Interannual abundance of nonnative fathead minnows ( Pimephales promelas ) in Upper Klamath Lake, Oregon

Since its introduction about 20 yr ago, fathead minnow ( Pimephales promelas ) has become very abundant in Upper Klamath Lake, Oregon. In 1991 mean bench seine catch per unit effort (CPUE) was 214, compared to 25 for native blue chub ( Gila coerulea ), the next most abundant species. In 45 trap-net samples collected in 1992, fathead minnow constituted 59% of the fishes caught in Agency Lake subbasin, 27% in Upper Klamath Lake, and 17% in tributary inflow habitats. From 1991 to 1995 fathead minnow declined and the abundance of some native fishes increased. Introduction to Klamath Basin was coincident with U.S. Environmental Protection Agency of fathead minnow as bioassay subjects. Upper Klamath Lake fathead minnow have incomplete lateral lines and males have mandibular tubercles, diagnostic of the northeastern subspecies. Although the origin, as bait bucket transfer, forage fish, or laboratory release, cannot be determined with certainty, the possibility of laboratory release suggest modification of bioassay protocols to require destruction of test or excess subjects.