Use of T-PODs for acoustic monitoring of Cephalorhynchus dolphins: a case study with Hector’s dolphins in a marine protected area

Abstract

The 4 dolphin species in the genus Cephalorhynchus have small populations, restricted distributions and are threatened by anthropogenic activities. It is therefore essential that tools be developed for assessing these threats and the effects of management actions. The T-POD is a com- mercially available acoustic data logger. We evaluated the efficacy of the T-POD for addressing habi- tat use questions relevant to management by passive acoustic monitoring of Hector's dolphins C. hec- tori in the Banks Peninsula Marine Mammal Sanctuary (BPMMS), New Zealand. Three T-PODs were deployed at 3 inshore locations in summer and winter over a 2 yr period (n = 431 d). Acoustic detec- tion data were summarised by detection-positive minutes per day. A linear regression model indi- cated that season had the largest effect on detection rate (F = 81.95, p < 0.001), with T-POD ID (i.e. variation in sensitivity between individual instruments) also having a significant effect (F = 9.45, p < 0.001). There was no significant difference between acoustic detection rates at night and during the day (paired t-test, t = 0.55, p = 0.59). The present study provides further evidence to support the year- round ban on all gill netting within the BPMMS. We demonstrated that T-PODs can be used to inves- tigate temporal differences in habitat use by Cephalorhynchus dolphins. However, several issues must be considered when designing such experiments; namely the mode of deployment, accounting for variation in sensitivity between individual T-PODs and the possibility of false positive detections from non-target species.