Distribution and prevalence of the Asian fish tapeworm, Bothriocephalus acheilognathi, in the Colorado River and tributaries, Grand Canyon, Arizona, including two new host records.

The Asian fish tapeworm, Bothriocephalus acheilognathi, has invaded the lower Little Colorado River (LCR), a tributary of the Colorado River, where it infects humpback chub (Gila cypha), speckled dace (Rhinichthys osculus), and fathead minnow (Pimephales promelas). This study examined the distribution of R. acheilognathi in the Colorado River and tributaries in Grand Canyon. In 1994, 22.5% of humpback chub, 10.3% of plains killifish (Fundulus zebrinus), 3.8% of speckled dace, and 2.2% of fathead minnow were infected. In 1995, 2.4% of fathead minnow and 1.4% of speckled dace were infected. Humpback chub, an endangered species, and plains killifish are new host records for this parasite. Nearly all (66.7 to 100%) infected fish were captured in areas near the LCR and were probably the result of infected fish emigrating from that tributary. However, 4 infected fish (1 plains killifish, 1 speckled dace, and 2 fathead minnows) were caught 92.8 to 202.1 km downstream from the LCR. Another speckled dace was caught in the lower section of Kanab Creek, a warm tributary, indicating a potential expansion of the parasite's range. Infection of humpback chub by B. acheilognathi is of concern due to the endangered status of this fish. Because B. acheilognathi requires high water temperature for completion of its life cycle, this species is largely confined to the LCR by the cold water of the mainstem Colorado River. The potential effects of plans to seasonally warm the Colorado River on B. acheilognathi are discussed.