RISE AND FALL OF THE POTOMAC RIVER STRIPED BASS STOCK - A HYPOTHESIS OF THE ROLE OF SEWAGE

The spawning and nursery area of striped bass Morone saxatilis in the Potomac River is located about 30 km downstream from the outfall of the Blue Plains sewage treatment plant in Washington, D.C. The area coincides with the fertile zone of the river that receives sewage from several treatment plants and has abundant phytoplankton and zooplankton. During the striped bass spawning season, April to June, there is a longitudinal downriver succession of sewage nutrients, phytoplankton, zooplankton, and striped bass larvae, suggesting the presence of a trophic link between sewage nutrients and larvae. We developed a transfer function-noise model of average daily biological oxygen demand loadings from sewage treatment plants and the total commercial landings of striped bass from 1938 to 1983. The model suggests a causal relationship between the sewage nutrients and the fishery. A multiplicative decomposition analysis of striped bass juvenile indices from 1958 to 1986 suggests that sewage nutrients play an important role in recruitment. We hypothesize that an increase in sewage raised the fertility of the spawning and nursery regions and was responsible for the abundance of striped bass from the 1940s through the 1960s. We hypothesize further that the improvements in sewage treatment processes since the early 1970s have greatly lowered sewage nutrient loading, lowered fertility in the spawning and nursery regions, and contributed to the recent decline of striped bass in the Potomac Estuary.