Estimating the Sockeye Smolt Production Capacity of Baker Lake, Washington

Baker Lake is one of two reservoirs created as part of Puget Sound Energy’s Baker Hydroelectric Project that support the largest native sockeye salmon (Oncorhynchus nerka) run in Washington State.  The Baker River sockeye salmon population became severely depressed during the twentieth century but recent recovery efforts have dramatically increased adult returns.  An understanding of the smolt production capacity of Baker Lake was needed to predict a target maximum population size.  All outmigrating smolts are captured and counted at a floating surface collector that provides downstream passage.  Annual smolt counts provide a direct measurement of current smolt production and eighteen years of zooplankton and water quality data were available from previous Baker Lake studies.  In 2009, we modified the sampling procedures used to measure the density, biomass, and species composition of the zooplankton community, euphotic zone depth, and nutrient concentrations.  Standard sockeye smolt production models were used to derive estimates of production capacity.  Smolt production capacity estimates ranged from 181 kilograms per square kilometer (developed using the zooplankton biomass model) to 806 kilograms per square kilometer (developed using the euphotic zone model).  Estimates of smolt production capacity required that smolt biomass estimates be converted to smolt numbers.  This conversion required the selection of a target smolt size (100 millimeters or 8.7 grams) that represented a tradeoff between the number of smolts and size-dependent effects on smolt-to-adult survival.  The observed biomass of sockeye smolts outmigrating from Baker Lake in 2010 was the highest on record (638 kilograms per square kilometer) and provided a means of verifying estimates of production capacity.  The euphotic volume model provided the most realistic estimate of production capacity available for Baker Lake.  The resulting production capacity estimate based on the euphotic volume model was 1.87 million target-sized smolts.  Age 1+ sockeye smolt lengths observed outmigrating in Baker Lake in 2010 were larger than the target smolt size suggesting that Baker Lake is currently underseeded.  However, the proportion of preferred zooplankton prey items was reduced in 2009, suggesting that juvenile sockeye may be affecting the Baker Lake zooplankton community.  The combination of production capacity models and empirical smolt biomass observations suggests a cautionary approach to increasing recruitment.  One approach to managing the juvenile sockeye population would be to gradually increase the number of fry released and monitor for subsequent indications of density-dependent responses in the form of reduced smolt size or condition factor.