Productive inland fisheries based on small fish are found in various parts of theworld. A well-known example is the fishery for the small cyprinid Mirogrex terrasanctaeon Lake Kinneret which dates from biblical times (Petr and Kapetsky, 1983). Another interestingexample is from the Philippines where highly productive fisheries were based on thegoby Mistichthys luzonensis which, at 12.5 mm mean length, is the smallest commerciallyexploited fish (Gindelberger, 1981).
Small fish have not been intensively exploited in African waters although there aresome traditional fisheries which utilize them. The long-established one based on the LakeTanganyika clupeids is well-known and Poll's (1952, p. 154) description of it as “de quelquesorte le sport national des indigènes du Tanganyika” suggests that is played an importantsocial as well as economic role. Clupeids were also taken in the West African rivers, asin the “Atalla” fishery of the lower Niger (Awachie and Walson, 1978). In Lake Chilwa, thesmall cyprinid Barbus paudinosus was a major component of the fishery (Eccles, 1970) andBarbus spp. are regularly exploited in African rivers.
There has been a recent upsurge of interest in the small pelagic fish of African lakesand their commercial value is becoming more apparent. There appears to be two main reasonsfor this. The first is that the rapid growth of Africa's human population has led toincreased exploitation of many inshore fish stocks. In Lake Victoria, for example, thetilapia stocks were thought to have been over-exploited as early as 1960 (Fryer and Iles,1972) and increased fishing pressure since then has left the pelagic stocks, mainly thecyprinid Rastrineobola (Engraulicypris) argentea, as the only possible avenue forfisheries expansion apart from Haplochromis.
The second factor is the considerable increase in surface water resources broughtabout by the construction of several large reservoirs during the last three decades. Insome of these, productivity was depressed because no indigenous fish could colonize theirpelagic waters. Pelagic fish of the great lakes seemed to be good candidates for introductioninto these lakes and the success of the Tanganyika sardine Limnothrissa miodon inLake Kariba (Marshall, 1984) showed the possibilities of translocating and commerciallyfishing such species. The establishment of such fisheries might compensate local populations,to some extent, for the loss of traditional homes and other social problems causedby large reservoirs (Balon, 1978).
Pelagic communities in African inland waters consist, as a rule, of small zooplanktivorousspecies and a group of predatory fish which feed upon them. This paper isconcerned mainly with the small fish which are the basis of the system. In addition tobeing small these fish usually have a short life-cycle and high productive potential.There are four major families which include species with actual or likely value as pelagicfish, and these are discussed in more detail below.
2.1 Family Characidae
Small characids, mostly Alestes spp., are widely distributed in African waters buthave generally not become pelagic. In Lakes Malawi and Tanganyika, for example, they tendto be restricted to inshore areas or affluent rivers (Jackson et al., 1963; Poll, 1953).Nevertheless they might have been expected to become pelagic in reservoirs, and Balon(1974) suggested that Alestes lateralis would have done so in Lake Kariba had it not beenfor the introduction of Limnothrissa. This view was disputed on the grounds that theoriginal samples were taken in shallow water, that there were no data to show thatA. lateralis was planktivorous and that ample time had elapsed for it to have become pelagicin the 10 years between Kariba's creation and the sardine's introduction (Marshall, 1984).
Diet is probably the main reason why Alestes have not become pelagic. Most speciesexamined ate a wide variety of food but were mainly insectivorous, with flying terrestrialinsects being a significant component (Poll, 1953; Reynolds, 1973; Marshall and vand derHeiden, 1977; Paugy, 1978, 1980/1980a). Although plankton was occasionally taken, onlyA. baremoze was able to utilize it extensively (Paugy, 1978). A consequence of this insectivoryis to confine these fishes to inshore areas where insects are likely to be mostabundant.
The breeding habits of Alestes are poorly known but may be another reason why theyhave not become pelagic. Some, like A. imberi, seem to be potamodromous and requirerunning water in which to breed (Bowmaker, 1973). Others, like A. lateralis, remain inweed beds throughout the year and may require vegetation in which to spawn (Balon, 1971;Bowmaker, 1973). Either of these reproductive strategies would inhibit the development ofpelagic forms.
