Population dynamics of shortnose sturgeon, Acipenser brevirostrum, in the Altamaha River, Georgia
Bednarski, Michael Stanley
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The endangered shortnose sturgeon, Acipenser brevirostrum, occurs in large tidal rivers along the Atlantic coast of North America, ranging from the St. John River, Canada, southward to the St. Johns River, Florida. Throughout their range, populations have declined in response to habitat degradation, overfishing, nutrient enrichment, and incidental harvest. Though the status of most populations is uncertain, available evidence suggests that populations south of the mid-Atlantic bight are in particularly poor condition; presently, only three appear to exceed 1,000 individuals. Unfortunately, long-term quantitative studies of southern populations are completely lacking, resulting in critical information gaps that hinder effective restoration. The objectives of this study were to assess recent trends in 1) abundance, 2) population structure, 3) recruitment, 4) habitat suitability and 5) apparent survival in the Altamaha River, Georgia, a large, relatively unaltered southern river system. I used anchored monofilament gill and trammel nets to sample shortnose from summer 2004-2010. Individual fish were measured, assigned to a specific life stage, PIT tagged, and released. I used the Huggins closed-capture model to estimate abundance for each life stage. Changes in size-structure were examined using a non-parametric multiple comparison procedure. Linear regression was used to identify the influences of high flow duration on age-1 recruitment. Correlation analysis was used to analyze the influences of summertime flow on temperature. I used a Cormack-Jolly-Seber model to assess the effects of drought on apparent survival. I captured 1737 unique shortnose (72 within-year and 153 among-year recaptures). Estimates of total abundance varied from 1206-5551. Population structure shifted from juvenile-dominated in 2004-2007 to adult dominated in 2008-2010. Age-1 recruitment ranged from 30-2976 and was strongly correlated with sustained high flow during the young-of-year period. Decreases in summer flow were negatively correlated with temperature. Drought appeared to negatively effect apparent juvenile survival. Our results indicate that the Altamaha River hosts the largest and healthiest southern population. However, given the population’s accelerated life cycle, highly variable recruitment, and apparent sensitivity to variations in flow, we recommend that management focus on maintaining flows at levels likely to maximize long-term population persistence.