Fishes of the Lower Lulua River (Kasai Basin, Central Africa): A Continental Hotspot of Ichthyofaunal Diversity under Threat

The ichthyofauna of the Lulua River, a large right bank tributary of the Kasai River in central Africa, is among the most poorly documented in the Kasai ecoregion. To remedy this lack of knowledge, sampling was carried out between 2007 and 2014 along the main channel and in many tributaries. A total of 201 species distributed in 81 genera, 24 families, and 12 orders are reported from the lower reaches of the Lulua. The species richness reported in this study represents a substantially improved documentation of the Lulua ichthyofauna (historically estimated at only 79 species). Here, 129 species are recorded for the first time, bringing the total number of known species to 208. Among these, five have recently been described: Raiamas brachyrhabdotos Katemo Manda, Snoeks, Choca Manda, and Vreven 2018, Distichodus kasaiensis Moelants, Snoeks, and Vreven, 2018, Distichodus polli Abwe, Snoeks, Choca Manda, and Vreven 2019, Epiplatys atractus Van Der Zee, Mbimbi, and Sonnenberg 2013, and Hypsopanchax stiassnyae Van Der Zee, Sonnenberg, and Mbimbi 2015; numerous additional taxa are currently under investigation. Recognized here as a continental hotspot of ichthyofaunal diversity, the Lulua is under extreme threat from exploitation of forest products for building materials, deforestation for shifting agriculture and charcoal exploitation, destructive fishing practices, and mining, all of which are rapidly increasing in the watershed. The present study provides baseline documentation for use in conservation planning and future developmental projects in the Kasai ecoregion in general and Lulua River basin in particular.