2.2 Family Cyprinidae
The cyprinids are widely distributed in Africa and occur in almost all water-bodies.Several pelagic species of Engraulicypris and Rastrineobola have developed in Lakes Malawi,Tanganyika, Victoria, Mobutu Sese Setu and Turkana (Turner, 1982a). The only two to havebeen commercially exploited are R. sardella of Lake Malawi and R. argenteus of Lake Victoria(Petr and Kapetsky, 1983).
The numerous small cyprinids of African rivers generally have not become pelagic inreservoirs but one species which has flourished in small dams in Zimbabwe is Neobola(formerly Engraulicypris) brevianalis. It has been translocated to provide food for blackbass and trout in some reservoirs but has not been commercially exploited (unpublisheddata).
The numerous small Barbus spp. have not produced pelagic forms in any large Africanreservoirs, although Reynolds (1973a) studied them in Lake Volta in the hope that theymight have some fisheries potential. In most lakes they are confined to inshore areas oraffluent rivers, possibly because most seem to be strongly rheophilic and require riverineconditions. An exception is Lake Le Roux in South Africa where B. anoplus occurs in allareas (Cambray, 1983). This reservoir is narrow and fjord-like with a mean breadth of only1.4 km (Allanson and Hahndiek, 1979); it is also very turbid and has extreme and irregularwater-level changes. It thus resembles a very large river and so is suitable for Barbus.
Most cyprinids seem to be strongly rheophilic which affects their distribution inlarge lakes and reservoirs (Poll, 1953; Jackson et al., 1963; Begg, 1974). Even those whichcan penetrate deep water, such as the mpasa Opsaridion microlepis of Lake Malawi stillhave to move up rivers to breed (Tweddle, 1983). The small Barbus spp. have catholicfeeding habits with zooplankton as a relatively minor component of their diet (Cambray,1983). These facts suggest that the evolution of pelagic cyprinids might be a relativelyrecent phenomenon and could explain why some cyprinids seem to be less efficient pelagicspecies than the clupeids (Turner, 1982a).
2.3 Family Cichlidae
The cichlids are widespread and very successful in African waters. The adaptiveradiation of Haplochromis in the great lakes is a distinctive feature of the Africanfreshwater fish fauna (Fryer and Iles, 1972) and they have been able to exploit almostevery food source, including zooplankton. Pelagic forms occur in several lakes and thebest-known are the “Utaka” of Lake Malawi, primarily Haplochromis quadrimaculatus butwith other Haplochromis spp. as well (Jackson et al., 1963). The “Utaka” are abundantin inshore areas but this decline rapidly with distance from shore and these fish do notfully occupy pelagic waters (Fig. 1).
The reason why cichlids have not become truly pelagic is probably their specializedbreeding habits. Haplochromis males, like most other cichlids, establish territoriesmarked by a “nest” to which females are attracted. After a brief courtship the femalestake the fertilized eggs into their mouths and move away (Fryer and Iles, 1972). Thisbehaviour means that cichlids will be restricted to inshore areas, especially as theirbreeding seasons may extend over several months. Cichlids can, of course, establishthemselves in the pelagic waters of shallow lakes, as in Lake George (Gwahaba, 1975). Therecent discovery of mid-water spawning by Haplochromis chrysonotus in Lake Malawi (Ecclesand Lewis, 1981) suggests that true pelagic behaviour may be developing in some cichlids.
2.4 Family Clupeidae
This is by far the most significant pelagic family and the one which has beenstudied in most detail. It is primarily a planktivorous marine family which has colonizedthe rivers of West Africa and the Congo system (Beadle, 1981) right up to Lakes Tanganyikaand Mweru where specialized pelagic forms have evolved. The most important group are the22 species of the sub-family Pellonulinae occuring throughout West and Central Africa(Table 1).
Two specialized lacustrine species, Limnothrissa miodon and Stolothrissa tanganicae,have developed in Lake Tanganyika and form the basis of the pelagic fishery there (Petrand Kapetsky, 1983). Three other lacustrine forms occur in Lake Mweru but virtually nothingis known about them although Jackson (1961) suggested they were neither large enough norabundant enough to be of commercial value. Further investigation of these species isclearly required.
