by
John D.R. Bayona
Tanzania Fisheries Research Institute
Kigoma Centre
Tanzania
Studies on the biological productivity of Lake Tanganyika are reviewed with emphasis on the assessment of the lake's trophic status and its sustained fish production potential. Results have suggested that the lake has a high carbon transfer efficiency at a low rate of primary production, thus it is capable of sustaining high fish yields. However, if sound decisions are to be made on the management of Lake Tanganyika's fisheries, further research will be required on the long term fluctuations in predator and prey abundance and on fish migration patterns.
The management of the fisheries of Lake Tanganyika will, rely largely upon thoroughly determined estimates of sustainable yield and knowledge of the lake-wide distribution of fish stocks.
Investigations into abundance, yield and distribution of pelagic fish in Lake Tanganyika were carried out by Johannesson (1974), Mathisen (1976), Chapman (1976), Mann et al. (1975), Coulter (1977 & 1981), Herman (1977), Roest (1978) and FAO (1978). Annual sustained fish yield was estimated to be in excess of 300,000 tons. Acoustic deployment by Johannesson (1974) resulted in an even higher estimate of fish yield (600kg/ha). These estimates suggest that Lake Tanganyika is exceptionally productive with potential yields even higher than those of the heavily exploited lake Kyoga and Edward. However, the high transparencies of the pelagic waters, limited algal biomass, and the stability of thermal stratification argued against the possibility of such high levels of fish production and it remained nebulous and inconclusive as to how the lake could sustain such high yields of fish. This paradox called for validative research into the relationship between primary production and fish production in order to help in the understanding of the trophic status of the lake which has remained so enigmatic for many decades.
The purpose of this paper is to review some of the important research contributions which have aided the understanding of biological productivity in the lake and to discuss some existing gaps in our knowledge of abundance, yields and distribution of fish which still exist. These areas of uncertainty are proposed for further reseach. Predation is discussed as an important aspect of the trophic network and one which influences species abundance and distribution.
Quantitative estimates of primary production in Lake Tanganyika became available only during the last decade. These estimates are based on research and published data by Melack (1980), Hecky et al. (1978), Hecky and Fee (1981) and Hecky et al. (1981). Previous assessments of the productivity of Lake Tanganyika were based on qualitative approaches, some of which are reported by Beauchamp (1939), Kufferath (1952), Van Meel (1954), Dubois (1958) and Coulter (1963).
Beauchamp (1939) observed that the lake had high thermal stability and noted the paucity of nutrients in the epilimnion. Open waters were blue with high transparency and day-time tows of plankton nets captured few planktonic organisms. These observations are characteristic of lakes with limited productivity and were the basis for Beauchamp's classification of Lake Tanganyika as oligotrophic (Beauchamp, 1939). Whilst Beauchamp's observations were confirmed by Kufferath (1952) and Van Meel (1954), the latter authors noted from observations of dense phytoplankton blooms and diurnal vertical movements of zooplankton and fish, that certain areas of the lake were productive. This led to the classification of the lake as pseudoeutrophic, i.e. possessing both oligotrophic and eutrophic characteristics (Van Meel, 1954). Coulter (1963) reported phytoplankton blooms in shallow inshore regions of the lake which contrasted with the oligotrophic appearance of the deep offshore waters. Because of the blue colour of the water (excluding the shallow bays of the lake), Hickling (1961) drew the conclusion that the waters of Lake Tanganyika were not very productive.
Thus, the initial estimations of the primary productivity and overall production potential of Lake Tanganyika were entirely qualitative and were based on the observation of such features as high water transparency, depth profile, thermal stability and abundance and distribution of plankton.
The quantitative estimation of primary production in Lake Tanganyika was first attempted in April of 1971 by Melack who estimated the pelagic primary production to be 0.5g cm2/day based on a single measurement, (Melack, 1980). Hecky and Fee (1981) gave an improved estimate of primary production by taking several measurements of primary production (by radioactive techniques) along N - S transects over the entire area of the lake during periods of high and low algal biomass.
