Recruitment of Estuarine-Dependent Alosines to Roanoke River and Albemarle Sound, North Carolina

Abstract

The deleterious effects of dams on alosine populations are widely documented in many rivers along the Atlantic coast. Alterations to the natural hydrologic regime can disrupt spawning, egg dispersal, and recruitment of larvae to nursery habitats. The goal of this study was to investigate the ecological processes that influence recruitment of river herring (blueback herring Alosa aestivalis and alewife A. pseudoharengus) to nursery habitats within lower Roanoke River and Albemarle Sound, North Carolina. It was hypothesized that variability in abiotic conditions and fluctuations in food abundance could structure nursery habitat and severely restrict recruitment. Ichthyoplankton and zooplankton samples were collected concurrently March through June 2008-09 at 19 stations, within three areas: River, Delta, and Sound. Significant spatial and temporal differences were observed for river herring abundances. Abundances (number/100m³ ± SD) were significantly higher in 2009 (30.8 ± 149.8) than in 2008 (4.1 ± 20.9). Across both years, abundances within the River (21.0 ± 127.6) were significantly higher than those in Delta (7.4 ± 35.4) and Sound (4.6 ± 24.8). Yolk-sac larvae were prevalent throughout samples (32%); however, larvae collected were predominantly preflexion stage (66%). Fish ages ranged from 4 to 19 days after hatch. Growth rates were similar for blueback herring (0.29 ± 0.16 mm/d) and alewife (0.30 ± 0.14 mm/d). Growth estimates were indicative of habitat quality and suggested riverine habitats supported the highest growth rates. Mortality estimates for blueback herring (0.76 ± 0.23 per day) were significantly higher than mortality estimates for alewife (0.64 ± 0.17 per day). High mortality for both years was probably related to larval dispersal and advective loss. Larvae do not appear to be food limited in this system as indicated by diet analyses and the spatiotemporal overlap between river herring and zooplankton. Decreasing zooplankton abundance was correlated with larval abundance and suggests foraging by larval alosines could negatively alter the structure of the zooplankton community. Diets varied little with early ontogeny and the smallest taxa (copepod nauplii and rotifers) accounted for over 85% of the diet. Because of a high-level of dietary overlap, intraspecific and interspecific competition is substantial for anadromous alosines. The result of long-term data analysis (1984 - 2009) for larval and juvenile river herring confirms Roanoke-Albemarle stocks are in decline. Larval fish abundance was negatively affected by spring river flow (r² = 0.62). High flows (> 300 m³/s) resulted in larval advection from Roanoke River. Spring river flow was positively correlated with juvenile abundance (r = 0.95) and best recruitment of juveniles occurs in years with moderate spring river flow (141 - 311 m³/s).