Year-Class Strength and Recovery of Endangered Shortnose Sturgeon in the Hudson River, New York

—Shortnose 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 1⁄4 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. Throughout their range, populations of shortnose sturgeon Acipenser brevirostrum have been negatively affected by anthropogenic changes to their habitats. Decreased water quality, habitat destruction, blockage of spawning runs, and incidental or intentional harvest (Kynard 1997; NMFS 1998; Collins et al. 2000; Secor and Niklitschek 2001; Root 2002) have caused reduced abundance or localized extirpations (e.g., Chesapeake Bay) in some instances (Secor et al. 2002). As a result, shortnose sturgeon was federally protected rangewide in 1973 pursuant to the Endangered Species Act in the United States and are considered a species of special concern under the Canadian Species at Risk Act. Yet, while many shortnose sturgeon populations number less than 5,000 (Table 1), the total Hudson River population may be as high as 61,000 fish (Bain 2001). Results from mark–recapture studies indicate adult population growth from about 13,000 in 1980 (Dovel et al. 1992) to about 56,000 in 1998 (Bain 2001), a 400% increase over an 18-year period. In the case of the Hudson River, population growth can arise through increased adult survivorship, an increase in recruitment, or a combination of the two processes. An elasticity analysis conducted by Gross et al. (2002) showed that rapid population growth of this and other sturgeon species is unlikely due to increased survival during the adult life stage. Rather, rapid population growth is most likely attributable to changes in firstyear survival and the formation of strong year-classes. Their results are based on the premise that interannual mortality rate during the first year of life is highly variable relative to later stage mortality and fecundity rates, and thus exhibit the greatest scope for positive