Inland Fisheries

3.1.3.1 Adaptations of fishes of potamon zones

There are two main adaptations which enable fish to survive the conditions during low waters.

· Species which is specifically adapted to resisting low dissolved oxygen concentration. The adaptations may be in the form of auxiliary respiratory organs for using atmospheric oxygen as in the case of such fishes as Clarias or Notopterus, or may be physiological as with Carassius or even behavioural as with many cyprinodonts. The same species often have a capacity to support high temperatures. They generally have complex breeding habits with multiple spawnings, a great degree of parental care, and only migrate laterally between the dry season habitat in the main river channel or the standing waters of the flood plain and the flood season habitat in the inundated area.

· Species which uses the rich habitat provided by the flood plain during the flood but escapes the severe dry season conditions by lateral movement off the plain and longitudinal migration within the main river channel to an alternative habitat. This is usually located in the deeper regions of the main river channel, but may also be in the sea or some other large water body adjacent to the river system. A certain proportion of these species move upriver, even as far as the rhithron zone. Such fishes show few adaptations other than a capacity for fast and sustained swimming. Their breeding strategy is generally simple, relying on a single release of a large number of eggs, either on the flood plain or in the headwater streams. To accomplish this they may undertake migrations for very long distances up-and down-river

Main characteristics of a river showing A-A Rhithron-like zone, and B-B Potamon zone