Spatiotemporal trends in cetacean strandings and response in the southwestern Indian Ocean : 2000–2020

On behalf of SIF, we would like to thank the Seychelles partners (Alphonse Foundation, Desroches Foundation, Island Conservation Society, Farquhar Foundation, Seychelles Islands Foundation, Silhouette Foundation) for providing financial support to acquire and grant use of their data. Collection of data in Reunion was funded by DEAL Reunion and Region-Reunion.The south-western Indian Ocean (SWIO) is a region of global importance for marine mammal biodiversity, but our understanding of most of the species and populations found there is still rudimentary. The Indian Ocean Network for Cetacean Research (IndoCet) was formed in 2014 and is dedicated to the research of all cetacean species across the SWIO. Since 2019, there have been efforts to create a regional network for coordinated response to stranding events as well as training and capacity building in the SWIO region. The present analysis represents a first investigation of stranding data collected by various members and collaborators within the IndoCet network, covering over 14,800km of coastline belonging to nine countries/territories. Between 2000–2020, there were 397 stranding events, representing 1,232 individual animals, 17 genera and 27 species, belonging to six families: four balaenopterids, one balaenid, one physeterid, two kogiids, six ziphiids and 14 delphinids. Seven mass strandings were recorded: two were composed of three to 20 individuals and five composed of > 20 individuals. Spatial analysis of stranding events indicated that local spatio-temporal clusters (excessive number of events in time and geographic space) were present in all countries/territories, except for the Comoros. The only significant cluster was detected on the southwest coast of Mauritius, just west of the village of Souillac. The SWIO region predominantly comprises relatively poor countries/territories, but imminent Ocean Economy developments are prevalent throughout the region. This study highlights the importance of establishing baselines upon which any future potential impact from anthropogenic developments in the region can be measured.Peer reviewe

Marine mammals as indicators of Anthropocene Ocean Health

The current state of marine mammal populations reflects increasing anthropogenic impacts on the global Ocean. Adopting a holistic approach towards marine mammal health, incorporating healthy individuals and healthy populations, these taxa present indicators of the health of the overall Ocean system. Their present deterioration at the animal, population and ecosystem level has implications for human health and the global system. In the Anthropocene, multiple planetary boundaries have already been exceeded, and quiet tipping points in the Ocean may present further uncertainties. Long and short-term monitoring of marine mammal health in the holistic sense is urgently required to assist in evaluating and reversing the impact on Ocean Health and aid in climate change mitigation

Common dolphin Delphinus delphis occurrence off the Wild Coast of South Africa

Despite their typical large group sizes, limited research exists on the occurrence of common dolphins Delphinus delphis because of the pelagic, offshore nature of this species and the lack of barriers to their movement in this environment. The main purpose of our study was to investigate the occurrence of common dolphins off the Wild Coast of South Africa (western Indian Ocean) and whether spatiotemporal and environmental conditions affected their encounter rate, relative abundance and mean group size. The annual sardine run in this region, during austral winter (May to July), is considered a main driver of dolphin occurrence; however, our boat-based surveys over the period 2014–2016 indicated that common dolphins occur and feed in this area outside of this time-frame. In terms of environmental factors, the largest group (~1 250 animals) was found in the deepest waters. Additionally, at Hluleka, dolphins were observed primarily feeding, which could suggest that this coastal area is highly  productive. As common dolphin distribution is thought to be correlated with prey distribution, our findings suggest that sufficient prey exists along the Wild Coast both during and outside the annual sardine run to sustain large groups of the dolphins and that their presence in the area is not solely a function of the sardine run. Keywords: boat-based survey, encounter rate, feeding behaviour, group size, relative abundance, sardine ru

Low mitochondrial genetic diversity in the Indian Ocean humpback dolphin Sousa plumbea in South African waters

The Indian Ocean humpback dolphin Sousa plumbea has been described as South Africa’s most endangered marine mammal due to its low abundance, reliance on coastal habitats with increasing anthropogenic threats and high rates of mortality from bycatch in bather protection nets (BPNs). Although the species has been well studied in South Africa, only a single study has examined its molecular ecology to date, and its population structure remains poorly understood. However, understanding population structure is vital for the conservation and management of a species. To address these research gaps for S. plumbea in South African waters, we analysed the mitochondrial D-loop of 157 museum skin and tooth samples collected between 1963 and 2017 from across the species’ geographic range in South Africa. Our data show that the humpback dolphin has extremely low mitochondrial diversity (haplotype diversity, HD = 0.47; nucleotide diversity, π = 0.2%) with only 3 haplotypes identified, which is comparable to the Critically Endangered Māui dolphin Cephalorhynchus hectori maui and the Critically Endangered Mekong population of Irrawaddy dolphin Orcaella brevirostris. Mitochondrial genetic diversity has not changed significantly in the last 50 yr, despite the high levels of bycatch in BPNs over this time period. Furthermore, we found no evidence of differentiation between dolphins from the KwaZulu-Natal Coast and the Cape South Coast (Western Cape and Eastern Cape). The extremely low mitochondrial diversity we found adds to the growing body of evidence that the humpback dolphin is becoming increasingly vulnerable and that urgent conservation efforts are required for the survival of the species