A mean daily integral rate of primary production of 1.4g cm2/day was estimated during October – November which is the period of high algal biomass and photosynthetic activity. During a period of low algal biomass (April – May), the authors estimated primary production to be 0.6g cm2/day. An overall annual rate of primary production of 0.8g cm2/day (equivalent to 290g cm2/year), was derived (Hecky and Fee, 1981). The same authors derived a growth rate of 1.2 per day for algal biomass.
After comparing the estimated annual rate of primary production in Lake Tanganyika (0.8g cm2/day) with that of other tropical lakes (4.5, 1.45, and 1.7g cm2/day for Lakes George, Titicaca and Lanao respectively), Hecky et al. (1981) concluded that the rate of primary production in Lake Tanganyika is not high. However, these authors considered the efficiency of carbon transfer from primary production to fish to be extremely high. Hecky and Fee (1981) made an estimate of 0.45% as the carbon transfer efficiency, derived as the percentage proportion of clupeid production to primary production.
Hecky and Fee (1981) suggested that loss of algal biomass by grazing needed to be fairly high for the efficient energy transfer from one trophic level to the next. Hecky and Kling (1981) noted that the estimated algal biomass or algal standing stock was 0.59g/cm2, which is low relative to most other tropical lakes. These findings suggest a high level of efficiency of carbon transfer from autotrophic organisms to fish. High growth rates lead to high turn-over rates of algal biomass. If loss of algal biomass is high due to grazing then the algal biomass at any one time remains low thus avoiding problems of self-limitation caused by depletion of nutrients.
It is argued by Hecky et al. (1981) that the efficiency of the trophic structure in Lake Tanganyika could be due to:
a relatively short food chain leading to harvestable fish and
the great age of the lake which has allowed for the selection of trophically efficient populations.
The study by Hecky and Kling (1981) on bacterial, phytoplankton and protozoan biomasses has shown that there are potential sources of fixed carbon other than primary production by algae. The authors conclude that bacterial biomass is almost equal to phytoplankton biomass and that it is greater than protozoan biomass in the euphotic zone. Bacteria, however, have comparable growth rates to phytoplankton, their estimated mean growth rate being 0.9 per day and their production 440mg cm2/day in the euphotic zone (Hecky et al., 1981). Hence, heterotrophic bacterial production should be an important source of fixed carbon to higher trophic levels in the euphotic zones. If the depth to the limit of detectable oxygen (100 – 200m) is considered, then the contribution of fixed carbon from heterotrophic bacterial production could be higher than that from phytoplankton production though it is important to note that within the euphotic zone bacterial production (440mg cm2/day) is about half the mean rate of algal production (800mg cm2/day).
Littoral periphyton and macrophytes, together with allochthonous particulate matter, were considered to be minor contibutors of additional organic carbon to secondary producers and their contribution is very low compared to phytoplankton production (Hecky and Fee, 1981).
Well over 90% of the annual harvested weight of fish in Lake Tanganyika is made up by six endemic species belonging to the pelagic fish community. These are two clupeids (Stolothrissa tanganicae and Limnothrissa miodon), which are the principal prey species, and four Lates species (L. mariae, L. microlepis, L. angustifrons, and L. stappersi) which are predators. A taxonomic revision of the genus Lates was made by Greenwood (1976). The exploitation of these species is based on light attraction, and three major fisheries, namely, traditional scoopnetting, artisanal lift-netting and industrial purse-seining, operate on the lake. These are discussed by Collart (1954 & 1958), Andrianos (1976) and Herman (1977).
Many studies aimed at assessing the yield and production potential of Lake Tanganyika have been made and these are cited above. From these studies, two general conclusions can be made about fish turnover rates and levels of harvestable biomass from the lake, viz:
Empirical evidence of the growth and production capacities of small clupeids and of production/biomass ratios suggest that lakes dominated by such species will have high rates of production.
