Effects of thermal discharges on mortality ofMercenaria mercenaria in Barnegat bay,New Jersey

Thermal discharges from the Oyster Creek Nuclear Generating Station do not affect mortality in natural populations ofMercenaria mercenaria (Linné) in Barnegat Bay, New Jersey. The analyses of daily growth increments and disturbance bands in shell cross-sections of death assemblages of the pelecypods collected at the mouth of Oyster Creek (strongly affected by thermal discharges) and at three control sites (unaffected by thermal discharges) in the bay indicate that similar mortality patterns exist in all assemblages. This is revealed by mortality rate curves, survivorship curves, and life tables, which are nearly identical for each assemblage. Each death assemblage results from natural and not census mortality, as is evident from its corresponding death-frequency histogram which shows that individuals have died at different times of the year. The peak frequency of stress and death occurs in older individuals of the populations and develops in the summer and winter. The high incidence of summer death may be associated with the effects of physiologic stress during spawning and with increased activity of predators and parasites during the warmer months of the year, whereas high winter mortality seems to be caused by harsh environmental conditions. Mortality data recorded on life assemblages ofM. mercenaria transplanted to the substrate for 1 year at the mouth of Oyster Creek and at a single control site in the bay show that mortality is significantly greater in the assemblage transplanted to the control site. Shell microgrowth analysis of the dead specimens collected from the transplanted assemblages reveals the following: (1) Maximum frequency of death in clams is between 50 mm and 65 mm in h′ (see text), and at 5 to 6 years of age; (2) peak frequency of death occurs in the summer; (3) no significant difference in the seasonal frequency of death exists between the two samples; and (4) natural instead of catastrophic mortality is evident. It is concluded that mortality ofM. mercenaria in Barnegat Bay is caused by the normal population dynamics of the species. The pattern of ontogenetic mortality in the bivalve is high-low-high. Mortality is high during the planktonic larval stages, low subsequent to spat settlement, and high again in the gerontic stage. Mortality rates rise significantly after sexual maturity is attained.