Abstract
Swimming behavior of Chinook Salmon (Oncorhynchus tshawytscha) smolts affects transit time, route selection and survival in complex aquatic ecosystems. Behavior quantified at the river reach and junction scale is of particular importance for route selection and predator avoidance, though few studies have developed field-based approaches for quantifying swimming behavior of juvenile migratory fishes at this fine spatial scale. Two-dimensional acoustic fish telemetry at a river junction was combined with a three-dimensional hydrodynamic model to estimate in situ emigration swimming behavior of federally-threatened juvenile Chinook salmon smolts. Fish velocity over ground was estimated from telemetry, while the hydrodynamic model supplied simultaneous, colocated water velocities, with swimming velocity defined by the vector difference of the two velocities. Resulting swimming speeds were centered around 2 body lengths/second, and included distinct behaviors of positive rheotaxis, negative rheotaxis, lateral swimming, and passive transport. Lateral movement increased during the day, and positive rheotaxis increased in response to local hydrodynamic velocities. Swim velocity estimates were sensitive to the combination of vertical shear in water velocities and vertical distribution of fish.
Highlights
Chinook Salmon (Oncorhynchus tshawytscha) populations across the Pacific Coast declined throughout the 20th century, and have remained low in the Central Valley of California in particular [1, 2]
Swimming behavior leads to a difference in how quickly smolts move downstream relative to the average velocity of the water, and the results show that smolt movement was slower than the water velocity
The swimming behavior of emigrating Chinook Salmon smolts is an important factor in route selection and transit time
Summary
Chinook Salmon (Oncorhynchus tshawytscha) populations across the Pacific Coast declined throughout the 20th century, and have remained low in the Central Valley of California in particular [1, 2]. Of the four recognized evolutionary significant units (ESUs) of Chinook Salmon in the Central Valley, two (winter-run and spring-run, named for the season when adults return to freshwater; [3]) are listed under the federal Endangered Species Act. Chinook Salmon exhibit anadromous life-histories, hatching and rearing in fresh water before emigrating to the ocean as juveniles. In the Central Valley, juveniles experience high mortality during emigration, due largely to geomorphic and hydrologic changes at the watershed scale that have degraded or eliminated habitat, as well as decreased river flows due to drought and water exports, entrainment in water diversions, predation by non-native species, decreased food supply, and presence of contaminants [3–5]. Survival varies strongly as a function of water year.
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