Introduction
The Congo Basin (CB) comprises an immense hydrographic network with a wide variety of habitats hosting an extremely diverse fish community currently estimated at 1270 species, of which over 75% are considered basin endemics [1][2][3]. Second only to the Amazon in terms of volume, the CB drains an area of about 3.7 million km 2 of Central Africa [4,5], and its location on both sides of the Equator results in the Congo River (CR) discharging an almost constant volume of 1308 km 3 [6] of freshwater into the Atlantic Ocean (AO). From its source in the savanna highlands of the Lualaba and Haut-Katanga provinces in southeastern Democratic Republic of the Congo (DRC) to its mouth in the western Kongo Central province, the main channel of the CR flows through 4374 km [5] generating a high diversity of macro-and microhabitats including falls, rapids, swamps, pools, floodplains, flooded forests, lakes, deep rocky substrates, and a short estuarine reach. Along its journey to the AO, in the western basin, the CR is joined by large tributaries such as the Lulonga, Ruki, Ubangi, Sangha, Alima, Lefini, Kwa-Kasai, N'sele, and Inkisi Province in the southeast of the Democratic Republic of Congo near the Angolan border at an elevation of about 1200 m asl [35], and it gradually descends the Katanga plateau (a northward extension of the Kalahari plateau) crossing the Central Kasai Province to merge with the main channel of the Kasai River at about 58 km downstream of the city of Luebo in the Kasai Province. From its headwaters, near the boundaries of the Upper Zambezi and Lualaba ecoregions (Figure 1), to its confluence (at about 386 m asl) with the KR, the Lulua flows over a distance of 1256 km, characterized by high geomorphological complexity [36], resulting in extensive habitat heterogeneity along its main channel including numerous rapids, pools, small and large tributaries, large floodplains, and perennial and permanent swamps [37,38]. The numerous rapids divide the main channel of the LR into several alternating sections of low (muddy and deep) and fast current (rocky and shallow). The LR is under a tropical savanna climate characterized by two main seasons: a longer wet season (from October to early May) and a shorter dry season from (mid-May to mid-September). The basin receives an annual rainfall that varies between 1259 and 1750 mm [39].
On the basis of its elevation profile, we divided the LR into three sections: Upper, Middle, and Lower Lulua ( Figure 2). The Upper Lulua is about 260 km long, flowing from its source near the Angolan border to Kusununu, about 160 km upstream of the city of The LR is under a tropical savanna climate characterized by two main seasons: a longer wet season (from October to early May) and a shorter dry season from (mid-May to mid-September). The basin receives an annual rainfall that varies between 1259 and 1750 mm [39].
On the basis of its elevation profile, we divided the LR into three sections: Upper, Middle, and Lower Lulua ( Figure 2). The Upper Lulua is about 260 km long, flowing from its source near the Angolan border to Kusununu, about 160 km upstream of the city of Sandoa (Sanduwa), where a significant expansion, from about 200 m to over 1000 m, of the river's floodplain is noted. Following the classification of McManamay and Derolph [40], that section of the river has a low stream gradient with an average of 2.19 m/km (0.22%).
With a length of about 660 km, the Middle Lulua is the longest section of the river and stretches from that first expansion of the floodplain to the Mbumba rapids (just upstream of Mwana Nanga at about 336 km from the Lulua outflow into the Kasai) marking the beginning of a long series of rapids. The Middle Lulua is characterized by a very low stream gradient with an average of 0.215 m/km (0.022%). From the Mbumba rapids, the river flows again in a low-gradient zone, marking the beginning of the Lower Lulua, before flowing through a very low stream gradient as it joins the Kasai. That stretch of the river is about 336 km long and can be divided into two subsections. The first subsection is about 200 km long starting from the Mbumba rapids to the vicinity of Muebela and has an average stream gradient of 1.62 m/km (0.162%), with several rapids and localized braiding of the channel. The last 136 km of the LR, which constitutes the second subsection of the Lower Lulua and its estuary, is characterized by a flattening of stream gradient with an average of 0.188 m/km (0.0188%). In this stretch, the LR is joined by its largest and longest tributary, the Kaluebo or Luebo River. Sandoa (Sanduwa), where a significant expansion, from about 200 m to over 1000 m, of the river's floodplain is noted. Following the classification of McManamay and Derolph [40], that section of the river has a low stream gradient with an average of 2.19 m/km (0.22%). With a length of about 660 km, the Middle Lulua is the longest section of the river and stretches from that first expansion of the floodplain to the Mbumba rapids (just upstream of Mwana Nanga at about 336 km from the Lulua outflow into the Kasai) marking the beginning of a long series of rapids. The Middle Lulua is characterized by a very low stream gradient with an average of 0.215 m/km (0.022%). From the Mbumba rapids, the river flows again in a low-gradient zone, marking the beginning of the Lower Lulua, before flowing through a very low stream gradient as it joins the Kasai. That stretch of the river is about 336 km long and can be divided into two subsections. The first subsection is about 200 km long starting from the Mbumba rapids to the vicinity of Muebela and has an average stream gradient of 1.62 m/km (0.162%), with several rapids and localized braiding of the channel. The last 136 km of the LR, which constitutes the second subsection of the Lower Lulua and its estuary, is characterized by a flattening of stream gradient with an average of 0.188 m/km (0.0188%). In this stretch, the LR is joined by its largest and longest tributary, the Kaluebo or Luebo River.

Notes on Collection Sites
A total of 43 sites were sampled along the lower reaches of the LR including 13 main channel sites, 20 tributaries, and 10 sub-tributaries, as reported in Table A1 and Figure 3. Most main channel sites are in areas of river braiding and rapids ( Figure A1), except sites 1 (Luebo), 3 (Nsanga Nyembo), and 9 (Ntumba). Typically, the sampled sites are characterized by shallow habitats with a rocky substrate and fringing sandy beaches. The river flow in these sections is very rapid, generating numerous cascades, rapids, and waterfalls, and vegetal cover is predominately forest islands and woody riparian savannas. Water pH is slightly acidic with an average of 6.65 but can drop to 5.5 (at Dijiba, Site 6). However, a basic pH of 8 was recorded at Site 12 (Kabeya Nsaka) during the dry season. Collecting sites located on tributaries and sub-tributaries are more diverse, ranging from large channels of 35-63 m width (Kaluebo, Miao, and Moyo Rivers) to small creeks of less than 5 m width (Kasonga, Nkalala, and Tukomba Creeks). The pH in these tributaries is generally slightly acidic but varies from 5.5 (Luyenga River) to 7.1 (Kapelekese River). The substrate is also highly diverse, alternating among rocky, muddy, sandy, and leaf covered depending on location along channels ( Figure A2).

