A Decade of Live California Sea Lion (<I>Zalophus californianus</I>) Strandings Along the Central California Coast: Causes and Trends, 1991-2000

Abstract

Stranded marine mammals offer a unique sample of relatively inaccessible wild animals that are more likely to represent the diseased segment of the population and are easy to examine thoroughly. Examination of these animals, therefore, offers a method to detect novel diseases in free-living aquatic mammals. Diseases in marine mammals may reflect environmental changes such as ocean pollution, prey shifts, and global warming. To detect spatial and temporal trends in prevalence of such diseases, we reviewed records for 3,707 California sea lions (Zalophus californianus) that stranded live between 1991 and 2000 along the central California coast. Reasons for stranding were determined from a combination of clinical examinations, hematology and serum biochemistry, radiography, gross necropsy, histopathology, microbiology, and biotoxin assays. Over the ten years, 74% of sea lions stranded in Santa Cruz, Monterey and San Luis Obispo Counties, and 83% of these were admitted between May and October each year. Malnutrition was the most common reason for stranding (32%), followed by leptospirosis (27%), trauma (18%), domoic acid intoxication (9%), and cancer (3%). Strandings caused by malnutrition were greatest during the El Nino events of 1992, 1993, and 1998, while strandings caused by leptospirosis accounted for over 60% of strandings in 1991, 1995, and 1999. Although domoic acid was first reported in California sea lions in 1998, there was a small stranding event in 1992 that, based on clinical examinations and histopathology, was probably also caused by domoic acid. The observed prevalence of cancer among stranded animals remained constant over the past ten years at 3%.