Distribution patterns of delphinids in the California Current Ecosystem observed through acoustic monitoring of species‐specific echolocation clicks

Abstract

Visual surveys along the U.S. west coast conducted in the 1980s‐90s suggested that Pacific white‐sided and Risso's dolphins likely undergo annual movement between California during winter‐spring, and Oregon‐Washington in the summer‐fall. Using high‐frequency autonomous acoustic recordings within the southern California, central California, and Washington regions of the California Current System, we evaluated the seasonal occurrence of these dolphin species through detection of their echolocation clicks. The clicks of Pacific white‐sided and Risso's dolphins are characterized by a unique combination of local frequency peaks for each species, providing a statistically robust means of identification from autonomous acoustic records. Based on these unique spectral characters, we extracted periods of clicking by each species from concurrently sampled coastal and shelf/slope locations off southern California and Washington, and a single offshore site off central California. The number of hours that each species was heard per day was quantified and compared among seasons and regions. Both species were heard year‐round at shelf/slope sites off southern California and Washington; however their patterns of occurrence at coastal sites differed somewhat. Our results indicate year‐round presence of Pacific white‐sided and Risso's dolphins in some regions, suggesting that the seasonal distribution of these species may have recently changed.