Implications of Floodplain Isolation and Connectivity on the Conservation of an Endangered Minnow, Oregon Chub, in the Willamette River, Oregon

Abstract

The objectives of this study were to determine the distribution and abundance of endangered Oregon chub Oregonichthys crameri, a small floodplain minnow endemic to the Willamette Valley of western Oregon; to describe the fish communities in current and historic chub habitats; and to establish new populations through introductions. Various sampling gears were used, including seines, minnow traps, dip nets, and a gill net. Oregon chub distribution was found to be restricted in comparison with their historical range, whereas nonnative fishes were widespread in the off-channel habitats preferred by Oregon chub. Oregon chub were absent, or low in abundance, when nonnative fishes were present, and several populations declined or were extirpated when their habitats were invaded by nonnative fishes. Isolated habitats with low connectivity supported larger populations of Oregon chub and were less likely to contain nonnative fish species than were habitats with high connectivity. In habitats that supported abundant Oregon chub populations, chub were often the numerically dominant fish species. Results suggest that increasing the connectivity of floodplain habitats in a system where nonnative fishes are widespread may be detrimental to the conservation and recovery of this species.