The influence of variations in background noise on Florida manatee (Trichechus manatus latirostris) detection of boat noise and vocalizations

Abstract

A manatee’s primary modality to detect a vessel on a possible collision course is hearing as underwater visibility is limited in many manatee habitats and their visual acuity is poor. We estimate a Florida manatee’s ability to detect the sound of an approaching boat and vocalizations in four different soundscapes in Sarasota Bay, FL. Background noise samples were collected every 5 minutes for a two-week period during winter and summer at each location (2019 or 2020). Sound levels in third octave bands (0.5, 1, 2, 4, and 8 kHz) were measured and compared to manatee auditory hearing thresholds and to sound levels of an approaching boat traveling at a slow, medium, or fast speed. Background sound levels in a wider band (1–20 kHz) were calculated to model vocal communication space at each location. We found that a manatee’s estimated ability to detect an approaching boat differs greatly among locations, with time of day, and by season, and that fast boats are predicted to be detected later than slow boats. Latency of boat noise detection is estimated to sharply increase when considering unusually loud background noise levels. We suggest that such uncommonly loud conditions (e.g. 95th percentile sound level), not just typical conditions (median sound level), are important to consider for understanding the problem of manatee-boat collisions. Additionally, background noise impacts estimated vocal communication space and may limit the ability of vocal-mediated mother-calf cohesion. Altogether, a manatee’s ability to detect acoustic signals of interest is expected to vary greatly spatially and temporally.

Manatees display diel trends in acoustic activity at two microhabitats in Belize

“…Similar adjustments in vocalization rates have been reported for dolphins (Tursiops truncatus) PLOS ONE [51] and humpback whales [52] in the presence of small crafts. Boat noise in manatee habitats varies by location and can reduce manatee communication space [53]. Manatees in our study may have compensated for boat presence by increasing vocal effort.…”

“…Foraging may also play a role in how sites are used. As suggested by Self-Sullivan et al ( 2003) [53] seagrass beds surround both sites, providing extensive foraging habitat for manatees. Studies of movements of GPS tagged Antillean manatees indicated they forage at different locations within a small geographic range (4-6 km) [56].…”

“…Declining populations suffer from high mortality rates due to watercraft collisions and habitat loss resulting from development [10]. Passive acoustic monitoring techniques can help bridge the gap in our understanding of manatee habitat use in turbid aquatic habitats by detecting their vocal activity [11,12]. Longitudinal records of underwater soundscapes can also reveal associations between manatee presence, ambient noise levels [13,14], and vessel-generated noise [15][16][17].…”

Anecdotal Accounts of Manatee Behavior: Conservation and Management, Behavioral Ecology, and Cognition

“…The anecdotes on anti-predator behavior might yield some insights to the degree that responses to vessels might be similar to responses to potential predators. Rycyk et al (2022) have recently reported acoustic differences in local habitats that might affect responses to vessels through differential sound transmission, but more work remains to be done to explain manatee behavioral variability.…”

“…Given their apparent cognitive flexibility and good hearing, why are the same manatees repeatedly struck by boats? Studies of masking by multiple boats and the effects of the acoustical environment (e.g., Rycyk et al, 2022) are needed to address this question.…”