Failure of Submersed Macrophytes to Provide Cover for Rainbow Trout throughout Their First Winter in the Henrys Fork of the Snake River, Idaho

Abstract

Abstract Submersed aquatic plants that are abundant in some stream reaches have a potential to provide winter concealment cover for juvenile salmonids. We monitored an index of macrophyte abundance in a portion of the Henrys Fork of the Snake River during two winters that differed in severity and assessed the densities of age-0 rainbow trout Oncorhynchus mykiss associated with the macrophytes. The macrophyte index averaged 84–87%, in November 1989 and 1992, and an average of 10–13 fish/100 m2 were concealed there. In 1990, macrophyte cover declined to 59% in January and 46% in early February; fish density declined by about one-third by January and dropped to nearly zero in February. In 1992–1993, the macrophyte index declined to an average of 39% following anchor ice formation in December and to 32% in January. Fish density in December was reduced to about half of the November density and to about 1 fish/100 m2 in January. Movement of marked fish in 1989–1990 was predominantly from macrophytes into cobble...