Year-class strength and recovery of endangered shortnose sturgeon in the Hudson River, New York

Shormose sturgeon Acipenser brevirostrum, a U.S. endangered species, has experienced a fourfold increase in abundance in the Hudson River in the past three decades. The age structure and trends in year-class strength of yearlings were investigated to evaluate the underlying pattern of annual recruitment that accompanied population recovery. Annuli in pectoral spine sections were used to estimate ages of specimens captured with gill nets bimonthly from November 2003 to November 2004. Age estimates (range = 5-30 years) were generated for 554 shortnose sturgeon ranging from 49 to 105 cm total length. Hindcast year-class strengths corrected for gear selectivity and cumulative mortality indicated high recruitments (31,000-52,000 yearlings) during 1986-1992. This interval was preceded and succeeded by approximately 5-year periods of lower recruitment (6,000-17,500 yearlings), suggesting 10-fold recruitment variability over the 20-year period. The pattern and relative magnitude of hindcast recruitment patterns were corroborated by shortnose sturgeon catch-per-unit-effort trends in an independent beam trawl survey. An analysis of hindcast year-class abundance and coincident environmental conditions indicated that flow volume and water temperature in the fall months preceding spawning were significantly correlated with subsequent year-class strength. Our results suggest that shortnose sturgeon of the Hudson River have experienced several strong year-classes concomitant with the observed population recovery during the 1980s and 1990s. The data indicate that population growth slowed during the late 1990s, as evidenced by a nearly constant recruitment pattern at depressed levels relative to the 1986-1992 year-classes.