Cumulative exposure of Hawaiian spinner dolphins (Stenella longirostris) to human activities: No rest for the weary

The spinner dolphins of Hawaii Island are a small 668 ± 62 SE (95% CI = 556-801), genetically isolated population that rely on four sheltered bays to rest during the day. These bays however, are also used by people for recreational, tourism and subsistence purposes. Consequently the dolphins are repeatedly exposed to human activities on a daily basis. This repeated exposure to human activities may affect individual vital rates and ultimately population viability. Concerns have been raised regarding the effect of human activities on the Hawaii Island spinner dolphin population. The National Oceanic and Atmospheric Administration (NOAA) are considering a number of mitigation approaches to reduce the number and intensity of human-dolphin interactions. Prior to the implementation of any mitigation approach however, baseline information on the exposure to human activities is required. Here, we modelled the daytime cumulative activity budget (resting, socialising and travelling) of dolphins under impact (human activities within 100 m) and control (no human activities within 100 m) conditions. First, a systematic photo-identification study of individual dolphins (n = 235) was used to record their presence/absence in each of four important dolphin resting bays, based on the proportion of time they were observed in each bay. Secondly, concurrent passive acoustic recordings (n = 2148 days) were used to document the daily presence/absence of dolphins within bays. Thirdly, data from land-based and boat-based group focal follows (n = 428 hrs) inside and outside resting bays were used to provide behavioural time series of dolphins under control and impact conditions. Simulations were used to estimate location specific (inside bays/outside bays) activity budgets for individual dolphins. Finally, the cumulative activity budget of the population was estimated. During the day, individual spinner dolphins spent between 49.5% and 69.4% of their time resting (mean=61.7%, SD=6.5) and were exposed to human activities for 82.7% of the time. Despite the high level of exposure, human activities seemingly did not have a significant effect on dolphin activity budgets. This result is, likely an artifact of the low level of control data available (< 18% of observations) to make robust comparisons between behavioural patterns in control and impact conditions Furthermore, intervals between interactions were short (median duration = 10 min), before dolphins were exposed again, which may prevent individuals recovering from disturbance and deprive them of rest. Control observations, as defined in this study, may not accurately represent true resting behaviour of spinner dolphins. Specifically, it is likely I quantified resting behaviour when dolphins were in a “light” rather than a “deep” sleep behavioural state, which may lead to rest deprivation, impaired cognitive abilities and ultimately effects on population viability. The chronic exposure of spinner dolphins to human interactions and the concurrent use of the resting bays for recreational, commercial and subsistence purposes must now be of major concern for management