RECENT DEVELOPMENTS IN THE FISHERIES OF LAKE KARIBA (ZAMBIA/ZIMBABWE) (continue)

4. FISHERIES ECOLOGY (continue)

4.5 STOCK ASSESSMENT, POPULATION DYNAMICS AND YIELD PREDICTION (continue)

4.5.2 Population dynamics

Some vital stock parameters of 22 species of fish from Lake Kariba are presented in Table 8 (Kolding, Tirasin and Karenge, 1992). Mortalities are generally high for short-lived species, like Brycinus lateralis.

A stock assessment working group was set up in March 1992 (Anon., 1992) to improve data collection and develop appropriate methods of stock assessment. Members of the group are fisheries scientists from both Zambia and Zimbabwe. The group reviewed all existing data on Limnothrissa miodon in Lake Kariba and made recommendations on methodological improvements, future research and data sampling.

The major conclusions were:

1. Both experimental and commercial catches showed very little variation in length composition over the year. This applied to the whole time-series data available (1970 to 1991) (Figure 6). The distribution pattern of the fish is size dependent and smaller groups are found in shallow water. The size groups increase along a vertical gradient from the shore to deep water (Mtsambiwa, 1989). It was not possible to derive consistent growth estimates from length frequency data alone.

2. Estimates of growth were made from readings of daily increments of otoliths (Figure 7). The estimate of L∞ = 135 mm, K = 0.95 per year, were obtained from reading 55 otoliths.

3. Mortality estimates could not be made from the length-converted catch curve due to the problem of bias in the length frequency distribution of commercial catches.

4. Estimates of fishing mortality were obtained from yields and total stock estimates obtained from acoustic surveys. These estimates were based on the assumption that stock estimates represent the average stock for the year. Fishing mortality estimates ranged from 1 to 1.5 per year.

5. Total mortality estimates made using empirical models, ranged from 4 to 6 per year. Natural mortality estimates were not derived. The main sources of predation were assumed to be the tigerfish Hydrocynus vittatus and cannibalism.

4.5.3 Yield prediction

Several estimations have been made of the potential production of the Kariba Fishery. Marshall (1984) gives an account of the earlier estimations, notably by Hickling (1956), Maar (1959) and Balon (1974b). Yield estimates from those studies ranged from 9 000 to 30 000 t/yr. Marshall (1984) also cites predictions from the morpho-edaphic index and Gulland's yield equation. The morpho-edaphic index gave an overall prediction estimate of 23.3 kg/ha. This is for both the inshore and pelagic fisheries. This estimate is far lower than the current pelagic production of 60 kg/ha. Gulland's yield equation estimate of 30 to 57 kg/ha is closer to current catches.

Recently, Moyo (1990) used several models (Schaeffer, 1954; Schnute, 1977; and Walter, 1986) to predict inshore potential yield using the inshore fishery catch and effort data collected between 1970 and 1986. These models gave a potential yield ranging from 570 to 333 t/yr. Moyo's predictions are far lower than the current yields of 6 749 t for the whole lake (Walter, 1988; and Murphree et al., 1989).

The discrepancies between prediction and actual production indicate some of the problems inherent in the use of prediction models. First, the models assume that catches and effort are at equilibrium, while in most cases this condition does not prevail. Second, Moyo did not include data from the Zambian inshore fishery.

4.6 MULTI-SPECIES MODELLING

Multi-species interactions in the pelagic food chain of Lake Kariba were investigated using the ECOPATH II model (Anon., 1992).

The ECOPATH box model (Christensen and Pauly, 1991) assumes that each ecotrophic group is in a steady state where total gross production (P=B*Z) is balanced by mortality (Z) so that the average biomass (B) remains constant. This a rigid assumption, especially in a young ecosystem such as Lake Kariba, which is known for its successive changes in plant and animal populations. On a shorter time scale (e.g., a year), however, the assumption of steady state can be considered reasonably fulfilled and the model can serve as a useful tool for exploring and evaluating the consistency of collected data and population statistics from the various groups.

Only three trophic groups were selected in the pelagic food chain of Lake Kariba: tigerfish, kapenta and zooplankton.

For each group, values of

1. the diet (% weight or volumetric composition),

2. average biomass (t/km²),

3. catches (t/km²), and

4. production to biomass ratio and gross growth efficiencies (GE),

were determined (Machena, Kolding and Sanyanga, in press).

For Kapenta, the 1991 catch of 28 600 t and a biomass value of 55 kg/ha obtained from hydro-acoustic surveys in January 1992 were used. Biomass and P/B ratio of tigerfish were obtained from Langerman (1984), assuming these had not changed significantly.

