INITIAL RESULTS OF THE ECOLOGICAL SURVEY OF THE CICHLID FROM THE MWANZA GULF OF (TANZANIA): BREEDING PATTERNS, TROPHIC AND DISTRIBUTION

WITH RECOMMENDATIONS FOR COMMERCIAL TRAWL-FISHERY

by

F. WITTE*) (HaplochromisEcology Survey Team, Mwanza, Tanzania; MorphologyDepartment, Zoolo- gisch Laboratorium, Universityof Leiden, The Netherlands)

SUMMARY

Ecological investigations on the haplochromine Cichlidae from the Mwanza Gulf of Lake Victoria revealed that more than 140 species are living in the habitats fished with bottom trawlers (i.e. all major habitats except rocky bottoms, dense stands and very shallow water). These species can be classified into 9 trophic groups. The phytoplankton/detritus eaters and the zooplanktophages (both belong- ing to the smaller species) form the dominant group in the sublittoral waters with a mud bottom, while eaters dominate in shallower water, especially over sand. Most haplochromine species appear to be strongly habitat-restricted throughout life. Breeding of the bulk of the haplochromine species (phytoplankton/detritus eaters, zooplanktophages and some insectivores) in the sublittoral and littoral waters with a mud bottom appears to be seasonal. Spawning takes place at the end of the rainy season. However, some species of the littoral sand habitat breed throughout the year. A discussion is given of the factors by which the seasonal breeding pattern might be imposed. For an optimal trawl fishery recommendations based on the present data are discussed.

INTRODUCTION

Lake Victoria, the third largest lake in the world, covers an area of approximately 69,000 km2. It lies across the equator and is shared by Tanzania, Uganda and Kenya. Approximately 80% of its demersal ichthyomass consists of haplochromine cichlids (KUDHONGANIA & CORDONE, 1974). In 1976 a commercial trawl fishery based upon these data was started in the Mwanza and the Speke Gulf of Lake Vic- toria. Little was known about the ecology of the haplochromine cichlids at that time: in the stock estimates the currently distinguished 250 + species were regarded as an ecologically homogeneous group and

*) c/o Drs. C. D. N. Barel, Zoologisch Laboratorium, Kaiserstraat 63, Postbus 9516, 2300 RA Leiden, The Netherlands. 176 knowledge of the biology and biological classification was restricted to a few museum and university scientists. In August 1977 the Ecology Survey Team (HEST) started research on the ecology of the haplochromine species in the Mwanza Gulf. A major aim of this study was to provide data on which elementary fishing instructions could be based, thus guaran- teeing a prolonged optimal exploitation of the haplochromine fishes. This paper is the first to present such data. It deals mainly with the trophic composition of haplochromine populations from various habitats, distribution patterns of haplochromine species and breeding patterns. It is also the first paper dealing strictly with the ecology of Victoria haplochromine cichlids. However, it should be emphasized that an ecological survey as presented in this article would not have been feasable without GREENWOOD'S studies. His 1974 book on the biology and evolution of the haplochromine cichlids from Lake Victoria will stand out as a pioneer work on the ecology of these fishes. Except for three, all habitats in a representative transect of the Mwanza Gulf have been investigated. The three habitats not presented in this paper (but known to have a particular community of haplo- chromines) are: 1) the rocky shores, 2) areas with dense plant-stands and 3) the littoral habitat (<2 m). The latter two are well-known from GREENWOOD'S research (see GREENWOOD, 1974 for references and a summary). The rocky shores were found to lodge a ""- like (cf. FRYER, 1959) community of (WITTE-MAAS, in prep. ; VAN OIJEN et al., this volume). Ecological investigations of such a complex system as the haplo- chromine species flock, comprising over 250 species in Lake Victoria only, eventually require studies at the species level. However, most species concerned in the present study were discovered in the course of our field-work and have not yet been described. Awaiting the taxonomic description before publishing on the ecology of a species, would certainly imply an unwarranted delay of many years. A com- promise was sought in grouping species in trophic or other ecologically relevant categories. However gross this approach may seem, due to the large number of species, it does provide interesting data for the trawl fishery and gives an idea of the ecological complexity of this speciose community of closely related fishes. In later papers the ecology of some of the numerically important species will be discussed separately.

MATERIALS AND TECHNIQUES

The investigations were made in the northern part of the Mwanza Gulf and its bays, mainly the area between Nyamatala Island and