The most recent studies on the primary production and fish production of Lake Tanganyika by Coulter 1981) and Hecky et al. (1981) concur that the potential yield is higher than the present yield from the highly exploited regions of the lake. Coulter (1981) further reports that, whereas pelagic fish biomass relative to area is comparable with many other standing waters, total annual production is exceptionally high.
Of the two clupeids species, Stolothrissa tanganicae is the more abundant species in the pelagic zone, contributing about 60–80% of the total annual exploitable biomass (Coulter, 1970 & 1977). Both the growth rates and reproductive capacity of this species suggest high turnover rates of the species biomass.
S. tanganicae is small (maximum size 93mm) and short-lived and a given cohort contributes to the fishery for only 12 months. The species spawns throughout the year though there are spawning peaks which vary in different sectors of the lake. In Zambia, Ellis (1971) reported peak spawning in November – December and March – June. Matthes (1967) suggested that peak spawning takes place during December – February and May – July whereas Coulter (1970) reported peak spawning in August – December. In Tanzania, Chapman and VanWell (1978) reported a spawning peak in January – April whereas in Burundi, in the far northern portion of the lake, a spawing peak is observed in February – May (Mann et al., 1975; Roest, 1978).
Although varying from year to year, the usual pattern is for spawning to start in the south and spread progressively northwards. This is linked to the production cycle of the lake and is synchronised with plankton abundance (Coulter, 1970; Chapman and VanWell, 1978). Coulter (1963) reported that there were primary and secondary peaks in plankton production in July – September and November – December. Thus both the survival of Stolothrissa recruits to open waters and of fry in nursery grounds benefits from the abundant plankton which is available. Instantaneous growth rate (k) is fairly high and is estimated to be 0.21 or 7mm per month (Chapman and VanWell, 1978; Roest, 1978 and Coulter, 1977). Fry normally grow rapidly and reach a size of 55 – 61mm at the age of 4 – 5 months at which time they are recruited (Roest, 1978). S. tanganicae also suffers a high mortality rate (about 99%) suggesting that there should be rapid replacement of its stocks (Coulter, 1977). Thomson et al. (1977) suggested that clupeids are possibly selected for maximal reproductive energetics (i.e. are r - selected).
Production/Biomass ratios are often used to express the production turnover rate or productivity. Coulter (1981) estimated a high P/B ratio of 3.5 for S. tanganicae in L. Tanganyika which is indicative of high productivity in the pelagic fish community.
Based on weight - survivorship data from Roest (1978), Coulter (1981) applied Allen's (1950) graphical method to derive a P/B ratio. The derived estimate was 3.9 over a 12 month period of cohort survival and was later reduced to an assumed value of 3.5. As P/B ratios are often functions of mortality rate (Allen, 1971), the validity of the estimate was assessed by comparison with known estimates of mortality.
In Burundi, the estimated instantaneous total mortality rates for Stolothrissa below and above 79mm are 5.48 and 2.77 respectively. In Tanzania, Chapman and VanWell (1978) estimated a total instantaneous mortality rate (Z) of 5.2 per year or .43 per month. These values are in close agreement with the estimated P/B ratio of Stolothrissa
Fish yield estimates in Lake Tanganyika have ranged from 10kg/hectare (Kufferath, 1952) to about 600kg/hectare (Coulter, 1977). Although various assessment techniques were employed, which accounts for some variability in the estimates, it is evident that high annual yields of fish are sustained in L. Tanganyika.
The methods for estimation of fish yields in the lake are grouped into four categories by Coulter (1977), namely:
application of trophic-dynamic relationships
The use of abiotic variables to estimate fish yield in L. Tanganyika was initally attempted by Kufferath (1952) who estimated a yield of 10kg/hectare or an annual harvest of 30,000 tons. This estimate was based on chemical compostition of the lake. The use of abiotic variables to estimate yield is discussed by Ryder et al. (1974) who proposed that fish yields were proportional to a ‘morpho-edaphic index’ of total dissolved solids divided by mean depth of the lake. Coulter (1977) calculated the MEI of Lake Tanganyika to be 8.71 from a mean epilimnion conductivity of about 610 umhos and mean mixing depth of 70m. This gave a yield value of 39kg/hectare which is low compared with most estimates from catch and acoustic data. Estimates of fish yield from catch, effort and biological data were reported by Capart and Kufferath (1956), Turner and Herman (1977) and Coulter (1977 & 1981). Based on commercial catches and echosounding, Capart and Kufferath (1956) estimated a total annual yield of 30–35kg/hectare or 100,000 tons per annum. In Burundi, Turner and Herman (1977) estimated a potential average fish yield of 7,000 tons and 10,000 tons from industrial and artisanal/traditional fisheries, respectively.
