Growth and migration of 0-year American shad (Alosa sapidissima) in the Hudson River estuary: otolith microstructural analysis

A fine-scale demographic study of young-of-year American shad (Alosa sapidissima) was undertaken in May-November 1990 using otolith microanalysis (N = 280 larvae and 1507 juveniles) to elucidate patterns of growth and seaward migration. Migration was both size- and age-biased. Upriver fish were consistently smaller and younger than downriver fish until late in the season. However, cohorts moved down the river as early as June, older cohorts tending to move earlier than younger ones. Maximum relative condition was attained at the up- and mid-river sites in early August and declined steadily thereafter; maximum values were high in mid- to late July in the downriver collections and declined in the fall. The year-class was established mainly by cohorts hatched after 1 June, whereas most spawning activity occurred in early to mid-May. This was probably due to (i) a severe weather event in mid-May that caused high river flows and low temperatures, thereby reducing available food and resulting in high larval mortality, and (ii) rapid warming in June and low river flows, allowing food sources to build up and promoting larval survival.