The remaining pellonulines are riverine but appear to have the potential for becomingpelagic. This is well illustrated in West Africa where the riverine clupeids developedpelagic stocks in Lakes Kossou (Pellonula afzeliusi), Volta (P. afzeliusi, Sierathrissaleonensis) and Lake Kainji (P. afzeliusi, S. leonensis and Cynothrissa mento) (Vanderpuye,1973; Otobo, 1979). This suggests that some of the other species listed in Table 1 mightdo the same and this should be borne in mind if reservoirs are proposed in rivers wherethey occur. Some might also be suitable for translocation and further research on thesespecies would be valuable.
The success of the clupeids as pelagic fishes is probably due to several factors.They seem to be primarily planktivorous, but able to utilize other food if neccessary(Matthes, 1968; Otobo, 1979; Begg, 1974a; Cochrane, 1978; Gliwicz, 1984; Reynolds, 1970). It isinteresting to note the Stolothrissa, which Matthes (1968) considered to be the mostspecialized planktivore, did not establish itself in either Lakes Kivu or Kariba whilst themore generalized Limnothrissa did (both species must have been present when the fry wereintroduced).
Another interesting aspect of clupeid biology is that they seem to be able to adjusttheir growth rates to the food supply. This is well illustrated by Limnothrissa which inKariba only grow to half the size that they do in Kivu or Tanganyika (Fig. 2). Marshall(1984) postulated that food supplies in Kariba were poorer because the reservoir was lessfertile than the other two (Kariba conductivity = 80 μS cm-1; Tanganyika = 500; Kivu =1 200). Linked to this is a reduction in breeding size; in Tanganyika Limnothrissa matureat 75 mm (Ellis, 1971) but in Kariba they rarely reach this size and probably mature at40 mm (Begg, 1974a).
It is not known if other clupeids show similar plasticity but it seems likely as themorphology of the South African estuarine species Gilchristella aestuarius varies accordingto its food supply (Blaber, Cyrus and Whitfield, 1981). The ability to change growth ratesand body form is probably a mechanism to enable these fish to maintain a high biomass inthe face of limited food resources. This would enable them to adapt to a wide variety ofwater-bodies.
The most important characteristic of small pelagic species is their very high potentialproductivity. This is the factor that makes their economic exploitation possible.Their high productivities is a result of their short life cycles and consequently veryhigh P/B ratios which are inversely related to longevity (Leveque, Durand and Ecoutin, 1977).The P/B ratio for Stolothrissa was 3.9 over a 12-month period but mortality rates are sohigh that only 2.5% of the original number were survivors at the end of this period(Coulter, 1981). The implication is that production is about 4 times greater than thebiomass and that very high yields are possible. Coulter (1970) suggested that commercialyield could well be equal to production for these species.
This concept can be quantified by using Pauly's (1982) equation, in which
MSY = 2.3 w-0.26 Bv | ||
| Where MSY | = | maximum sustained yield (kg ha-1yr-1) |
| w | = | mean weight (g) |
| Bv | = | unexploited biomass (kg ha-1) |
It can be seen that yield increases exponentially as individual weight decreases,assuming Bv to be equal at all size ranges (Fig. 3).
This can be seen in most pelagic communities where unfished stocks support very highpredator populations. In Lake Tanganyika predators made up about 60% of the catch tobegin with (Coulter, 1970) whilst in Lake Malawi predatory Rhamphochromis spp. were aboutas abundant as their prey, Engraulicypris (Walczak, 1982). The predators in Lake Tanganyikadecreased under fishing pressure although the total catch increased. These were presumablythe fish that had previously been consumed by the predators (Coulter, 1970). Thisinterpretation is, however, somewhat open to question because the period of observationwas not sufficently long to include cyclic fluctuations in abundance of predators. Roest(pers. comm.) notes that over the period 1955–80 the abundance of Lucio lates, actuallyincreased while Lates decreased in the exploited areas.
Small pelagic species may also have an impact on nutrient dynamics, especially inreservoirs. Some evidence suggests that Limnothrissa plays a major role in Lake Karibaas a nutrient store and so reducing losses throught the outflow (Marshall, 1970; 1984).It has also been postulated that this fish contributed to the decline of the floating fernSalvinia molesta, which had been so troublesome in the early days of Kariba, by retainingnutrients which would otherwise have been available to the plant (Marshall and Junor, 1981).Much more research needs to be done on this aspect but it is interesting to speculate onthe contribution small fish might be able to make to the control of troublesome plantgrowths.
