by
M.J. Pearce
Department of Fisheries
P.O. Box 350100
Chilanga, Zambia
Data collected from pelagic and demersal fisheries since 1960 suggest that the abundance of Lates species in both communities has an important affect on the abundance of prey species. In the pelagic community the initial decline of large Lates species was followed immediately by an increase in the clupeid population. This was later followed by an increase in the L. stappersi population and a drop in S. tanganicae abundance. These latter species undergo mutual inverse cyclical fluctuations with a period of three years. The life histories of L. stappersi and S. tanganicae ensure a minimum of contact between the two species. When adult, L. miodon is too large to be preyed upon by L. stappersi. S. tanganicae, since its population size became reduced, has increased its growth rate and maximum size (L∞). In the demersal fishery, an inverse exponential relationship exists between L. mariae abundance and the abundance of small prey species.
In Lake Tanganyika, the highly predatory genus Lates is represented by four species. Two species, L. microlepis and L. stappersi, are members of the pelagic community. L. microlepis is the top predator in this community and L. stappersi is the most abundant predator. The other two species of Lates belong to the demersal community where L. angustifrons is the top predator, and the largest species in the lake, and L. mariae is the most abundant predator. In addition to their role in the demersal fishery, adults of both L. mariae and L. angustifrons move into the pelagic zone for part of the year (Coulter, 1976), and play an important predatory role there.
In the pelagic fishery, the abundance of L. microlepis, L. mariae and L. angustifrons has declined wherever industrial fishing occurs, while L. stappersi has become more abundant. In Burundian waters, the abundance of L. stappersi fluctuates with that of its principal prey, Stolothrissa tanganicae. The fluctuations are reported to follow a cycle of approximately six years (Herman, 1977).
With the exception of the Zambian waters of the lake, the demersal fishery has received less attention than the pelagic. In Zambia, extensive shelf areas and a deep anoxic layer have permitted a full demersal community to develop (Coulter, 1963), and this is exploited both by subsistance and commercial fisherman. Three surveys of this community have been conducted at approximately ten year intervals (Coulter, 1966; Kendal, 1973; Pearce, 1985) and considerable information is now available.
In this paper I have selected material collected from the pelagic fisheries and from demersal gill net surveys conducted since 1960. This material has been used to demonstrate that the abundance of the genus Lates has a considerable effect upon the species composition and/or abundance of both fish communities.
Two fisheries have existed for more than twenty years. The industrial fishery, which uses purse seines or ring nets and large boats, remained at a low level for a long time, but since 1983 has expanded considerably. The artisanal fishery has employed a constant number of fishermen over the same period but fishing methods have become more efficient during the last eight years. In the industrial fishery, daily catch records from all companies have been maintained throughout its history though kapenta catches were not broken down into component species. Length frequency samples of selected species from the catch have been made irregularly (normally several times per month) during this period. Gonads and stomach contents of some species have also been examined from time to time.
In the artisanal study, departmental fishing units have used methods identical to those employed by artisanal fishermen. Catches have thus been considered typical of artisanal catches, and have been used for length/frequency and catch per effort measurements.
Gill net surveys were conducted in 1960–63, 1969–72, and 1980–83. In each survey, fleets of nets of different mesh sizes were set at a number of locations in each of two regions and the catches recorded. Mesh sizes varied from 37mm to 192mm, but in each survey and region the actual number of nets and mesh sizes used were different. The two regions that were studied were the shelf areas surrounding Mpulungu and Nsumbu. The former has a high fisherman density and can be considered to be heavily exploited, while Nsumbu has a small fisherman density and is only lightly fished.
Methodology and gear varied between surveys. When comparing catches between surveys, number rather than weight was used, and only data from sample sites and mesh sizes common to all three surveys analysed. However, at Nsumbu, in the 1960–63 survey, the transect sampled was different from the ones used in later surveys and this may have introduced errors into the comparison.
Coulter (1970) detailed the changes that occurred in the pelagic fishery up to 1967. Briefly, the predator stocks declined and those of the clupeid (kapenta) prey increased. A relationship was found between these changes.
Table 1 and Fig. 1 summarize the changes in the catch rates of the industrial fishery since 1963. Despite changes in species composition, the total catch rate has remained fairly steady at about one metric tonne per haul. Since 1972 the abundance of kapenta has been declining irregularly and the abundance of large Lates spp. has continued to decline. Between 1973 and 1977 the abundance of L. stappersi increased greatly, and has remained at a high level. At the present time, L. stappersi provides about 35% of the annual catch, but in some years it has reached 60% of the catch
Superimposed on the general pattern are large annual fluctuations in abundance of both kapenta and L. stappersi. Since 1969, catch rates of both species have undergone cyclic fluctuations with a period of three years. Peaks of abundance of prey and predator alternate with each other, and a strong inverse relationship is apparent.
