Evaluating Spatial and Temporal Overlap Between Larval Alosines and Potential Zooplankton Prey in Lower Roanoke River and Albemarle Sound, North Carolina

Abstract

Spatial and temporal overlap between zooplankton and larval American shad (Alosa sapidissima), river herring (alewife A. pseudoharengus and blueback herring A. aestivalis), and hickory shad (A. mediocris) was evaluated in lower Roanoke River and Albemarle Sound, North Carolina. Zooplankton abundances in this system have historically been lower than those found in other coastal river systems. It was hypothesized food limitation during the early life history of alosines was contributing to recruitment failure. Zooplankton and ichthyoplankton 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 alosine abundances. Abundances (number/100m[superscript]3 ± SD) were significantly higher in 2009 (30.8 ± 149.8), than in 2008 (4.1 ± 20.9). Across both years, River (21.0 ± 127.6) alosine abundances were significantly higher than those in Delta (7.4 ± 35.4) and Sound (4.6 ± 24.8). Zooplankton abundances were higher than observed in previous studies and did not differ significantly between years. Zooplankton abundances exhibited the opposite spatial trend of alosines with significantly higher abundances (number/m[superscript]3 ± SD) observed in the Sound (16,547 ± 14,678) than in the River (4,934 ± 3,806) and Delta (4,647 ± 2,846). Differences in zooplankton composition were evaluated using analysis of similarity. Composition in the Sound significantly differed from the River and Delta. Canonical correspondence analysis explored the relationship between zooplankton and the environment and found that some differences in composition could be explained by salinity preferences of zooplankton taxa. Zooplankton size distribution was evaluated and the most common taxa segregated into two groups based on size. Rotifers and copepod nauplii comprised the small size group and Daphniidae, Bosminidae, calanoid copepods, and cyclopoid copepods composed the larger size class. Mouth gape models were developed for each alosine species and used to estimate maximum prey size at first feeding. At first feeding, alewives, blueback herring, and hickory shad are primarily able to consume copepod nauplii and rotifers. Larval American shad are larger and have a wider potential prey breadth including Bosminidae, cyclopoid copepods, copepod nauplii and rotifers, at first feeding. During both years, there was a high amount of overlap between larval alosines and size-appropriate zooplankton, suggesting larval alosines in this system are not food limited.