The biomass of zooplankton was back-calculated from the mean daily production rate over a one-year sampling period (3 mg (d.w.)/m³/day) obtained from Masundire (1992) and integrated over the mean lake depth of 29 m (H.M. Masundire, pers. comm.) and using his estimates of mean replacement time of 10 days (equals Z_zoo = 36.5). This mortality rate of zooplankton corresponds closely with the average value of 10% per day given in Gliwicz (1986: Table 2) from the comparable Cabora Bassa reservoir, 300 km down the Zambezi River.

The diet composition of tigerfish was compiled from Kenmuir (1975a) and Mitchell (1976). The diet has shifted from predominantly cichlids to largely kapenta (Begg, 1974b; and Cochrane, 1984). This was based on data collected before 1975/76. In order to simulate the effect of the slight cannibalism which has been observed for larger fish in Lake Kariba (Begg, 1974) and is regularly recorded in Lake Kivu (Marshall, 1991), a 2% fraction of the kapenta was assumed to be a victim of cannibalism (Table 9). The gross growth coefficients (GE) were estimated from the literature, with approximately 20% for piscivores and approximately 15% for lower trophic levels. An overall 20% of consumption was assumed non-assimilated (Christensen and Pauly, 1991).

All input values are shown in Tables 10 and 11 (Anon., 1992). All units are averaged over the whole lake surface area and flow rates are per year. From these input parameters, the ECOPATH II programme computes an array of output parameters (Christensen and Pauly 1991), of which the most important for the purposes of this paper are the ecological efficiencies.

Table 9 Diet matrix in percentages of 3 trophic levels in the pelagic food chain of Lake Kariba

(Source: Anon., 1992)

Table 10 The ECOPATH II run of the Lake Kariba pelagic food chain based on a Kapenta mortality rate of 5

(Input values are underlined)

(Source: Anon., 1992)

Table 11 The ECOPATH II run of the Lake Kariba pelagic food chain based on a Kapenta mortality rate of 12

(Input values are underlined)

(Source: Anon., 1992)

The EE values in Tables 10 and 11 are the ecotrophic efficiencies and give the fraction of the total production which is either consumed by predators included in the system (M₂) or caught by fisherman (F) (EE = F + M₂)/Z). M₀ is all other mortality (M⁰ = Z - F - M²).

The model was run with different estimates of the kapenta turnover rate: P/B or Z = 5 and P/B = 12 (Anon., 1992).

Three interesting results arose from this analysis. First, the natural predation on kapenta by the tigerfish is apparently very low (M² due to tigerfish = 0.5) compared to the total mortality. Secondly, the relatively low assumed proportion of cannibalism (2%) incurred a much higher mortality than that due to tigerfish predation (M² due to cannibalism = 0.7 when Z = 5, and 1.6 when Z = 12). Thirdly, the EE value of zooplankton exceeds 1 (M² > Z) with a kapenta mortality of 12. This means that more zooplankton is consumed than produced, which indicates that a Z value of 12 for kapenta is probably an overestimation.

5. FISHERIES DEVELOPMENT

5.1 DEVELOPMENT ASPECTS

5.1.1 The Zambia/Zimbabwe SADC Fisheries Project

Lake Kariba is shared between Zambia and Zimbabwe and the collaborative arrangements that existed between the two countries between 1963 and 1965 have been covered by Marshall (1984). Due to political problems, the arrangements broke down in 1965.

Contacts between biologists from the two countries have now been re-established through a formal bilateral project which is funded by NORAD and DANIDA. This is the Zambia/Zimbabwe SADC Fisheries Project. This project, which started in 1988, aims to strengthen the research capabilities of the research institutions of the two countries on the lake. The project is also promoting joint research programmes. It is also hoped that a joint management committee with a uniform management policy for the shared Limnothrissa fishery will be instituted.

5.1.2 Management aspects

The management set-up of the inshore gill-net fishery was reviewed in Marshall (1984). Recently, Chifamba (1991) has tried to give a definition of fishing effort in the pelagic fishery. The fishing unit is currently defined as a fishing net that has a maximum circumference of 30 m. Each vessel carries one such net. Despite vessels carrying a similarly shaped net, large variations can occur in the fishing performance of the vessels in relation to the size of the net, size of the engine, presence of fish finding devices, use of power winches, and type and wattage as well as number of lights used during fishing.

Statistical analyses of performance looking at the various variables showed that the size of the vessel, the height of the net, the presence of an echo sounder, mobility of the vessel, and the number and power of underwater lights were important. It also became apparent that vessels from the same company had a similar performance, suggesting that company management considerations were also important.

