Fish populations, gill net catches and gill net selectivity in the Lower Orange River, Namibia, from 1995 to 2001

Abstract

The Orange River: The lower part of the Orange River forms the border between Namibia and South Africa from the mouth of the river and 580 km upstream. The river origins in the Lesotho Highlands, and runs for approximately 2300 km from the source to the Orange River Mouth at Oranjemund (Namibia) and Alexander Bay (South Africa), where it discharges into the Atlantic Ocean. The total Orange River catchment is approximately 1000000 km2. The fish diversity in the Lower Orange River is relatively low.

Objective: The objective of this report is to provide baseline information about the fish resources in the Lower Orange River to form the biological foundation for recommendations for a sustainable management of the fish resources. Based on fish survey data from the period 1995-2001, the fish resources are described through studies of species diversity, relative impor- tance of the different species, life history parameters, catch per unit effort and gill net selectivity.

Methods: Fish were collected at ten stations with survey gill nets (multifilament, 22–150 mm stretched mesh size) and eight other sampling methods, such as seine nets, cast nets, electrofishing apparatus and rotenone. These additional gears are collectively called “other gears” in this report. The gill nets were used at seven of the stations to survey open, deep-water habitats in the main stream near the shore and deep backwater areas with some aquatic vegetation. Other gears were used at all ten stations and targeted mainly small species and juveniles of long-lived species in shallow, vegetat- ed and rocky habitats. Monofilament gill nets were in addition used during one of the surveys, but for standardisation and comparison with studies in oth- er Namibian rivers, these results were only used for analyses of number of species recorded, body length at maturity and length-mass relationships. Surveys were carried out in the spring in 1995 (low flood) and in the autumn in 1996, 1998, 1999 and 2001 (high flood). A total of 18082 fish were caught; 3644 with multifilament gill nets, 294 with monofilament gill nets and 14144 with other gears. The most impor- tant species in the catches were identified by using an index of relative importance (IRI), which is a measure of the relative abundance or commonness of the spe- cies based on number and biomass of individuals in the catches, as well as their frequency of occurrence.

Results: A total of 19 fish species from eight different families were recorded during the surveys, of which 13 spe- cies were freshwater species. The fish families repre- sented with the highest number of species were the Cyprinidae and the Cichlidae, with 8 and 3 species, respectively. Further additional species were recorded by the Ministry during surveys between 2002 and 2005. These include the freshwater species Labeobarbus cf. kimberleyensis (hybrid yellow fish), Tilapia rendalli (intro- duced) and Labeo umbratus. The additional marine spe- cies recorded were Argyrosomus inodorus, Pomatomus saltatrix and Lithognathus lithognathus. These marine species were all recorded in the estuary.