Physical characterization of the cage culture of Oreochromis niloticus L. 1758, in the Bukavu sub-basin on Lake Kivu, Democratic Republic of the Congo

doi:10.21203/rs.3.rs-2569011/v1

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Physical characterization of the cage culture of Oreochromis niloticus L. 1758, in the Bukavu sub-basin on Lake Kivu, Democratic Republic of the Congo

https://doi.org/10.21203/rs.3.rs-2569011/v1

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This work consisted in physically characterizing the floating cages used for farming Tilapia Nile in four bays of the sub-basin of Bukavu on Lake Kivu. The measurement of several parameters such as the depth of the water column and the distance of the cage taken by D-173 Bricodis decameter. The result shows that there are 218 cages of which 156 are exploited and 62 non-exploited. 76 cages are installed at 50 m and 58 cages are located at 6–49 m of the coast. The fact that the water of Lake Kivu is essentially pelagic pushes the fish farmers to place the cages on the littoral. Local cages are more abundant than those offered by FPI/IITA. 54% of the Oreochromis niloticus fry came from the D.R. Congo and 46% from Rwanda. Anthropogenic activities on a catchment scale cause benthic modification and reduce site characteristics. The diverse origin of fry should be discouraged to avoid new exotic species. Environmental impact studies of this culture on the biology and physicochemical parameters in the bays would help to address the environmental challenges of this culture in the Lake Kivu ecosystem.

Physical characterization

cage culture

Oreochromis niloticus

Bukavu sub-basin

Lake Kivu.

Cage culture in Lake Kivu was recently introduced by the Congolese government with the aim of boosting fish production in the region (Présidence de la République Démocratique du Congo 2019), although the activity started earlier on the Rwandan side (Kampayana et al. 2016). The country relies on fish imports from China which contributes up to 5,000 tons per year despite the great potential of water ecosystems in the country (Rothuis et al. 2014). Public and private partners have therefore found an opportunity to invest in the Nile Tilapia (Oreochromus niloticus) venture, one of the most farmed species worldwide with commercial production (Ronald et al. 2014, Prabu et al. 2019). The increasing demand for food to sustain the growing population and the declining fish stocks have motivated the introduction of alien species in aquatic ecosystems to support farming (Villanueva et al. 2009, Carpenter et al. 2011). However, the introduction of alien species may become a threat to the natural habitat when they invade and strive with indigenous species (Singh 2005).

The cages were installed on the shoreline of the city of Bukavu, one of most populated cities in the region subject to high anthropogenic pressure and pollution (Basima et al. 2006, Hyangya et al. 2021, Bisimwa et al. 2022). These activities discharge their wastes directly or indirectly into the littoral zone (Lina 2016) of the lake as the result in to reduce the bottom of littoral. These aspects may affect the physical characteristics of the lake including the depth of the water column and the nature of the bottom. Physically, CMFRI (2012) recommends that cage sites should be between 5 and 20 m deep to allow for oxygen exchange and the nature of the bottom should have a firm substrate with a combination of fine gravel, sand and clay. The FAO (2004) also recommends that cages meeting the standards should be located 20–50 m from the coastal zone. The platform infrastructures are variable usually made of immersed cages, floats, nets and ropes (Fitridge 2012). Net cages are designed to prevent fish escape and ensure good health and optimum water quality for fish (Moe et al. 2010). Criteria related to the type of cage and the choice of site was not made. A craze for cages of different shapes is present but not described. At present, there is no clear guideline for site selection for cage farming on Lake Kivu. Also, the origin of Nile tilapia (Oreochromis niloticus) fingerlings cultivated in the cage is not known. This study investigated to inventory the cages in exploited and non-exploited, the occupation of the platforms in relation to the water level and the column, the type of cages and the origin of the Nile Tilapia fingerlings. This helps to understand the environmental challenge of aquaculture in Lake Kivu.

Study area

This study was conducted on Lake Kivu in Bukavu sub-basin one of its four major basins during January to February 2022. Lake Kivu is one of the East African Great Lakes of the Albertine Rift, draining into Lake Tanganyika (774 m) via the Ruzizi River in the south (Beadle 1981). Lake Kivu is subdivided into four major basins (Degens et al. 1973; Tietze et al. 1980), namely the North Basin, East Basin of Idjwi Island, West Basin and the South Basin. In the latter, there are southern basin (area 108 km²), which includes the Bukavu and Ishungu sub-basins (Hyangya et al. 2021).