Notes on Collection Sites
A total of 43 sites were sampled along the lower reaches of the LR including 13 main channel sites, 20 tributaries, and 10 sub-tributaries, as reported in Table A1 and Figure 3. Most main channel sites are in areas of river braiding and rapids ( Figure A1), except sites 1 (Luebo), 3 (Nsanga Nyembo), and 9 (Ntumba). Typically, the sampled sites are characterized by shallow habitats with a rocky substrate and fringing sandy beaches. The river flow in these sections is very rapid, generating numerous cascades, rapids, and waterfalls, and vegetal cover is predominately forest islands and woody riparian savannas. Water pH is slightly acidic with an average of 6.65 but can drop to 5.5 (at Dijiba, Site 6). However, a basic pH of 8 was recorded at Site 12 (Kabeya Nsaka) during the dry season. Collecting sites located on tributaries and sub-tributaries are more diverse, ranging from large channels of 35-63 m width (Kaluebo, Miao, and Moyo Rivers) to small creeks of less than 5 m width (Kasonga, Nkalala, and Tukomba Creeks). The pH in these tributaries is generally slightly acidic but varies from 5.5 (Luyenga River) to 7.1 (Kapelekese River). The substrate is also highly diverse, alternating among rocky, muddy, sandy, and leaf covered depending on location along channels ( Figure A2).

Ichthyofauna Sampling
A total of six field expeditions, each a month long, were undertaken between 2007 and 2014: the first, during the rainy season in December 2007; the second, at the beginning of the dry season in June 2008; the third, during the rainy season in February 2010; the fourth, during the dry season in July 2010; the fifth, at the end of the dry and the beginning of rainy season in September 2011; the sixth, in September 2014. At each site, a stretch of 100 m was defined, and standard fishing techniques were employed [41]. Depending on habitat and conditions, these techniques included dip nets, cast nets, monofilament gill nets, seine nets, and some local fishing gears ( Figure A3). In isolated stretches of the main channel and some tributaries, with permission, the controlled use of the isoflavone ichthyocide rotenone, was employed to sample species not readily captured using other methods [42]. Fishes were euthanized, with MS222, prior to tissue sampling and body preservation in accordance with recommended guidelines for the use of fishes in research [43,44].

Ichthyofauna Sampling
A total of six field expeditions, each a month long, were undertaken between 2007 and 2014: the first, during the rainy season in December 2007; the second, at the beginning of the dry season in June 2008; the third, during the rainy season in February 2010; the fourth, during the dry season in July 2010; the fifth, at the end of the dry and the beginning of rainy season in September 2011; the sixth, in September 2014. At each site, a stretch of 100 m was defined, and standard fishing techniques were employed [41]. Depending on habitat and conditions, these techniques included dip nets, cast nets, monofilament gill nets, seine nets, and some local fishing gears ( Figure A3). In isolated stretches of the main channel and some tributaries, with permission, the controlled use of the isoflavone ichthyocide rotenone, was employed to sample species not readily captured using other methods [42]. Fishes were euthanized, with MS222, prior to tissue sampling and body preservation in accordance with recommended guidelines for the use of fishes in research [43,44].

Anthropogenic Activities
Anthropogenic activities were surveyed along the LR during our sampling trips. We recorded human activities, both in and outside collecting sites, that likely had direct or indirect impacts on land cover, water quality, fish populations, and hydrology of LR and its tributaries.

Fish Diversity
A total of 3825 individuals were collected between 2007 and 2014. Examination of these specimens resulted in the identification of 201 species, belonging to 24 families and 12 orders with their distribution among the main channel and tributaries indicated in columns 1-21 (Table A2). The distribution of species among all sites is provided in Supplementary Table S1. The order Siluriformes, with 49 species, is the most dominant and represents about 24% of all identified species (Figure 4). It is followed by Cypriniformes (20%), Characiformes (19%), Osteoglossiformes (18%), and Cichliformes (6%). These five orders together represent 87% (177 species) of the species diversity currently known from the LR basin.

Anthropogenic Activities
Anthropogenic activities were surveyed along the LR during our sampling trips. We recorded human activities, both in and outside collecting sites, that likely had direct or indirect impacts on land cover, water quality, fish populations, and hydrology of LR and its tributaries.

