Monitoramento acústico passivo através de matriz de arrasto: detecção, localização, perfil de mergulho e estimativa da densidade de baleias cachalote (Physeter macrocephalus) na plataforma continental externa e talude sul brasileiro

Abstract

In face of new potential threats from anthropogenic activities introduced in all oceans worldwide, the monitoring of marine fauna species, such sperm whales, is imminent. This species typically spends 70 to 75% of its time in foraging dives, becoming difficulty to observed. However, while diving individuals produce foraging vocalizations, which make them amenable to acoustic monitoring. In recent years, Passive Acoustic Monitoring (PAM) have been increasingly applied. Results from the first sperm whale PAM effort carried out in the southern Brazilian outer continental shelf and slope are presented here. Three ship-based surveys were conducted using a 3-element towed array. Sperm whale click trains were detected and located using PAMGuard. This species acoustic encounters occurrence and distribution in relation to stationary and anthropogenic features were spatially assessed, and its abundance estimated through Conventional Distance sampling (CDS) analysis. Moreover, data from five Time-Depth Recorders (TDR’s) attached in sperm whales off Brazil were used to assess this species dive profile and five Digital Tags (Dtags) placed in individuals around Azores were used to directly estimate the percentage of time individuals were acoustically available. The joint assessment of both tag types allowed to estimate the detection probability at zero horizontal distance g(0). From 21 acoustic encounters recorded, 57% were beyond the 1000 meters (m) isobath, with 85.71% (n = 18) occurring between the slope limits of the 200 m and 2000 m isobaths. Seven recordings (33.33%) occurred in oil and gas blocks. In contrast, all acoustic encounters were recorded in Priority Areas for Conservation (PACs). The best-fitted generalized linear model (GLM) indicated depth as the only covariate with a significantly positive relationship to sperm whale acoustic encounters distribution. A total of 139 complete dives were used to assess this species dive profile. V-shaped shallow dives (40.29%) were the most frequent dive type, followed by intermediate-depth (29.50%), deep dives (17.27%) and U-shaped shallow dives (12.95%). Twenty-seven TDR’s dives were identified as foraging, from which, together with the 60 foraging dives from Dtags, were estimated an expected value of acoustical availability correspondent to 38.10 minutes (min). With a 30.58 min silent time and a finite time-window of 27.71 min, the estimated g(0) was 0.96. A density of 0.0146 whales/ km2 and an abundance of 1654.35 whales (CV: 0.379, CI: 778.36 – 3516.24) for the surveyed area were estimated, from the acoustic data recorded during Spring 2014, considering a g(0)=1. The number of sperm whales was underestimated in 4% compared to the resulting abundance when considered the g(0) estimated from Dtags and TDR’s. Although PACs cover the entire area of sperm whale occurrence, they are only instruments that may support the implementation of future management actions in the study area. Despite a minority of individuals have been recorded in oil and gas exploration blocks, due to the sperm whale’s proximity to such areas, monitoring this species’ distribution, as well as the process associated with oil and gas activities is recommended. Although still in development, acoustic monitoring presents as an alternative or complementary method to visual monitoring, being able to access useful information on sperm whale distribution and abundance, while its applicability potentially contribute to the continuous improvement of such method.