Fountain darter parasites and conservation.

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The Comal River in central Texas begins rather abruptly, gushing forth clear water from fissures at hundreds, if not thousands, of gallons per second. This stream is unique in many ways, but perhaps its most intriguing quality is a rare fish, the fountain darter (Etheostoma fonticola).

The historically large volume, steady flow, and warm temperature (74[degrees]F or 23[degrees]C) of the Comal River created habitat for the fountain darter and, unfortunately, a non-native snail, the red-rimmed melania (Melanoides tuberculatus), and its accompanying parasitic trematode or flatworm. The trematode parasitizes the fountain darter, manifesting itself as cysts in the gills. These cysts are not benign. They can do extensive damage to a fish's gills, disrupting respiration. Aquatic biologists at the National Fish Hatchery and Technology Center in San Marcos, Texas, are staying atop the learning curve with respect to the threat this parasite poses for fountain darter conservation.

Fountain darters live only in the Comal and San Marcos rivers. At present, only Comal River fish seem to be threatened by the trematode. Luckily, the parasite has occurred at only very low numbers on San Marcos River darters. But what the parasite means for the security of the species is not known yet. It's certainly something that concerns us.

That concern is directed toward scientific inquiries into this very question. A group including graduate students and Dr. David Huffman of Texas State University, Dr. Robin Overstreet of the University of Southern Mississippi, and Andrew Mitchell of the U.S. Department of Agriculture-Harry K. Dupree Stuttgart National Aquaculture Research Center in Stuttgart, Arkansas, is compiling information to better understand the impact of the trematode infestation.

The group is not just interested in what is happening in central Texas. The snail has been found in 15 states. Tropical fish production in Florida was being seriously affected until a Florida group developed methods to control the snail in production ponds. In spring systems of west Texas and Nevada, the snail and the parasite affect numerous native fish species including the largemouth bass (Micropterus salmoides), and at least four listed species, including the Devils Hole pupfish (Cyprinodon diabolis).

The group associated with the San Marcos Technology Center has been involved in many studies. As a result, the snail's habitat requirements are now better understood. We know the snail prefers spring runs, and that it is sensitive to cold temperatures and temperatures above 90[degrees]F (32[degrees]C). The snail's range in the U.S. will probably be restricted to the southern and western states because of its thermal tolerance. The snail resists most disinfectants but fishery equipment can be successfully treated with commonly used dimethyl ammonium chloride compounds.

The interaction of the parasite and several fishes has also been studied, revealing that some fish species are much more resistant to the parasite than others. Some young fishes are more sensitive to the parasite than older fishes. Methods for monitoring the parasite level within a stream or river are being developed.

The information compiled during the last couple of years will be used to study potential methods for controlling the spread of the snail and its parasite and for managing the habitats where they can not be completely removed.

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Meanwhile, the Technology Center maintains a standing stock, or refuge population, of fountain darters on site. Should the worst happen in the wild--a chemical spill or drying of the springs that serve as darter habitat--the station holds more than 500 adult fountain darters at any given time for future restocking into the wild.

Holding darters in captivity under the watchful eye of biologists has led to ancillary benefits. These adult fish produced over 10,000 young last year, and they were used for several studies. The longer we have darters on station, the more we learn about their biology. In the end, we're better equipped to deal with threats that may arise with darters in the wild.

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Some very serious threats challenge the fountain darter, but with the capable help of university, state, and federal biologists, this native fish may someday be removed from the endangered species list.

Dr. Brandt is the director of the National Fish Hatchery and Technology, Center in San Marcos.