In general fish are concentrated in permanent bodies of water in the dry season and spread over the floodplain in the floods. Some species, however, appear to remain in the main river channel throughout their life in some systems. In the Niger, for example (Daget, 1954), there are a number of pairs of homologous species within the different genera, one species of which breeds in the floodplain, the other in the river channel.
Because of the considerable differences in dissolved oxygen concentration in the system, both at low water and during the floods (see 4.2) this factor appears to play an essential role in determining the distribution of fish within the system. In general the more active the species the more it tends to avoid de-oxygenated areas, but many species commonly encountered on the floodplain have adaptations for survival in low dissolved oxygen conditions. These include: cocoon formation in Protopterus sp. and dormant eggs among various Nothobranchius and Aphyosemion species, both of which permit the survival of the species over dry periods. External gill filaments are present in the young of both Heterotis and Gymnarchus and complete external gills are found in Protopterus and Polypterus. Species of Clarias, Heterobranchus, Ctenopoma, Parophiocephalus and Protopterus all have accessory breathing organs which permit aerial respiration. Such species form a group that is well adapted to swamp life and tend to concentrate in the more de-oxygenated small pools and swamps of the floodplain during low water, when other more active species are to be found in the river or larger lagoons. An example of such associations is given in Table V for three size groups of floodplain pools in the Ouémé River (data from unpublished personal observations).
Migrations of fish within tropical river systems are a well established phenomenon and have been widely described from all areas of Africa. Daget (1960) identifies two components of such movements:
Each of these requires a different type of behaviour and probably different sets of physiological stimuli and both must be regarded as “active” migrations. The movement of fish upstream against currents is well known (Whitehead, 1959; Matagne, 1950; Welcomme, 1969), but fish entering the floodplain frequently have to move against equally strong current (Johnels, 1954).
Various authors (Blache, 1964; FAO/UN, 1970; Williams, 1971) classify movements into four main phases correlated with flood regime:
rising water - waters confined within banks; at this time fish may undertake longitudinal migrations within the river channel, either from parts of the river with no floodplain or from adjacent lakes
rising water - waters extending over floodplain; the fish spread by lateral migration over the floodplain
falling water - water draining from floodplain; as the flooded area diminishes there is a movement to the river and other permanent water bodies
falling water - once arrived in the river the fish disperse to the dry season habitats.
Following the dispersal there is apparently a period of stability with little movement.
TABLE V
Differences in species composition of catches from permanent floodplain pools of different areas
| Species | Percentage representation by weight | ||
| Small pools (up to 500 m2) | Medium pools (500–5 000 m2) | Large pools (over 5 000 m2) | |
| Swamp dwelling | |||
| Clarias ebriensis | 72.2 | 20.0 | 1.3 |
| Clarias lazera | 5.0 | 13.6 | 3.4 |
| Ctenopoma kingsleyae | 0.9 | 7.2 | P |
| Gymnarchus niloticus | - | P | 2.1 |
| Heterotis niloticus | - | 26.0 | 2.6 |
| Parophiocephalus obscurus | 23.8 | 27.2 | 1.6 |
| Polypterus senegalus | P | 0.3 | 0.7 |
| Protopterus annectens | P | 0.8 | 0.7 |
| Xenomystus nigri | P | 0.2 | P |
| Generally distributed | |||
| Citharinus latus | - | 0.1 | 1.2 |
| Distichodus rostratus | - | 0.7 | 8.1 |
| Hepsetus odoe | - | 2.3 | 2.6 |
| Hemichromis (2 species) | P | P | P |
| Hyperopisus bebe | - | P | 5.4 |
| Lates niloticus | - | - | 10.1 |
| Labeo senegalensis | - | - | P |
| Mormyrops deliciosus | - | - | 18.4 |
| small mormyrids | - | - | 18.4 |
| Pelmatochromis güntheri | P | P | P |
| Schilbe mystus | - | - | 6.0 |
| Synodontis (2 species) | - | - | 15.2 |
| Tilapia (4 species) | - | 1.6 | 2.2 |
P = Present at less than 0.1 percent
The longitudinal migratory phase in the river may be particularly spectacular, and intense migrations of potamodromous species, particularly of the genus Labeo have been recorded both for the Nzoia and Yala Rivers, Kenya (Cadwalladr, 1965) and for the Luapula (Matagne, 1950), where the fish ascend the rivers from lakes. In these cases there is an additional lacustrine component to the migration.