Catch data were analysed using Gulland's (1970) yield model. Gulland (op. cit.) established a yield equation of the form:
Cmax = X.M.Bo
where: Cmax = Maximum sustainable yield in kg/hectare/year
Bo = Virgin biomass
M = Instantaneous natural mortality coefficient
X = A constant representing the fraction of total
annual production to be taken by the fishery
Henderson et al. (1973) suggested that the above yield equation was appropriate in the early stages of a developing fishery. Coulter (1977) used values of L, K and M for Stolothrissa to estimate exploitation levels of the species using yield tables by Beverton and Holt (1964). He estimated maximum sustainable annual yield to be 60–80% of the Stolothrissa biomass. Taking catches from all the fisheries he estimated Cmax to be 120kg/hectare and using Gulland's (1970) equation, biomass estimates were derived from which an estimated yield of 125 kg/hectare was calculated.
Using a different application of this technique, Coulter (1981) calculated production for all prey species in Burundi using an P/B ratio estimate of 3.5 and he derived an annual production estimate of 700kg/hectare/year. Hecky et al. (1981) consider that the estimate is far too high to suit management applications because the underlying assumptions are unrealistic. It should be noted that Coulter (1977) implicitly assumed that:
the foodweb upon which clupeids depend would not be altered by removal of predators
Application of trophodynamic relationships in assessing fish yields was attempted and reported by Hecky et al. (op. cit.). The authors applied Melack's (1976) and Oglesby's (1977) equations which directly relate fish yield to primary production. Melack's (1976) equation, based on data from 8 large African lakes, takes the form:
Log FY = 0.113PG - 0.91
where: FY = Fish yield in kg/ha/yr
PG = Gross primary production as gO2/m2/day
Oglesby's (1977) equation, based on data from 15 Indian lakes takes the form:
Log Yf c = 2 log PC - 6.0
where: PC = Photosynthetic radio carbon uptake in gC/m2/year
The latter equation gave a yield estimate for L. Tanganyika of 8kg/ha/yr based on a PC estimate of 290 gC/m2/year by Hecky and Fee (1981) whereas the former equation predicted a fish yield estimate of 20kg/ha/yr. These are both clearly underestimates as shown by the current actual yield and the stability of the fisheries. Hecky et al. (1981) discounted these results because he considered that the applied regression equations are inappropriate for Lake Tanganyika. Hecky et al. (op. cit.) suggest that the great age of Lake Tanganyika together with the observed high efficiency of carbon transfer mean that it is not comparable with any of the lakes from which the two functional relationships were developed.
Other yield assessments for Lake Tanganyika emanate from the results of acoustic surveys (as reported by Coulter, 1977). Only three acoustic estimates of pelagic ichthyomass in the lake are available for further management applications. A standing stock of 2.8 × 106 tons of fish or 880kg/hectare is reported from a survey which was conducted during November 1973 (Johannesson, 1974). A similar survey which was conducted during November – December 1976 (the period of the seasonal peak in fish abundance) yielded an estimate of 0.68 × 106 tons of fish or 210kg/hectare. A third survey which was conducted in May 1975, a period of low fish abundance, yielded 0.47 × 106 tons of fish or 144kg/hectare (Mathisen, 1976).
