Cohort‐Specific Growth and Mortality of Juvenile American Shad in the Pamunkey River, Virginia

Abstract

AbstractWe estimated the variation in instantaneous rates of growth (G) and mortality (M) between intraannual cohorts of juvenile American shad Alosa sapidissima in the Pamunkey River, Virginia. The ages of juveniles captured by push net during the juvenile abundance index surveys in 1998 and 1999 were estimated by counting daily rings on the sagittal otoliths. Weight‐at‐age and abundance‐at‐age data were used to generate instantaneous daily rates of growth and mortality for 5‐d cohorts. In 1998, the peak hatch date lagged behind the peak spawning periods that had been inferred from collections of American shad broodstock and occurred after peak spring flow events. In 1999, the correspondence between the hatch date distribution and the peak spawning periods was greater than in 1998. The instantaneous daily growth rate was relatively constant between cohorts, ranging from 0.037 to 0.064 in 1998 and from 0.046 to 0.066 in 1999. The instantaneous daily mortality rate was more variable between cohorts, ranging from 0.047 to 0.084 in 1998 and from 0.044 to 0.093 in 1999. The physiological mortality rate, or M/G, was calculated for all cohorts. Most cohorts in 1998 and 1999, including the largest cohorts in both years, had an M/G value close to 1.0, indicating that these cohorts were barely maintaining or losing biomass during the early juvenile stage. The results of this study indicate that in both 1998 and 1999 the year‐class declined in biomass during the early juvenile stage and underwent demographic structuring that affected the composition of the population that ultimately migrated to sea.