Fish Diversity
A total of 3825 individuals were collected between 2007 and 2014. Examination of these specimens resulted in the identification of 201 species, belonging to 24 families and 12 orders with their distribution among the main channel and tributaries indicated in columns 1-21 (Table A2). The distribution of species among all sites is provided in Supplementary Table S1. The order Siluriformes, with 49 species, is the most dominant and represents about 24% of all identified species (Figure 4). It is followed by Cypriniformes (20%), Characiformes (19%), Osteoglossiformes (18%), and Cichliformes (6%). These five orders together represent 87% (177 species) of the species diversity currently known from the LR basin. At the family level, the ichthyofauna of the LR is predominated by Cyprinidae, which is the most speciose family with 41 species (20%), followed by Mormyridae with 35 species (17%), Distichodontidae with 19 species (9%), Alestidae with 18 species (8.9%), Mochokidae with 17 species (8%), and Cichlidae with 12 species (6%). The remaining families are less abundant, each with less than 5% ( Figure 5). Overall, Cyprinidae, Mormyridae, Distichodontidae, Alestidae, Mochokidae, and Cichlidae account for about 70% of the ichthyofaunal diversity in the lower LR. At the family level, the ichthyofauna of the LR is predominated by Cyprinidae, which is the most speciose family with 41 species (20%), followed by Mormyridae with 35 species (17%), Distichodontidae with 19 species (9%), Alestidae with 18 species (8.9%), Mochokidae with 17 species (8%), and Cichlidae with 12 species (6%). The remaining families are less abundant, each with less than 5% ( Figure 5). Overall, Cyprinidae, Mormyridae, Distichodontidae, Alestidae, Mochokidae, and Cichlidae account for about 70% of the ichthyofaunal diversity in the lower LR.

Anthropogenic Activities
Overall, the economic situation of people in the LR basin is cause for considerable concern. Economic activity is almost entirely based on unsustainable exploitation of natural resources and minerals, and this, along with rapid demographic growth, is placing increasing pressure on ecosystems throughout the LR valley. During our sampling trips, we identified four main categories of human activity with direct or indirect impact on fish communities in LR: (1) activities affecting land cover, (2) water quality, (3) river hydrology, and (4) fish populations ( Table 1). The first three categories contribute to habitat loss and degradation, which are the greatest threats to fish communities in central Africa [10], whereas the last category affects fish population equilibrium and contributes to population fragmentation and decline. Table 1. Human activities identified in the LR valley grouped in categories following their impact on the system.  Figure A8) and is a primary source of income and food for numerous households in the area. This activity is the major cause of deforestation and land-cover alteration throughout the valley. Additionally, settlement into the region is very active and characterized by the rapid expansion of preexisting towns and villages, as well as the creation of new villages and camps. In the absence of electricity, most households rely on charcoal and firewood for cooking and lighting, further contributing to deforestation.

Category (1) Land Cover (2) Water Quality (3) Hydrology (4) Fish Populations
Artisanal alluvial diamond mining (Figures A7E,F) is widespread ( Figure A8) and remains the most lucrative activity for local populations throughout the valley. Mines are

Anthropogenic Activities
Overall, the economic situation of people in the LR basin is cause for considerable concern. Economic activity is almost entirely based on unsustainable exploitation of natural resources and minerals, and this, along with rapid demographic growth, is placing increasing pressure on ecosystems throughout the LR valley. During our sampling trips, we identified four main categories of human activity with direct or indirect impact on fish communities in LR: (1) activities affecting land cover, (2) water quality, (3) river hydrology, and (4) fish populations ( Table 1). The first three categories contribute to habitat loss and degradation, which are the greatest threats to fish communities in central Africa [10], whereas the last category affects fish population equilibrium and contributes to population fragmentation and decline. Table 1. Human activities identified in the LR valley grouped in categories following their impact on the system. Shifting agriculture ( Figure A7A,B) is consistently observed along the LR valley ( Figure A8) and is a primary source of income and food for numerous households in the area. This activity is the major cause of deforestation and land-cover alteration throughout the valley. Additionally, settlement into the region is very active and characterized by the rapid expansion of preexisting towns and villages, as well as the creation of new villages and camps. In the absence of electricity, most households rely on charcoal and firewood for cooking and lighting, further contributing to deforestation.

