River fisheries
Rivers drain all but the most arid areas of the earth through channels that are regu- lated by physical laws that impose on them certain forms. The ideal form is rarely encountered in practice and represents an end point to which geographic process tend. In general a river may be divided into two principal zones, the steep and fast flowing rhithron upstream and the sluggish and flat potamon downstream. While conditions in an individual system are highly variable along its length, similar reaches of different rivers differ much less even between continents and at different latitudes. All continents have a series of major river systems which consist not only of the river channels but also the swamps, lakes and seasonally flooded lands associated with them.
Most rivers are highly conditioned by the patterns of precipitation in their basins. Differences in rainfall intensity throughout the year generate a flood wave that progresses downstream in the majority of rivers (flood rivers), although singular geographic circumstances may distribute discharge more evenly throughout the year in some systems (reservoir rivers). The number of reservoir rivers is increasing through flow regulation and dam building. Although the basic nature of the river is determined by the rocks over which it flows, the flood regime seasonally modifies the physical and chemical conditions within the river particularly in the tropics. In higher latitudes other features of climate, such as insolation or air temperature exert an increasing influence.
Seasonal changes in discharge, nutrient concentrations, pH, temperature and dissolved oxygen in their turn influence the composition and abundance of the plant and animal communities inhabiting the river. These changes are particularly marked in the floodplain area of the potamon where the rise in water during the floods inundates extensive areas of land flanking the main channels. This increase in living space, together with the release of nutrients associated with the submersion of the soil produces an annual surge of primary productivity closely followed by an expansion in biomass of animal communities.
The number of fish species inhabiting rivers is a function of the size of the river, with larger basins such as the Amazon having well over 1000 species. The individual species are highly adapted to the conditions in the type of river reach in which they live. Such adaptations are not only morphological but also behavioural and some species have developed extensive migrations to avoid adverse conditions or for breeding and feeding. Alternatively elaborate breeding mechanisms have also evolved.
Other features of the biology of fishes are linked to the hydrological cycles within the river. Thus, the flood is associated with spawning in the majority of species when the abundance of living space and food provides the best conditions for the survival and growth of the young fish. Such is the influence of these factors that in years of more intense flooding survival and growth are so improved that the total biomass of the fish community rises and a strong year class is produced for transmission on to other years. In reservoir rivers and in the rhithron seasonal and year-to-year differences of this type are not so marked.
Fish communities in rivers provide the basis for fisheries which are pursued with a great variety of gear. Fishing intensity is also seasonal and is tied either to variations in temperature or to the flood. Because the fish community can vary in abundance with the fluctuations in flood strength catch is similarly correlated with years of high catch following after years of particularly good flooding. Fish communities respond to increases in fishing pressure in a number of ways. In general catch in rivers having simple fish communities follow a typical yield curve whereas those in which communities are complex show a plateau in catch which may persist over a great range of effort. This plateau masks changes in the composition of the community with a drift from large, slow growing to small, fast growing forms.
Rivers and their basins are used for many purposes other than fisheries. Many of these maodify the qualit or quantity of water in the system and thus interact with the fish communities in the river to their detriment. Management of the river for fisheries therefore becomes increasingly important as the intensity of use of the river rises. Fisheries themselves also require management which may be accomplished either by direct interventions on the fish stock or by legislative or economic activities on the fishermen themselves. As the river becomes increasingly modified the capture fisheries originally pursued there become less viable. Although the water courses may continue to provide food or recreation the major developmental emphasis is towards replacement activities such as aquaculture on the former floodplain or the creation of new fisheries in the reservoirs associated with the main channel.