Kapenta consists of two species, Stolothrissa tanganicae and Limnothrissa midon. Fig. 2 shows the relative contribution of the two species to the kapenta catch. In most years S. tanganicae makes the greater contribution, but its abundance fluctuates over much wider limits than that of L. miodon. Comparison of Fig.1. and Fig.2 shows that the greater part of the annual fluctuation in the kapenta catch is due to changes in abundance of S. tanganicae. The relationship between the catches of S. tanganicae and L. stappersi over the period 1980–1983 is shown in Fig.3. An exponential relationship between the catch rates of the two species is apparent.
By taking logarithms of the catch rates, a linear correlation was obtained, (Fig. 3b). The correlation coefficient is - 0.968, which was significant at P = 0.05. Regression of L. stappersi abundance on S. tanganicae abundance gives the formula :-
Log catch/haul L. stappersi = 1.05 Log catch/haul S. tanganicae -1.42
and the converse regression gives :-
Log catch/haul S. tanganicae = -0.89 Log catch/haul L. stappersi -1.32
An inverse relationship between the abundance of L. stappersi and S. tanganicae is also apparent on a seasonal level. Fig.4 shows that catches of L. stappersi are high during the rainy season (November-May) when kapenta catches are lowest. Conversely, high kapenta catches in the dry season (June-October) occur when L. stappersi is scarce.
Superimposed on the seasonal relationship between catches of L. stappersi and kapenta is a difference in the distribution pattern of the two taxa. Table 2 indicates average catch rates in two areas for L. stappersi and kapenta during the first and second halves of 1983. L. stappersi congregates along the west coast, whereas kapenta is more abundant along the east coast.
Using the microcomputer program ELEFAN I (David and Pauly, 1981), analyses of the growth parameters of L. miodon and S. tanganicae were made for the periods 1963–69 and 1980–83 (Pearce, 1985). Results for S. tanganicae are given in Table 3. There was a significant increase (P = 0.05) in the parameter L∞ between the two periods but the growth coefficient K showed no significant change. L. miodon showed no change in growth parameters, which were estimated as:-
L∞ = 16.4cm
K = 0.96 year
Results of the 1980-83 survey indicate that the Nsumbu shelf region had a much larger fish population than the Mpulungu shelf. Fig.5 shows that Nsumbu catch rates were nearly double those at Mpulungu (3.23 Kg/net and 1.71 Kg/net respectively). Catch rates of L. mariae were nearly four times as high at Nsumbu (1.57 kg/net) as at Mpulungu (0.39 kg/net). L. mariae comprises nearly half (48.5%) the catch at Nsumbu but only one quarter (22.8%) at Mpulungu. L. angustifrons gave only slightly higher catch rates at Nsumbu, and the populations of this species in the two regions are probably similar. A number of small species (here grouped together as “small prey species”) were twice as abundant in the Mpulungu region (Table 4). Hemibates stenosoma, a medium sized cichlid reported to be a major prey species of L. mariae (Coulter, 1976), gave similar catch rates in both regions. Chrysichthys spp. formed similar proportions of the catch in both regions, as did several small predatory species.
Fig.6 compares catch rates and catch composition of the two regions in each of the three surveys. This figure should not be compared with Fig.5 as different catch criteria are used.
Fig.6 shows that in the heavily exploited Mpulungu region, catch rates have dropped steadily from 12.6 fish/net in 1960–63 to 7.0 fish/net in 1980/83. Catch rates of L. mariae fell substantially from 2.0 fish/net to 0.4 fish/net while catches of small prey species almost doubled during the same period, increasing from 1.2 fish/net to 2.2 fish/net. H. stenosoma maintained a fairly constant percentage of the total catch, but its abundance in the 1980/83 survey was rather lower than in the previous two surveys.
At Nsumbu there was a slight drop in the catch rate from 1960/63 to 1969/72, but the catch rates during the second and third surveys were the same. The pattern of L. mariae catches is similar to that of the total catch, but the drop between 1960/63 and 1969/72 is more marked. Small prey species increased in abundance over the period from 0.2 fish/net to 1.3 fish/net. H. stenosoma increased markedly in abundance from a low level of 0.6 fish/net in 1960/63 to 3.0 fish/net in the two later surveys. This was a similar catch rate to that made for this species in the first two surveys in the Mpulungu region. Changes, if any, in abundance of D. cunningtoni could not be followed because of the extremely low numbers that were caught.
