Van Doornik et al.: Stock-specific distribution of juvenile Oncorhynchus mykiss during early marine migration 
105 
We observed a considerable difference in both CPUE 
and location of juvenile steelhead between 2006 and 
the other years of our study period. It is possible that 
some of the variability in our sampling success was a 
result of sampling essentially in the right place at the 
right time, a time and place that varies from year to 
year. For example, in 2007, we caught no steelhead 
from the Middle and Upper Snake River and very few 
steelhead from the Middle and Upper Columbia Riv¬ 
er or Lower Snake River. The numbers of releases of 
hatchery-reared steelhead from those areas in 2007 
were similar to those in other years (Fish Passage Cen¬ 
ter, hatchery releases of steelhead during 2006-2016, 
available from website, accessed February 2017); there¬ 
fore, the timing of our sampling may have resulted in 
our failure to catch fish from those stocks. We know 
that juvenile steelhead do not stay in our study area 
for long. McMichael et al. (2013) found evidence that 
juvenile steelhead from the Columbia River migrate 
away from the mouth of the Columbia River at a faster 
rate than Chinook salmon and that they exit the plume 
area soon after entering the ocean, mostly within 3 d. 
In another study, few juvenile steelhead were caught in 
trawl hauls conducted 1 month later in June along the 
same transects used in our study (Daly et al., 2014). 
It’s likely that annual variability in the timing of ma¬ 
rine entry and quick migration away from coastal wa¬ 
ters, evidenced by the fact that we caught most of our 
samples at the sampling stations farthest from shore, 
combined to cause variability in our sampling success. 
A thorough evaluation of both freshwater and marine 
environmental drivers of migration behavior could help 
describe the interannual and inter-stock differences 
we found. Flow can influence river migration rate and, 
therefore, timing of ocean arrival. Temperature and food 
conditions can alter growth rates, resulting in disparity 
in size of fish among years or stocks and influencing 
migration behavior. Importantly, differences in survival 
can influence the spatial and temporal distributions of 
the surviving individuals, which are then represented in 
our catches. However, initial comparisons with some ba¬ 
sic measures of environmental variables (river volume, 
river plume location and volume, and marine tempera¬ 
ture and salinity) did not highlight obvious drivers of 
interannual and inter-stock abundance and distribution 
(data from these comparisons are not shown). 
The baseline of genetic data that we assembled al¬ 
lowed us to identify stock-specific distributions of ju¬ 
venile steelhead upon their entry into the ocean. The 
ability to identify the stock of origin of both tagged 
and untagged steelhead will be invaluable for future 
research into the possibility that forces that affect the 
survival of steelhead during early marine migration in¬ 
fluence different genetic stock groups differently. 
Acknowledgments 
We thank the captain and crew of the F/V Frosti for 
providing a safe and professional sampling platform 
and the many people who assisted with sample collec¬ 
tion efforts. Thanks also to B. Beckman, D. Teel, and 
L. Weitkamp for providing helpful reviews of previous 
versions of this manuscript. 
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