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  • Writer's pictureS Liefferink & R Tate

Chiloango Drainage, Cabinda - Angola

Updated: Nov 13, 2022

Figure 1: Dense gallery forest confined to a river valley in Cabinda Province

The catchment area studied in this post is set 50 km north east of the city of Cabinda. The climate of the area is tropical with approximately 900 mm of rainfall a year. The ecoregion is located within the Guineo-Conglian Region and thus dense moist forest typically characterises the terrestrial habitat. However, within the project area, which is in the Mayombe region, savannah habitats are also known to occur. The region experiences bimodal flood regimes with the first flood period in November and the second in April. The setting of the catchment studied in this post was set within rolling savannah hill-land’s, with swamp-forest woodlands within the valley areas (Figure 1). Dense tropical forest also occurs in the lower elevations in this area (Figure 2). Topography within the study area consisted of undulating gradients/slopes in which the watercourses were present in valley bottom gallery and swamp forest systems between islands of savannah habitat (Figure 1). The overall position of the study area in relation to the longitudinal zonation of the watercourses can be described as the source zone - foothill tropical river systems. The hydrological setting of the area considered in this post was within a catchment of an unnamed tributary of the lower Chiloango River system which drains into the Atlantic Ocean (Figure 3). Two surveys were undertaken into the area described above, where six sites were studied.

Figure 2: Dense gallery forest in Cabinda Province
Figure 3: The Chiloango River, Cabinda Province-Angola

Flat areas in the upper reaches of the tributary considered in this post were observed to be composed of dense swamp habitat. The lower strahler order watercourses consisted of shallow systems with deep detritus layers and herbaceous cover (Figure 4). Higher order watercourses in the flat areas consisted of extensive swamp forest habitat with thick detritus layers (+1m) under flowing water column.

Figure 4: A headwater shallow swamp
Figure 5: The upper reaches of a river in the study area within a source zone tributary of the Chiloango River

The higher strahler watercourse in the study area was a channeled system with gravel and sand substrates. The channel was approximately 3-5m wide with undercut banks, submerged woody vegetation and root wads the primary cover for fish species (Figure 6; Figure 7). Marginal vegetation and intermittent areas with aquatic vegetation such as Nymphae sp. were also noted.

Figure 6: The primary flow path in the study area
Figure 7: Instream and marginal habitats in the main river assessed

The swamp forest habitat type was noted to occur in all areas that were either periodically or permanently inundated and corresponded to the lower lying channels and landscape depressions of the study area. The average canopy cover for this habitat type was determined to be 88% with a minimum recorded cover percentage of 73%. Typical swamp and riparian forest specialist species are associated with this habitat type and included Mitragyna ledermannii, Mitragyna stipulosa, Anthocleista vogelii, Uapaca spp. and Alchornea cordifolia. Evidence of extensive use of the watercourses by forest elephant was observed throughout the drainage considered in this assessment. It was hypothesized that the channelled systems are used as a corridor for the movement of the animals. In addition, fine clays in the river banks and plant root structures were found to be excavated by the elephant.

Water chemistry was found to be slightly acidic, with moderate dissolved solid content and low turbidity (Figure 6). The pH was found to fluctuate between sites, with the lowest pH observed in the head-water swamp sites at pH 5.0. The primary flowpath was found to be closer to neutral at pH 6.5. Electrical conductivity was higher than would be expected in a tropical system. The elevated conductivity is hypothesized to be a result of the high concentration of humic substances. Turbidity was noted to be low during both surveys despite significant rainfall events.

Figure 8: Water chemistry (confirmed in laboratory)

The diversity of freshwater biology is typically closely related to that of the watercourse geomorphological history. The watercourses considered in this post were located in the Chiloango (Shiloango) River basin, which is within the southern boundary of the Ogooue, Kouilou-Niari, and Nyanga Freshwater Ecoregion (Darwell et al. 2008). Historically the watercourses in this ecoregion were connected to the Lower Congo River system and therefore the freshwater biodiversity show some similarities. The watercourses in this freshwater ecoregion drain from inland plateaus with rolling hills. According to Darwell et al. (2008), the fish fauna biodiversity is rich with more than 230 known species, and approximately 55 endemic species. The level of taxonomic exploration in this ecoregion is considered poor. Using a Haltech electroshocker, a fish community assessment of the wadable sites were made.

Twenty three fish species representing eleven families were captured during the survey. There were no clear dominant families observed during the survey. Benthopelagic species such as Bryconalestes longipinnis were observed at each flowing sites considered in the assessment and was determined to be the most abundant fish in the project area. Where submerged logs and undercut banks were observed, sampling the area always yielded Mormyridae typically represented by Breinomyrus brachyistius. Swamp habitats in the upper reaches of the watercourses as well as the larger bodies of still water typically yielded smaller bodied fishes such as Epiplatys singa, Neolebias ansorgii and Nannaethiops cf. unitaeniatus.

Photographs of the various fish species are presented below.

Alestidae: Bryconalestes longipinnis (top) and Brycinus kingsleyae (bottom)
Mormyridae: Paramormyrops kingsleyae (top) Marcusenius moori (middle) Brienomyrus brachyistius (bottom)
Nannocharax parvus (top), Nannathiops cf. unitaeniatus (middle), Neolebias ansorgii (bottom)
Cichlidae: Hemichromis elongatus (top), Oreochromis cf. niloticus (middle), Pelvicachromis subocellatus (bottom)

Cyprinidae: Labeo cf. lukulae (top); Nothobranchiidae: Epiplatys singa (bottom)
Clariidae: Channallabes apus (top); Protopteridae: Protopterus cf. annectens (bottom)
Anabantidae: Microctenpoma cf. fasciolatum (top), Microctenpoma ansorgii (bottom)
Channidae: Parachanna obscura (top): Eleotridae: Eleotris cf. melanosoma (bottom)

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