Ecology of Winter Concealment Behavior of Juvenile Spring Chinook Salmon in the Grande Ronde River Basin, Oregon

Abstract

We characterized winter concealment behavior (WCB) for juvenile spring Chinook salmon (Oncorhynchus tshawytscha) from October through January in three study streams of the Grande Ronde River Basin, Oregon. The Nocturnal Index (NI), the frequency of fish using WCB, increased significantly as mean daily water temperature decreased. WCB was negatively associated with mean daily water temperature in all three streams, and positively associated with fish size in Catherine Creek and the Lostine River. WCB was not significantly associated with fish density in any of the three streams. Both the NI and detections of concealed fish indicated that fish were concealing amid interstitial spaces during the day and emerging at night as early as October–November, yet no population fully exhibited WCB during any month sampled. Although low water temperature influenced WCB in the Grande Ronde River Basin, other ecological factors affected the behavior because not all fish used WCB even when mean daily water temperatures were < 1°C.