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 build.
