Estimating Chinook and Coho Salmon Smolt Abundance and Migration Timing Using a Rotary-Screw Trap in the Anchor River, Southcentral Alaska

Existing habitat inventory data and assessments throughout Alaska are incomplete.  This limits the capacity of resource managers to understand, anticipate, and prepare appropriate responses to changes in watershed processes that can result from anthropogenic and climate change.  To address this need, the Kenai Fish & Wildlife Field Office is conducting field investigations to parameterize and validate the application of a GIS-based model to assess current habitat conditions and predict population potential for Chinook Oncorhynchus tshawytscha and coho O. kisutch salmon.  A major component of the field work involves estimating smolt abundance and migration timing using a rotary-screw trap.  In 2010, we used a stratified mark-recapture study design that included daily releases of marked smolts upstream of the trap.  Totals of 14,073 Chinook and 8,922 coho salmon smolts were captured during 2010, of which 3,022 Chinook and 2,772 coho salmon smolts were marked and 728 and 544 were recaptured, respectively.  Nearly all smolt passage occurred at night.  The project encountered challenges common to other researchers including limited options for trap placement, variation in capture efficiencies, changes in smolt size over time, dynamic river conditions and debris load, and gaps in data collection.  We were unable to sample during the early part of the Chinook salmon smolt outmigration because of unsafe river conditions and had to model missing data.  We also modeled missing data on three occasions when the trap could not be fished because of high water events, two of which were periods of potentially high smolt passage.  Mark-recapture assumptions were probably met in most strata even though capture efficiencies, river conditions, and smolt size varied over the course of the smolt outmigration.