Spatial and temporal variability in instantaneous rates of growth and mortality rates of larval alewife and blueback herring in the Tar-Pamlico River, Pamlico Sound, North Carolina

We estimated the variation in instantaneous rates of growth (G) and mortality (M) between cohorts of larval alewife A. pseudoharengus and blueback herring Alosa aestivalis in the Tar-Pamlico River, Pamlico Sound, North Carolina. Ages of larvae captured by push net were estimated by counting daily rings on sagittal otoliths. Weight-at-age and abundance-at-age data were used to generate instantaneous daily rates of growth and mortality for 7-d cohorts. Instantaneous daily growth rate was relatively constant between cohorts, ranging from 0.103 to 0.277 for alewife and from 0.105 to 0.200 for blueback herring. Instantaneous daily mortality rate was more variable between cohorts, ranging from 0.064 to 0.270 for alewife and from 0.100 to 0.251 for blueback herring. All but one blueback herring cohort, had an M/G value >1.0, indicating that these cohorts were losing biomass during the early larval stage. For alewife, M/G values were more variable where 50% of the cohorts had values <0.37. The effect of habitat was consistent between species where M/G values were higher and closer to 1.0 in the backwater areas. Overall M/G values were 0.57 for alewife and 1.6 for blueback herring indicating that the environmental conditions in the Tar-Pamlico River are more favorable for alewives.