Factors influencing the behavior and duration of residence of adult Chinook salmon in a stratified estuary

The duration of residence, behavior, and thermal experience of adult Chinook salmon (Oncorhynchus tshawytscha) in a stratified lagoon-type estuary over a four-year period was determined using acoustic and radio biotelemetry transmitters and archival temperature tags. Results did not support the hypothesis that adult Chinook salmon would hold extensively and migrate slowly through the estuarine lagoon with return trips to the sea and use of the salt wedge for behavioral thermo-osmoregulation in response to high summer water temperatures. Passive tidal transport was not observed as predicted and entry from the sea to the estuarine lagoon occurred during all tidal phases suggesting that conserving energy was not a priority for fish during the estuarine phase of their migration. An unexpected finding was that the persistent pursuit pressure of pinnipeds, especially from California sea lions (Zalophus californianus), appeared to negate the suitability of estuarine lagoon as holding habitat. This finding led to the hypothesis that the risk of pinniped predation was primarily responsible for the brief residence (< 24 h on average) and rapid migration (mean 1.2 km/h, 0.42 body lengths/s) of adult Chinook salmon in the estuarine lagoon, especially given its relatively small size (7 km). This hypothesis will be difficult to test without exclusion of pinnipeds. Thermal records of fish that retreated back to the sea after tagging suggested that use of the marine river plume could be important for osmotic adaption to allow rapid migration through estuaries, which often contain concentrations of predators anticipating the return of migrating salmon. Alternatively, use of the marine river plume could have occurred independent of predators. Fish retreating back to the sea should be considered a tagging effect as downstream movement commonly occurs when adult salmonids are released after tagging. There was no evidence of any other tagging effects or biased behavior by fish tagged in this study. Regardless of the reasons, the successful migration of fish that held for weeks in the sea after tagging indicates that there could be considerable flexibility in the river entry timing of some salmon stocks. Tagging adult salmonids in the sea as they approach their natal rivers is ideal but the associated logistical challenges and expense make tagging fish immediately upon entrance to river mouths the next best option when possible.