The Biology and Culture of Tilapias

Microhabitats and the Dynamics of Spatial Separation

Information on spatial separation (horizontal, vertical) of numerous
tilapias and their preferences for a habitat has been obtained either by
analyzing statistics of commercial and artisanal fisheries, or by employing
different sampling methods: trawling, gill nets, traps or rotenone (Ita 1978;
Bruton and Boltt 1975; Balon 1974), echo-sounding (Bruton and Boltt
1975; Capart 1955), direct underwater observations by diving (Bruton and
Boltt 1975) and also by electric fishing in lakes and rivers (Lamarque et al.
1975 ; Lamarque and Closset 1975 ; Pienaar 1968).
The general characteristic of the spatial distribution of tilapias in lakes is
their restriction to the shallow littoral zones (see Table 4). Thus in Lake
Kainji, Nigeria (maximum depth ca. 50 m), S. galilaeus, S. niloticus and 7'.
zillii are caught in the littoral zone, at a depth of 0 to 7 m. The biomass
there attains 107 kg/ha (Ita 1978).
The littoral distribution of lacustrine tilapias originates fundamentally
from their physiological incapacity to descend to great depths (Caulton and
Hill 1973, 1975), a characteristic which supports the riverine origin of
tilapias. But ecological factors also affect this:

• temperature, through on one hand the existence of tolerance limits and
  specific temperature preferences, and on the other hand a direct in-
  fluence on the physiological capacity of tilapias to adapt to depth
  (Caulton and Hill 1975), or to withstand certain unfavorable conditions
  of DO, toxic gases, etc.;


• DO and the presence of dissolved toxic gases (H,S, NH3, C02) which
  restrict the depth penetration to varying degrees according to the
  species and their tolerance and preferences;


• demands and specific preferences for a habitat or determined biotope in
  accordance with feeding (feeding grounds), nesting (spawning grounds),
  guarding the young in mouthbrooders (brooding areas) and protection
  against predators (refuges and shelters: escape range, see Gerking 1959).
  On the whole, substrate-spawners with macrophyte feeding habits are
  more dependent on shoreline habitats (macrophytic vegetation has a maximum
  depth of 7 to 8 m) than are the mouthbrooders with their microphagous
  feeding and more pelagic habits, especially in waters rich in plankton (Lowe-
  McConnell 1959). It is interesting to note that in certain shallow lakes very
  productive of plankton, for example, Lake George, Uganda (maximum depth
  2.5 m), the tilapias (S. niloticus and S. leucostictus) preferentially occupy a
  littoral band of 50 m even though the plankton food is abundant throughout
  the lake (Burgis et al. 1973; Gwahaba 1975). Caulton (1978a, 1978b,1978c)
  has put forward a bioenergetic interpretation of this phenomenon.
  The microseparation of tilapias in rivers is, as in most species of fish, in-
  fluenced by current speed (in accordance with the swimming capacity) and
  by ecological factors which are closely associated with this: the depth, the
  nature of the substrate and the cover of vegetation (Hynes 1970; Brown
  1975; Huet 1959). Gaigher (1973) showed that S. mossambicus avoided the