In the meteoritic crater lake—L. Bosumtwi—and its inflowing streams there are 11 species of fish of which four are cichlids and one of these is endemic. The zonal distribution, feeding and reproductive habits, as well as morphological adaptations of the fish are described. The species show a remarkable zonation from the upper reaches of the permanent streams to the limnetic zone of the lake, with each species occupying a well defined habitat. The cichlid populations dominate the lacustrine environments, while the non-cichlids predominate in the riverine situations. This distribution pattern is related to the distribution of food resources, the availability of breeding sites and also to the possession of suitable body form or some behavioural characteristics. Despite the presence of two mouth brooding planktonic feeders in the lake, the need to spawn on a firm substrate, together with the surface feeding habit of the juvenile cichlids, makes none of the fish completely independent of the shallows. Cohabiting species show well marked differences in their feeding habits and the near rigidity of these feeding habits throughout the year may be attributed to the stability of the environment.
The fish populations in a natural tropical lake basin—Lake Bosumtwi, Ghana were studied over a period of 14 months. The basin has a radial drainage pattern with 37 inflowing streams, five of which are permanent, and has no outflow. This situation has produced, over the geological history of the lake, two very different chemical environments: fairly dilute inflowing streams and a relatively concentrated soda lake.
A combination of fishing methods was employed, and these included the use of gill nets, cast nets, hand nets, basket net traps, and hook and line fishing. The fish captured were identified, counted and their total lengths and maximum depths measured. The percentage composition of the food was estimated by a points scheme, and by the number method. Eleven species of fish recorded were from the lake basin. Three of these—Roloffia petersii, Epiplatys chaperi and Amphilius atesuensis—have not been recorded from this basin before.
The distribution of Aufwuchs, phytoplankton, aquatic macrophytes, zooplankton, benthos and allochthonous fauna and plant debris—(the major food items of the fish)—were found to be zoned in the lake basin, with autochthonously produced food items predominating in the lacustrine zones, whilst the allochthonously produced items predominate in the riverine zones.
The distribution of the fish populations was found to be related to the distribution of their food items, with the non-cichlid populations, which feed on allochthonous food resources, predominating in the riverine zones, whilst the cichlid populations, which feed on autochthonously produced food items, predominate in the lacustrine zones.
The juvenile and adult populations offish species investigated rely on very different food resources. The exception to this is Chromidotilapia güntheri.
The fish populations occupying any of the riverine or lacustrine zones, show well marked differences in not only the level at which they feed, but also in their food preferences, especially where two or more co-habitating species occupy the same feeding level. Conversely it was found that species with very similar feeding habits do not occur in the same zone.
Of the five cichlid fishes in the lake two, Sarotherodon multifasciatus and Chromidotilapia güntheri are mouth brooders, whilst the remaining species, Tilapia discolor, T. busumana, and Hemichromis fasciatus are substratum brooders.
The zonal distribution of the fish in the lake basin was found to be related not only to the distribution of their food items, but also to their body form, the behaviour of the fish, availability of breeding sites, and possibly the chemistry of the water.