Towed passive acoustic monitoring complements visual survey methods for Heaviside's dolphins Cephalorhynchus heavisidii in the Namibian Islands Marine Protected Area

The genus Cephalorhynchus contains four dolphin species, of which three are classified as Near Threatened or Endangered and one subspecies is close to extinction. Understanding the species' abundance, distributions and habitat preferences is necessary for effective management to prevent further...

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Bibliographic Details
Published in:African journal of marine science 2020-10, Vol.42 (4), p.495-506
Main Authors: Gridley, T, Martin, MJ, Slater, J, Roux, J-P, Swift, RJ, Elwen, SH
Format: Article
Language:English
Subjects:
Acoustic data
Acoustic surveying
Acoustics
Aquatic mammals
Centroids
Cephalorhynchus
Cephalorhynchus heavisidii
Cetacea
click detection and classification
Coastal ecology
Coastal zone
Data collection
Dolphins
Dolphins & porpoises
Echolocation
Ecological distribution
encounter rate
Environmental monitoring
Habitat preferences
Habitat selection
Islands
line-transect survey
Marine mammals
Marine parks
Marine protected areas
Methods
Monitoring
Narrowband
narrowband high-frequency clicks
Offshore
PAMGuard
Population decline
Porpoises
Protected areas
southwestern Atlantic
Species extinction
Surveys
Taxonomy
Threatened species
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Summary:The genus Cephalorhynchus contains four dolphin species, of which three are classified as Near Threatened or Endangered and one subspecies is close to extinction. Understanding the species' abundance, distributions and habitat preferences is necessary for effective management to prevent further population declines. Heaviside's dolphin C. heavisidii is endemic to the Benguela ecosystem off southwest Africa, and like other Cephalorhynchus species these dolphins produce narrowband high-frequency (NBHF) echolocation clicks with a centroid frequency around 125 kHz. We conducted dedicated visual and acoustic line-transect surveys within and adjacent to the Namibian Islands Marine Protected Area in 2012-2014. Acoustic data were processed in the passive acoustic monitoring software PAMGuard, using the default porpoise click detector and classifier to identify NBHF echolocation clicks. Click detection and classification in PAMGuard included a large excess of false positives, which were easily identified by manual verification of events, and ultimately provided 52 definite detections. The acoustic methods provided data in offshore areas and during overnight periods, but were imperfect and not suitable for ecologically important shallow coastal areas. While demonstrating the utility of passive acoustic monitoring in line-transect surveys targeting Cephalorhynchus species, the study shows that both visual and acoustic methods were needed to collect data throughout the range of Heaviside's dolphin.
ISSN:1814-232X
1814-2338
DOI:10.2989/1814232X.2020.1848925