Bukavu sub-basin is located between 28° 2′ 24″ and 28° 3′ 0″ South latitude, 2° 1′ 44.4″ and 2° 1′ 58.8″ East longitude (Kaningini 1995), forms a natural boundary between the Democratic Republic of the Congo to the west and the Republic of Rwanda to the east (Descy et al. 2012). It is surrounded by the Mitumba Mountains to the southwest, the Rwandan ridge and the Ruzizi River to the east and the northern basin of Lake Kivu (Masilya et al. 2005). Bukavu sub-basin that borders the Lake is subdivided into five bays: Nguba, Ndendere, Nyofu, Nyalukemba, and "Société Nationale des Chemins de Fer du Congo (SNCC). The Fig. 1 shows the four bays of fish farming in sub-basin of Bukavu in Lake Kivu.

Data Collection for physical characteristics of floating cages sites

The depth of the site where cage instaled and distance of the cage to the coast were taken by using a D-173 Bricodis decameter and a 100 m graduated rope. Sediment at the bottom was taken by using a sediment trap. An average of 3 gatherings were taken at the floating cages were installed. The rope with a plunge of 3 kg was thrown to the bottom of the water column to measure the depth of the water column of the site with the floating cage. This measurement of this parameter was repeated 3 to 4 times depending on the number of cages installated at that site. The type of net and cage materials were obtained by observation of each cage and by consulting the literature on cage construction (CMFRI 2012, FAO 2004). An interview based on a survey questionnaire among the operators of the floating cages was carried out to find out the origin of the strain of Oreochromis niloticus fingerlings used in this culture. The data were collected for 2 months, from January to February 2022.

Data processing and statistical analysis

Data processing was done using R software (R Core Team 2019) and Excel. Student's t test and Chi² test were performed to find out if there is a significant difference between the physical parameters or between the numbers of exploited and non-exploited cages at the 5% significance level (Millot 2009).

Counts of exploited and non-exploited cages in the studied bays of the Bukavu sub-basin

A total of 218 floating cages were inventoried in the 4 bays, of which 156 cages are in operation (72%) and 62 cages are not in operation (28%). Ndendere Bay has the highest number of cages, i.e. 120 cages of which 91 cages are in operation and 29 are not in operation. Nyofu Bay has the next highest number of cages at 52, of which 35 are in operation and 17 are inactive. Nyalukemba Bay is poorly represented with 10 active cages. It can be seen that active cages are more abundant than inactive cages. The comparison of active cages to inactive cages did not show a significant difference (t = 1.23, df = 3.6, p = 0.29) between cages in terms of numbers at a risk greater than 5%. Figure 2 shows the numbers of active and inactive cages in the four bays of Bukavu town.

Occupation of exploited cages in relation to the coastal zone of the studied bays in the Bukavu sub-basin

In total 156 floating cages operated in the bays were recorded, Ndendere and Nyofu have 22 cages located at 5m (L5m) of the coast. The number of floating cages between 5and 20m wide (L6-49m) is higher in Ndendere with 35 cages following by Nguba with 18 cages. Cages located at the distance beyond 50m (L + 50m) are more numerous in all 4 bays. Statistical analysis shows that there is a relationship between the bays and the different cage occupancy thresholds (χ² = 25.78, df = 6, p = 0.0002) in relation to the odds in the Oreochromis niloticus rearing sites. The numbers of active floating cages in relation to the different sills they occupy in the bays according to the coastal zone are shown in Fig. 3.

Position of the floating cages operated in relationship to the depth of the water column in the studied bays of the Bukavu sub-basin

In the bays of the Bukavu sub-basin, the location of the cages in relation to the depth of the water column shows that 22 floating cages are in a depth of 5 m with a submerged net depth of 4 m (F4m). It is observed that Ndendere bay contains a large number of floating cages located at a depth of 5 to 10 m, i.e. 27 cages, while Nguba bay has the largest number of cages located beyond 10 m. Ndendere bay has 51 cages in operation, or 56% with a net depth of 4 m and 40 cages in operation, or 44% with a net depth of 6 m. The frequency of the number of cages operated in relation to the depth of the submerged net depends on the number of cages in each bay. There is no relationship between cages with 4 m and 6 m submerged nets depth as the difference is not significant (p > 5%) (Table 1).

Table 1

Location of floating cages operated according to the depth of the column in the bays of the Bukavu sub-basin. *F4m: 4 m submerged net depth, F6m: 6 m submerged net depth, P5m: cages at 5m depth, P6-10: cages between 6-10m depth, F + 10m: cages below 10m depth.*
Bays	Wet Filet	P5m	P6-10m	P + 10m	n	F(%)
Nguba	F4m	0	2	2	4	20
	F6m	0	0	16	16	80
Ndendere	F4m	20	27	4	51	56
	F6m	0	8	32	40	44
Nyalukemba	F4m	0	3	3	6	60
	F6m	0	0	4	4	40
Nyofu	F4m	2	18	3	23	66
	F6m	0	0	12	12	34
Total		22	58	76	156

Types of floating cages in operation installed in 4 bays of the Bukavu basin

Among the floating cages operated in the different bays, 92 cages type or 59% are manufactured local, and 64 cages type are assembled by FPI/IITA. The FPI/IITA cages type are grouped into platforms of 4 cages. Ndendere bay has the largest number of floating cages operated of the local and FPI/IITA. Nyalukemba bay has 4 FPI/IITA cages and one local cage. It is the least represented compared to the other bays. There is no relationship between the numbers of FPI/IITA cages and local cages operated (χ² = 8, df = 6, p = 0.23), as the difference is not significant. Figure 4 shows the numbers of FPI/IITA and locally operated cages in the bays of the Bukavu sub-basin.

