Differential effects of human activity on Hawaiian spinner dolphins in their resting bays

Abstract

Hawaiian spinner dolphins display predictable daily behavior, using shallow bays to rest during the daytime, bays that are also frequented by humans. All previous research on the potential response of Hawaiian spinner dolphins to human activity has been conducted visually, at the surface. In this study we take a different approach by using passive acoustic monitoring to analyze dolphin behavior and assess whether human activity affects the behavior of the animals. We used days (n=99) and hours (n=641) when dolphins were confirmed present in visual surveys between January 9, 2011 and August 15, 2012 and metrics generated from concomitant 30-second sound recordings (n=9615). Previous research found that the dolphins were predictably silent during rest and that acoustic activity matched general activity of the dolphins with higher acoustic activity before and after rest, and silence during rest. The daily pattern of dolphin whistle activity in Bay 2 and 4 (Kealakekua and Kauhako) matched what would be expected from this earlier work. However, in Bay 1 and 3 (Makako and Honaunau) there was no drop in dolphin whistle activity during rest. After assessing the relationship between time of day and dolphin acoustic activity, data on human presence were used to determine how variability in the dolphins’ acoustic activity might be explained by human activity (i.e. the number of vessels, kayaks and swimmer snorkelers present). Bay 2, the bay with the most human activity, showed no relationship between dolphin whistle activity and human presence (either vessels, kayaks, or swimmer/snorkelers). Although the relationships were weak, Bay 1 displayed a positive relationship between dolphin whistle activity and the number of vessels and swimmer/snorkelers present in the bay. Bay 4 also showed a positive relationship between dolphin whistle activity and the number of swimmer snorkelers. We also documented less sound being added to the soundscape with each additional vessel in Bay 2 when compared to Bay 1, a bay with dolphin-focused activities. We hypothesize it is not the magnitude of the activity but the focus of the activity that matters and suggest that the effect of human activity on spinner dolphin acoustic behavior should be explored in future studies. These results have implications for designing future studies as well as for ongoing efforts to protect Hawaiian spinner dolphins in their resting bays.