Summary of Part I: The Trophic Interrelationships and Ecology of Some Littoral Communities of Lake Nyasa, with Especial Reference to the Fishes
An account is given of the fauna of part of the littoral zone of Lake Nyasa, at Nkata Bay, and of the ecology and interrelationships of its individual constituents.
The main physiographical features of the littoral zone studied are outlined. The shore at Nkata Bay is partly rocky and partly sandy and includes a small zone in which "intermediate" conditions prevail. A small swampy estuary (Crocodile Creek) is also present.
Each major habitat is quite distinct and each has a distinctive fauna. These faunas are described and the feeding habits and general ecology of the individual species, and particularly the fishes, are described. The fishes, most of which belong to the family Cichlidae, are numerous, both specifically and numerically, and many of them show striking adaptations to their mode of life, particularly in the structure of the mouth and in the dentition. These adaptations are described and illustrated.
Food webs are constructed for the major habitats and their structure is briefly discussed.
As each major lacustrine habitat, and particularly the rocky shore, harbours a large number of closely related species, many of which take similar, or even identical foods, competition might be expected. Data bearing on this problem are analysed.
Whilst most insect eating fishes on the rocky shore take broadly similar foods each appears to show certain preferences which tend to minimise competition. These differences are in some cases accentuated by differences in the vertical or horizontal distribution of different species.
Many herbivorous fishes on the rocky shore take similar or identical foods though in some cases they have different feeding mechanisms. Some species are spatially isolated by virtue of their ecological preferences, but several closely allied species co-exist in the same microhabitat and take the same foods. Such co-existence is at variance with the so-called Gaussian hypothesis and seems to be possible because of a superabundance of the algal food on which these species feed.
On the sandy shore the fishes are usually either specialised feeders occupying an otherwise almost unexploited feeding niche or are general feeders which can probably shift their emphasis from one kind of food to another should any one kind become scarce.
In the intermediate zone the situation is similar to that on the sandy shore but two of the most important species, which are related to rocky shore species, are probably restricted to this zone because of an inability to compete with their relatives on the rocky shore and because of a lack of features suited to life on sandy shores.
In Crocodile Creek the number of species is small and almost all are general feeders.
The importance of predators in maintaining a dynamic balance on the rocky shore is pointed out.
Most species of all groups are restricted to one major habitat. Demand for a certain type of food or substratum appear to be the main factors restricting distribution. The significance of such inter-habitat movements as take place is indicated.
Brief comparison with other areas in the lake is made.
A note on the productivity of Lake Nyasa is given and attention is directed particularly
to the great production of algal Aufwuchs on rocky shores and to the accompanying rich faunas.