HOLLOWS, PEEPERS, AND HIGHLANDERS: AN APPALACHIAN MOUNTAIN ECOLOGY

12. Small Fishes in Shallow Headwaters

81 12 Small Fishes in Shallow Headwaters A s drivers of Appalachian dirt roads, we are at least subliminally aware of the schools of fish that dash madly about as we splash through a ford. The most common fishes (two or more fish species) flushed by our tires are members of the minnow, sucker, sunfish, and perch families. Within these comparatively few taxa, Appalachia’s numerous small, steep streams have catalyzed the evolution of a great fish diversity. Many fishes native to Appalachia live in the alternating pools and riffles of headwater streams. These habitats are shallow, subject to torrential spates and bone-dry droughts; the fish dwellers are exposed to predation by raccoons, snakes, and herons. How do small fish persist in shallow headwaters? During spring and autumn, when streams flow abundantly, fish can move freely among pools before settling down for the season. Once established, fish seldom wander, even when moderate water levels allow them to explore other pools. One study found that after summer flash floods, 75 percent of the fish could still be found in their original, pre-flood locations. Darters and sunfishes in particular exhibit this kind of site loyalty. If winters are so cold that fish risk being frozen in shallow water , they migrate from their springtime spawning grounds in shallow headwaters to deeper overwintering reaches farther downstream. Presumably, natural selection has favored young adults that return to 82 breed in their own home pool — one of the best indicators of future reproductive success is that one’s parents successfully reproduced there. Each creek, even each pool in a single reach, probably holds a unique cocktail of chemical odors, part of which becomes indelibly imprinted in the memory of juvenile fish during their first summer. We know that coho salmon imprint on naturally occurring chemical cues for homing. The salmon’s homing response is highly specific: molecules with atomic structures that differ slightly from the stimulating molecule do not elicit the homing response. Assuming that small fish in Appalachian streams use chemical cues to return to their home pool, an important part of the story still remains a mystery — what molecules provide pool-specific cues? Root exudates, the organic molecules released by the roots of trees, are a possibility. The number of combinations of different organic molecules comprising the exudate constellation in soil and water must be enormous, and their tree sources remain relatively stable over the long term. In Pine Cabin Run and thousands of other Appalachian headwaters , adult creek chubs prey on blacknose dace and cannibalize juvenile creek chubs. Adult creek chubs themselves fall prey to belted kingfishers and green herons. How do small fish in shallow headwater pools escape larger predators? Where shelter is limited, adult creek chubs aggregate under cover during the day. At night, when the birds are gone, they disperse to find their prey. Where shelter is plentiful, adult chubs do not aggregate. Young chubs employ a combination of camouflage and stillness, finding cover beneath leaves and other small objects. As you might expect, in pools that host predators, prey density increases with the complexity of cover provided by rocks, limbs, and overhangs . Both predator and prey choose pools that minimize their odds of falling prey. Countershading, body coloration that combines a dark back and light belly, also reduces the odds of being eaten. Viewed from below, a fish with a pale belly presents less of a contrast against the sky than one with a dark underside. The dark back camouflages a fish Hollows, Peepers, and Highlanders 83 from aerial fish-eating birds. A fish with a light back and dark belly would be obvious to predatory fishes from all directions. Yet another way to avoid being eaten is to hide behind another individual. The random movements and indistinguishability of individuals within a school preclude the predator from fixating on one individual. A fascinating adaptation for reducing one’s odds of being eaten is the alarm system, a method of communication in which a fish emits an alarm substance that elicits fright in pool mates. Two major groups of bony fishes — one including chubs, dace, and catfishes; and the other containing perch and darters — rely on alarm systems . Large, specialized cells, called club cells, produce the alarm substance, rupturing and releasing their contents when the skin is injured. Nearby fish of the same species smell the alarm...