Spatial Use of Pantropical Spotted Dolphins in Relation to Stock Boundaries and Environmental Features in Hawaiian Waters

Abstract

Genetic studies and sighting distributions were used to delineate four stocks of pantropical spotted dolphins in Hawaiian waters: a pelagic stock and insular stocks off Hawaiʻi, Maui Nui, and Oʻahu. Information on short-term movements and spatial use in Hawai‘i is limited, yet such information is critical to their conservation, particularly in light of recent documentation of frequent interactions between fishing vessels and spotted dolphins throughout the main Hawaiian Islands. To examine spatial use, we satellite-tagged eight individuals from 2015 through 2018 (Kauaʻi (pelagic), n=2; Oʻahu, n=2; Maui Nui, n=2; Hawaiʻi, n=1) and tracked their locations from 7-21 days (median=14 days). The pelagic dolphins and one Oʻahu individual (SaTag004) moved over a wide range (199-324km maximum displacement from tagging location) while all other insular dolphins remained near their tagging location (maximum displacements: 54-80 km) despite moving cumulative straight-line distances up to 1,490km. Insular spotted dolphins used nearshore waters associated with island slopes (median distances from shore: 11.2-29.5km), although exhibited some inshore/offshore movements (maximum distances: 29.2-63.5km), whereas pelagic dolphins used waters farther offshore (median distances: 45.3-54.2km; maximum distances: 114.8-148.3km). Pelagic dolphins, the Hawaiʻi dolphin, and one Oʻahu dolphin (SaTag002) used deeper waters (median depths: 2,645-3,701m) compared to individuals tagged within the Maui Nui stock and SaTag004 (median depths: 657-798m). Four of five insular dolphins frequently moved across recognized stock boundaries with one individual moving across all three insular stock boundaries, and pelagic dolphins overlapped ranges of insular dolphins off Oʻahu. Insular dolphins were predominantly characterized by low move persistence (persistence in speed and direction), while pelagic dolphins more frequently exhibited periods of high move persistence. Our results advance understanding of pantropical spotted dolphin movement patterns and spatial use in Hawaiian waters and their variability among stocks and indicate that current stock boundaries do not appropriately represent the range of these populations.