That movement in rivers can be extensive is demonstrated by Williams (1971) who tagged fish in the Kafue River some of which were returned from up to 120 km away. Fish from the Central Delta of the Niger also migrate for great distances ((up to 400 km - Daget, 1957) and reached the area around Bamako before the construction of the Markala Dam interrupted their passage (Daget, 1960a). Similar long-distance migrations have been noted for the Chari/Lake Chad system (Durand, 1970) and the Nzoia/Lake Victoria system (Cadwalladr, 1965).
There is evidence that migrations onto and off floodplains are not random and that the different species tend to arrive at characteristic times and in particular groups. Thus, Williams (1971) and the University of Idaho (1971) note that Clarias spp., Schilbe mystus, Barbus spp., and Tilapia spp. 1 begin migration earlier than other species in the Kafue River and in some Tilapia females tend to migrate earlier than males. Migrations off the floodplains are even more specifically segregated. There is firstly a tendency for adult fish to leave the plain early, as soon as there are indications of diminishing water level (FAO/UN, 1970). Secondly the young fish leave in a precise order. Fishes sensitive to deoxygenated conditions, such as Alestes, leave the plain first and other more resistant genera such as Clarias or Polypterus at a later date (Daget, 1957). Different species of the same genus may also have different migration times and Welcomme (1969) has shown that two species of Barbus behave very differently in this respect. Durand (1971) also demonstrates that fish leaving the Yaérés floodplain by way of the El Beid River are associated in groups each with a characteristic time of migration.
The reproductive cycle of riverine fish species in Africa is closely allied to the seasons, reproduction occurring almost universally just prior to or during the floods (Blache, 1964; Daget, 1954; Svensson, 1933; Carey and Bell-Cross, 1967; FAO, 1968; Durand, 1970). Fish normally reproduce in the grass swamps at the edge of the advancing floods (Daget, 1957) although a number of species including Heterotis niloticus, Gymnarchus niloticus and Hepsetus odoe, construct floating nests which permit deeper-water spawning. Some species, however, may spawn in the river channel before the water overflows onto the floodplain. This has been noted particularly for Clarias and Tilapia species in the Niger (Daget, 1957). Cichlids from the Kafue River may reproduce before the floods and carry eggs and fry onto the floodplain (Williams, 1971; University of Idaho, 1971; Dudley, 1972) and personal observations also shows this to be true of Hemichromis and Tilapia in the Ouémé River, Dahomey.
Reproductive activity continues for a number of weeks and in some species throughout the entire flood season although it clearly peaks during the earlier phases of flooding and stops entirely by the end of the floods (Daget, 1957). Flooding appears to be essential to the completion of the reproductive cycle of most species and the failure of the floods due to the Sahelian drought has brought about a decline in reproductive success of fish, from the Central Delta of the Niger, the Senegal River and Lake Chad (Stauch, pers. comm.).
There are some indications that the intensity of flooding influences reproductive success as stronger year classes have been noted from those years when particularly intense floods occurred in the Kafue River (Dudley, 1972).
1 Including Sarotherodon sp. (Trewavas, 1973)
The feeding habits of many species of riverine fish have been widely described (Daget, 1954; Svensson, 1933; Blache, 1964; Carey, 1971; FAO/UN, 1970) and the adults cover the complete range of trophic types from piscivorous predators to planktonophages, although species feeding solely on plankton are generally rare in rivers. Juvenile fish appear to feed mainly on periphyton, detritus, zooplankton and small insects. There is considerable seasonal variation in feeding intensity correlated with the flood regime. During the floods the release of nutrients, rapid growth of vegetation and increased availability of other sources of food such as seeds, young shoots, leaves, insects and molluscs form the basis for a particularly intense feeding activity (Blache, 1964). Most fish are in peak condition during this season and this is reflected in high condition factors and abundant fat deposits in the body cavity (FAO/UN, 1970). During low water feeding intensity is reduced (Johnels, 1954; Daget, 1957, 1960) and in certain areas may cease altogether. This phenomenon, at least in part, is responsible for the formation of clear rings on the scales of many riverine species.
According to some authors (Johnels, 1954; Kapetsky, 1974) piscivorous predators are an exception to the above pattern, showing reduced feeding during the initial stages of flooding when fish are widely dispersed on the floodplain and are protected by the heavy growth of vegetation, and increased activity during low water when individuals are crowded together into a diminishing volume of water.
Mean growth rates are given for several species of riverine fish by FAO/UN (1970) and Kapetsky (1974). However, these are very variable and change both seasonally and from year to year.