Category (1) Land Cover (2) Water Quality (3) Hydrology (4) Fish Populations
Artisanal alluvial diamond mining ( Figure A7E,F) is widespread ( Figure A8) and remains the most lucrative activity for local populations throughout the valley. Mines are mostly located along riverbanks, and sometimes whole tributaries are dammed or diverted to mine their beds. Mined materials are directly sifted or washed in the river, releasing large amounts of clay, silt, sand, and gravels. Clay mines ( Figure A7C) are generally smaller and provide clay for brick fabrication ( Figure A7D) destined for local construction.
Currently, the only major hydroelectric project in the LR valley is the Grand Katende hydroelectric dam, with a planned capacity of 64 MW, which is in an advanced planning stage.
Fishes play a central role in the regional food supply and are often the only source of animal protein. Fishing activities in the LR are extensive and practiced by men, women, and children primarily for subsistence. Many fishing techniques are used including netting, trapping, damming and dewatering ( Figure A7G), long lines, hook and line, and a locally produced vegetal ichthyocide ( Figure A7H). Fishing practices are generally unregulated and often destructive, utilizing extremely small mesh-sized nets targeting small fish, as the larger fish are becoming increasingly scarce.

Discussion
The dominance of Siluriformes, Cypriniformes, Characiformes, Osteoglossiformes, and Cichliformes in the LR reflects a similar composition as found in the entire CB [12,61]. At the family level, the CB ichthyofauna is dominated by Cyprinidae, Mormyridae, Cichlidae, Mochokidae, Alestidae, Distichodontidae, Amphiliidae, and Nothobranchiidae, accounting for 73% of basin-wide diversity [62]. A similar family composition is also found in the LR. We note that the apparent shift in taxonomic dominance in the lower LR is likely due to the fact that representatives of the Siluriformes are distributed among seven families, while Cypriniformes are represented by members of a single family. When compared to the family composition of the northeastern tributaries of the CR in the Cuvette Centrale ecoregion [26], we noticed the dominance of the same families in both systems. However, the most speciose family is no longer the Cyprinidae but the Mormyridae. Such a shift in dominance suggests that a difference of dominant habitats available in these river networks may be the cause. The rivers of the Cuvette Centrale ecoregion flow through dense tropical forests, with numerous blackwater swamps and small forested streams, likely offering more suitable habitats for mormyrid species than for most cyprinids. While the Mormyridae is also predominant in another river in the Kasai ecoregion, the Kwilu [27], as well as in proximate ecoregions [28,34,63], the dominance of Cyprinidae in the lower Lulua is noteworthy and likely reflects the influence of surrounding savannah and grasslands, and a riparian vegetation restricted in width along its course. Additionally, historical connections between the Upper Lulua with the headwaters of the Upper Zambezi ecoregion (see below), with its high number of cyprinid species, may contribute to the large number of cyprinids collected in the present study. These factors, combined with the presence of some characteristic and endemic species, suggest that the LR basin may represent a distinctive ecoregion within the CB; however, additional collecting within the basin is needed before definitive conclusions can be reached.
While the family composition of the LR is similar to that of the CB as a whole, species composition indicates some affinities with the Upper Zambezi ecoregion to the south of the basin ( also been reported from the Upper Zambezi system [25,50,62]. Such affinities with the Upper Zambezi can be explained by the existence of intermittent connections between the headwaters of the two systems during their geological evolution and currently in times of extreme humidity [25,[64][65][66][67]. From a taxonomic point of view, the lower LR is characterized by a remarkably rich and diversified ichthyofauna. In addition to the 201 fish species inventoried in this study, seven species reported by [14,47,[68][69][70], but not observed during our study, need to be added to the list. These species are Petrocephalus simus Sauvage 1879, Brachypetersius cadwaladeri (Fowler 1930 (Boulenger 1912). It is likely that these species are rare, are very localized, or occupy specialized habitats and were able to escape capture during our campaigns. Adding these seven species, the total number of species known from the LR basin reaches 208 species. This number will very likely be exceeded in the coming years as we intensively sample more habitats in unexplored areas of the basin, particularly the Middle and the Upper LR. Indeed, of the 1256 km of the main channel, only a 200 km stretch was intensively prospected within the framework of this study. Thus, the present results suggest that the Lulua catchment harbors an extraordinarily high number of fish species in comparison to other CR tributaries, such as the Inkisi River basin where 61 fish species have been reported [31], the Lefini River basin where 140 fish species have been inventoried [30], the N'sele River with 148 fish species [28], the Kwilu River with 150 species [27], the Luilaka and Salonga Rivers, both tributaries of the Ruki River, in the Salonga National Park where 152 species have been inventoried [29], and the Lindi River where 187 fish species have been reported [26]. Overall, only the Itimbiri (232 species) and Aruwimi (246 species) Rivers, in the Cuvette Centrale ecoregion, have been reported to harbor a higher number of species [26] than that reported in the present study. These differences could, of course, be due to many factors including the size of each river catchment, the diversity of habitats, and their respective productivity, to which we can add sampling techniques, effort, and gear used in different studies. Furthermore, according to Worm and Duffy [71], there is a reciprocal influence among the productivity of an ecosystem, its stability, and its biodiversity; thus, the most productive ecosystems would have high diversities.