Cranial variation in common dolphins Delphinus spp. off South Africa, with the inclusion of information from the holotype of Delphinus capensis

The common dolphins (genus Delphinus) are widely distributed in all temperate and tropical oceans. Over this wide geographical distribution they show considerable range in morphological variation, which has led to descriptions of several species in the genus. Until recently, only two species of Delphinus were accepted, but this classification has become contentious. This study investigated the occurrence of morphologically different types of Delphinus in South African waters and assessed geographic and morphometric variations within each type. A total of 296 skulls of Delphinus spp., obtained from the Port Elizabeth Museum and the Iziko South African Museum, were photographed in their dorsal and ventral aspects for geometric morphometric analysis. Our results revealed three clusters of specimens based on analysis of the dorsal aspect of the skull, and two clusters for the ventral aspect. Significant differences in cranial size were found between clusters. Both cranial aspects showed that the main variations in skull shape occurred in the rostral region and braincase area, indicating divergent adaptations relating to these features. There was a substantial difference between the composition of the dorsal-aspect and ventralaspect clusters, suggesting the presence of only one species in South African waters. Significant differences between morphological clusters associated with the three regions within the study area (cold-temperate, warm-temperate and subtropical) are probably mediated through differences in local environmental conditions (e.g. different water temperatures and productivity).Keywords: cranial morphology, Delphinus delphis, geographical variation, geometric morphometrics, rostral index, sexual dimorphism, taxonom

Long-term trends in cetacean occurrence during the annual sardine run off the Wild Coast, South Africa

During the austral winter, cetaceans and other apex predators follow the annual northeastward movement of shoaling sardines, known as the sardine run, along the southeast coast of South Africa, including a 400-km stretch called the Wild Coast. In total, 131 opportunistic aerial surveys were conducted between May and July, from 1996 to 2014, to monitor sardine movement. Cetacean-sighting data from these surveys were analysed, focusing on long-term trends in frequencies of the cetaceans. In total, 630 sightings involving five cetacean species were recorded: 268 (approximately 32 400 individuals) of Indo-Pacific bottlenose dolphin Tursiops aduncus, 108 (approximately 79 400 individuals) of long-beaked common dolphin Delphinus capensis, 242 (approximately 670 individuals) of humpback whale Megaptera novaeangliae, 1 (two individuals) of southern right whale Eubalaena australis, and 11 (16 individuals) of Bryde’s whale Balaenoptera edeni. The occurrence of common dolphins, typically associated with sardines, decreased significantly in average group size over the study period (p = 0.0343); bottlenose dolphins, considered generalist feeders, demonstrated no such trend (p = 0.916). Humpback whales were most frequently sighted between 2010 and 2014, and with significantly larger groups observed towards the end of the study period (p = 0.0121). For all these species, more sightings were made inside than outside of marine protected areas (>70% of the dolphin species, and >65% of the humpback whales), both pre- and post-2005 (from 2005 the size of the survey area increased). The results indicate that movements of the common dolphin may be employed as a proxy for sardine occurrence. Long-term trends evident in the data also demonstrate the importance of this coastal region for bottlenose dolphins as well as use as a migratory corridor for humpback whales.Keywords: aerial survey, Balaenoptera edeni, Delphinus capensis, Eubalaena australis, Megaptera novaeangliae, mysticetes, Transkei region, Tursiops aduncus, spatial and temporal distributio

Profiling and spatial variation analysis of persistent organic pollutants in South African delphinids

The continuous disposal of persistent organic pollutants (POPs) in South Africa (SA) warrants concern about their detrimental effects on humans and wildlife. We surveyed six dolphin species (<i>n</i> = 90) incidentally captured in shark net installations or stranded off the SA east and south coast from 2005 to 2009 to study the POP exposure. <i>Sousa plumbea</i>, an inshore and estuarine species, was found to be the most contaminated by total POPs (21 100 ng g^–1 lw) of all the dolphins off SA, followed by <i>Tursiops aduncus</i> (19 800 ng g^–1 lw), <i>Lagenodelphis hosei</i> (13 600 ng g^–1 lw), and <i>Delphinus capensis</i> (5500 ng g^–1 lw), whereas POP levels in the offshore or pelagic delphinids were much lower. In all delphinids, dominant pollutants were dichlorodiphenyltrichloroethanes (DDTs), which represented more than 60% of the total concentration of total POPs, followed by polychlorinated biphenyls (PCBs, 30%). Concentrations of DDTs in <i>S. plumbea</i> and <i>T. aduncus</i> off SA were among the highest levels reported in delphinids globally. Approximately half of the adult <i>T. aduncus</i> had PCB concentrations above the effect threshold for impairment of immune functions. The concentrations of Mirex and Dieldrin in SA delphinids were higher than those found in species from other regions of the Southern Hemisphere