Occurrence and distribution of cetaceans in Namibian waters

Occurrence and distribution of cetaceans in Namibian waters

A summary of Pauline Glotin’s MSc thesis, 2016

 

The Benguela upwelling, situated in the west coast of Angola, Namibia and South Africa, is a region highly productive and biologically diverse. Namibia was one of the world’s largest whaling areas in the 20th century and with at least 25 species known to occur here, hosts more than 60% of the world’s whale and dolphin species (Best, 2007). Despite this – knowledge about cetacean fauna is remarkably poor.

Figure 1: Map of the Benguela current upwelling system (Kirkman et al., 2015)

Figure 1: Map of the Benguela current upwelling system (Kirkman et al., 2015)


The main aims of my study were to provide an updated description of cetacean diversity within Namibian waters, especially within and adjacent to the Namibian Islands Marine Protected Area (NIMPA), and to predict the spatial and seasonal distribution patterns of the cetaceans in coastal and offshore Namibia.

picture 1 sandra

Figure 2: Map of Namibia showing the Namibian Islands Marine Protected area and its associated islands

The NIMPA (Figure 2) was created in 2009 to protect the marine resources and the unique island environments, especially the seabird populations. It covers 10 000 km², and could therefore protect and be of benefit to other species, particularly cetaceans.

Cetaceans in the wild are difficult to observe, range across vast areas which are expensive to survey, therefore it is often challenging to record sightings. Only a handful of studies have been conducted relating to the presence and the distribution of cetaceans in Namibian waters.

Reports on the occurrence of some single species have been documented over the past decade (e.g. pygmy right whales – Leeney et al., 2013; pygmy and dwarf sperm whales – Elwen et al., 2013; southern right whales – Roux et al., 2015 and humpback whales – Elwen et al., 2013), however the last study on the diversity and the distribution of cetaceans (only concerning the small odontocetes) occurring in Namibia was published in 1992 (!!) by Findlay et al.

My study investigated the distribution of six cetacean species (Heaviside’s dolphin, dusky dolphin, bottlenose dolphin, pilot whale, humpback whale and southern right whale) using a species distribution modelling approach called maximum entropy (Maxent)

Maxent predicts the presence probability of a species using the correlation between presence records of this species and environmental variables. The environmental variables thought to influence the cetacean species distribution in the Namibian waters used in this study are: sea surface temperature, chlorophyll a concentration, habitat depth, seabed slope, habitat complexity, distance to shore and mean dynamic topography.

Cetacean records were collected from a variety of sources such as dedicated surveys (boat anand aircraft), platforms of opportunity and citizen science.  In total, 2831 sightings of 22 species were recorded from 2008 to 2015 (Figure 3).

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Figure 3:Cetacean sightings recorded between 2008 and 2015 in Namibian waters

 

The figures below are two examples of habitat suitability of two different species in summer, predicted by Maxent. In general, the predictive maps produced by the models revealed seasonal changes in habitat for all the six species analysed. Moreover, we observed that some species are seasonal visitors or year-round residents and some have either a near shore (Figure 4) or an offshore distribution (Figure 5).

This is the first study to spatially quantify the areas of highest suitability of cetacean species in Namibian waters. Species distribution models (SDMs) have been widely used for marine species over recent times as they can provide a good alternative for providing information about species distributions when dedicated surveys are difficult and where species sightings are poorly documented.

 

References

Best, P.B., 2007. Whales & Dolphins of the Southern African Sub region. Cambridge University Press, Cape Town.

Elwen SH, Gridley T, Roux J-P, Best PB, Smale MJ (2013) Records of kogiid whales in Namibia, including the first record of the dwarf sperm whale (Kogia sima). Mar Biodivers Rec 6:e45

Elwen SH, Tonachella N, Barendse J, Collins T, Best, PB, Rosenbaum HC, Leeney RH, Gridley T (2013) Humpback whales in Namibia 2005-2012: occurrence, seasonality and a regional comparison of photographic catalogues and scarring rates with Gabon and West South Africa. Pap SC/65a/SH24 to Sci Comm Int Whal Comm

Findlay KP, Best PB, Ross GJB, Cockcroft VG (1992) The distribution of small odontocete cetaceans off the coasts of South Africa and Namibia. South African J Mar Sci 12:237–270

Kirkman SP, Yemane D, Atkinson LJ, Kathena J, Nsiangango SE, Singh L, Axelsen BE, Samaai T (2015) Regime shifts in demersal assemblages of the Benguela Current Large Marine Ecosystem: A comparative assessment. Fish Oceanogr 24:15–30

Leeney RH, Post K, Hazevoet CJ, Elwen SH (2013) Pygmy right whale Caperea marginata records from Namibia. African J Mar Sci 35:133–139

Roux JP, Braby RJ, Best PB (2015) Does disappearance mean extirpation? The case of right whales off Namibia. Mar Mammal Sci 31:1132–1152

 

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About seasearchafrica

The Sea Search group is a collective of scientists and students with a strong academic background in the area of marine mammal science. Our primary focus is the production of peer-reviewed scientific research and student training. We also provide specialist consultancy services and work with industry and government to promote conservation through effective management.

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