SOME OBSERVATIONS ON THE PREDATORY FEEDING HABITS OF HYDROCYNUS VITTATUS CASTELNAU IN LAKE KARIBA

by

M. Takano
and
S.P. Subramaniam
Fisheries Research Division
Department of Fisheries
P.O. Box 35100
Chilanga
Zambia

ABSTRACT

The feeding habits of tiger-fish (Hydrocynus vittatus) in Lake Kariba were studied. The fish was mainly piscivorous, with Limnothrissa miodon constituting the major part of the diet. Diet composition showed spatial and temporal variations. The occurrence of juvenile cichlids in stomachs was higher in in-shore waters. Tiger-fish of less than 300mm showed a positive preference for L. miodon. The maximum length of the prey was about 42% of the predator length. Tiger-fish predation is a major factor contributing to the natural mortality of L. miodon. Increased fishing pressure on the sardines in recent years has resulted in an increase in natural mortality. as light-fishing operations concentrate sardine shoals and increase their visibility thus making them more prone to easy predation by tiger-fish. The study also suggests that tiger-fish take a substantial toll on the juveniles of commercially important cichlids.

1. INTRODUCTION

The tiger-fish, Hydrocynus vittatus is a common carnivorous species in Lake Kariba and more than 80% of its diet is composed of fish (Kenmuir, 1972). H. vittatus is one of the most abundant species in the lake and contributes about 50% by weight to the monthly gill-net catches in the Sinazongwe area of the lake (Annual Reports, Fish. Res. Divn. 1981 – 84). Limnothrissa miodon (Kapenta), which was introduced into the lake in 1967/68, supports a commercial fishery of great magnitude in Zambia and Zimbabwe. Kenmuir (1983) reported that there was an increase in the abundance of tiger-fish in the lake, following the establishment of the Limnothrissa population. Kenmuir (1972) also compared the changes in the feeding habits of tiger-fish during pre-and post-introduction phases of Limnothrissa. The effect of H. vittatus on the natural mortality of Limnothrissa stocks through predation is considered to be substantial. The main objective of this study was to determine the current food preference of H. vittatus in Lake Kariba and to gather information on its predation upon commercially important fish species, including L. miodon and cichlids, in the Sinazongwe area of the lake.

2. MATERIALS AND METHODS

Fleets of gill-nets ranging in mesh size from 85mm to 178mm with an interval of 13mm were used for sampling H. vittatus from June, 1983 to May, 1985 at Charlets and Nchete/Samaria island stations. The Charlets station is a shallow weedy area providing good cover for juvenile fish whereas Nchete/Samaria island station is relatively more exposed with a steeply sloping bottom. The nets were set continuously for three consective days at these stations on a monthly basis and the catches removed between 0600–0800 hours. A total of 4536 specimens of H. vittatus ranging in size from 85mm to 725mm (fork length) were caught during the survey. Of these, 1231 had full stomachs and were used in the stomach-content study. After recording length and weight measurements of each tiger-fish, the stomachs were removed and preserved in 10% formalin for subsequent analyses of the contents. The size of prey fish, when found intact, was also recorded.

3. RESULTS AND DISCUSSION

The contents of tiger-fish stomachs consisted mainly of fish, though insects, molluscs and crustaceans were also recorded. Fish were found in about 97% of the full stomachs examined and comprised mainly L. miodon and cichlids followed by unidentified and digested fish. The occurrence of Synodontis zambezensis, Alestes spp. and juvenile H. vittatus was insignificant (<1% each).

A comparison of the stomach contents of H. vittatus from the two stations indicated that there was spatial variation in its feeding habits. While L. miodon occurred in 41.5% of the stomachs of tiger-fish from Nchete/Samaria island station, it was recorded from only 29.5% of those examined from Charlets station. Conversely, cichlids were more abundant in the stomachs of fish from Charlets station (33.2%) than those from the island station (28.2%). This may be explained by the greater abundance of juvenile cichlids at Charlets station due to the weedy habitat there. The cichlids preyed upon were mainly Tilapia rendalli, Oreochromis mortimeri and Pseudocrenilabrus philander. The incidence of S. zambezensis and H. vittatus in the stomachs was negligible.

