STREAM-BED SCOUR, EGG BURIAL DEPTHS, AND THE INFLUENCE OF SALMONID SPAWNING ON BED SURFACE MOBILITY AND EMBRYO SURVIVAL

Bed scour, egg pocket depths, and alteration of stream-bed surfaces by spawning chum salmon (Onchorhynchus keta) were measured in two Pacifi c Northwest gravel-bedded streams. Close correspondence between egg bu rial depths and scour depths during the incubation period suggests an adaptation to typical depths of bed scour and indicates that even mino r increases in the depth of scour could significantly reduce embryo su rvival. Where egg burial depths are known, expressing scour depth in t erms of bed-load transport rate provides a means for predicting embryo mortality resulting from changes in watershed processes that alter sh ear stress or sediment supply. Stream-bed alteration caused by mass sp awning also may influence embryo survival. Theoretical calculations in dicate that spawning-related bed surface coarsening, sorting, and form drag reduce grain mobility and lessen the probability of stream-bed s cour and excavation of buried salmon embryos. This potential feedback between salmon spawning and bed mobility implies that it could become increasingly difficult to reverse declines in mass-spawning population s because decreased spawning activity would increase the potential for bed scour, favoring higher embryo mortality. Further analysis of this effect is warranted, however, as the degree to which spawning-related bed loosening counteracts reduced grain mobility caused by surface co arsening, sorting, and redd form drag remains uncertain.