Coho Salmon Growth in Relation to Natural Turbidity Regimes in a Coastal Stream of Northern California

Abstract

AbstractWe examined how the growth rate and food consumption of juvenile Coho Salmon Oncorhynchus kisutch varied in relation to natural turbidity regimes in a coastal stream of northern California. Instream sensors were used to continuously monitor the location, turbidity, and temperature exposure of juvenile Coho Salmon fitted with passive integrated transponder tags. We observed that overwinter growth rate and food consumption varied in relation to the duration and magnitude of turbidity and temperature exposure. Growth rate and food consumption were positively associated with low‐to‐moderate turbidity exposures that ranged from >3 NTU to >20 NTU and negatively associated with elevated turbidity exposures that ranged from > 55 NTU to >150 NTU. This shift to negative associations for fish that experienced long exposures at turbidity levels > 55 NTU suggests a threshold for assessing potential risk of impairment. However, our analyses show that the influence of turbidity on consumption and growth rate is complicated by a fish's temperature exposure history, which varies among fish depending on individual movement patterns and duration of residency within different stream reaches. Consequently, Coho Salmon growth reflects the net effect of turbidity, temperature, and other environmental factors that are associated with specific life history patterns. These findings advance our understanding of how natural turbidity regimes may influence Coho Salmon growth and offer insight for assessing biological impairment in streams. Moreover, our findings indicate how knowledge of environmental context is crucial for understanding the applicability of laboratory‐derived turbidity thresholds for fish populations in streams. Such findings corroborate other field‐based studies and add to evidence that laboratory‐derived turbidity thresholds alone may be inadequate predictors of biological impairment to stream fish populations.