FAO FISHERIES TECHNICAL PAPER 262
River fisheries
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FAO FISHERIES TECHNICAL PAPER 262 River fisheries |
by
R.L.Welcomme
Senior Fishery Resources Officer
FAO Fishery Resources and Environment Division
The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.
M-42
ISBN 92-5-102299-2
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FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome, 1985
© FAO
PREPARATION OF THIS DOCUMENT
This technical paper presents an updating and extension of material already published externally to FAO in ‘Fisheries Ecology of Floodplain Rivers’ (Longman, 1979). It is intended as a general summary of the current thinking based upon the literature on all aspects of river fisheries from the physical and biological environment in which they are pursued to their management. As such it is of interest not only to students, scientists and administrators working in the fisheries sector, but also to biologists, ecologists and geographers working on aquatic organisms other than fish and more general aspects of natural resources development and management.
The cover illustration shows a portion of the floodplain of the Magdalena river and illustrates the complexity of the aquatic system in some floodplains.
| Distribution: FAO Fisheries Department FAO Fisheries Regional Offices Selector SI CIFA COPESCAL EIFAC | For bibliographic purposes this document should be cited as follows Welcomme, R.L., River fisheries. FAO Fish. 1985 Tech.Pap., (262): 330 p. |
| SUMMARY |
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. |
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Rivers have formed nuclei for human settlement from the origins of mankind. Many of the earliest civilizations emerged upon the fertile floodplains and since about 5000 b.p., when the earliest systematic colonization of the Nile, Mesopotamic, Indus and Chinese rivers occurred there has been a concentrated effort aimed at the domination of the hydro-logical regimes for the benefit of agriculture. The Roman culture and later that of Western Europe impounded many smaller rivers for water power. In many of the more arid parts of the world streams and rivers were manipulated to provide water for irrigation. These trends have increased until the present day efforts at impoundment, deviation and canalization have left few rivers with undisturbed channels. To the environmental impacts of hydraulic engineering must be added the hazards of the contamination of the waters with a variety of agricultural, domestic and industrial chemicals. In addition bad, or non- existent basin management, deforestation and farming of marginal hill slope lands has increased erosion and the silt loads of rivers resulting in rapid modification of the lowland reaches of the river. These changes not only modify the environment, depriving the fish of living space and access to parts of the river necessary for the completion of their life cycles, but also changes in the quality and quantity of the water in which they live.
The fish communities of rivers have provided the basis for fisheries presumably from the earliest phases of human occupation of river valleys. The FAO Yearbook of Fishery Statistics (FAO 1984) the shows the nominal catch of the worlds freshwaters to have grown from 7.1 million tons in 1977 to 8.9 million tons in 1983 representing 10.4 and 11.6 percent of total world catch respectively. The relatively slow rate of increase of 3 percent per year would seem to indicate that catch levels are reaching a maximum. Much of the present inland catch still derives from rivers or the seasonally inundated ground associated with them especially in Latin America and South East Asia where large lakes are rare. The increase in the number of uses competing with fisheries has brought about the disappearance of some long established fisheries and others are on their way to extinction.
In general studies of fish communities have lagged behind those of lakes and reservoirs although there has been an increase in interest in this topic in the last decade. Practical concern with the management of rivers for fisheries began towards the end of the last century in North America and Europe and led to research in such waters in support of stocking and physical improvement programmes principally in support of srort fisheries for salmonids. These led to early classifications of rivers into zones in Eastern Europe where commercial fisheries for coarse fish were also economically important. One of the earliest systematic studies on large rivers were those of Antipa (1910) whose original work on the Danube was continued by other workers until it has become one of the most extensively studied of the worlds major rivers. Antipa's general conclusion that the fisheries production of the Danube was directly proportional to the extent and duration of flooding in any particular year (Botnariuc, 1968) has proved to be equally applicable to all other flood rivers investigated. The work on the Danube also illustrated that the floodplain cannot be considered in isolation but must be treated as an integral part of the larger system (Botnariuc, 1967; Balon, 1967). Somewhat later Russian workers commenced studies on the Volga river although this work intensified only after the creation of the cascade of reservoirs in that system and only part of the literature is available in translation. Detailed studies of the Mississippi-Missouri system were further delayed and it is only in the last decade that understanding of that system has been obtained. Modern work on these and other temperate rivers is now highly complex and deals with many biological and ecological issues especially those connected with the conservation of riverine habitats.
