Ecology of juvenile chinook salmon in a small non-natal stream of the Yukon River drainage and the role of ice conditions on their distribution and survival

We investigated the ecology of juvenile stream-type chinook salmon (Oncorhynchus tshawytscha) in Croucher Creek, a small non-natal tributary of the upper Yukon River, in 1998 and 1999. Underyearling (age 0+) salmon enter Croucher Creek from the Yukon River in June, and by midsummer reached an average density of >0.5/m(2). Fish were most commonly found in small pools. Their mean size increased until the end of August, but growth virtually ceased after that, when water temperatures fell. Juveniles remained in the stream through winter, and their distribution and survival were strongly influenced by aufeis, a thick layer of ice that develops from the freezing of groundwater. Over-winter survival was not dependent on fish size. Those fish that survived the winter grew rapidly and doubled in body mass in the spring. About 900 yearling fish emigrated from Croucher Creek in late June and early July at a mean length of 89 mm and mass of 7.2 g. Most of the migrants overwintered in a 700 m long reach of the creek that was downstream from groundwater sources and did not experience severe icing conditions. We suggest that small streams may be important habitats for juvenile salmon in the Yukon drainage, especially if there is a year-round source of groundwater flow that creates conditions suitable for overwintering.