Vessel performance has increased over the years and this is largely due to increased mechanization. Fishing effort from both Zambia and Zimbabwe has almost doubled since 1979, yet CPUE has tended to remain constant (Figure 8) and this is presumed to be due to increased catching power.

5.1.3 Commercial catch statistics

Marshall (1984) described the system of enumerating inshore fish catch data on the Zimbabwean side of Lake Kariba. This system is also described by Thorsteinsson, Sanyanga and Lupikisha (1991). Basically, data are collected by enumerators over a 7-day period in a number of villages per month. These data are then extrapolated over the whole lake and over the whole year.

The system on the Zambian side is different. Unlike Zimbabwe, where fishermen settle in fixed fishing camps, fishermen on the Zambian side settle and fish anywhere along the shore. For the purpose of sampling, the Zambian shoreline is divided into four zones or “strata”. This zonation is arbitrary. Each zone is further divided into sub-strata based on the size of the villages, as reflected by the number of boats per village. The villages with 1 to 5 boats belong to the first sub-stratum; those with 6 to 10 boats belong to the second; and those with 11 or more belong to the third (Thorsteinsson, Sanyanga and Lupikisha, 1991). Three villages are selected at random from each sub-stratum. There are 36 sampling villages (4 strata × 3 minor strata × 3 sampling villages) along the Zambian shore. Two teams of recorders conduct the enumeration. This enumeration is carried out three times per year. A new list of 36 villages is prepared for each survey.

(Source: Anon, 1992)

Figure 8 Historical trends in the kapenta fishery for the Zimbabwean side, 1974 to 1991

Table 12 Estimated annual fish production and CPUE of Lake Kariba fishery (1970–1991) (Zambia)

Note: The period of no fishing activities was due to the liberation war in Zimbabwe

(Source: Sanyanga, Lupikisha and Muchabaiwa, 1992)

Table 13 Catch and effort in the inshore fishery (Zimbabwe)

(Source: Sanyanga, Lupikisha and Muchabaiwa, 1992)

At each sampling village, catches are enumerated for 3 days, including fishing effort (number of fishermen and boats). CPUE from each substratum based on the number of boats is used for extrapolation.

The Zambian and Zimbabwean inshore catches are shown in Tables 12 and 13 respectively. These catches are at variance with those reported by Walter (1988) for Zambia and by Murphree et al., (1989) for Zimbabwe. These authors report a yield of 4 537 t on the Zambian shore and 2 212 t on the Zimbabwe shore, but they conducted their own enumeration. The possibility of official under-enumeration is currently under investigation.

Both the Zambian and Zimbabwean inshore fisheries were reported to be much more productive in the early stages of the fisheries.

Pelagic catches in both Zambia and Zimbabwe are shown in Tables 14 and 15. Effort (boat nights) for both countries are also shown in Tables 16 and 17. Pelagic fishing started much earlier in Zimbabwe. Pelagic fishermen in both countries are required to submit monthly returns showing daily catches of sardine.

Pressure on the fish resources is increasing. It is much easier to control effort in the pelagic fishery than in the inshore fisheries, as the nature of the inshore fisheries makes it easy for new unregistered entrants. Most of the fishing villages are in remote areas which are difficult to monitor. Capital input requirements are also low. The inshore fishery is an occupation of last resort, and in times of poor harvests, people easily move into fishing.

Table 14 Landings (tonnes) of the sardine Limnothrissa miodon, 1974–1991 (Zimbabwe)

(Source: Sanyanga, Lupikisha and Muchabaiwa, 1992)

Table 15 Landings (tonnes) of the sardine Limnothrissa miodon (Zambia)

(Source: Sanyanga, Lupikisha and Muchabaiwa, 1992)

Table 16 Total effort (boat-nights) in the sardine fishery (Zimbabwe)

(Source: Sanyanga, Lupikisha and Muchabaiwa, 1992)

Table 17 Total effort (boat-nights) in the sardine fishery (Zambia)

(Source: Sanyanga, Lupikisha and Muchabaiwa, 1992)

5.1.4 Integrated planning and management of Lake Kariba and its environs

One of the recommendations of the study Integrated Planning and Management of Lake Kariba and its environs (Hutton (Pvt), Ltd, 1991) addresses the establishment of a Regional Planning Council for the Zimbabwe side of Lake Kariba. The objectives are primarily to produce a master plan for the lake environs to facilitate integrated development and management of the area.

