Timing and Location of Spawning by Nonnative Wild Rainbow Trout and Native Cutthroat Trout in the South Fork Snake River, Idaho, with Implications for Hybridization

Abstract

Our goal was to assess the likelihood of hybridization between introduced rainbow trout Oncorhynchus mykiss and native Yellowstone (YS) cutthroat trout O. clarki bouvieri, based upon habitat use and temporal overlap during spawning. We used radio transmitters in 1996 and 1997 to describe the spawning behavior of rainbow trout, hybrids of rainbow trout and YS cutthroat trout, and YS cutthroat trout (1997 only). Fish displayed two distinct spawning strategies, either spawning in side channels of the main stem (9 rainbow trout, 14 hybrids, and 10 YS cutthroat trout) or in tributaries (5 rainbow trout, 3 hybrids, and 7 YS cutthroat trout). Within the main stem, the majority of rainbow trout and YS cutthroat trout migrated to the same 8-km section to spawn, whereas hybrid trout spawned throughout the study site. The median spawning date for main-stem-spawning YS cutthroat trout (June 9) was significantly later than for rainbow trout (May 19) and hybrids (May 18). However, long spawning periods for rainbow trout (94 d), hybrids (113 d), and YS cutthroat trout (71 d) allowed for considerable overlap. The amount of spawning overlap varied among the four tributaries. In one tributary, complete spatial and some temporal overlap occurred (three rainbow trout, two hybrids, and two YS cutthroat trout); in another tributary no spatial or temporal overlap occurred (four rainbow trout and three YS cutthroat trout); and only YS cutthroat trout used the remaining two tributaries (one YS cutthroat trout in each). Molecular analyses verified that females of both rainbow trout and YS cutthroat trout were hybridizing and showed that the genetic composition of hybrid trout was more similar to rainbow trout than to YS cutthroat trout (mean = 64% rainbow trout markers). These results suggest that the majority of YS cutthroat trout (12 of 17) experience spatial and temporal overlap with rainbow trout and hybrids, but three tributaries may still provide some reproductive isolation for native fish.