THE EFFECT OF AGRICULTURAL DISCHARGE ON STRIPED BASS (MORONE-SAXATILIS) IN CALIFORNIA SACRAMENTO-SAN-JOAQUIN DRAINAGE

The striped bass (Morone saxatilis) population of the Sacramento-San J oaquin Delta has declined approximately 80% since the mid-1970s. This decline has been attributed to factors such as water diversions, pollu tion and reduced abundance of food organisms. One source of potential pollutants is agricultural return water. The Colusa Basin Drain discha rges 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 ti me striped bass are spawning, early developmental stages could be adve rsely affected. Toxicity studies conducted over a 3 year period consis tently demonstrated acute toxicity to striped bass larvae and to oposs um 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 d id a model based on historically important river flows and delta diver sions. These studies indicate that agricultural return water should no t be disregarded when considering potential causes of the decline of s triped bass.