Seasonal movements of fluvial westslope cutthroat trout in the Blackfoot River drainage, Montana

摘要

I studied the seasonal movements and habitat use of fluvial westslope cutthroat trout Oncorhynchus clarki lewisi from 1997 to 1999 in the Blackfoot River drainage in western Montana to help guide restoration efforts and lead to a better understanding of this subspecies. Of 22 radio-tagged fish, 16 migrated during the spawning period (mean length, 371 mm). Ten of the 22 fish were tracked over a 2-year period, and 2 of these fish migrated in both 1997 and 1998. Migrations to tributaries occurred during the rising limb of the hydrograph in both years and lasted for an average of 10 d (range, 1-14 d) in 1998. Migratory fish moved both upriver and downriver to reach spawning tributaries during both years. In 1998 the mean distance traveled to access tributaries was 3 1 km (range, 3-72 km). Fish staged at the mouths of tributaries for up to 14 d before entering near the peak in the hydrograph. They remained in tributaries for an average of 27 d (range, 4-63 d), the duration varying with size of tributary and flow year. Once in tributaries, fish generally remained within a 200-m reach, but frequently moved within the area. In four tributaries to the Blackfoot River, actively spawning fish were observed in May 1998 as flows subsided after the peak discharge. Neither of the two repeat migrants spawned within 3 km of their previous year's spawning location, though both spawned in the same tributaries. After leaving tributaries, fish moved both up- and downriver to overwintering areas and did not move more than 100 m thereafter. Fish did not exhibit fidelity to their prespawning main-stem locations. At least six fish died after spawning (38%). Westslope cutthroat trout movements, prespawning and postspawning, exhibited a plasticity not previously reported in Montana and demonstrate the large spatial extent to which fluvial westslope cutthroat trout utilize aquatic resources. To enable continued improvement of the westslope cutthroat trout population in the Blackfoot River drainage, I recommend riparian timber management that continues long-term input of large woody debris to tributaries, continued closure of the Blackfoot River watershed to angling harvest, and the use of culvert designs that will pass spawning fish under most flow conditions.