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

Type Report
Title Fish populations, gill net catches and gill net selectivity in the Lower Orange River, Namibia, from 1995 to 2001
Publication (Day/Month/Year) 2007
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.
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 importance
of the different species, life history parameters,
catch per unit effort and gill net selectivity.
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, vegetated
and rocky habitats. Monofilament gill nets were
in addition used during one of the surveys, but for
standardisation and comparison with studies in other
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 important
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 species
based on number and biomass of individuals in the
catches, as well as their frequency of occurrence.
A total of 19 fish species from eight different families
were recorded during the surveys, of which 13 species
were freshwater species. The fish families represented
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 (introduced)
and Labeo umbratus. The additional marine species
recorded were Argyrosomus inodorus, Pomatomus
saltatrix and Lithognathus lithognathus. These marine
species were all recorded in the estuary.
Thirteen species were caught in the multifilament gill
nets, of which three were marine species (Liza richardsoni,
Mugil cephalus and Lichia amia). The two most
important species, Labeobarbus aenus and Labeo capensis,
contributed 90% of the total IRI. The Cyprinidae
family was the dominating family in the gill net catches
(94% of the total IRI). Labeo capensis dominated
the gill net catches in the upper parts of the Orange
River, whereas Labeobarbus aeneus dominated the
catches closer to the river mouth. Labeo capensis,
Labeobarbus aeneus, Labeobarbus kimberleyensis and
Clarias gariepinus were the only species found at all
the gill net stations.

Related studies