One of the causes of the decline in tilapia stocks in Lake Kariba was attributed to predation by H. vittatus (Matthes, 1968; Kenmuir, 1971 and Coulter et al., 1965). Coke (1966) also observed 50% occurrence of tilapia in tiger-fish stomachs. However, Donnelly (1971) found that Alestes lateralis formed 54.1% and tilapias only 2.1% of the tiger-fish diet, which lead him to conclude that since the establishment of aquatic plants which provide cover for tilapia juveniles, the influence of H. vittatus on tilapias was minimal. The present study suggests that tilapias still constitute an important item of food for H. vittatus in Lake Kariba despite the weed cover, although L, miodon remains the major prey.

As L. miodon has soft fins and bones relative to cichlids and many other prey species, the rate of digestion of L. miodon may be more rapid than for other prey. The unidentified and digested fish particles recorded were therefore probably mainly L. miodon making the estimate of their actual contribution to the diet very conservative.

Fig. 1 shows the variation in the dietary composition of tiger-fish. Although the sampling at Nchete/Samaria island station was irregular, sufficient samples were taken for it to be apparent that there were pronounced seasonal changes in the composition of the stomach contents. Terrestial insects formed a significant portion of the food during the month of December and this may be attributable to the characteristic decline in the L. miodon population during this month (Subramaniam, unpublished data) and the seasonal phenomenon of the nuptial flights of termites at the onset of the rains. This observation tends to suggest that H. vittatus is an opportunistic feeder, although the general dietary preference is for fish. Limnothrissa abundance is usually greater during the period June/July – September (Subramaniam, unpublished data) and this should have been reflected in the stomach contents data. Unfortunately, lack of data for the island stations for this time of year has meant that this was not demonstrable though heavy predation on L. miodon was shown for the month of June. The percentage contribution of L. miodon to the diet during December to February was relatively lower than that for the period March to June.

Table 2 shows the frequency of occurrence of different prey in the stomachs of H. vittatus of varying lenghts. From these data, some indications of specific food preferences for various length-groups of tiger-fish may be derived. Limnothrissa and cichlids were found in the stomachs of all sizes of tiger-fish sampled. Small specimens of H. vittatus (<300mm) showed a positive preference for L. miodon. A Chi-squared test showed that occurrence of L. miodon was significantly higher than that of cichlids in H. vittatus of lengths up to 290mm (r=18.62, P<0.001%). Similary, the occurrence of cichlids was more conspicuous in the stomach contents of the 300–400mm size group. H. vittatus and S. zambezensis were recorded only in the stomachs of larger specimens (>500mm). Tiger-fish of less than 80mm were not normally sampled by the gill-net fleet although specimens of 50–80mm obtained by occasional beach-seineing, contained predominantly zooplankton and fish-fry in their stomachs (Bell-Cross, 1965; Kenmuir, 1975). Molluscs were represented in the stomach contents of the 250–490mm size group and insects were invariably found in specimens smaller than 50mm.

The relationship between predator length and prey length (Fig. 2) showed that the maximum length of the prey was about 42% of the predator length. Kenmuir (1972) also observed that the length of prey of tiger-fish does not generally exceed 40% of the predator length. The length-frequency distributions of various prey items recovered from tiger-fish stomachs indicated that the length composition of ingested L. miodon, cichlids and unidentified fish was more or less similar with a peak at about 40mm (Fig.3). The length-frequency distribution of L. miodon from commercial catches peak at 49–51mm (Subramaniam, unpublished data) though it appears that in inshore waters tiger-fish feed mainly upon smaller specimens of about 40mm, which is the size at which the species start to mature in Lake Kariba (Subramaniam, unpublished data). Cochrane's (1976) observation on the gut contents of the tiger-fish by-catch from the commercial Limnothrissa fishery also indicated that there was no selective feeding on larger sardines by the bigger tiger-fish.

