Ecotoxicology 3, 123-142 (1994)

The effect of agricultural discharge on striped bass (Morone saxatilis) in California's Sacramento-San Joaquin drainage H . C . B A I L E Y 1, C. A L E X A N D E R 2, C. D I G I O R G I O 1, M. M I L L E R 2, S.I. D O R O S H O V 2 and D . E . H I N T O N 1 ~Department of Medicine, School of Veterinary Medicine, Universityof California, Davis, CA 95616, USA 2Department of Animal Science, School of Agricultural and EnvironmentalSc&nces, Universityof California, Davis. CA 95616, USA Received 9 June 1993; revised and accepted 16 December 1993

The striped bass (Morone saxatilis) population of the Sacramento-San Joaquin Delta has declined approximately 80% since the mid-1970s. This decline has been attributed to factors such as water diversions, pollution and reduced abundance of food organisms. One source of potential pollutants is agricultural return water. The Colusa Basin Drain discharges water from over 150 000 acres and can account for over 20% of the flow of the Sacramento River. Because discharge occurs at the same time striped bass are spawning, early developmental stages could be adversely affected. Toxicity studies conducted over a 3 year period consistently demonstrated acute toxicity to striped bass larvae and to opossum shrimp (Neomysis mercedis), an important food organism for juvenile striped bass. Acute toxicity was also demonstrated with striped bass embryos. In addition, a model based on pesticide use more effectively predicted striped bass recruitment during the period of decline than did a model based on historically important river flows and delta diversions. These studies indicate that agricultural return water should not be disregarded when considering potential causes of the decline of striped bass. Keywords: striped bass; opossum shrimp; agriculture; toxicity; bioassay

Introduction Striped bass (Morone saxatilis) are one of the most important sport fishes in California and the most important sport fish in the Sacramento-San Joaquin Estuary and San Francisco Bay (Moyle 1976; IESP 1987). However, in recent years, the adult population has declined approximately 80% from levels present in the early 1970s. This decline has been largely attributed to failure of recruitment (Chadwick et al. 1977; Stevens et al. 1985). Changes in the pattern of recruitment from 1960 to 1990 are shown in a plot of the annual 38 mm striped bass index (Fig. 1, solid line). The 38 mm index is a measure of year class abundance obtained by the California Department of Fish and Game ( C D F G ) from tow net surveys conducted in July at fixed sampling stations following the spawning period (Stevens 1977). The index averaged 75 in the 1960s, 48 in the 1970s and 24 in the 1980s. The index has not exceeded 6 from 1987 to 1991. 0963-9292 © 1994 Chapman & Hall

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