Systematic study of the fisheries ecology of tropical rivers began on the Niger when a laboratory was set up in the Central Delta (Blanc et al., 1955) whose output through the numerous publications of Daget clarified much of the taxonomy and biology of fish in that river several FAO projects have been associated with the study of this river in Niger, Benin and Nigeria. The Nile Sudd in Sudan has been studied by a series of missions including the Jonglei Investigation Team (1954) and Mefit Babtie (1984). Intensive but short term duration studies on the Kafue river by the Universities of Idaho and Michigan did much to shed light on the biology of the fishes of the Kafue flats. The ORSTOM team studied the Yaeres floodplain of the Logone River during the 1970's and elsewhere workers have been gathering information on the fisheries and general ecology of the Shire river, the Okavango delta (Botswana Society, 1976) and components of the Zaire river.
In South America most of the river systems have been examined to a certain extent. The numerous works of Bonetto and his team have provided a great amount of information on the Parana river and its tributaries while Godoy (1975) summarized an extensive amount of work on the Brazilian Mogi Guassu tributary of the same system. In the Amazon staff of the Instituto Nacional de Pesquisas Amazonicas have studied the area around Manaus and an FAO funded project enabled the Peruvian authorities to collect information on the same river at the level of Iquitos. Work on the Orinoco was begun by Mago-Leccia (1970) and has been continued by Novoa and his co-workers. Lowe-McConnell carried out fundamental studies on the ecology of tropical river fish communities in the Rupununi river. Surveys of the fisheries of the Magdalena river were started by INDERENA and intensified through the activities of an FAO project.
Studies on Asiatic rivers have been somewhat more limited although work on the Mekong from Chevey et Le Poulain (1940) onwards have contributed to the general understanding of large tropical systems. Otherwise occasional studies have been carried out on rivers in Peninsular Malaysia, Borneo, India, Sri Lanka and the Mesopotamic rivers. Doubtless there is abundant literature on the major Chinese systems but this is not available in translation at this time although some Soviet studies on the Amur river have been published.
All these works and many others combine to give a body of information on the fish and fisheries of the world as presented in this Technical Paper. This information indicates that while there is a considerable amount of diversity in rivers, some general conclusion can be drawn. Firstly, although potamon and rhithron differ considerably the form and behaviour of these two major river zones appears consistent irrespective of continent or latitude. In other words rhithron reaches resemble one another wherever they are as do potamon reaches and each may be considered as forming a set permitting information and data to be pooled irrespective of geographic origin. Secondly, the dynamics and behaviour of fish communities in the potamon of flood rivers is different from those of reservoir rivers (and those whose flows have been artificially modified by man). Thirdly, the biology and ecology of many of the fishes in flood river is so finely attuned to the seasonal flood that modifications to the hydrological regime produce changes in the composition and productivity of the fish community consistant with the conversion from flood to reservoir conditions.
This technical paper summarizes the present state of knowledge on the fish and fisheries of river systems although certain limitations are imposed by the need to contain the size of the document. It emphasizes the role of rivers as food producers and, although other uses of the fish communities are referred to, detailed analysis of the extensive literature on sport fishery management and practice is omitted. The first section deals with the physical and chemical environment and briefly summarizes those aspects of primary and secondary productivity of rivers in so far as they are significant for fisheries. A complete treatise on the limnology of running waters is inappropriate here and would require a volume this size for this topic alone. The analysis of the biology and ecology of the fish concentrates on larger rivers as these are the locations of the major fisheries and, although mention is made of the biology of rhithronic communities, the extensive literature on salmonid rivers is not developed to the full as this has been published elsewhere. Consideration of the fisheries of rivers is of necessity speculative as the quantity and quality of the statistical data available impose limits on the depth of analysis. Nevertheless fishery scientists are increasingly called upon to make rapid proposals for management strategies or evaluations of possible impacts of interventions by ather users within the river basins so some generalized models are needed. Clearly such models must be open to criticism and eventual modification in the light of future experience. Similarly, the section on management tends to examine the effects of management on the fish community and does not explore fully the social and economic implications of the various approaches to the regulation and improvement of fisheries in rivers, a more detailed treatment of these topics being found in more specialized works.