A number of lakes and reservoirs have pelagic stocks, but their fisheries are unevenlydeveloped (Table 2). Various factors have promoted or inhibited this process and eachcan be considered in more detail.
4.1 Lake Tanganyika
The oldest and best-developed African pelagic fishery is on this lake and currentlyproduces about 73 000 t or 22.5 kg ha-1 annually (Petr and Kapetsky, 1983). The establishmentand expansion of this fishery was undoubtedly facilitated by the fact that a traditionalfishery already existed (Poll, 1952) and that very high yields were possible.
Stolothrissa tanganicae is the most important species in the fishery and has consequentlybeen studied in greatest detail (Matthes, 1968; Ellis, 1971; Chapman and Van Well,1978; Roest, 1978). The biology of Limnothrissa is less well-known except for some detailson growth and breeding (Matthes, 1968; Ellis, 1971). Some workers have investigated thebiology of the four predatory Lates spp. (Chapman and Van Well, 1978a; Coulter, 1977; Ellis1978) whilst relationships within the pelagic community as a whole were described byCoulter (1970). Productivity and yield estimation have also been investigated (Coulter,1981) as have the economic aspects and future prospects of the fishery (Herman, 1978).
The most outstanding feature of the Tanganyika fishery is its very high potentialproductivity. Coulter (1977; 1981) attempted to estimate production by using Gulland's(1971) equation in which
| Y = M · X · B | |
| where | Y = yield (kg ha-1) |
| M = Natural mortality | |
| X = proportion of the stock available to the fishery, usually X = 0.5 | |
| B = unexploited biomass (kg ha-1) |
Sardine biomass is estimated to be about 160 kg ha-1 and M = 5.2 (Coulter, 1977) whichwould suggest that the yield might be about 400 kg ha-1. The P/B ratio is about 4 andproduction has elsewhere been estimated at 350 kg ha-1 (Coulter, 1981). The yield mightthen amount to 1.1 million tons for the whole lake, which is far in excess of the presentyield. However, much depends on how representative early estimates of biomass really are.Given long-term cyclic fluctuations in biomass and the possibility that the estimates weremade during the cycle when biomass was at a peak, then the 1.1 million ton estimate foryield could be too high. Coulter (1977) suggested that the stock might be sufficientlyresilient for yield to be determined empirically by fishing without causing long-term transformations.It may not, of course, be economically feasible to establish and operate suchan intensive fishery again, because long-term fluctuations in stock availability wouldcreate economic difficulties for fishing intensively when on the low side of the abundancecycle.
4.2 Lake Malawi
The fishery on this lake is poorly-developed except in the south, but studies suggestthat an annual yield of 30–40 kg ha-1 may be possible (FAO, 1982). In inshore waters mostof this would be “Utaka” (Haplochromis spp.) but Engraulicypris sardella would be mostabundant in open waters.
The potential of Lake Malawi seems very low in comparison to Lake Tanganyika andTurner (1982a) postulated that this was because Engraulicypris was not an efficientplanktivore. He concluded that most pelagic production was being channelled into largezooplanktonic animals or the lake fly Chaoborus, neither group being preyed upon byEngraulicypris. By contrast, these organisms were absent from Lake Tnaganyika havingprobably been eliminated by the pelagic clupeids.
Turner (1982a) felt that the introduction of the Tanganyika clupeids into Lake Malawimight greatly improve its productivity. He acknowledged that this might have unforeseeneffects on the lake's fauna and was not a step to be lightly contemplated.
4.3 Lake Victoria
The fishery for Rastrineobola (Engraulicypris) argenteus on this lake would appearto be potentially very valuable but is poorly-developed. Provisional figures show thatabout 9 500 t were taken from the Kenyan sector in 1982 but it was not apparently exploitedin Uganda (Ssentongo, 1983).
