The various species of fish found in inland waters are used by man for a number of purposes. Among these three main uses can be identified: (i) food, (ii) sport, and (iii) ornament.
(i) Food: This is by far the most important use, especially in tropical and sub-tropical areas
far removed from the coast where freshwater fish often contributes a considerable
proportion of the total animal protein within the diet.
Fish for food is either captured directly from natural waters or is reared through a variety of extensive or intensive aquaculture techniques.
After it is landed, fish is either consumed fresh or is treated in a variety of ways to ensure longer preservation and a range of subsidiary alimentary products, such as fish oils, pastes or sauces may also be prepared.
The location of the main river fisheries of the world is indicated in Fig. 5.
(ii) Sport: Recreational fishing is widespread in industrial nations of the temperate zone and the practice is now spreading to other areas where there is heavy urbanization. To some extent this use of the fishery resource overlaps with food as in most countries fish caught by angling is also consumed.
(iii) Ornament: The beauty and interesting behaviour of many species of freshwater fish coupled with their small size has favoured their use for ornament or hobby. Several hundreds of species of small tropical species are regularly caught and exported for this purpose and provide a relatively small and rather localized source of income to the exporting countries.
(iv) Other uses: There are a number of minor uses for inland water fish species, including: disease vector control, where fish such as Gambusia or Lebistes have been introduced to control malarial mosquito larvae; and aquatic vegetation control, where higher plant eaters such as Ctenopharyngodon idella or Tilapia zillii have been introduced into canals or enclosed water bodies to keep down vegetation which has become a nuisance.
In most river systems there are three main classes of fishermen: (i) occasional fishermen, (ii) part-time fishermen, and (iii) full-time fishermen.
(i) Occasional fishermen: Among riverine inhabitants of any river system it is probably true to say that the majority of individuals fish at some time during the year. In rivers where exploitation is mainly for food, occasional fishing activities are likely to take the form of basket fisheries for migrating fish in small streams operated by women or small boys or an annual communal fish drive in a particular floodplain water body that happens to be the traditional common preserve of their village. Such fisheries are almost always subsistence and only rarely produce a surplus for sale on the market and the individuals concerned have very little dependence on the fishery. In the industrial countries of the temperate zone recreational fishermen can best be described by this category.
(ii) Part-time fishermen: This category comprises those elements of the fishing community who have an alternative occupation. This is usually farming and the fisherman/farmer combination is especially common in the larger flood plains of the tropics, where the two activities can be readily combined. Possibly the majority of fishermen exploiting river systems belong in this category and during their fishing phase they produce sufficient fish surplus to their own requirement to be sold on the market.
(iii) Full-time fishermen: Fishermen of this type are found on some rivers but, because of the seasonality of the fishery, they cannot be employed throughout the year at any one location. For this reason they are highly mobile and follow the fish up and down river, from the river bank to the margin of the floodplain, or even shift from system to system depending on the availability of the stock.
Fig. 5 Map showing location of major rivers of the world. Principal floodplain food fisheries are indicated although most rivers shown support either food or sport fisheries.
A great variety of fishing methods has evolved for use in rivers. This is in response to the numerous species in riverine fish communities, the seasonal variability of conditions and the many different types of fishing ground. As species and habitat diversity are apt to increase both with the size of river basins and with decreasing latitude the corresponding diversity of fishing gear reaches its fullest range in the large tropical floodplain fisheries of Africa and Asia.
There are various systems for classifying fishing gear but in rivers a primary division into active and passive gears is perhaps most useful.
(i) Passive gear: Passive fishing methods rely on the movements of the fish to effect their own capture. They are thus particularly useful during times of year when fish are actively migrating, although during times when the fish are not moving certain types of gear such as traps or hooks can be made more attractive by baiting. Typical passive gears are shown in Fig. 6. These include barrages, hook lines, traps and set nets such as gill nets, stow nets and fyke nets. In recent years, the ready availability of cheap twine has favoured the use of gill nets, to the extent that many newly established fisheries are based on this gear and it has come to replace other traditional gears in old established fisheries. This predominance of gill nets has resulted in an increase in the selectivity of many fisheries and a failure to exploit all elements of the community.
(ii) Active gear: Active fishing methods rely on the movement of the gear to effect capture. They are used throughout the fishing year but are most frequently encountered during those times when the fish are quiescent and, therefore, less liable to capture by passive gears. The most widespread traditional active methods are various kinds of dip nets and lift nets (Fig. 7). Large dip nets can be used either as surface nets to catch pelagic species or as trawls for bottom-living forms. As gill nets are tending to replace many passive gears, so cast nets and seine nets are supplanting the active gears. Cast nets are particularly common and a variety of mesh sizes are available which allow them to capture all sizes of fish. Because of their size and cost, seine nets are less common and are restricted to teams of professional fishermen operating from specially cleared beaches.
Because of the differences in capacity of the various types of fishing gear there are well established seasonal patterns of gear use especially in tropical floodplain fisheries. Here, the flood cycle can effectively be divided into four main periods each with its own characteristics.
(a) Rising water: At this time fish are moving actively in the river channel and on to the flood plains. Fishing mainly centres on barrage traps, fyke nets or stow nets installed in the main river channels and in the channels leading on to the flood plain. Gill nets are also used during rising water although they are vulnerable to current and to floating islands of vegetation. In some countries, for instance China, fry produced by spawning in the early flood are collected as they move onto the floodplain for stocking into ponds and reservoirs for aquaculture. Ripe running adult fish are also captured for artificial spawning for the same purpose.
