[MARMAM] Two new publications about using PAM in distance sampling and estimating abundance of Heaviside's and dusky dolphins

Morgan J. Martin mjmartin at sandiego.edu
Sun Dec 20 14:55:29 PST 2020

Dear MARMAM Community,

My co-authors and I are very pleased to share with you two recently
published papers in *Frontiers in Marine Science* and the *African Journal
of Marine Science*. The first paper provides the first abundance estimates
for Heaviside's and dusky dolphins off Namibia. The second paper is a
methods paper describing how we used PAM from a towed hydrophone array in
combination with visual observers on line transect ship surveys to increase
the detectability of Heaviside's dolphins.

*First Abundance Estimates of Heaviside’s (Cephalorhynchus heavisidii) and
Dusky (Lagenorhynchus obscurus) Dolphins Off Namibia Using a Novel Visual
and Acoustic Line Transect Survey*
Morgan J. Martin, Tess Gridley, Jean-Paul Roux and Simon H. Elwen
To link to this article: https://doi.org/10.3389/fmars.2020.555659
*Abstract:* Knowledge of a population’s abundance is of primary importance
for conservation management. However, robust estimates of abundance are
often difficult to obtain, especially for cetaceans which spend most of
their lives submerged. Cetacean abundance is commonly estimated using
aerial or vessel-based line transect surveys and distance sampling methods.
During 2012–2014, the first line transect surveys to estimate cetacean
abundance were conducted in Namibian waters. Surveys took place in the
Namibian Islands’ Marine Protected Area (NIMPA), a large MPA located along
the southern Namibian coastline. A combined visual and acoustic
double-platform survey configuration was used to investigate the factors
affecting detectability of the endemic Heaviside’s dolphin (*Cephalorhynchus
heavisidii*) and dusky dolphin (*Lagenorhynchus obscurus obscurus*). The
present analysis estimates the probability of detection on the transect
line (g(*0*)) for these two species and generates density and abundance
estimates which incorporate a correction for both animals missed on the
transect line and attractive responsive movement. The average annual
baseline density and abundance estimates for Heaviside’s dolphins in the
NIMPA region during 2012–2014 were 0.08 individuals/km2 (CV = 28.6%, 95% CI
= 0.04–0.15 individuals/km2) and 1594 individuals (CV = 28.6%, 95% CI
=776–3275), respectively. The average annual baseline density and abundance
estimates for dusky dolphins in the NIMPA region during 2012–2014 were 0.16
individuals/km2 (CV = 26.2%, 95% CI = 0.10–0.28 individuals/km2) and 3493
individuals (CV = 26.2%, 95% CI: 2015–6052), respectively. A discussion on
the distribution of Heaviside’s and dusky dolphins is provided for this
region where such information is urgently needed. Based on existing
knowledge of the species and area, these estimates are regarded as
reasonable. They indicate moderate sized populations of animals within the
NIMPA and provide an important first baseline on which future estimates can

*Towed passive acoustic monitoring complements visual survey methods for
Heaviside’s dolphins Cephalorhynchus heavisidii in the Namibian Islands
Marine Protected Area*
Tess Gridley , Morgan J. Martin , Jeffery Slater , J-P Roux , Rene J. Swift
and Simon H. Elwen
To link to this article: https://doi.org/10.2989/1814232X.2020.1848925
*Abstract:* 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.

Please feel free to email me for a pdf copy of either paper. Thank you for
your interest in our research!

Merry Christmas,

Morgan J. Martin, PhD
Postdoctoral researcher
University of Victoria, B.C.
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