Effect of Spinner Dolphin Presence on Level of Swimmer and Vessel Activity in Hawai'ian Bays

Abstract

Questions have been raised about the effects human activity in Hawai'ian bays has on dolphins. Concerns about the effects of this activity have led the National Marine Fisheries Service to begin the process of enacting regulations to reduce the impacts of swimmers and vessels on Hawai'ian spinner dolphins (Stenella longirostris). One step in evaluating potential effects is to determine if dolphin presence attracts swimmers and vessels into bays. In this study, numbers of vessels and swimmers in Kealake'akua, Honaunau, and Kauhako Bays were measured and related to spinner dolphin presence. In Kauhako Bay, mean number of swimmers per scan sample was significantly higher when dolphins were present, and in Honaunau Bay, mean number of kayaks per scan sample was significantly higher when dolphins were present. In addition to measuring the relationship between dolphin presence and vessel and swimmer presence, it is important to track vessel and swimmer numbers over time and to determine patterns of use in individual bays. This establishes trends in human use of bays and allows management on a more individual bay basis. During this study, Kealake'akua Bay experienced significantly more vessel and swimmer activity than Kauhako Bay. Numbers of one- to three-person kayaks, motorboats <6 m, and zodiacs were highest in Kealake'akua Bay. Numbers of swimmers from shore were higher in Honaunau Bay than in Kauhako Bay. Overall, numbers of vessels and swimmers in the bays were higher than in previous decades, and swimmers comprised the majority human activity in the bays.