A major drawback is that there are presently very little data available on E. argenteusor any pelagic Haplochromis spp. This is urgently required before any further developmentscan take place. Productivity is difficult to estimate but a conservative estimate of30 kg ha-1 is probably realistic; the total yield would then reach 200 000 t annually.Turner's (1982a) comments about introducing clupeids into Lake Malawi may also apply toLake Victoria and this, again, is an area that would repay further study.
4.4 Lake Kivu
This lake is comparable to a reservoir in many respects as it is geologically veryyoung, with few fish species of which none were pelagic. The first artificial introductionof Tanganyika clupeids was made into Lake Kivu, between 1958 and 1960, when large numbersof fry were stocked (Collart, 1960). It was hoped that Stolothrissa would become establishedbut it was, in fact, the less specialized Limnothrissa which did so (Frank, 1977).Recent studies have shown that it is well-established and the artisanal fishery that hasbeen established has attained yields of 42 kg ha-1 which would amount to 13 500 t if thewhole lake was fished (Spliethoff, de Iongh and Frank, 1983).
The population around the lake has no tradition of fishing, fish are not popular asfood and fishing material is scarce and expensive (Petr and Kapetsky, 1983). These factorshave inhibited rapid development of this fishery, but once they have been overcome itmight be possible to attain the yield of 30 000 t (or 110 kg ha-1) estimated by Welcomme(1972) since Kivu would appear to be at least as rich as Lake Tanganyika.
4.5 Lake Kariba
This was the first reservoir to be stocked with clupeids and the successful fisherythat developed is now well-known (Marshall, 1984; Marshall, Junor and Langerman 1982).Limnothrissa was the only species introduced; it colonized the lake within 2 years andcommercial fishing began after 6 years.
Sardine production reached 12 000 t in 1981 but declined in 1982 and 1983 (Fig. 4).This suggests that the population was fully-exploited but it may also be a result of thehydrological regime during this period. Sardine abundance in the lake seems to be closelyrelated to river inflow and nutrient supplies (Marshall, 1982). The 1981–82 and 1982–83rainy seasons were very poor and the river flows the lowest on record. Because sardineabundance decreases along with river flows the reduced catches were expected (Marshall,1981).
Studies to estimate Kariba's potential are in progress and some data suggest that itis very high (Marshall, 1984). These data should be treated with caution, however, as theyare still to be verified. In the last good year, 1982, the eastern Sanyati basin produced7 400 t or about 74 kg ha-1 (Marshall and Shambare, 1983). If this was extrapolated overthe whole lake then the catch would reach 40 000 t or about 4 times the present yield.
A secondary effect of the sardine in Kariba was that the Tigerfish Hydrocynus vittatusbecame partially pelagic, preying on the small fish in open water. Although it declinedunder intensive fishing pressure, it increased in the gillnet catches (Junor and Marshall,1979). Other species, such as Synodontis and Eutropius, also now feed on sardines andthe survival of these fish has probably been enhanced which may contribute to an improvementof inshore productivity. This is an important factor in Kariba because productionfrom this zone is low (Marshall, Junor and Langerman, 1982).
4.6 Cahora Bassa/Reservoir
The colonization of this reservoir by Limnothrissa from Kariba is an interestingexample of its hardiness and adaptability. Soon after their establishment live sardineswere found in the Zambezi river below the Kariba dam, having survived passage through thehydroelectric turbines, and it was suggested that they might move down the river to CahoraBassa (Junor and Begg, 1971; Kenmuir, 1975). This in fact happened and there is now asubstantial stock in the lake (Bernacsek and Lopes, 1984) but for various socio-economicreasons a fishery has not yet been established.
The potential yield has been estimated to be about 8 000 t or 30 kg ha-1 (Bernacsekand Lopes, 1984). A recent acoustic survey produced estimates of a 2 800–17 600 fish perhectare (Lindem, 1983); since these fish weigh about 1.0 g this amounts to 2.8–17.6 kg ha-1.The mean was about 10 kg ha-1 which was very low and suggests that production might notexceed 30 kg ha-1 (assuming their P/B ratio to be about 3.0).
Two factors may have influenced this estimate, however. The first is thatLimnothrissa may be as highly seasonal in Cahora Bassa as it is in Kariba (Marshall, Junorand Langerman, 1982). February is a period of low abundance in Kariba and this may alsobe the case in Cahora Bassa. On the other hand productivity may in fact be this lowbecause of the heavy clay load in the water which reduces light penetration and mayinhibit primary production (Gliwicz, 1984; Bernacsek and Lopes, 1984).