Frequency of origin of fry used in fish cage culture in the studied bays of the Bukavu sub-basin

With regard to cage farmers, 84 fish farmers are pulling in fingerlings from Rwanda (54%) and 71 respondents are pulling in fingerlings from D.R. Congo (46%). The quantity of fingerlings from organizations of D.R. Congo is a bit low compared to those from Rwanda. Statistically, there is no significant dependence between fry from D.R. Congo (χ² = 12, df = 9, p = 0.21) and those from Rwanda. Table 2 shows the origin of the strains of fry used in the rearing of fish in floating cages in 4 bays of the Bukavu sub-basin.

Table 2

**Origin of fingerlings used in floating cage culture in the four bays studied (***n = number of survey respondents*)
Origin	Ndendere	Nguba	Nyalukemba	Nyofu	n	F(%)
PROLASA (RDC)	18	0	0	26	44	28
RWANDA	35	32	2	15	84	54
KAMANYOLA (RDC)	2	0	2	2	6	4
SENAQUA (RDC)	0	21	0	0	21	14
Total	55	53	4	43	155	100

In January 2022, we carried out a count of floating cages in the bays. The results showed 218 cages. This number seems to be less than satisfactory in relation to the demand for aquaculture products in Bukavu. An increase in the number of cages would still be favorable to meet the demand of the urban population, estimated at 1,625,785 inhabitants as of 2019 (Ulengabo 2020). It was observed that of all cages, 156 are being exploited (i.e. 72%) and 62 not being exploited (i.e. 28%), which shows that farmers are abandoning cage cultivation despite the support of IITA. The farmers, which would make another analysis in the following days on the causes of abandonment of cage culture in Lake Kivu are not listed many farmers mention the high cost of the investment compared to the cost of production, the water quality and feed nutrition. However, Hambrey (2008) remarks that in Africa, failures have been frequent, especially in the case of support by the government or aid projects. This is partly due to the haphazard nature of cage culture, which requires encouragement, motivation and understanding of local conditions to succeed.

Even though the physical conditions of the studied bays are favorable for cage culture (CMFRI 2012), 62 cages are already not in use. These are cages that float on the surface of water without fish, which means a loss of potential fish yield, are also a source of chemical pollution from the materials from which the cages are made. Eliminating unused cages would be important to reduce their effects on the aquatic environment of the bays of Lake Kivu.

Our results show that only Ndendere Bay has the highest number of cages compared to the other bays (Fig. 2). This bay is much appreciated by the operators in view of the number of anglers who frequent it compared to the other bays. The security of their cage would be guaranteed by the involvement of the fishermen in the culture. The physical parameters collected inform us that few of the cages are less than 5 m wide (Fig. 3). The cages that are wider than 5 m are more numerous. The study by Zaghoudi (2013) on the impact of floating cage fish farming on sensitive coastal sites in the Khniss lagoon and the Kuriates islands in Tunisia shows that the location of the cages should not coincide with traditional fishing areas. In the bays of Bukavu, the cages can be kept at a distance to the coast of 21–50 m. These areas are less threatened by the concentration of plastic waste near the shoreline and would avoid the destruction of macrophytes. The international standards of the FAO (2004), placing the cages at a distance of 20–50 m from the coastal zone, are to be strictly observed in the bays of Lake Kivu. This strengthens the cohesion with anglers who are concentrated in the 2–10 m wide zone. The bays are not the same in terms of morphology explain the significate difference (p < 5%).

Cage culture is practiced in areas with a benthic depth of 5–20 m (CMFRI 2012, https://www.fao.org/3/x5744e/x5744e0f.htm). These authors report that this depth maximizes water exchange avoids wave formation and reduces the risk of contamination in order to avoid oxygen depletion, accumulation of debris and the build-up of certain harmful gases generated by the decomposition of waste deposited at the shore. All the studied cages in the bays of the Bukavu sub-basin are at a depth favorable for cultivation, ranging from more than 5 m from the benthic zone. This is because Lake Kivu has a rather shoreline than the other Albertine Rift lakes. Whatever the local cages when the submerged depth of the net is 4 m, the difference of the cage to the bottom zone is acceptable. The nature of the benthic bottom observed in the studied bays consists of mud, sand and plastic waste. This does not meet the criteria of an ideal site for cage culture, including a firm substrate with a combination of fine gravel, sand and clay (CMFRI 2012).