(b) The flood: Throughout high water the fish are very dispersed and fishing is at a minimum, although baited traps and single hook lines are set among the vegetation of the plain, as well as gill nets which are set in more open water lagoon areas.
(c) Falling waters: Fisheries during falling waters concentrate mainly on the young fish returning to the river from the flood plain. Simple barrage traps and earth bunds are used to block major drainage channels and active gears such as cast nets and small seines are particularly common at this time. In addition to the fry caught on the rising flood, young fish caught at this time may be used to stock fish ponds and lakes and in these cases catching methods concentrate on those which catch fish unharmed.
(d) Low water: This period is the most productive for inland fisheries usually giving high catch levels just after bankfull, which decline slowly as stocks are depleted. Active methods such as gill nets, dip nets and small trawls are especially common together with baited longlines or traps. As this period advances there is a tendency for the river channel to break up into a series of pools in the rhithron or into a series of deep and shallow reaches in the potamon. Similarly, water bodies on the flood plain become increasingly isolated and in many cases begin to disappear. In such areas, the use of poisons is common and these are often coupled with massive fish drives using a range of traditional gears such as dip nets, hand nets or pots. In some areas of the world electric fishing is used instead of poisons.
Fig. 6 A selection of passive gear:
Fig. 7 Traditional active gears:
In some rivers, especially those of the North Temperate Zone, fish stocks are used essentially for recreation. Anglers generally concentrate on a few fish species of major sporting interest and these fish are favoured by management policies. Fisheries for favoured species are often maintained by stocking additional fish into the waters and, less frequently, by removing undesirable species. In consequence the population dynamics of such systems are somewhat confused and the effects of the fishery little understood. The behaviour of food fisheries based on a few species is better known and is generally supposed to follow the surplus yield model of Schaefer (see Ricker, 1975). This assumes that the yield from a fish population is related to the abundance of the population and the total fishing effort and predicts that sustainable catch will rise with increased fishing effort until it reaches a maximum value and then will decline (Fig. 8).
In fish communities composed of many exploitable species the situation is somewhat more complicated, because fishing produces effects within the fish community as well as within the stocks of individual species (Henderson and Welcomme, 1974). As shown in Fig. 9, fishing tends to reduce the average age of the stocked fish, which because of the higher growth rate of the younger fish tends to increase the efficiency of food utilization. Acceleration in growth rate and reduction in size of maturation of exploited species is also common but this is eventually not sufficient to counter the effects of fishing pressure. Consequently, as fishing is further intensified, the large, slower-growing and longer-living species tend to be replaced by species of higher turnover rates, which raise productivity by a more rapid throughput of nutrients. During this phase total catch by weight may change little, giving a flattened yield curve, but the fishery is based on a succession of smaller and smaller species. This process is often intensified where size-selective gears are common, as the disappearance of larger species leads to progressive reduction in mesh size.
Eventually, the limits of the fish community to absorb further increases in fishing pressure will be surpassed with the result that the abundance and biomass of fish declines to a point that the fishery collapses at least in its intensive form.
Fig. 8 Theoretical Schaefer curve relating sustainable catch to various levels of fishing effort
Fig. 9 Diagram of changes in multi-species fisheries brought about by increasing intensity of fishing
In the multi-species community, there are always certain elements that are more vulnerable to exploitation because of their behaviour. In rivers the migratory species, which congregate seasonally to undertake extensive movements, often form the subject of specialized fisheries. As a result, stocks of such fishes have been seriously depleted in many areas and in some have become effectively extinct. Apart from such specialized fisheries, fishing in rivers tends to be broad spectrum, using a large range of gears for catching all sizes and species of fish. This means that the process described above may not follow this pattern so strictly as all species are, in theory, affected by the fishery to more or less the same degree. However, because there is a tendency for the fisherman to select for the larger species in the community and because smaller species with short life cycles are more resistant to fishing pressure, the net effect is still the progressive elimination of larger and slower-growing elements of the stock.
Because many other sources of stress to the fish community such as climatic variations, pollution or other environment disturbance, produce similar effects, the various pressures may be cumulative. Thus, a fishery which can resist a given level of exploitation under a certain climatic regime or level of pollution will rapidly collapse if less favourable conditions of rainfall or water quality ensue.
When the intensity of fishing is a direct function of the number of fishermen per unit length of river or unit area of flood plain, some conclusions can be drawn as to the relationship between the fishermen and the fish stock. This is the case in artisanal fisheries, where at any given level of technology the catching power of the individual may theoretically be regarded as standard. Substituting number of fishermen for fishing intensity in the yield curve allows the mean catch per fisherman to be calculated (Fig. 10). Such a yield curve (A) derived from a group of African rivers was somewhat flatter than an equivalent curve, derived from a group of lakes. It also reaches its maximum yield at about 10 fishermen/km² of flood plain, as opposed to the lake fisheries, which reached their maximum at about 1.5 fishermen/km² (Henderson and Welcomme, 1974). The reason for this may be that a considerable proportion of the river fishery is exploited by part-time fishermen who adapt the time they spend in the fishery to its relative rewards.
The curves in Fig. 10 illustrate another principle; if a new factor is introduced into the fishery to improve the performance of individual fishermen the yield curve (B) is displaced to the left resulting in the same total catch from less fishermen. This implies that any development in a fishery which is already approaching full exploitation can only be made at the cost of reducing the number of fishermen or the time spent by the individual fisherman in the fishery. Alternatively, should the same number of fishermen continue in the fishery for the same amount of time total catch will be less than maximal and individual catch rates will also be reduced.
Fig. 10 Theoretical relationship between number of fishermen and catch (above) and catch per fisherman and number of fishermen (below)