Analysis of an estuarine striped bass population: Effects of environmentalconditions during early life

Estuarine fish populations are exposed to a variety of environmental condit ions that cause both shortterm variability and long-term trends in abundanc e. We analyzed an extensive data set for striped bass (Morone saxatilis) in the San Francisco Estuary to refine our understanding of how environmental variability influences recruitment. We examined the effects of environment al variability during early life stages on subsequent recruitment (age 3 yr ), and the degree to which conditions in early fife may have contributed to a long-term decline in abundance of adult striped bass in the San Francisc o Estuary. Survival from egg to young-of-the-year: varied strongly with fre shwater flow; this effect apparently occurred within the first week or two of life, a time period that encompasses transport of eggs and larvae from t he rivers to rearing areas and the onset of feeding. The rate of freshwater flow to pumping facilities that export freshwater from the system had smal l or sporadic effects on survival during the first month or two of life. Al though many young striped bass between ages 2 and 8 mo were entrained in ex port pumping facilities, the resulting high mortality was unrelated to tota l mortality rates determined from field data on young striped bass. Tins la ck of effect was apparently due to strong density-dependent mortality occur ring between ages 1 mo and 3 yr (Kimmerer et al. 2000). The available data do not support previously suggested relationships between recruitment and f reshwater flow during early life, or between gross estimates of pesticide i nput and survival of early life stages. We used a simple life-cycle model t o show that various combined factors could have led to a decline in adult a bundance, particularly a large and increasing adult mortality, but that eve nts early in life probably did not contribute substantially to the decline. These results demonstrate that several decades of monitoring data from num erous life stages are needed to distinguish among alternative hypotheses ab out environmental influences on populations of estuarine fish.