Novel aspects of stocked juvenile steelhead emigration patterns as revealed using dual-frequency identification sonar

We used a Dual-Frequency Identification Sonar (DIDSON) near the mouth of a small Lake Erie tributary to monitor juvenile steelhead emigration. Data were recorded continuously over two stocking seasons (April-May 2014; April-June 2015) and total emigration counts were determined every hour. Fish lengths at time of emigration were estimated using the DIDSON fish measuring tool and quantified through time. Percent survival to emigration was high with > 71% of individuals reaching the river mouth in both years. Photoperiod was the best predictor of peak emigration timing, and steelhead were most likely to emigrate at night. Larger individuals were more likely to emigrate shortly after stocking; however, the size range of fish that emigrated throughout the study period was wide. Residual population estimates performed after the DIDSON sampling period revealed that in both years < 6% of the steelhead that were stocked remained in Trout Run by the end of June, > 2 months after stocking. Most of these fish (63%) were < 150 mm TL, below the typical size threshold for steelhead to emigrate. These findings provide useful information on the survivorship and residence time of stocked juvenile steelhead and indicate that multiple factors, including photoperiod and length at stocking, should be considered when determining best practices.