The cages used in the Bukavu bays are made of High Density Polyethylene (HDPE) but the type differs between partners. The local cages have a surface area of 20 m² and are rectangular in shape, generally 3 m wide and 6 m long, or a volume about 80 m³. These cages are made up of 8 plastic drums with a capacity of 250 litres and nylon nets for a platform of two cages, one of which is a cage for fingerlings with a 4-mesh net and the other a pre-growth net (6 meshes). This net is also used for grow-out in local cages. The latter are mounted with soft metal pipes x40x40. Our observations show that in some cages mosquito nets are used and in others cloths for the fingerling cages. These practices can reduce the circulation of oxygen in the cage. The farmers report that these cages are capable of holding 10,000 fingerlings.

The FPI/IITA cages have a surface area of 36 m², i.e. a volume of 216 m³, and are made of HDPE plastic pipes with a diameter of 200 mm. In the four studied bays of the Bukavu sub-basin, 4 cages make up a platform of which 3 are used: the first cage for fry rearing (4 meshes net), the second for pre-fattening (6 meshes net) and the last for fattening (7 mesh net). All cages with fish are covered by a protective net to avoid predators. They can hold 30,000 fry per platform. Compared to Lake Victoria, a study confirms that the viability of cage culture for Nile Tilapia is a function of large volume (Musa et al. 2021). This suggests that production would be low in Lake Kivu.

It is easily understood that cage culture in the bays of Lake Kivu lacks institutional arrangements and coordination between public sector institutions to establish the necessary regulations that can accommodate cage aquaculture development. The 2004 FAO recommendations on cage culture in Africa are guiding principles that would assist institutions responsible for managing marine and inland water resources to regulate aquaculture.

In the studied Bukavu sub-basin bays, the species Oreochromis niloticus is used in culture. Interview surveys with farmers show that 54% of fingerlings come from Rwanda and 46% from D.R. Congo. Some mentioned that the Rwanda fingerlings are preferred because they are mono-sex types compared to those from D.R. Congo. Others report of delays in the supply of fry from the Congolese causing the net to be left in the waters. Despite this preference, dependence is not significant. Observers continue to wonder about the real strain used to better monitor this culture. However, a recent study in the Bukavu basin revealed that the imported strains of Oreochromis niloticus, currently being cultured in cages in Lake Kivu, have six times more monogenic (new to the lake) parasites than the native strains of the lake (Muhindo 2020). Leakage is likely to occur, which could be a vector for the introduction of species into Lake Kivu (Rana & Telfer 2008) and the risk of an invasion of these new parasites that would decimate Oreochromis niloticus species and other fish native to the lake, if these groups proliferate (Muhindo 2020). Control fisheries should be encouraged in habitat monitoring and in the use of a strain native to Lake Kivu. This strain would have little impact on the aquatic environment.

In the studied bays of the Bukavu sub-basin of Lake Kivu in eastern D.R. Congo, floating cage culture continues to be practiced clandestinely. The sites meet the required physical conditions for cage farming. Proper regulation of this culture is an approach that limits the disruption of the installation of cages in certain sites that do not meet the criteria. The protection of macrophytes and riparian vegetation around the bays is a crucial asset for the promotion of this culture. The bays of Bukavu are also influenced by erosion and landfill activities from the watershed. These actions contribute to the deterioration of the physical quality of the sites. Biological and physicochemical studies of the sites in the bays would complement this research focusing on the environmental impact of cage culture on the Lake Kivu ecosystem.

AKNOWLEDGEMENT

We thank Mr. Toussaint MUSHAMALIRWA for drawing up the map of this swamp and sampling stations. To the members of the Lab of Hydrobiology, Aquaculture and Resources Management at the Université Officielle de Bukavu for their support in the field.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interest

AUTOR CONTRIBUTIONS

Baguma G collected the data and wrote the article. Irenge B.A and Furaha N participated in the field activities. Cubaka K.A, Balagizi A.D and Mbalassa M corrected the draft many times. Cocquyt C supervised the team and reviewed the manuscript draft many times.

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No competing interests reported.

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Physical characterization of the cage culture of Oreochromis niloticus L. 1758, in the Bukavu sub-basin on Lake Kivu, Democratic Republic of the Congo

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Abstract

Figures

Introduction

Methods

Study area

Data Collection for physical characteristics of floating cages sites

Data processing and statistical analysis

Results

Counts of exploited and non-exploited cages in the studied bays of the Bukavu sub-basin

Types of floating cages in operation installed in 4 bays of the Bukavu basin

Discussion

Conclusion

Declarations

References

Additional Declarations

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