LATES SPP AND OTHER PREDATORS AS A CAUSE OF MORTALITY IN SMALL PELAGIC FISH SPECIES IN THE AFRICAN GREAT LAKES

by

Richard Ogutu-Ohwayo
Uganda Freshwater Fisheries Research Organisation
P.O. Box 343
Jinja
Uganda

1. INTRODUCTION

Small pelagic species are commercially important in many African lakes and reservoirs. They include Limnothrissa miodon and Stolothrissa tanganicae in Lake Tanganyika, Limnothrissa miodon in Lakes Kariba and Cahora Bassa, Pellonula afzeliusi and Cynothrissa mento in Lake Volta (Petr and Kapetsky, 1983; Marshall, 1988) whilst Alestes spp. are among the most important commercial fish in Lake Albert. The small pelagic cyprinid Rastrineobola argentea became one of the most abundant fish in Lakes Victoria, Kyoga and Nabugabo after most of the other species in these lakes were exterminated by Lates niloticus. It is the only native species that has been able survive in the presence of this predator and is now one of the three most important commercial species in Lake Victoria (Wandera, 1992).

Large piscivorous fish are found in most of the African lakes and reservoirs where small pelagic species are abundant. Amongst them are species of Hydrocynus and Lates in, for example, Lakes Kariba, Tanganyika, Albert, Turkana, Victoria, Kyoga and Chad. To a great extent these fish depend on the small pelagics for much of their diet. Thus Lates feeds on Rastrineobola in Lakes Victoria, Kyoga and Nabugabo, on Alestes spp. in Lake Albert, on Engraulicypris stellae in Lake Turkana and on Alestes dageti and Micralestes acutidens in Lake Chad (Coulter, 1970, 1981; Hopson, 1972; Ogutu-Ohwayo, 1984, 1990a; Ogari and Dadzie, 1988; Ligtvoet and Mkumbo, 1990).

This paper examines the importance of Lates niloticus as a predator on Rastrineobola Lakes Victoria, Kyoga and Nabugabo and on Alestes spp. in Lake Albert. The extent to which these predators contribute to the total mortality will be emphasised. Fisheries management normally attempts to regulate fishing mortality but death from natural causes can contribute significantly to the total mortality in a fish stock. The importance of piscivory by large predatory fishes is demonstrated by events in Lake Victoria. There was about 600 000 t of haplochromines in the lake before the introduction of the Nile perch, L. niloticus (Kudhongania and Cordone, 1974) but after it became established the haplochromine stock rapidly disappeared (Ogari and Dadzie, 1988; Ogutu-Ohwayo, 1990a, 1990b; Ligtvoet and Mkumbo, 1990). Data from the commercial catches in the Kenyan waters of the lake show that predation by Lates depleted the haplochromine stocks faster human exploitation could have done.

2. METHODS

Data on the food and feeding habits of Lates in Lakes Victoria, Kyoga, Nabugabo and Albert, collected between 1968 and 1992, will be used to illustrate the importance of mortality from predation. Each fish collected during this time was measured (total and standard length) and weighed and the sex and maturity noted. Stomach fullness was determined by pressing it from the anterior to the posterior end and they were rated as empty, quarter, half, three-quarters and full. The stomach contents were preserved and the weight of the prey recorded. The prey items were identified and the numbers, length and weight of each individual were also recorded.

3. RESULTS

At first, haplochromines were the most important prey taken by Lates in Lakes Victoria and Kyoga. Following the collapse of the haplochromine stock they have taken to feeding on the shrimp Caradina nilotica, anisopteran nymphs and Rastrineobola and both their own juveniles and those of Oreochromis niloticus. Their prey is similar in Lake Nabugabo except that Caradina does not occur there. In Lake Albert they feed on Caradina, anisopteran nymphs, Alestes spp. and haplochromines (Worthington, 1928; Gee, 1969).

The type of prey taken by Lates varies in relation to the size of the predator. Caradina are the main prey item for fish below 20 cm in total length. In all the lakes, anisopteran nymphs become most important in fish from 20–59 cm long. In Lake Albert fish longer than 60 cm feed mainly on Alestes whilst in the other lakes they take Rastrineobola. Cannibalism occurs in Lates over 20 cm in length but they do not feed on tilapiines until they reach 60 cm. In Lake Kyoga Lates of 5–79 cm in length fed extensively on Rastrineobola.

In Lake Kyoga the proportion of Lates with food in their stomachs declined during the day from 0700 to 1800 but rose rapidly between 1800 and 1900 and remained high during the night. During the day there was an increase in the proportion of stomachs with digested food in them. It seems that Lates feed once a day, beginning late in the evening and progressing into the night.

Since Lates appear to feed only once a day, the amount of food found in a stomach can be regarded as the daily ration of that fish. On average, they took from 1∙2–6∙8 % of their own weight as food per day.

4. DISCUSSION

The small pelagic fish species are resilient and can withstand high levels of predation and fishing because of their high turnover rates (Mannini, 1992). Nevertheless, predators do appear to have some effect. For example, in Lake Tanganyika there appears to be an inverse relationship between the abundance of predators and their prey; an increase in the abundance of Lates stappersii is followed by a reduction in that of its main prey, Stolothrissa tanganicae (Coulter, 1970; Roest, 1988). When the population of predators was reduced by fishing that of the clupeids increased (Coulter, 1965–66). By contrast, in Lake Kariba changes in the abundance of Limnothrissa were closely followed by similar changes in that of Hydrocynus vittatus (Marshall, 1987). In Lake Volta an increase in the abundance of clupeids was followed by an increase of Hydrocynus spp and Lates niloticus (Petr and Kapetsky, 1983).

The effect of predatory fish might be different from those caused by a fishery (Popova, 1978). This is because fisheries take mature fish in good condition whilst predators mostly take immatures or those individuals with a slow growth rate and in poor condition. This does not apply in the case of large predators feeding on small pelagic species. Thus, Lates feeds on the entire size range of Rastrineobola, including that selected by the fishery.

Data on the total quantity of fish eaten by predators like Lates are lacking but it is likely to be very high. An understanding of the mortality caused by predators is an important factor to be considered in the population dynamics of small pelagic fish species. It should also be taken into account when efforts are made to determine optimum levels of fishing effort.

5. ACKNOWLEDGEMENTS

I am grateful to the FAO/UNDP IFIP project for inviting me to this symposium and providing an air ticket and to the British Council for meeting my living expenses. The work reported here was supported by a grant from the International Development Research Centre (IDRC), Ottawa, Canada.

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