In each survey period, the Nsumbu region produced much higher catch rates than the Mpulungu region. Nsumbu still has larger stocks, particularly of L. mariae, than Mpulungu had 20 years ago. However, the catch rates of small prey species and of H. stenosoma in the 1980/83 survey at Nsumbu were very similar to those of Mpulungu in the 1960/63 survey.
Fig.7 shows that there is an inverse exponential
relationship between the catch rates of L. mariae and of small
prey species. Catches from both regions follow the same
relationship. The data from the last two surveys in the Nsumbu
region were pooled because of the similarity between them. A
linear regression of log transformed data produced the following
relationship:-
Log No. small prey species = -0.614 (Log No. of L. mariae) + 0.221
R = 0.91 This regression is significant at P = 0.05
Coulter (1976) discussed the likely end result of intensive fishing on the pelagic community. A change in species composition following exploitation, without any large change in overall yield, has been shown to have occurred in the Zambian waters of Lake Tanganyika. The pelagic community is now composed of only three important species, L. stappersi, S. tanganicae and L. miodon. The population sizes of L. stappersi and S. tanganicae, which are inversely related, fluctuate violently with an oscillation period of 3 years whereas L. miodon seems to have a fairly stable population level.
The difference in the period of oscillation in the size of the stocks of L. stappersi and S. tanganicae between Burundian waters (Herman, 1977) and Zambian waters is notable. Since the annual cycle of water turnover is initiated in the south by upwellings (Coulter, 1963), it is possible that nutrient release is greater in Zambia than further north, and that this causes more ‘explosive’ population increases. Coulter (1976) postulated that these oscillations are primarily the result of environmentally caused survival success of S. tanganicae and juvenile L. stappersi which, because of the different growth rates of the two species, is manifested in different catch rates of the two species in different years. If this were the case, then the period and amplitude of the oscillations in abundance would be irregular, unless the environment itself was subject to regular cyclic changes and there is no evidence that this is so. The regular, well marked nature of the fluctuations and the high inverse correlation between the catch rates of the two species suggests that predator-prey relationships are the primary cause of these changes.
The application of current mathematical predator-prey models (Larkin, 1966; Pope, 1979) may not be suitable for this system, as such models are applicable to situations where predators and prey are captured together. In the case of L stappersi and S. tanganicae, the two species are generally not caught together, and boats make a choice before leaving harbour whether to fish for L. stappersi or kapenta, depending on season and current catch rates. The amplitude of the fluctuations in size of the catches may therefore be exaggerated by catch selection. Despite this possibility, there can be no doubt that fluctuations in population size do exist and that the recent increase in the population size of L. stappersi has had an appreciable effect on the S. tanganicae population. A recent analysis of population parameters of the three major pelagic species (Pearce, 1985) has indicated that catches of S. tanganicae of around 2000 metric tonnes per year represents the maximum yield of this species, and that the stocks of S. tanganicae in Zambian waters are not as large as had previously been assumed. This being the case, it is not unreasonable to expect that an increase in the abundance of the major predator would have an important effect.
If there is an inverse relationship between the abundance of these two species, it probably operates through cyclical changes in their natural mortality rates. When L. stappersi is abundant the natural mortality of S. tanganicae increases as a direct result of predation, and similarly, when the population size of S. tanganicae is low, the natural mortality rate of L. stappersi increases because of the scarcity of the food resource.
Adult populations of L. stappersi and S. tanganicae, though closely linked in a predator-prey relationship, maintain separate centres of population density. The populations of the two species do overlap and, from time to time, high densities of one species may be found in the “territory” of the other though generally the two species intermingle only at the margins of their populations. There is a seasonal and geographic exclusion effect and in most years this follows a similar pattern. At the beginning of the rainy season (November), adult L. stappersi move into waters adjacent to the west coast of Hore Bay, where they form spawning (or gonad maturation) concentrations. At the same time, adult S. tanganicae move north into open waters where they presumably spawn. At the end of the rainy season L. stappersi move back into pelagic waters, while juvenile S. tanganicae move into the inshore waters near the south coast. Such individuals of S. tanganicae that did not move north in the rainy season tend to concentrate along the east and south coasts. These patterns of movement would appear to have evolved as a mechanism to improve the spawning success in both species. Juvenile L. stappersi are reported to occur together with kapenta in the north of the lake (Ellis, 1978; Chapman and van Well, 1978) but this does not normally occur in Zambian waters. In two recent years of exceptional L. stappersi abundance (1977 and 1984), juveniles of this species have been caught together with kapenta. Whether this is due to ‘spillover’ from a particularly successful spawning or to environmentally caused changes in distribution is unclear.