The variability of these results is partly due to natural and biologically induced fluctuations in the clupeid populations (Coulter, 1981) and partly to the methodology in acoustic calibration (Bayona, 1978). However, by assuming that the highest target contribution to acoustic integrated voltages was from clupeids, yield estimates (as derived from Gulland's (1970) equation) fall within the reported range by Coulter (1977) of 0.4–0.5 million tons of fish (this excludes the first acoustic results from Johannesson (1974).
Coulter (1977) considers an acoustic biomass estimate of 880kg/hectare or a yield of 600kg/hectare to be either a large over-estimate or to represent a seasonal pulse of very high productivity.
In L. Tanganyika, the two clupeid species are the main prey species, and the four species of Lates the principal predators. This observation is based on the published research of Poll (1953), Matthes (1967), Henderson et al. (1972), Herman (1977), Mann et al. (1975), Chene (1975), Ellis (1978), Chapman and VanWell (1978) and Coulter (1981).
Henderson et al. (1972), who observed large fluctations in clupeid catches, suggested that such variation is related more closely to predation than to exploitation by fishermen. They concluded that the observed fish harvest is only a quarter of the consumption by predators. Ellis (1978) commented that L. stappersi is man's most efficient competitor for clupeids. These statements implicitly imply that predation is intense in Lake Tanganyika. Evidence for and consequences of predation are reflected in morphological adaptations, fish gut-content and predator-prey interactions.
Information on form and function from Poll (1953) and Ellis (1978) suggests that L. stappersi is a fast moving predator, capable of capturing its prey by sudden attack at high speed.
This species has a compressed fusiform body with fairly long, subfalcate pectoral fins which are characteristic of fast moving, continuously swimming fish. Turbulence is reduced and stability and speed improved by a forked caudal fin with a high aspect ratio. The dorsal fin is a friction reducing device. According to Ellis (1978) the vertebral column of L. stappersi is not highly flexible. However, the possession of a large amount of red unstriated red muscle near the tail is indicative of a fast swimming species. The mouth of L. stappersi, which is extensible and has a mobile premaxilla, suggests that it is capable of capturing its prey while steering at high speed.
L. stappersi was reported to attain a maximum size of 450mm at an age of 5 years although individuals occasionally live longer (Mann et al., 1975; Chapman and VanWell, 1978a and Ellis, 1978).
Examination of the stomach contents of this species suggests changes in feeding habits during its lifetime. Below 70mm, L. stappersi is exclusively planktivorous, feeding mainly on copepods (Mesocyclops spp.), prawns (Limnocaridina spp.) and other zooplankton (Ellis, 1978, Chene, 1975). However, it becomes piscivorous at about 130mm, feeding mainly upon Stolothrissa. Ellis (1978) observed that 95% of the stomach contents of L. stappersi comprised of Stolothrissa.
Coulter (1970), Mann et al. (1975), Chapman et al. (1974), Chene (1975), Herman (1977) and Turner (1977) reported some variations in the catches of predators and prey species.
From ring-net catch data it became evident that the abundance of L. stappersi and Stolothrissa were negatively correlated. However, Stolothrissa catches correlated positively with catches of L. microlepis. It is believed that L. stappersi is not capable of the rapid vertical and lateral movements which are exhibited by Stolothrissa but that such movements can be successfully made by L. microlepis.
In addition to seasonal variations, long term cyclical variations in catches with a period of 6 years have been observed for L. stappersi and Stolothrissa (Chene, 1975, Herman, 1977). According to catch data from Burundi, there were successive peaks in abundance of L. stappersi in 1956 – 57, 1962 –63, 1968 – 69 and 1976 – 77 (Herman, 1977). These long term variations are probably predator - prey related. Henderson et al. (1972) confirmed that centropomid predators have a major influence on clupeid abundance and that the observed variations are consistent with the predator - prey model of Larkin (1966).
Most researchers consider the marked schooling behaviour of Stolothrissa to be an adaptive mechanism for avoiding severe predation (FAO, 1978; Coulter, 1981). Other predator avoidence mechanisms displayed by this species include extensive diel vertical migrations (Coulter, 1966) and lateral movements.
