Slippery customers for conservation: Distribution and decline of anguillid eels in South Africa

• Published in: Aquatic conservation 2022-08, Vol.32 (8), p.1277-1290
• Main Authors: Hanzen, Céline C., Lucas, Martyn C., Weyl, Olaf L.F., Marr, Sean M., O’Brien, Gordon, Downs, Colleen T.
• Format: Article
• Language: English
• Subjects: Catadromous fishes; Catadromous species; Conservation; dams; Data analysis; Distribution; Eels; Exploitation; extent of occurrence analysis; Extremities; Fisheries; Fisheries management; Fishery regulations; Freshwater; History; Inland water environment; Introduced species; Marine fishes; Migrations; Museums; Pattern analysis; Pollution; Population decline; Protected areas; Restoration; river connectivity; Rivers; Sampling; southern Africa; threat to migratory species; water resource management
• ISSN: 1052-7613; 1099-0755
• DOI: 10.1002/aqc.3823

Four anguillid eel species occur in the western Indian Ocean rivers of Africa: Anguilla bengalensis, Anguilla bicolor, Anguilla marmorata and Anguilla mossambica. These catadromous fishes face multiple stressors, including habitat alteration and deterioration, barriers to migration, pollution and the adverse impacts of alien species, but knowledge of eel species occurrence, abundance and ecology in Africa remains poor. This study investigated the present and historical distribution of anguillid eels and the potential associated drivers of declines at the southern extremities of their ranges in South Africa. Data analysed included sampling conducted in KwaZulu–Natal and Eastern Cape between 2015 and 2020, and secondary data extracted from databases, museums and local management agencies. The median extent of inland penetration increased as follows: 22 km for A. bicolor, 29 km for A. marmorata, 94 km for A. bengalensis and 293 km for A. mossambica. The median altitude followed a similar pattern. Extent of occurrence analyses were carried out at the regional level in KwaZulu–Natal. The sampling data on present distribution (2015–2020), compared with historical data, suggests declines in the extents of occurrence of the four eel species in KwaZulu–Natal, ranging between 31 and 48% in the last 30 years and between 35 and 82% since the 1950s. With increasing human threats in the region, especially from watercourse modification and water ion, further declines for these species are expected. Conservation measures recommended include the maintenance or restoration of the ecological connectivity of important rivers and the implementation of freshwater protected areas. Although eels are at present not widely exploited in South Africa, there is a need for fisheries regulations to manage sustainable commercial exploitation.