Compensation mechanisms and ‘rejuvenescence’ of reduced populations (Coulter, 1976) has recently been shown to exist for S. tanganicae, L. mariae and L. microlepis (Pearce, 1985). In each of these species, there appears to have been an increase in L∞. Maximum sizes of adults of all species in the catch are now regularly larger than previously calculated or assumed L∞ values, (Coulter, 1976; Roest, 1977; Chapman and van Well, 1978).
L. miodon has been shown to maintain a relatively constant level of abundance throughout the twenty years of industrial exploitation in Zambian waters. Neither the initial decline of the larger Lates spp. nor the subsequent increase in L. stappersi appear to have affected its population size. The explanation for this probably lies in the basic difference between L. miodon and other species in choice of habitat. In the south of the lake, L. miodon spends the whole of its life within the coastal waters which overlie the shelf areas. Only when mature adults of L. miodon in their second year congregate off the west coast during the rainy season does this species come into contact with L. stappersi and, by this time, the former are too large to be preyed upon by the latter. It therefore seems probable that the size of the L. miodon population is affected more by fishing pressure and the predators of the shelf community than by pelagic predators. The apparent decline in L. miodon abundance during the last few years (see Fig.2) is probably the result of industrial fishing effort directed at the west coast during the rainy season. Adults in their second year, which were previously not exploited, are now caught in considerable quantities.
The life histories of L. miodon and S. tanganicae have been shown to differ considerably, as have their reactions to the increase in abundance of L. stappersi. Population parameters and potential yields in Zambian waters are also very different for the two species (Pearce, 1985). It seems unlikely that production studies which treat these two species as a single entity (Coulter, 1977, 1981) can be relevant in Zambian waters, where L. miodon makes an equal contribution to the kapenta catch, and has greater potential than S. stappersi for further exploitation.
In the demersal community, the abundance of L. mariae has been shown to have a considerable effect upon the abundance of some species and an inverse relationship has been demonstrated between the catch rates of L. mariae and of small prey-species. Where L. mariae abundance has been reduced by fishing pressure, populations of small prey-species have increased.
There has been no convincing evidence that the population size of H. stenosoma is related to that of L. mariae although the former species constituted a major part of the diet of L. mariae during the 1960/63 survey period (Coulter, 1976). At Nsumbu, 1960/63 catch rates of H. stenosoma were very low when compared with its catch rates at Mpulungu. Possibly the high population level of L. mariae at Nsumbu in 1960/63 resulted in a low population of small prey species so that L. mariae also fed intensively on H. stenosoma. At lower abundance levels of L. mariae, small prey species were sufficiently common for H. stenosoma to be ignored as a food source and its population size became controlled by other factors. The lower abundance of H. stenosoma at Mpulungu in 1980/83 is possibly due to fishing pressure directed at this species in the absence of profitable catches of L. mariae.
In the period 1960/63 the Nsumbu region is considered to have been a virgin fishery, and the abundance and species composition of the demersal community in this survey is probably representative of the natural population. Although this region still gives high catch rates, the reduction in L. mariae catches and increase in H. stenosoma and small prey populations indicates that even light exploitation has affected the demersal community. Comparison of catch rates of the 1969/72 and 1980/83 surveys with those of 1960/63 necessitated using data from transects situated within the National Park. Areas outside the National Park have lower total catch rates, much lower catch rates of L. mariae, and higher catches of small prey-species than is indicated in Fig.6. The role of the National Park in creating inaccessible waters that can help to stabilize the fish population and buffer the effects of fishing cannot, as yet, be reliable assessed.
In the 1960/63 survey, the Mpulungu region was considered to be lightly exploited but since then fishing has increased and catch rates, particularly of L. mariae, have declined considerably. Today the Mpulungu region is heavily fished and indeed over fished. Differences between catch rates at Mpulungu and Nsumbu have given rise to the speculation that the Nsumbu region is more productive than Mpulungu. Since data from both regions have been included in a single, significant regression (Fig.7) it would seem that there is no real difference between the two regions and that Mpulungu region was already undergoing considerable exploitation in the period 1960/63.
The reaction of the demersal fishery to exploitation seems to be of a different nature from that of the pelagic fishery, in that there has been no emergence of an alternative predator to the large Lates spp. and total yield has declined considerably. However, in a gill net fishery, catches do not necessarily represent true abundance within the community. Abundance and species composition of the catch is not only a function of abundance and species composition of the community but also of the catchability of the individual species.
