River basins
Rivers may be conveniently divided into fast-flowing, upland reaches and slow-flowing reaches on the lowlands. Rapids reaches tend to divide into pools and riffles, which alternate along the river channel. The lowland reaches are normally flanked by floodplains, which often extend over large areas. To a great extent the ecology of a river is conditioned by its flood regime and the living organisms of the system including the fish are highly adapted to these seasonally repetitive events. Because of the extent of the flooded zones and the increase in production which occurs during flooding, the lowland reaches of rivers usually support fisheries of exceptional value.
The fishing communities which exploit rivers are equally adapted to the changes in flood regime and migrate from one area to another within the system or have alternative occupation for the flood periods when fish are less available. The fishing gear also tends to be adapted to the seasonal changes occurring within the fish communities, particularly where such fisheries have existed for a long period of time. Normally, when fish are moving actively within the system during the period of the rising flood or when the waters are returning to the main channel, fishing with passive gears is preferred, whereas active methods are more common during the low water period when the fish themselves remain relatively inactive.
There are many methods which may be used for the evaluation of fisheries or of fish stocks. Without doubt because of the linear character of rivers and the great spatial dispersion of the individual elements of the fishery (fishermen, villages, canoes, etc.), evaluation is difficult and requires much time and money. In general, there are three main levels at which fisheries or fish stocks may be evaluated. The first consists in using rapid methods for the approximate evaluation of fish stocks based on extrapolation from other similar river systems. The second consists in the evaluation of the fishery through frame surveys and catch assessment surveys. The third level consists in more traditional methods for assessment of the stocks by experimental fishing and traditional stock assessment methodology.
In addition to fishing, there are many other communities which use the water and, consequently, affect in one form or another the fish communities and the fisheries based upon them. Changes in the quality of water through pollution exercises a direct effect on the fish, whereas other processes such as sedimentation can produce changes in the morphology of the river and thereby less directly the fish populations. Because the fish communities are so well adapted to seasonal flooding and depend on it for their reproduction and feeding, whatever activity tends to alter flow also influences the behaviour and abundance of the fish. Thus, hydraulic works, in general, whether carried out for the generation of electricity, flood control, facilitation of navigation or agriculture, are all prejudicial to the fishery in one form or another.
There is a range of management techniques to compensate for the effects of these changes and to ensure that the stocks of fish within a river be utilized in the best way. These include initially the careful control of the fishery within the natural environment but tend to become increasingly concerned with the control of those abiotic factors whereby the aquatic system is controlled. In systems that are completely controlled it is normally necessary to consider new types of fisheries whether these be in the new reservoirs created by the dams or by other forms of intensive or extensive aquaculture in the plains downstream.