The sustained high yields of fish from Lake Tanganyika are evidently due to extremely high rates of algal growth and to recycling of organic material to the fishery. It is established that standing stocks of phytoplankton at the base of the food chain usually remain low (Hecky et al. 1981) which gives the pelagic waters a deceptively oligotrophic appearance.
Because biological productivity is fairly low around May (Coulter, 1963 & 1969), estimations of potential yield obtained during this low point in the production cycle should give a conservative figure, and this should be borne in mind when making exploitation and management decisions. Thus the yield estimate of 125kg/ha or 400,000 tons/year for the whole lake, which was based on acoustic deployment in May of 1975 (Mathisen, 1976) is probably an underestimate of the actual yield potential. This estimate is in good agreement with the estimated yield potential of the lake based on actual fish landings in Burundi (Coulter, 1977).
Division of the estimated total potential yield of fish into estimates for each of the riparian states is not yet practicable because of lack of information on fish movements, distribution and long term variations in predator - prey species abundance.
Johannesson (1974) conducted two acoustic surveys with an interval of two weeks in the northern region of Lake Tanganyika. He found that the resulting biomass estimate from the first survey was 120,000 tonnes whereas that from the second was 260,000 tonnes. Chapman (1975) reported pronounced changes in biomass distribution within as short a period as 24 hours. During fishing tests in 1974, Chapman et al. (1974) found that Stolothrissa appeared to be absent from the southern part of Lake Tanganyika whereas during the corresponding period in the previous year, Johannesson (1974) found that clupeids were spread throughout the lake. These varied results indicate that there are extensive lateral movements by clupeids which may be related to the distribution patterns and movements of their primary predator, Lates stappersi. Further evidence of fish migrations is provided by the recovery near Mpulungu of one tagged L. stappersi which had been tagged in Bujumbura (FAO, 1978).
Some observations, however, contradict the preceeding indications of fish migrations and suggest that movements of fish are highly localized. Chapman and VanWell (1978) reported that the pattern of peak abundance for L. stappersi is similar throughout the lake. The abundance of this species peaks during the first half of the year (December to June). Therefore fish migration cannot be explained on the basis of this observation. Coulter (1970) suggested that the steady nature of the changes in size composition and predator - prey abundance of the pelagic fish populations in the south east arm of the lake, indicated that they were isolated from the stocks in the rest of the lake. Similar isolation is reported for Stolothrissa in the northern end where the species exhibits only localized movements (FAO, 1978).
These rather contradictory findings on fish migrations complicate decision-making with regard to management of the fisheries. Research into fish movements and biomass distribution in Lake Tanganyika should therefore be pursued as a priority. It is also suggested that both seasonal and long term variations in the population structures of L. stappersi and Stolothrissa be studied more precisely.
Recent data from the Tanzanian waters of Lake Tanganyika have shown that the Stolothrissa fishery has become highly seasonal. Both fishermen and government bodies have expressed concern over the decline in abundance of Stolothrissa over the last eight years.
The 6 year alternating cycles of abundance for Stolothrissa and L. stappersi proposed by Herman (1977) would appear to be an oversimplification and there has not been a major recovery of Stolothrissa since its decline in 1976. During this year the mean catch of Stolothrissa by the commercial purse-seiner m.v. Ruvua fell to 127kg/boat/night which constituted only 9% of the total catch, whereas the mean catch of L. stappersi was 1,021 kg/boat/night or 72% of the catch. If a 6 year cycle was in evidence, a recovery of Stolothrissa would have occurred in 1982 but by 1983 the contribution made to the catch by Stolothrissa was down to 2% (as opposed to 86% by L. stappersi). By 1984 Stolothrissa had increased its contribution to 8% and L. stappersi decreased to 80% of the catch though there is not yet sufficient data available to indicate whether these changes herald a substantial upsurge in the population size of Stolothrissa and a corresponding decline in L. stappersi.
It is therefore clear that the duration of the cyclical interaction between the two species is highly variable and further research is required into the predator - prey relationships that exist between them.
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