Long‐term catch trends and risk assessment of the Critically Endangered white‐spotted wedgefish (Rhynchobatus djiddensis) from South Africa

Abstract

The white‐spotted wedgefish (Rhynchobatus djiddensis) is a Critically Endangered shark‐like ray in the family Rhinidae. Throughout its Western Indian Ocean distribution, it is targeted for its valuable meat and fins and is reported to have undergone major population declines. However, there remains a need for species specific time‐series data to accurately assess localized population declines. This study used two independent long‐term (37 and 40 years) time‐series catch data from competitive shore angling and shark nets to investigate the size composition and catch per unit effort (CPUE) and conduct a risk assessment for the population on the east coast of South Africa. From 1977 to 2017 the competitive shore fishery captured 7,703 individual R. djiddensis, whilst shark nets in the same region captured 2,856 individuals from 1981 to 2017. Individuals captured in the nets had a sex ratio of 1.8:1 females to males, and were larger than those caught by the anglers. Although the mean annual sizes of net‐caught individuals were above the size of reported sexual maturity, there was little evidence to suggest that any individuals captured were reproductively active. Both the competitive shore fishery and shark net catches exhibited strong seasonal trends with the majority of R. djiddensis catches occurring from October to May peaking in austral summer. Standardized CPUE from the competitive shore fishery declined substantially between 1977 and 2017 and shark net catches exhibited a significant (p < 0.05) fourfold decline in annual nominal CPUE from 1981 to 2017. Ultimately, a risk assessment showed a 65.1% decline in abundance over a period of three generation lengths, which indicates that the sampled population of R. djiddensis in South Africa should be classified as Endangered according to the IUCN Red List using criterion A2b. The conservation implications of this are discussed.

Application of DNA mini-barcoding reveals illegal trade in endangered shark products in southern Africa

“…A recent trend shows that the fins of wedgefishes are becoming more common in the shark fin trade (Fields et al 2018). Declines of R. djiddensis have been observed in South Africa (Daly et al 2021) and in Mozambique, where this species was previously reported to be abundant (Pierce et al 2008;Hopkins 2011). In South Africa, R. djiddensis was caught as bycatch by demersal prawn trawlers operating on the Thukela Bank (located off central KwaZulu-Natal), until the fishery closed in 2002 (Jordaan et al 2021).…”

“…Additionally, wedgefishes are targeted by foreign vessels off eastern Africa (offshore of Mozambique, Tanzania and Madagascar) (Kyne et al 2020). Thus, for R. djiddensis the threat to the population seems not to be within South Africa, where this species is protected, but rather in neighbouring countries where the species is under severe threat of exploitation (Kyne et al 2020;Daly et al 2021). Recently, species in the family Rhinidae have shown severe population declines globally, resulting in 9 of the 10 species (90%) being assessed as Critically Endangered on the IUCN Red List (Kyne et al 2020), including R. djiddensis (Kyne et al 2019).…”

Seasonal changes characterise the shark and ray assemblages in a subtropical shallow sandy habitat in the iSimangaliso Wetland Park, South Africa

“…In SA, its population has been described as “Endangered”, with the main threats in KZN being fisheries— e.g. , shore angling—and the bather protection nets (locally known as shark nets) located south of the study area ( Daly et al, 2020 ; Pradervand et al., 2007 ). Since the mature individuals of this species can move long distances, they are also vulnerable to threats across the border in Mozambique ( Jordaan et al, 2021 ; SAAMBR, 2022 ).…”

“…Rhynchobatus djiddensis is known to be most abundant in central and southern KZN during summer and to concentrate northwards, possibly into Mozambique, during winter ( Daly et al, 2020 ; Jordaan et al, 2021 ). The current findings suggest that at least some wedgefishes remain in SA waters during winter, where they are under the protection of the iSimangaliso MPA, rather than being exposed to threats in Mozambique.…”

Evaluating the effects of a SharkSafe Barrier™ shoreline deployment on bull shark (Carcharhinus leucas) behaviour

“…In response to potential negative shark–human encounters and the increasing socio‐economic pressure associated with these encounters, shark culls are often government‐instituted programmes that involve killing sharks with the use of beach nets (also known as shark nets) and drumlines (or other types of baited hooks) to maximize beachgoer safety (Ikehara, 1961; Tester, 1968; Tester, 1969; Government of Western Australia, 2014; Guyomard et al, 2019). However, with continued trends in elasmobranch population decline (Worm et al, 2013; Pacoureau et al, 2021) and the substantial by‐catch of other endangered, threatened, or protected (ETP) species in association with beach nets (Brazier et al, 2012; Atkins, Cliff & Pillay, 2013), such as the Indian Ocean humpback dolphin ( Sousa plumbea ; Atkins et al, 2016) and the white‐spotted wedgefish ( Rhynchobatus djiddensis ; Daly et al, 2021), there is an urgent need to develop and implement more ecologically responsible solutions. One such approach is the implementation of less‐invasive exclusion barriers, such as exclusion nets (as noted in McPhee, 2012) and the SharkSafe Barrier™ (O'Connell et al, 2014a).…”