Size and age distributions of juvenile Connecticut River American shad above Hadley Falls: influence on outmigration representation and timing

Age‐ and size‐based habitat use and movement patterns of young‐of‐year American shad in rivers are not well understood. Adult females reach their natal rivers at different times and ascend the river at different rates, which may lead to variation of hatch dates at a single location. Also, shad are serial spawners, so eggs of the same female may be released at different distances from the river mouth. It has long been hypothesized that juvenile shad emigration is a function of size or age, and not necessarily keyed only to a decrease in water temperature during the fall. We seined three sites in the Connecticut River biweekly to collect pre‐migrant shad during river residence (spring to fall). During emigration, samples were also collected weekly at two hydroelectric facilities. Otoliths were removed from ∼20% of the fish to obtain age and growth rate information. We found increases in length and age over time until late in the season, after which such increases were mostly insignificant. Cohorts collected early in the year as pre‐migrants were never sampled as migrants later in the year at the hydroelectric projects. Cohorts collected late in the year as migrants were never collected earlier in the year as pre‐migrants. Only during a narrow window of time were fish collected as both pre‐migrants and migrants. Fish that were hatched later in the season exhibited higher growth rates than fish that were hatched earlier in the season. Copyright © 2008 John Wiley & Sons, Ltd.