When the river redhorse turns its energies to spawning, all of its fins turn a brilliant, bright red (they can be at least partially red the rest of the year). The redhorse also develops pearl organs, or tubercles, on its skin around this time. These organs give the skin the coarse, raspy texture needed for spawning. The adults make runs upstream, moving mostly at night to find good breeding habitat. Most suckers seek out rocky riffles in the shallow water of small streams, but some local biologists believe some river redhorse may actually be breeding in larger river mainstems in Chicago Wilderness. The males move onto the riffles and either excavate gravel with their tails in a sweeping motion or plow through it with their heads, all in an effort to free up silt so oxygen-rich waters can percolate through the gravel where the eggs will incubate.
Facing into the current, males lie in wait for females. When one approaches, the male shows his worthiness, darting back and forth in a sweeping courtship dance. Ripe females are attended in the spawning act by one, sometimes two, males. The pearl organs allow the male and female to cling together and maintain a station over the excavation while the eggs — upwards of 50,000 of them — are simultaneously fertilized and dropped among the clean gravels.
The parents promptly abandon the area and head back downstream, but soon swarms of newly hatched fish take temporary station in the slow-moving shallows. Here they provide food for predatory fishes, such as black bass and sunfish. Herons, too, eat the smaller fish. Those lucky enough to move into deeper waters could reach two feet long and eight pounds at the end of their 12-year lifespan.
In Illinois, the river redhorse is protected as a threatened species. Though biologists have found the fish in stretches of the Fox, Des Plaines, DuPage, and Kankakee Rivers, pollution and sedimentation from agriculture and urbanization compromise many of the clean spawning gravels the fish needs and reduce populations of the filter-feeding animals it depends on for food. For these reasons, scientists now treat the sensitive redhorse as an indicator species to show where river conditions have declined or improved. – C. Springer