Relating Salmonid Population Health to Natural Turbidity Regimes in Northwest Streams

We investigated the utility of a field-based approach to evaluate the relationship between turbidity and salmonid population health and to establish biologically linked reference turbidity regimes for NW streams. We used state-of-the-art water quality monitoring and fish tracking technology to continuously monitor the location and turbidity exposure of juvenile coho fitted with PIT tags. The relationship between the turbidity exposure history and growth and survival for each tagged individual were examined with linear and generalized linear regression models. We identified several turbidity exposure metrics that are related to fish growth and survival. Also, we found turbidity exposures commonly exceeded published impairment thresholds. Studies relating biological impairment to natural event suspended sediment/turbidity regimes typical of NW streams are rare. Consequently, we don’t really know what turbidity regimes are associated with healthy (unimpaired) populations and we don’t know the threshold for impairment. We suggest that field-based approaches be used to examine the biological significance of existing or proposed turbidity standards and that such data are needed to inform the development of biologically linked criteria.