It should be noted that about 14.4% (29 species) of the species reported in the lower LR were exclusively collected in tributaries and have yet to be found in the main channel (species in bold type in Table A2). Tributaries share an additional 83 species with the main channel bringing the total number of species collected in tributaries to 112 (55.7% of the lower LR fish diversity). This suggests that the LR tributaries harbor a significant proportion of species in the system. It is likely that the real proportion of species finding shelter in tributaries is considerably higher as our investigation was halted due to insecurity in the Kasai provinces, and we undoubtedly did not capture the entire species richness of certain tributaries and sub-tributaries since only a single campaign was undertaken in localities outside of the main channel. Furthermore, several tributaries of the lower LR were not surveyed at all. Nonetheless, the main channel harbors the highest number of species and provides shelter to about 85% (172 species) of the basin's ichthyofauna.
The species richness reported in this study represents a substantial improvement over the documentation of the LR ¶ichthyofauna historically estimated at 79 species [14,47,[68][69][70]. In columns 22 and 23 of Table A2, distribution data culled from published IUNC assessment of the status and distribution of central African fishes [72], the Kwilu River [27,73], the freshwater fishes of Angola [25,49], and FishBase [62] were used to investigate which of the species reported in this study have previously been recorded as present in the LR basin and/or the Kasai ecoregion as a whole. We found that 129 of the species reported here have not previously been recorded in the LR (Table A2, column 22). Representatives of families such as Malapteruridae, Tetraodontidae, Bagridae, Kneriidae, Clupeidae, Pantodontidae, and Notopteridae have not previously been reported from the LR, and 32 species reported here were previously unrecorded from the Kasai ecoregion (Table A2, column 23). Adding these species to the list of 367 species compiled from existing publications and databases [25,27,60,62,[72][73][74][75] brings the total number of species currently known from the Kasai ecoregion to 399. On the basis of this assessment, the Kasai ecoregion occupies the first position, in terms of species richness, in the entire CB (exclusive of Lake Tanganyika). It is followed by the Cuvette Centrale ecoregion with 360 species [26,29], the Lower Congo rapids and Lower Congo ecoregions with a combined 328 species [76], and Pool Malebo estimated at 316 species [3]. However, a total of 399 species for the Kasai ecoregion needs further investigation as some of the species in these lists, such as Labeo ansorgii, Labeo cylindricus, and Labeo annectens, may have far more restricted distributions than currently indicated, and it is highly likely that additional species will be documented in the Middle and Upper Lulua (personal observation). Additionally, it has become clear that molecular analyses are needed throughout the entire basin to ensure that species are not under-split due to rampant cryptic speciation [77][78][79] or over-split due to morphological plasticity (Liyandja, in prep.).
The highly diversified ichthyofauna of the lower LR and that of the entire basin is threatened by a combination of exploitation of forest products for building materials, deforestation for shifting agriculture and charcoal production, destructive fishing practices, and mining (particularly for diamonds), as presented in the results. According to Nguimalet [80], alluvial diamond mining is responsible for regressive and lateral erosion resulting in a complete restructuring of river geomorphology in the Central African Republic. Such changes lead to significant habitat loss and, consequently, to fish population fragmentation and/or decline. In addition to geomorphological modifications, water quality is impacted by clay, silt, and other materials released into the water, by sifting or washing operations during diamond mining, which further increase turbidity and conductivity. Soil erosion caused by deforestation and increase in runoff also contribute to the modification of water quality by carrying more materials (sediment runoff) into the water. This excess of particulate matter in suspension affects fish physiology in numerous ways, including suffocation by clogging their gills [81]. As human populations increase, each of these activities will continue to intensify, and threats are expected to worsen throughout the region. Additionally, intensification of mining and dam construction projected to support the planned Congolese Government developmental programs is underway. A prime example is the Grand Katende hydroelectric dam which is now in an advanced planning stage (https://www.afrik21.africa/en/drc-kinshasa-to-restart-work-on-the-katendehydroelectric-dam-64-mw; accessed on 20 June 2021). If completed, the Katende dam will inundate the lower LR where the present study has been carried out, and it will undoubtedly lead to major habitat degradation and likely the extirpation of many fish species [82], numerous of which are of central importance to the fisheries of the region. Subsistence fisheries in the LR valley are not monitored, and no data on the number of fish produced from these activities are available to allow an accurate assessment of their impact on the fish communities. However, according to observations made on the field and the types of fishing gear and techniques used by fishers, we can hypothesize that fish resources in the LR valley are under very high pressure and possibly already overexploited. Some of these techniques affect movements of migratory species (Labeo spp., Labeobarbus spp.) and have an impact on their reproduction cycle. Additionally, the repetitive use of locally produced organic ichthyocides, such as Tephrosia spp., have a drastic impact on fish communities. They are already known to be responsible for the decline in species richness and abundance in tropical streams [83].
Among identified species, only one, Teleogramma monogramma, has been evaluated as vulnerable by the IUCN Red list of threatened species [84] in Central Africa. According to Stiassny et al. [10], a congener, Teleogramma brichardi, is critically endangered because of urbanization around its restricted range near Kinshasa. The populations of T. monogramma reported here have only been collected near the City of Kananga and at the Katende site, suggesting that this species may be at higher risk than assessed by the IUCN. Twentyseven (13.4%) species reported in this study have not been evaluated, and 18 (9%) have been reported as data-deficient by the IUCN. Some of these species such as Hypsopanchax stiassnyae, Amphilius maesii, Nannocharax uniocellatus, and Clypeobarbus mediosquamatus appear to have highly localized distributions along the lower Lulua, suggesting that a reevaluation of their conservation status is urgently needed. The remaining species (77%) have been evaluated as of least concern in Central Africa. However, given the intensity of human activities in the LR valley, there is clearly a strong imperative to carry out local evaluation of the conservation status of the LR species.

