Hypolimnetic oxygenation in Twin Lakes, WA. Part I: Distribution and movement of trout

Abstract

Summer temperature/dissolved oxygen squeeze conditions have negatively impacted coldwater fisheries, including rainbow (Oncorhynchus mykiss) and brook trout (Salvelinus fontinalis), in North and South Twin lakes of northeastern Washington. In 2009, line diffuser hypolimnetic oxygenation (HO) was initiated in North Twin to prevent anoxia and increase summer trout habitat. Suitable trout habitat volume increased from a low of 0% preoxygenation to 100% of the North Twin hypolimnion following HO. To assess if trout utilized the expanded habitat following oxygenation, summer trout movements and habitat utilization were assessed from 2006 through 2012 using various combinations of ultrasonic telemetry, archival tags, gillnetting, and hydroacoustics. Trout utilized North Twin hypolimnetic habitat more frequently compared to pre-HO years and to unoxygenated South Twin, and swim speeds significantly decreased with oxygenation. Chaoborus zooplankton distributions have also changed following HO; water column densities were significantly lower with oxygenation compared to preoxygenation and to unoxygenated South Twin.