L. mariae is a species with particularly high catchability, both because of morphological (Coulter, 1970) and behavioural characteristics. Its dominance in gill net catches exaggerates its true abundance in the demersal community, and its high susceptibility to capture is the reason for its rapid decline relative to that of other components of the total catch. As the L. mariae population declined, the overall efficiency of nets necessarily also declined thus it is possible that the current true abundance of the demersal community at Mpulungu is not very different from that of Nsumbu in 1960/63.
Differential susceptibility to gill-net capture may also explain the apparent lack of a dominant predator in the Mpulungu region. Three possible contenders, D. cunningtoni, Malapterurus electricus and Boulengerochromis microlepis, all have very low catchabilities and are almost certainly more common in the demersal community than their presence in gill net catches indicates. Redirected artisanal and subsistence fishing effort towards the capture of smaller species is likely to be sufficient at present to prevent any of the smaller predatory species (e.g. Bathybates spp.) from increasing in abundance. However, the possibility of such an increase is not ruled out in the future as gill net fishing for very small fish becomes increasingly unattractive to fisherman.
The similarity in the response of prey species to predator abundance in both pelagic and demersal communities is notable and suggests that the abundance of Lates spp. plays a more important role in determining the abundance of their prey than do other factors. The inverse situation in which abundance of the predator is determined by the abundance of its prey is seen only in the pelagic fishery and applies only to S. tanganicae and L. stappersi. This is likely to be a result of the decline of the population of larger Lates spp., so that a simplified food web has resulted. The loss of the species interactions that tend to maintain a stable species composition has resulted in the present situation in which population fluctuations prevail. In the demersal community, a speciose fauna and a fishery which is directed at the predator rather than the prey has prevented a similar situation from occurring.
I wish to acknowledge my indebtedness to the Department of Fisheries, Government of Zambia, under whose auspices this paper was produced. I am particularly grateful to the staff at Mpulungu Field Station who did much of the physical work in collecting data. I also wish to acknowledge the work of G.W. Coulter, and R. Kendall whose data I have used extensively in this paper.
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Table 1. Catch per haul of major pelagic species in the industrial catch.
| Species | Metric tonnes | |||
| 1963–67 | 1968–72 | 1973–77 | 1978–83 | |
| Kapenta | 0.70 | 0.80 | 0.65 | 0.54 |
| L. stappersi | 0.11 | 0.04 | 0.24 | 0.34 |
| L. microlepis | 0.18 | 0.09 | 0.08 | 0.04 |
| L. mariae L. angustifrons | 0.11 | 0.05 | 0.03 | 0.03 |
| Total | 1.10 | 0.98 | 1.00 | 0.95 |
Table 2. Catch rates of Kapenta and L. stappersi from two areas in the Mpulungu region during 1983.
| Species | Sampling period | Metric tonnes/night | |
| West coast | East coast | ||
| L. stappersi | Jan. – June | 3.11 | 1.99 |
| July – Dec. | 1.16 | 0.23 | |
| Kapenta | Jan. – June | 0.04 | 0.76 |
| July – Dec. | 0.77 | 1.32 | |
Table 3. Growth parameters of S. tanganicae calculated by ELEFAN (Pauly and David, 1981)
| Parameter | 1963 – 69 | 1979 – 83 |
| K | 1.59 | 1.56 |
| L∞ | 11.0cm | 12.2cm |
Table 4. Species included under the heading “small prey species”. (N.B. Many small species, eg Simochromis spp., occur only in the littoral zone and were thus not recorded in the surveys.
| Trematocara spp.* | Limnochromis spp.* |
| Limnotilapia spp. | Xenotilapia spp. |
| Grammatotria lemairei | Haplotaxodon spp.* |
| Chrysichthys sianenna* | Leptochromis spp. |
| Bathybagrus tatranema |
* Important genera and species
 | Figure 1. Catch per haul of Kapenta and L. stappersi by Sopelac |
Figure 2. Catch per haul of L. miodon and S. tanganicae by Sopelac |  |
 |  |
Figure 3. Relationship between abundance of S. tanganicae and L. stappersi |
Figure 4. Seasonal variation in catch rates of Kapenta and L. stappersi by Sopelac (1963–1978) |  |
Figure 5. Catch rates of selected species and groups during the 1980/83 survey
Figure 6. Catch rates in Nsumbu and Mpulungu regions in 3 surveys
Figure 7. Relationship between catch numbers of L. mariae and small prey species
