A systematic review of the agents and factors influencing California sea lion health

Marine ecosystems are increasingly affected by climate-driven disturbances such as sea-level rise, marine heatwaves, ocean acidification, and pollution. Monitoring sentinel species is essential for detecting long-term environmental changes and informing evidence-based management. The California sea lion ( Zalophus californianus ) serves as a sentinel because its population dynamics and health reflect both environmental anomalies and anthropogenic stressors. This systematic review synthesizes nearly a century of research on factors affecting California sea lion health. Following PRISMA guidelines, 244 studies published between 1929 and 2024 were included, representing 284,627 individuals and 298,607 documented health-related occurrences. Environmental anomalies (external factors) accounted for 88.21% of all disorders, followed by nutritional and metabolic disorders (3.70%), infectious and parasitic diseases (3.17%), trauma and entanglements (2.38%), and toxins or contaminants (1.89%). Sex and age were frequently unreported, with 50.80% of individuals lacking sex identification and 61.4% lacking age classification, reflecting the limitations of stranding-based data. Geographically, 89.88% of individuals were reported from the United States and 10.12% from Mexico. Despite these biases, the synthesis reveals consistent patterns indicating that climate anomalies, particularly marine warming events, are the predominant drivers of adverse health outcomes. This review provides a comprehensive baseline to date for understanding health threats to California sea lions and offers critical insights for future monitoring and conservation strategies.