Northern Range Expansion of California Coastal Bottlenose Dolphins (Tursiops truncatus)

Abstract

The California coastal stock of bottlenose dolphins (Tursiops truncatus) expanded its range north from the Southern California Bight, its historical range, into Central California coincident with the 1982-1983 El Niño event. Since the late 1980s, bottlenose dolphin sightings north of Central California have been increasingly reported. To determine the present-day northern range limit for these dolphins, photo-identification efforts were carried out from 2007 to 2018 in San Francisco Bay and nearby coastal waters during which 84 individuals were identified. The results demonstrate a significant range expansion along the Northern California coast at least as far as Sonoma County (38.7º N). Comparisons with photo-identification catalogs compiled south of San Francisco from 1981 to 2015 revealed that 92% of the 84 dolphins were matched to Monterey Bay (n = 77), Santa Barbara (n = 27), Santa Monica Bay (n = 29), Orange County (n = 9), Corona Del Mar (n = 2), San Diego (n = 31), and Ensenada, Mexico (n = 1). Many of the 84 dolphins (54%) showed long-range movements across the stock’s range between the Southern California Bight and the San Francisco Bay Area. The greatest movement distance recorded was by two individuals first observed in San Diego, California, in the 1980s and subsequently in Puget Sound, Washington (47º N), in 2017, setting a coastal bottlenose dolphin long-distance movement record of at least 2,500 km.