Changing Patterns of Sea Lion Strandings along the Central California Coast

Abstract

Marine mammal health is a proxy for the state of the environment. California sea lions (Zalophus californianus californianus, or CSL) heavily populate the California coast and have provided stranding data since 1975. From 2001 to 2010, at least 5,334 CSLs stranded along the central California coast. The main causative stranding factors included strong El Niño Southern Oscillation events in 2003 and 2009, domoic acid toxicosis (16% of all stranding cases and noted especially in 2005 and 2009), unknown infectious diseases (18.9%), leptospirosis (15.9% of all stranded CSLs and found mostly in 2004 and 2008), and malnutrition (22.3%). Male CSLs were more often reported as stranded (57.9%), and the majority of the population (70%) accrued during the summer breeding months of May through August. Since 1991 on, the species has stranded further south and has recently begun to favor female strandings. While the CSL population continues to grow, so, too, does its annual number of stranded individuals. However, as environmental factors become more variable in both their strength and timing due to threats like climate change, prospective predictions of CSL strandings will become less reliable.