Conclusions
The ichthyofauna of the lower Lulua River is continentally outstanding in terms of fish diversity, and the entire Lulua basin may constitute one of the most species-rich fish communities of the CB. Such an elevated level of fish diversity is likely a result of the basin's extensive habitat diversity ranging from numerous rapids, falls, pools, small and large tributaries, large floodplains, and perennial and permanent swamps, in addition to its location at the intersection of the Kasai and Lualaba ecoregions. Additionally, its shared headwaters with the Upper Zambezi ecoregion have likely also contributed to the high diversity of its ichthyofauna. The present study, which reports more than 200 fish species in the lower LR, represents a baseline documentation that can be used in conservation and future development projects in the Kasai ecoregion in general and in the LR in particular. In the long term, the outcomes of this study are intended as a starting point for monitoring and protecting the biodiversity of the entire basin. Further investigations are necessary to capture the true fish diversity of this exceptional river catchment, and such knowledge will also contribute to a better understanding of the specific composition and biogeographical history of the ichthyofauna of the Kasai basin and that of the CB as a whole.

Supplementary Materials:
The following are available online at https://www.mdpi.com/article/10 .3390/d13080341/s1: Table S1. Species distribution among sampling sites.  Our thanks also to Barbara Brown (in memoriam), Radford Arrindell, Tom Vigliotta, and Chloe Lewis (AMNH) for their help with accessioning and cataloging specimens. We are also grateful to BEBUC, through the Else Kroner-Fresenius-Stiftung foundation, for their financial and moral support to T.L.D.L.

Conflicts of Interest:
The authors declare no conflict of interest.