Fish

Abstract

Monthly trawl samples of fishes were taken in Chesapeake Bay in the vicinity of the Calvert Cliffs Nuclear Power Plant from 1969 through 1981. Analyses of variation in abundance between individual samples were used to investigate correlations between physical factors (temperature, salinity and dissolved oxygen [DO]) and fish abundance. While significant relationships were found, they may reflect common correlation with other factors that vary with season, station and depth rather than direct causal relationships. Low summer concentrations of DO in deeper water (9 and 12 m) were more frequent during the later years of the study. The distribution of striped bass with respect to DO and temperature was consistent with models of habitat limitation by DO and temperature.Sample abundances within each year were used to estimate annual abundance and age classes of major species. Decreases of several species in the early 1970s may have been due to Tropical Storm Agnes and to decreases in aquatic vegetation; however, the declines in these fish species were not coincident. Measures of overwinter survival of juvenile croaker were correlated with winter severity. Overwinter survival of juvenile spot was lowest in the severe winter of 1976–1977, but there was no consistent relationship between severity of weather and overwinter survival in all years of study. Correlations among annual species abundances did not reveal either strong positive correlations, suggestive of common response to environmental variation, or strong negative correlations which might indicate competitive complementarity.Analyses of relative and absolute abundance of several feeding groups showed decreases in zooplanktivores, species for which mysids are important, piscivores, and feeders on small epifauna and benthos.Size-frequency analyses of menhaden, croaker and spot showed the appearance of juveniles in the fall samples, followed by the appearance of smaller fish in early winter. These smaller fish may have been more slowly growing fish which left shallow nursery grounds later than the other juveniles or may have been late-spawned fish from a different offshore spawning area than the main group of juveniles.