Whole‐river manipulation of olfactory cues affects upstream migration of sockeye salmon.

This study experimentally manipulated olfactory cues in a river by adjusting hydropower operation and monitored behavioural responses of 193 adult sockeye salmon, Oncorhynchus nerka (Walbaum), using radio telemetry. Experimental dilution of olfactory cues in a tributary to 72% natal water relative to non‐natal water was associated with 80% reduced odds of fish entering the tributary from a river mainstem and increased migration times by an average of 12.2 hr, with females taking 4.6 hr longer than males to enter the tributary. Additionally, males delayed 2.7 hr longer than females at a diversion outlet in the river mainstem that discharged a natal water source during experimentally diluted olfactory conditions. Increased migration time associated with dilute olfactory cues would decrease energy available for reproduction and potentially reduce fitness. Hydropower managers should continue to regulate natal water conditions in the Seton River during sockeye salmon migrations to avoid negative impacts on migration timing and success. [ABSTRACT FROM AUTHOR]