Spawning Habitat of Hatchery Spring Chinook Salmon and Possible Mechanisms Contributing to Lower Reproductive Success

摘要

Spawning is a critical stage in the life history of salmonids. Spawning location has been reported to be a significant factor in the fitness differences between natural- and hatchery-origin (natural and hatchery, respectively) female spring Chinook Salmon Oncorhynchus tshawytscha, although the mechanisms responsible remain uncertain. The objectives of this study were to compare the spawning distributions of hatchery and natural female spring Chinook salmon within the Chiwawa River, Washington, and determine whether spawning characteristics differ between hatchery and natural female Chinook Salmon or between channel types within the Chiwawa River. Over a 10-year period, locations of female carcasses were examined to assess differences in the spawning distribution of hatchery and natural females. Additionally, a suite of metrics was measured from redds of known origin females to characterize the spawning location and redd characteristics. A greater proportion of hatchery females spawned in the lower watershed within plane-bed channels than spawned upstream. Conversely, the majority of natural females spawned farther upstream in pool-riffle channels. The distribution of hatchery fish was presumably strongly influenced by the hatchery acclimation-release location in the lower river. Given the geomorphic characteristics of plane-bed channels, salmon were unable to build redds there similar to those in more typical pool-riffle channels. Redds constructed in plane-bed channels were smaller, shallower, and closer to stream banks than redds in pool-riffle channels. Limited differences were detected in redd characteristics between natural and hatchery females in pool-riffle channels. Smaller hatchery females constructed smaller redds with lower tailspill relief than did larger natural fish. As the results of differences in spawning distribution and fish size, redds constructed by hatchery females were more susceptible to environmental sources of mortality than were redds constructed by natural females. This study provides insight into the possible mechanisms responsible for the reported lower reproductive success among hatchery females spawning in the natural environment.