Bárbara Stefânea da Silva

Conventional visual surveys of the cryptic franciscana dolphin Pontoporia blainvillei are constrained by turbid near-shore waters and the species’ tendency to cluster in small groups. We combined passive acoustic monitoring (PAM) with drone counts to calibrate an acoustic footprint detection model and infer group size from acoustic data. Click trains were detected in PAMGuard, localized, and clustered with HDBSCAN into click-train clusters (CTCs). Drones provided visual group sizes during synchronized sections. A 2-step spatiotemporal matching paired CTCs to visual groups. An independent manual audit estimated a 6.4% false-positive rate, used to correct automated counts. For paired clusters, we fit GLMs with an offset for animal-seconds and alternative distance terms; AIC selected a linear negative binomial model. The calibrated baseline acoustic footprint at the trackline was r ₀ = 0.028 click trains animal ^-1 s ^-1 (95% CI: 0.014-0.056; CV = 0.34; R ² = 0.54), equivalent to 1.68 (CI: 0.84-3.36) min ^{- 1} . We obtained 28 paired CTCs from 8 visual groups and applied the calibration to 267 unpaired CTCs. Predicted group sizes had a median of 1.34 individuals (95% CI: 0.79-2.57; CV = 0.15), consistent with predominantly small units. This framework provides a pathway from clicks to counts and delivers an important step required for passive acoustic density estimation of this species.

Beaked whales, deep-diving cetaceans from the family Ziphiidae, exhibit cryptic behaviors, and data on these species in Brazilian waters are limited to strandings and isolated sightings. This study characterizes the occurrence and acoustic behavior of beaked whales in the Foz do Amazonas Basin using combined visual and passive acoustic monitoring along the Brazilian Equatorial Margin. Audio files were analyzed to identify clicks with frequency-modulated pulses, a diagnostic characteristic of beaked whales. The PAMpal package in R was used to calculate acoustic parameters, including peak frequency, 3- and 10-dB bandwidths, and inter-pulse intervals. Three sightings, eight acoustic detections, and one combined visual-acoustic detection were recorded. Based on diagnostic features, sighting D01 likely corresponds to Mesoplodon europaeus, D05 may represent Mesoplodon, and D06 could belong to either Ziphius or Mesoplodon. Unsupervised clustering based on peak frequency and inter-pulse intervals revealed three distinct acoustic clusters, suggesting that at least three different beaked whale species were detected. Further acoustic classification studies are needed to determine the specific species associated with each cluster. These findings contribute to a broader understanding of the distribution and acoustic ecology of deep-diving beaked whales in the Foz do Amazonas Basin, a region where information on marine megafauna remains scarce.