Abstract
AbstractWe estimated the variation in the instantaneous rates of growth and mortality between cohorts of larval alewife Alosa pseudoharengus and blueback herring A. aestivalis in the Tar–Pamlico River, Pamlico Sound, North Carolina. The age of larvae captured by push net was estimated by counting the daily rings on sagittal otoliths. Weight‐at‐age and abundance‐at‐age data were used to generate instantaneous daily growth (G) and mortality rates (M) for 7‐d cohorts. The instantaneous daily growth rate was relatively constant between cohorts, ranging from 0.103 to 0.277 for alewives and from 0.105 to 0.200 for blueback herring. The instantaneous daily mortality rate was more variable between cohorts, ranging from 0.064 to 0.270 for alewives and from 0.100 to 0.251 for blueback herring. All but one blueback herring cohort had an M/G value exceeding 1.0, indicating that these cohorts were losing biomass during the early larval stage. For alewives, M/G values were more variable, with 50% of the cohorts having values less than 1.0. The effect of habitat was consistent between species, with M/G values being higher and closer to 1.0 at sites in tributary creeks and backwater areas of Tar River. The overall M/G values were 0.57 for alewives and 1.60 for blueback herring from both backwater and main‐channel sites, indicating that the environmental conditions in the Tar–Pamlico River are more favorable for alewives.Received March 25, 2011; accepted November 10, 2011
Highlights
Alosa pseudoharengus and blueback herring A. aestivalis in the Tar–Pamlico River, Pamlico Sound, North Carolina
Several studies have reported little evidence for the differential timing of spawning between the two species (Street et al 1975; O’Connell and Angermeier 1997; Walsh et al 2005), but some studies suggest that alewives spawn before blueback herring (Jones et al 1978; Loesch 1987)
The back-calculated hatch day frequency data showed that spawning extended from mid-March through midMay for blueback herring and from early April through June for alewives
Summary
Alosa pseudoharengus and blueback herring A. aestivalis in the Tar–Pamlico River, Pamlico Sound, North Carolina. The variability in patterns of recruitment success for fishes that have a protracted spawning season may be due to the habitat type and quality experienced by the early life stages This is especially the case for anadromous species whose drifting eggs and larvae encounter many different habitats and conditions, ranging from swiftly moving water to estuarine habitats. Spawning overlap is high both spatially and temporally for alewives and blueback herring and can have important effects on survival, growth rates, recruitment, and predator avoidance (Fortier et al 1995; Gotceitas et al 1996) Factors such as competition and predation by adults and juveniles play an important role in the spawning and recruitment success of these species (Schmidt et al 1988)
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