The study recognized the increasing conflict between a number of activities: fisheries development, wildlife utilization, tourist development, urban development, population expansion, etc. The development and promotion of these is uncoordinated because the four authorities with jurisdiction in the area - the Department of National Parks and Wild Life Management and the three local authorities - all have independent planning agendas. The intended re-organization and management of the inshore fishery can best be done in the context of an increased understanding of the complex environment within which the fishing communities operate.

The master plan will address strategic issues of the lake and its environs and will foster a coordinated implementation of local plans.

A similar exercise is being instituted on the Zambian side of the lake.

5.2 SOCIO-ECONOMIC ASPECTS

5.2.1 Distribution, numbers and ethnic identity of inshore fishermen

Walter (1988) and Murphree et al., (1989) give socio-economic accounts of both the Zambian and Zimbabwean inshore fisheries. There were 892 and 1 939 artisanal fishermen in Zimbabwe and Zambia respectively. The Zimbabwean fishermen were located in 40 fishing camps along the shore. Cooperative fishermen in Zimbabwe constituted 29% of the total. Between 1983 and 1988, the number of Zimbabwean fishermen increased at a rate of 5.4% per year. The trend was the same in Zambia, with an increase of 7.4% per year between 1986 and 1988. The increases related to demographic growth and the movement of people into fishing areas from the drought-stricken communal lands. In Zambia, there were no organized fishing camps. Fishermen were at liberty to camp and fish where they pleased: thus they were widely dispersed over the whole shoreline. This created problems of enumeration and regulation.

In Zimbabwe, 69% of the fishermen were Tonga, i.e., the ethnic group close to lake, whereas in Zambia the Tonga amounted to between 45 and 50%.

5.2.2 Fishing in the household economy

Murphree et al., (1989) reported that 91% of the Zimbabwean fishermen fished on a fulltime basis throughout the year. This aspect is not clear from Walter's (1988) report on the situation in Zambia.

In Zimbabwe, fishing provided the household's main source of income, other inputs being from subsistence agriculture carried out by wives, income from livestock, trading and a variety of enterprises. In Zambia, fishing was one source of income for 45% of the households and the main source for the other 55%.

5.2.3 Fishing gear

Walter (1988) recorded 6 360 nets of 100 m in the Zambian waters, representing an average of 3.28 nets per fisherman. In the Zimbabwean waters there were a total of 1 633 nets, or 1.83 nets per fisherman.

There were 342 boats belonging to fishermen in the Zimbabwe waters, or a ratio of 0.38 boats per fisherman overall. Of these, 42% were dugouts and the rest were of metal or fibreglass construction. Walter (1988) reported 1 427 fishing craft on the Zambian, side making a ratio of 0.85 craft per fisherman. Motorized craft were rare. According to Walter, there were some 25 to 30 engines in the Zambian waters. Murphree et al., (1989) reported only 5 in Zimbabwean waters.

5.2.4 Processing, marketing and distribution

Recent accounts of processing and distribution of inshore catches are given in Walter (1988) and Murphree et al., (1989) for both Zambia and Zimbabwe.

After landings, catches were gutted, fish for home consumption was removed and the balance sold. About 10% was retained for home consumption. Another 10% was considered to be lost through spoilage.

In both countries, fishermen relied heavily on traders who purchased fish for either drying or marketing fresh. In the event that traders were not available, fishermen processed the fish themselves. Conditions were markedly different on the Zimbabwe shore in areas close to Kariba town owing to the existence of a fresh fish marketing chain. A large company - Irvin & Johnson (Pvt) Ltd (I&J) - operated an ice box collection network. Some private traders also had ice boxes.

Fish that was not sold fresh was purchased by the traders themselves for smoking and drying at fish camps.

Of a total of 562 traders recorded in Zimbabwe, 59% were men and 41% women. Of these traders, 17% were from the district in which the camps were located and 83% from outside the districts. These traders come from different parts of the country. I&J marketed their fish in major urban areas.

5.2.5 Constraints

The artisanal fishermen from both shores operate in a remote and poorly accessible environment and face a number of constraints, with negative impacts on the procurement of supplies and nets, and the marketing of produce.

Both Murphree et al., (1989) and Walter (1988) considered that limitations of infrastructure, craft, gear and fishing techniques led to high spoilage and low productivity. Most artisanal fishermen lived below the subsistence level.

The Zambia/Zimbabwe SADC Fisheries Project aims to address some of these problems. Dependence on several incomes results from lack of economic security on the lakeshore. Only by restructuring the fishery in such a way that economic security is improved can the lakeshore communities have improved social welfare benefits. This is being looked at in the context of lakeshore integrated planning and development. Fisheries resource management plans need to be integrated with land use and lakeshore management plans, and political participation within a socio-cultural framework is crucial.

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