Species distribution modelling of Bryde’s whales, humpback whales, southern right whales, and sperm whales in the southern African region to inform their conservation in expanding economies

Abstract

In southern African waters, information about species distribution and habitat preferences of many cetacean species is limited, despite the recent economic growth that may affect them. We determined the relative importance of eight environmental variables (bathymetry, distance to shore, slope, chlorophyll-a, salinity, eastwards sea water velocity, northwards sea water velocity and sea surface temperature) as drivers of seasonal habitat preferences of Bryde’s whales (Balaenoptera brydei), humpback whales (Megaptera novaeangliae), southern right whales (Eubalaena australis) and sperm whales (Physeter macrocephalus). Using presence only data from multiple sources, we constructed predictive species distribution models (SDMs) consisting of ensembles of seven algorithms for these species during both summer and winter. Predicted distribution for all cetaceans was high in southern Africa and, in particular, within the South African Exclusive Economic Zone (EEZ). Predictive models indicated a more pronounced seasonal variation for humpback, sperm and southern right whales than for Bryde’s whales. Southern right whales occurred closer to shore during winter, humpback whales were more likely to occur along the east coast in winter and the west coast in summer, and sperm whales were more concentrated off the shelf in winter. Our study shows that ensemble models using historical, incidental and scientific data, in conjunction with modern environmental variables, can provide baseline knowledge on important environmental drivers of cetacean distribution for conservation purposes. Results of this study can further be used to help develop marine spatial plans and identify important marine mammal areas.

Clicking throughout the year: sperm whale clicks in relation to environmental conditions off the west coast of South Africa

“…The high acoustic occurrence of sperm whales obtained from AARs 2 and 3 deployed at 1118 m water depth suggests that this water depth is one of the preferred and more important habitats, as indicated by whale catches where more whales were caught inhabiting water depths of 1000 m than 4000 m (Best 1999). AAR4 deployed at 4481 m water depth produced the lowest percentage of sperm whale acoustic occurrence, indicating that a small proportion of this species occupies this deep sea area, as similarly indicated by whale catches (Best 1999) and species distribution modelling (Purdon et al 2020b). Furthermore, sperm whale clicks produced at great depths could have been deflected by the thermocline (an acoustic barrier) and prevented from reaching AARs 1 and 4, as these 2 recorders were sometimes positioned above or below the thermocline depth (~200 m depending on season; Shabangu et al 2020c).…”

“…For example, AARs 3 and 4 had the same sampling protocol and were deployed within the same period but at different water depths (1118 m for AAR3 versus 4481 m for AAR4); however, AAR3 produced more hours with sperm whale clicks than AAR4. Purdon et al (2020b) found water depths between 750 and 1500 m and a distance to shore of approximately 130 km to have the highest effect on sperm whale distribution in the southern African region. Moreover, AAR3 had a higher detection range than AAR4 due to variations in ambient noise between the 2 positions.…”

“…8). Correspondingly, Purdon et al (2020b) found sperm whale distribution to be influenced by warm SST. Results of Purdon et al (2020) and the present study indicate that this species might be vulnerable to climate warming, not only in the high latitudes, but also in low latitude habitats such as the west coast of South Africa.…”

“…1) that promotes biological productivity (Andrews & Hutchings 1980, Brown 1992, Shannon 2009. Purdon et al (2020b) is the only study to have investigated the relationship between sperm whale distribution and environmental conditions in the southern African region. It is important that relationships between sperm whale occurrence and environmental conditions be established specifically in South African waters in light of the recently estimated global in crease in climate change-related effects on environmental variables such as sea surface temperature (Halpern et al 2019).…”

Acoustic occurrence, diel‐vocalizing pattern, and detection ranges of southern right whale gunshot sounds off South Africa's west coast

“…Moreover, the high number of detections of gunshot sounds from AAR1 (the more coastal listening station deployed at 855 m water depth) could be due to high prey densities and favorable environmental conditions associated with that depth (Elwen & Best, 2004a, b;Prieto et al, 2017;Purdon et al 2020;Shabangu et al, 2019). For example, areas shallower than the 1,000 m isobath on the west coast of South Africa are known to have elevated productivity to support high biomasses of prey due to increased upwelling (Andrews & Hutchings, 1980;Lamont et al, 2015Lamont et al, , 2018.…”