Abundance and Size Structure of Shortnose Sturgeon in the Altamaha River, Georgia

Abstract

Endangered Shortnose Sturgeon Acipenser brevirostrum inhabit large tidal rivers along the Atlantic coastline of North America, ranging from the St. John River, Canada, southward to the St. Johns River, Florida. Currently, longterm assessments of the abundance and age structure of southern populations are completely lacking. To address this information gap, we assessed recent changes in Shortnose Sturgeon abundance and age structure by sampling with anchored entanglement gear in the Altamaha River, Georgia, during summer in 2004-2010. To estimate abundance, we used the Huggins closed-capture model in Program MARK. We assessed size structure by interpreting the first, second, and third quartiles of the FL of individuals captured during each year. In total, we captured 1,737 Shortnose Sturgeon (72 recaptures). Total estimated abundance was variable, ranging from 1,206 individuals (95% lognormal confidence interval [CI] = 566-2,759) in 2009 to 5,551 individuals (95% CI = 2,804-11,304) in 2006. Much of the annual variation in total abundance was attributable to wide variations in juvenile abundance, which ranged from a low of 62 individuals (95% CI = 24-181) in 2009 to a high of 3,467 individuals (95% CI = 1,744-7,095) in 2006. Annual shifts in size structure were indicative of rapid population turnover, probably resulting from a combination of mortality and permanent emigration. Although the Altamaha River Shortnose Sturgeon population shares several characteristics with northern populations (e.g., variable juvenile abundance and stable adult abundance), our results suggest that southern populations are more susceptible to decline because of their accelerated life cycle and inherently lower abundances.

Status of the Shortnose Sturgeon Population in the Savannah River, Georgia

“…DISCUSSION Abundance estimates for the Savannah River population of Shortnose Sturgeon are consistent with the prevailing hypothesis that populations of the species tend to be smaller at the southern end of the range (Kynard 1997;Peterson and Bednarski 2013). For example, Dadswell (1979) estimated that the Saint John River, New Brunswick, contained 18,000 adults, and Bain et al (2007) estimated that the Hudson River, New York, contained 56,708 adults.…”

“…For example, Dadswell (1979) estimated that the Saint John River, New Brunswick, contained 18,000 adults, and Bain et al (2007) estimated that the Hudson River, New York, contained 56,708 adults. In the only comparable study of a southern Shortnose Sturgeon population, Peterson and Bednarski (2013) documented a threefold variation in adult abundance (707-2,122 individuals) over a 7-year period in the Altamaha River. Those authors attributed large annual swings in adult abundance to a combination of variable annual recruitment coupled with a rapid population turnover following years of high recruitment.…”

“…Capture probability AIC c ΔAIC c w i K to better understand long-term recruitment patterns and casual mechanisms within the Savannah River, these results are not necessarily cause for concern given that Peterson and Bednarski (2013) documented successful recruitment in the Altamaha River population in only 3 of 7 years from 2004 to 2010. In that study, the authors also documented a strong positive relationship between late winter or early spring discharge and annual age-1 recruitment in the following year (Bednarski 2012).…”

“…Data analysis.-Individual Shortnose Sturgeon within the catch were first assigned to demographic groups based on the modal distributions that were apparent in the length-frequency histogram for each year, as described by Peterson and Bednarski (2013). Because Shortnose Sturgeon in southern rivers typically begin to reach sexual maturity at 500 mm FL (Dadswell et al 1984;Kynard 1997;Peterson and Bednarski 2013), modal distributions from length-frequency analyses are not typically useful for age assignments beyond age 1. This is particularly problematic for southern populations because males begin maturing at age 2 while females do not do so until at least age 3 (Dadswell et al 1984).…”

“…The inclusion of models that allowed capture probability to vary by time (i.e., m t , m t+g , and m t×g ) accounted for variability in weekly sampling effort and provided the best model fit during all 3 years of the study (Table 2). Furthermore, the modeling methods used in this study were nearly identical to those used by Peterson and Bednarski (2013) to estimate the Altamaha River population. Consequently, the results from this study provide the first directly comparable estimates for the Savannah River population.…”