Riverine salmonid egg burial depths: review of published data and implications for scour studies

Abstract

Published data on salmon, trout, and charr egg burial depths are highly variable and inconsistent. Primary sources of variation include elevation datum and portion of the egg pocket referenced to; differences in spawning behavior and the number, thickness, and location of egg pockets; relationships between egg depth, fish species, and corresponding size of female and spawning substrate and velocity characteristics; sampling method; presence of excavation barriers; redd superimposition; and scour and fill by hydraulic and other mechanical processes. Such sources of variability in the reported data have important implications for studies of scouring processes in salmonid spawning areas that require accurate identification of egg burial depths for predicting and preventing potential scour impacts. Cumulative measurement error and unexplained variation may amount to 5-20 cm or more in published values. The most relevant data for scour impact assessments are depths from the original stream bed elevation down to the top of the main egg pocket. Frequency distribution data are needed for determining probabilities and cumulative levels of scour impacts and for managing genetic diversity as well as population size. Preliminary depth threshold criteria are proposed for use now, pending further research.