Interactions between natural and hatchery chinook salmon parr in a laboratory stream channel

Abstract

Pre-smolt (fry and parr) hatchery chinook salmon Oncorhynchus tshawytscha are increasingly being used in supplementation projects to augment natural but diminished salmon populations, but little is known of the behavioral interactions between sympatric hatchery and wild chinook salmon populations and how those interactions may affect success of a supplementation program. We used experiments conducted in a laboratory stream channel to document behavioral interactions that occur when hatchery salmon are stocked to streams containing existing natural chinook salmon juveniles. We found that natural fish aggression was suppressed when hatchery fish were larger than natural fish in the spring and summer, and the aggressiveness of natural fish increased when paired with similar sized hatchery fish during fall. Types of micro-habitat used by natural chinook salmon were mainly affected by hatchery fish during the fall trials when more natural fish were found in open pool areas, and fewer used cobble cover when hatchery fish were present. Observed behavioral shifts by natural chinook salmon parr could increase energy expenditures and exposure to predators when hatchery fish are present, but there were few consistent effects from hatchery salmon on the growth or movement patterns of natural chinook salmon during this study. Relative fish size, increased stream densities, and the behavior of the hatchery salmon appeared to be a major factors influencing observed changes in the behavior of juvenile natural chinook salmon in the stream sections.