Intra-Annual and Diel Variation in Catch Rates of Juvenile Chinook Salmon in the Sacramento-San Joaquin River Delta

There is growing recognition that diel activity patterns exist in juvenile salmonids and that patterns within a single species can vary spatially and temporally.  The purpose of this study was to determine whether diel patterns in activity level exist in juvenile Chinook salmon in the Sacramento-San Joaquin River Delta, and whether these patterns change intra-annually.  We examined catch rates of Chinook salmon in trawls conducted continuously over 24 hour periods during spring and fall/winter months at multiple locations in the Delta.  We found that catch per unit effort was greatest during the day in spring sampling periods and greatest at night in fall/winter sampling periods.  Potential reasons for variation in activity patterns include differences among times of year in water temperature, photoperiod, and fish size.  Identifying temporal patterns in salmon activity and understanding their causes can provide resource managers with more accurate estimates of salmon abundance on which to make decisions.