish, so we examined it closely. Although we'd never seen such a thing before, we speculated it was teeth from the carp, and the prominent grooves in each tooth (below left) were adaptations related to a Grass Carp's main food preference--submerged vegetation.

Grass Carp, Ctenopharyngodon idella, are non-native fish originally imported from Asia to eat aquatic plants; they're also called "White Amur" because they occur naturally in the Amur River that forms the border between China and Russia. Introduced to Arkansas in 1963 by the U.S. Fish & Wildlife Service, Grass Carp eventually were placed in ponds across the Southeast to help eliminate "water weeds" that foul the hooks of sport fishermen and supposedly upset a pond's oxygen balance. As often happens with introduced species, Grass Carp got out of hand in some locales, stripping ALL vegetation from various bodies of water and completely destroying those ponds' natural ecology. Results were so alarming some states outlawed introduction of Grass Carp, while others permitted only "triploid" carp that were sterile and incapable of reproducing or overrunning an area.

In about 1990, a neighbor placed ten sterile Grass Carp in Hilton Pond to "improve fishing." The young carp were about ten inches long--big enough to avoid being eaten by Largemouth Bass that also populate the pond--but not too small to devour plants. These little fishes soon went about their business of eating almost every available piece of submerged vegetation, which they used in turn to increase their own body mass to five to seven pounds by the end of the first year, and to 15-20 pounds by year four. Several of these carp eventually died off; by the winter of 2002-2003 only four were left from the original batch but they had grown to prodigious lengths of nearly four feet and individual weights of 30-35 pounds! Swimming slowly about on Hilton Pond with their dorsal and caudal fins breaking the surface, these beasts looked for all the world like sharks trolling the water for their next meal--and could be quite startling when they jerked their tails and propelled away quickly at our approach.

Never fear this shark-like appearance, however, because Grass Carp--at least when adult--eat no animal matter, contenting themselves to dine on whatever vegetation might be available. And that brings up the function of the grooves on the teeth we found during the EEASC field trip to Hilton Pond Center. It turns out Grass Carp, among the largest members of the Minnow Family (Cyprinidae), have "pharyngeal teeth" that form on bony arches in the fish's throat. These structures grind food against a hard plate beneath the lower skull, and--in the case of Grass Carp--the grooves help shred tough vegetable matter and separate out softer, more nutritious plant tissue. By contrast, the Common Carp, Cyprinus carpio--another problematic European introduction--has pharyngeal teeth that are "molariform," i.e., molar-like and adapted for crushing shells of snails and other mollusks that make up the diet of this mud-probing, bottom-feeding, carnivorous relative of the vegetarian Grass Carp. (A Common Carp's molariform teeth and pharyngeal arch show in the photo at above right by Vince Travnichek, Missouri Department of Conservation.)

In addition to tooth differences, Grass Carp (below left) are distinguished from Common Carp (below right) and other carp species by their cylindrical body shape, a mouth that opens anteriorly (rather than ventrally), lack of barbels (whiskers) at the edge of the mouth, and the absence of hard, stiff spines in their dorsal and anal fins. The scales of both species are quite noticeable--the size of a 50-cent piece in old specimens.

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Images © American Nature Guides: Freshwater Fish
by H.W. Robison, 1992

In our estimation, Grass Carp have been a mixed blessing at Hilton Pond Center. At first they did help eliminate submerged plants that interfered with fishing and swimming, but they weren't content to stop there; they kept eating until most vegetation was gone, and--oddly enough--seemed to have left behind only those aquatics that were pernicious and non-native. Instead they ate grasses that would have provided shelter for future generations of baby bass and bluegills or tadpoles and frogs and even dined on surface plants preferred by waterfowl.

Fortunately, these underwater behemoths are near the end of their normal lifespans. Most Grass Carp die before 15 years of age and, since they are no longer growing, ours now have appetites that are greatly diminished. With Hilton Pond finally back to its normal depths after four years of drought, we're hopeful the submerged vegetation will rebound, that the balance of aquatic animals will return, and that sometime in the not-to-distant future these last three Grass Carp will succumb to old age, become vulture food, and provide us with several more sets of nicely grooved pharyngeal teeth to share with ever-curious members of the Environmental Education Association of South Carolina. "Carp diem."

A nearly four-foot, nearly 35-pound Grass Carp

SPECIES BANDED THIS WEEK
American Goldfinch--20
Chipping Sparrow--4
Dark-eyed Junco--2
Song Sparrow--1
Eastern Towhee--1
Northern Cardinal--1
White-throated Sparrow--3
House Finch--1

* = New species for 2003

WEEKLY BANDING TOTAL
8 species
33 individuals

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