Predicting large-scale habitat suitability for cetaceans off Namibia using MinxEnt

Abstract

Knowledge of the occurrence and distribution of cetaceans is particularly important for conservation and management, but is still limited within Namibian waters. We collated 3211 cetacean records from the Namibian Exclusive Economic Zone (EEZ) for the period 2008 to 2016 and applied the principle of minimum cross entropy (referred to as MinxEnt) to predict habitat suitability. MinxEnt is a generalised form of maximum entropy modelling allowing incorporation of additional information such as sampling bias. The habitat suitability of nine cetacean species or species' groups (five odontocete species, two mysticete species, two taxonomic groups: pilot whales Globicephalus spp and balaenopterids Balaenopterid spp) were predicted per season, in relation to environmental variables likely to drive cetacean presence: sea surface temperature, chlorophyll a concentration, water depth or distance to shore, seabed slope and habitat complexity. The environmental variable which most frequently influenced species habitat suitability was depth, and it was the main environmental driver of six species or species' groups: dusky Lagenorhynchus obscurus and bottlenose dolphins Tursiops truncatus, pilot, humpback Megaptera novaeangliae and southern right whales Eubalaena australis and the balaenopterid group. Further, Heaviside's dolphin Cephalorhynchus heavisidii habitat was best predicted by distance to shore in all seasons, while common dolphin Delphinus delphis habitat was best predicted by habitat complexity and sperm whale Physeter macrocephalus habitat by chlorophyll a concentration. We identify distinct spatial patterns in species' habitat suitability and provide baseline maps which can be used by managers of wildlife resources.

First Abundance Estimates of Heaviside’s (Cephalorhynchus heavisidii) and Dusky (Lagenorhynchus obscurus) Dolphins Off Namibia Using a Novel Visual and Acoustic Line Transect Survey

“…Resultant baseline density and abundance estimates and a discussion of the distribution of Heaviside's and dusky dolphins are provided for this region where such information are needed. Distribution was examined using the water depth at time of detection to investigate if our survey detections were consistent with the proposed differences in habitat preference of the two sympatric species (Findlay et al, 1992;De Rock et al, 2019).…”

“…The only other NBHF clicking species potentially in the survey region are dwarf and pygmy sperm whales (Kogia sima, K. breviceps), but these typically occur at very low densities and are found in much deeper water than the surveyed area off the continental shelf (Best, 2007). Dusky dolphins are observed frequently in the NIMPA (De Rock et al, 2019) and have distinctive click characteristics (Martin, 2019), differentiating them from other potentially encountered local broadband clicking species. In addition, local broadband clicking dolphin species prolifically produce whistles except dusky dolphins (Au et al, 2010) and presumably southern right whale dolphins (Lissodelphis peronii) (Rankin et al, 2007b), the latter of which are known to occur occasionally in a small area offshore of the Lüderitz upwelling cell (Findlay et al, 1992).…”

“…The visual and acoustic detection platforms were compared between species to examine differences between mean water depth at time of detection. The purpose was to examine if our survey detections were consistent with the proposed differences in habitat preference of the two sympatric species (Findlay et al, 1992;De Rock et al, 2019). Acoustic detections were only counted once even if multiple visual detections occurred during a single acoustic event.…”

“…They are small in body size (<1.7 m) and are typically observed in groups averaging three to four individuals (Findlay et al, 1992;Elwen et al, 2010;Martin et al, 2019) which can make them challenging to observe at sea. Their range is limited to coastal and shelf waters generally < 100 m depth (Elwen et al, 2006;De Rock et al, 2019) and they have small home ranges within this area (Elwen et al, 2006;Davis et al, 2014), which plays a role in generating some level of population structure within the species' range (Gopal et al, 2019). The Heaviside's dolphin conservation status was recently updated from 'Data Deficient' to 'Least Concern' within South Africa (Gopal et al, 2016), as they appear to be locally abundant in the southern part of their range (Elwen et al, 2009).…”

“…Animals within these four main populations are recognized as separate subspecies by the Society for Marine Mammalogy's Committee on Taxonomy 2018and on the IUCN Red List (Alfaro-Shigueto et al, 2019). Within the Benguela Current ecosystem, dusky dolphins have a slightly broader range than Heaviside's dolphins and are typically found in deeper waters up to 500 m depth (Findlay et al, 1992;De Rock et al, 2019). Group sizes range from 2 to 800 individuals (x = 35.3;Findlay et al, 1992;Best, 2007), although most encountered groups consist of fewer than 40 individuals (Best and Meÿer, 2010).…”