4.7 Lakes Kainji and Volta
Riverine clupeid populations existed in the Niger and Volta rivers before thesereservoirs were constructed. Once the lakes formed, these fish populations grew andoccupied the pelagic waters (Vanderpuye, 1973; Otobo, 1979). These fish are taken to asmall extent in the littoral artisanal fisheries but large-scale industrial fisherieshave not yet been established. It will probably be very difficult to establish such afishery on Volta because the lake bed was never cleared of trees and these would interferewith large nets.
Clupeid biomass on Volta is unknown (C.J. Vanderpuye, personal communication) butan estimate of 3 140 t mean biomass was obtained from Kainji (Otobo, 1979). Assuming thatthe P/B ratio of these fish is similar to those in Tanganyika then the yield might be ashigh as 10 000 t (or 79 kg ha-1). This is close to the yield from the Sanyati Basin ofLake Kariba and so is probably realistic.
Production in Volta might be similar or possibly even higher since it has a highermorpho-edaphic index than Kainji (Henderson and Welcomme, 1974). Thus a yield of 60 000 tmight be possible (at least when the lake is at its full surface area of 8 482 km2).
From the foregoing it is clear that pelagic populations in most African waters arestill greatly under-exploited. There are also other pelagic populations, such as theRastrineobola (Engraulicypris) spp. in Lakes Turkana, Edward and Mobutu Sese Seku or theclupeids in Lake Mweru, but very little is known about their biology, abundance orcommercial potential.
The possibility of introducing clupeids into some of the Great Lakes in order toimprove their pelagic production has already been mentioned. The dangers of interferingwith the biota of these lakes has been discussed (Fryer, 1972) and, although the argumentfor introducing clupeids into them is sound, most fisheries biologists still have reservationsabout doing so (Turner, 1982a).
These inhibitions are less strong where reservoirs are concerned as reservoirs haveextensive environmental impacts in any event (Petr, 1978). The success of Limnothrissa inKariba has shown the value of such introductions and Eccles (1975) has suggested that anumber of other fish from the Great Lakes could also be introduced. There seems to beno reason why these fish should not be stocked into those large reservoirs which do nothave them, like Nasser-Nubia. Medium-sized reservoirs, such as Itezhi-Tezhi and KafueGorge in Zambia, Nyumba ya Mungu in Tanzania or Kamburu in Kenya might also be suitable.
Indeed, in view of the adaptability of the clupeids it might be possible to introducethem into any reservoir regardless of size. There is an urgent need for further studieson this family especially to assess their commercial potential and suitability for stocking.Most clupeids occur in relatively warm waters and their temperature tolerances are notknown. This needs to be investigated and some may be suitable for high altitude lakes,such as Lake Tana or the reservoirs in Madagascar, which presently have a very limitedfish fauna.
This brief review of small pelagic fish has shown that they have an important rolein increasing supplies of fish in Africa. The clupeids are the family with the mostpotential, and further studies on this group are urgently required. The clupeids are alsomore easily harvested because they are readily attracted to lights, which is the commonestmethod of catching these fish.
The need to use light attraction is perhaps one of the major drawbacks in exploitingpelagic fish. Large, relatively expensive boats are needed to increase catches significantlyand to be able to venture into the open waters of large lakes. These capitalrequirements are often beyond the means of local inhabitants and in many countries theprohibitive cost or non-availability of gear such as lighting plants or netting materialsconstrains development. These factors have probably retarded industrial-scale fisheriesin many areas. Artisanal nearshore fisheries can be practised with relatively simplegears, such as kerosine lights in place of electric plants for fish attraction.
There are many advantages in utilizing these fish. Their high productivity meansthat catches will be good enough to repay the initial capital investment in a short time.A very palatable and easily marketed product can be prepared at very low cost by sundryingand costly processing facilities are not required. Finally, the bycatch ofpredators, such as Lates in Tanganyika or Hydrocynus in Kariba, is often more valuablethan the catch of the small species as they are sought-after and highly palatable fish.
