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Movement of resident rainbow trout (Oncorhynchus mykiss) transplanted below barriers to anadromy in Freshwater Creek, California
I conducted an experiment to determine if resident rainbow trout (Oncorhynchus mykiss) isolated upstream of a barrier to anadromy in Freshwater Creek, California would exhibit migratory behavior after individuals were relocated to a downstream reach with access to the Pacific Ocean. Between 2005 and 2006 a total sample of 131 age 1+ trout upstream of a 5-m-high waterfall were captured and individually marked with 23-mm passive integrated transponder (PIT) tags. Genetic analysis determined that above-barrier individuals have extensively hybridized with coastal cutthroat trout (Oncorhynchus clarki clarki). At each sampling event, half of the tagged individuals (n=22 and n=43 for trout in 2005 and 2006 respectively) were transplanted approximately 10 km from tidewater. Analysis of otolith microchemistry indicated that above-barrier trout were derived from residential parental lineages. An equal number were released at the point of capture above the barrier. Tagged individuals in above- and below- barrier reaches were subsequently relocated and/or recaptured to track their movement. The majority of transplanted trout displayed little downstream movement from the transplant location. Forty percent of transplanted individuals remained within 500 m of the release location in all sightings. The percentage of transplanted trout that moved into tidally influenced water (6%, or 4 of 65 individuals) did not appear to be different from the percentage of prior downstream resident (4%, or 9 of 210 individuals) that were captured and tagged in the vicinity of the transplant release location and later captured in tidally influenced water. Five, tagged individuals from above the barrier-were detected in below barrier reaches. Four of the transplanted individuals were last detected within the tidally influenced reach of the lower river. Downstream migrant traps captured seven tagged individuals, of which, two were determined to have smolted (one of which was not transplanted below the waterfall) and one was a pre-smolt. The similarity in movements observed in the transplant group and the below-barrier population, the smoltification of transplanted individuals, and the leakage of above-barrier fish downstream suggests the potential for resident trout to exhibit migratory behavior and contribute to breeding populations of steelhead.