Although no attempt has been made to quantify the impact of tiger-fish predation on L. miodon in this study, it appears that the removal, particularly of immature and maturing Limnothrissa, by tiger-fish predation is substantial and is a major factor contributing to the natural mortality of Limnothrissa in the lake. Commercial light-fishing in the lake concentrates shoals of Limnothrissa during fishing activity. This, coupled with enhanced visibility, subjects the sardines to heavy predation by tiger-fish during the fishing operations and tiger-fish of 270mm–310mm dominate the purse seine by-catch (Cochrane, 1976). It may therefore be concluded that the increased fishing pressure on L. miodon in recent years has increased its natural mortality by promoting a higher intensity of predation by tiger-fish.

Cichlids in general form a significant part of the artisanal fish catches of Lake Kariba. The present study suggests that tiger-fish take a substantial toll of juvenile cichlids, particularly in the marginal waters. There is no evidence to support the belief that the predation pressure on tilapias was reduced as a result of the semi-pelagic habit assumed by H. vittatus consequent to the establishment of a large Limnothrissa population in the open waters of the lake.

4. ACKNOWLEDGEMENTS

The authors are very much indebted to the field and technical staff based in Sinazongwe Research Unit for the assistance received in the collection and analyses of the samples.

5. REFERENCES

Anon., 1981–83 Annual reports of the Fisheries Research Division, Dept. of Fisheries, Chilanga, Zambia.

Cochrane, K.L., 1976 Catches of Hydrocynus vittatus Castelnau during sardine fishing operations in Kariba. Kariba Stud., 7:98–108

Coke, M., 1966 Progress report - Nov. 1966 L.K.F.R.I. (mimeo)

Coulter, G.S., D. Harding, D.H. Eccles and G. Bell-Cross, 1965 Unique opportunities for research in the Great Lakes of Central Africa. Nature Lond., 206:4

Donnely, B.G., 1971 The fish population changes in Lake Kariba between 1960 and 1968. Part II. Characidae and Citharinidae. L.K.F.R.I.Rep.

Kenmuir, D.H.S., 1971 Some aspects of Hydrocynus vittatus Castelnau (Tiger-fish) research at Lake Kariba. News Lett. Limnol.Soc.South.Afr., 17:33–8

Kenmuir, D.H.S., 1972 Report on a study of the ecology of the tiger-fish, Hydrocynus vittatus Castelnau in Lake Kariba. L.K.F.R.I.Rep

Kenmuir, D.H.S., 1975 The diet of fingerling tiger-fish, Hydrocynus vittatus Cast. in Lake Kariba, Rhodesia. Arnoldia Rhod., 7:1–8

Kenmuir, D.H.S., 1983 Fishes of Lake Kariba. Harare, Wilderness Publication.

Matthes, H., 1968 The food and feeding habits of the tiger-fish Hydrocynus vittatus (Cast. 1861) in Lake Kariba. Beaufortia 15:143–53

Subramaniam, S.P., (ed.) 1984 Annual Report, Fisheries Research Division, Dept. of Fisheries, Chilanga, Zambia. 82p.

Table 1. Frequency occurrence of food items in the stomachs of H. vittatus at Charlets and Nchete/Samaria island stations (June, 1983 – May, 1985)

Table 2. Frequency occurrence (percentage) of food items in various sizes of H. vittatus

Fig. 1 Percentage frequency of food items by number from H. vittatus samples of (A) Charlets (B) Nchete/Samaria island stations (1984 – 1985).

Fig. 2 Predator and prey length relationship.

Fig. 3 Length frequency distribution of prey in H. vittatus. Shaded histogram shows length frequency of commercial catch at Siavonga area, data from Subramaniam (1984).