How Common is Non-textbook Migration in Hudson River Blueback Herring?

Anadromous fishes are commonly thought to use inland waters for spawning and for rearing in the first growing season. Thereafter, they emigrate seaward to feed, grow, and mature. However, yearling (age 1+) alosine herrings have been observed in the Hudson River estuary during the spawning season. We quantified the frequency of this behavior for Blueback Herring (Alosa aestivalis) in two contrasting time periods: the 1980s-1990s, a period of relatively high abundance, and 2012-2013, a period of low abundance. Using otolith chemistry, we (1) assayed whether yearlings captured during the spawning run had egressed to sea and then returned to fresh water, and (2) examined adult spawners and retrospectively estimated the frequency of yearling returns to fresh water in these recruited fish. At least 36 % and up to 90 % of yearlings showed evidence of overwintering at sea or very far down the estuary prior to returning to fresh water with the spawning adults. Of the adults, 67-75 % showed evidence of returns up to fresh water as yearlings, despite large changes in population size and demographic characteristics. We conclude that this cryptic, non-textbook migratory behavior is common, and suggest that the importance of estuarine and freshwater nurseries may extend beyond the young-of-year period.