Seasonal fluctuations: Changes in growth rate may be expected during the year due to the expansion and contraction of the aquatic environment, seasonality of temperature and rainfall and flood-associated changes in the abundance of food (Kapetsky, 1974). Growth rates are usually slower in the dry season (Daget, 1957; Johnels, 1954) and Dudley (1972) found that about 75 percent of the expected first-year growth of juvenile Sarotherodon andersoni and S. macrochir occurs within six months after spawning (about six weeks after peak floods on the Kafue River). Kapetsky (1974) also found little growth in length during the dry season, especially during the period of maximum drawdown. Such differences in growth rate coincide with what is known of food abundance and feeding activity and correlate with the appearance of rings on the scales of most riverine species.
Annual fluctuations: Variations in intensity and duration of flooding give differences in growth from year to year. University of Idaho (1971), Dudley (1972 and 1974) and Kapetsky (1974) have all found high correlations between various indices representative of flood intensity (FI and index 1 - see 4.1), severity of drawdown (indices 2 and 3) and temperature in the Kafue system on first and second-year growth increments of various Cichlid fishes (Table VI). In general growth is positively correlated with the intensity of flooding and poor growth results from more severe drawdown conditions.
Very little detailed information is available on mortality rates of riverine species in Africa, particularly with regard to seasonality.
Annual: Daget et al., (1973) found annual values of Z = 2.691 and 2.656 for two zones of the Bandama River, but few definite conclusions as to the true mortality rate of the fish could be drawn from these figures as they represent total loss from the area, including loss by emigration. In fact a third zone of the river had a negative value of Z, indicating considerable migration into the area. Figures are quoted by Kapetsky (1974) for several cichlid species ranging from 99 percent (Z = 4.6090) for third year T. rendalli to 48 percent (Z = 0.6486) for fourth-year S. andersoni, although it is felt that mortalities as high as 92–99 percent are slightly over-estimated. Nevertheless these figures, together with those of Daget indicate the high mortality rates that are to be encountered in riverine species.
TABLE VI
Parameter estimates for simple and multiple linear regressions of first and second year growth increments of various cichlid species on temperature and on hydrological indices
| Species | Year of growth | Sex | Model | R |
| S. andersoni | 1 | M | Growth (cm) = 0.02 FI + 12.87* | 0.92 |
| 1 | M | TL (mm) = 146.51 - 0.11(HI2)** | 0.94 | |
| 1 | F | Growth (cm) = 0.014FI + 13.4* | ||
| 2 | M | TL (mm) = -29.47 + 1.98(TI)** | 0.90 | |
| 2 | F | TL (mm) = 38.24 - 0.30(HI3) +0.83 (TI)** | 0.93 | |
| S. macrochir | 1 | M | Growth (cm) = 0.2FI + 11.02 * | 0.9 |
| 1 | M | TL (mm) = 130.39 - 0.13(HI2)** | 0.92 | |
| 1 | M | TL (mm) = 130.13 - 0.32(HI2)** | 0.85 | |
| 2 | M | TL (mm) = 74.72 - 0.10(HI3)** | 0.58 | |
| 2 | F | TL (mm) = 14.69 - 0.18(HI3)** | 0.95 | |
| T. melanopleura | 1 | M | Growth (cm) = 0.029FI + 12.8* | 0.80 |
* Dudley, 1972
** Kapetsky, 1974
Seasonal variations: Due to the severity of conditions when water areas are diminishing and fish populations becoming more concentrated, mortality rates in the dry season can be expected to be higher than during the flood phase. At this period fish are more vulnerable both to natural mortalities from predation and environmental stress, and to fishing mortalities induced by the greater accessibility of the stocks. Observations by the University of Michigan (1972) indicate the high losses encountered in floodplain lagoons (between 75 and 85 percent over 3 months) and an overall decrease in biomass of 40 percent is estimated by these workers for a three-month period (July-September 1970) on the Kafue system. The Kafue figures and those of Daget et al., (1973) are very similar, indicating dry season losses of about 15 percent per month.
Kapetsky (1974) models growth and mortality for floodplain systems by rotating and reversing traditional exponential models. This gives low mortality rates for the initial part of the year (flood season) and an increasing mortality toward the end of the year (dry season). While this model is a useful approximation for the purpose of calculating production, it appears likely that mortality curves consist of two or three different components corresponding to the different phases of the flood cycle.
Variation between years: There is, at present, no information on variations in mortality rate between years. It is however known that periods of more intense flooding, which produce better recruitment and growth, also produce improved survival (University of Idaho, 1972). It also appears likely that differences in the amount of water remaining in the system in the dry season, together with its duration, would strongly influence survival from one year to the next. This would also depend on the intensity of the preceding flood as an intense flood with good spawning success followed by a prolonged dry period may result in a greater mortality than would occur during the same dry period if preceded by poor spawning.
In general, little is known of the seasonal and annual variations in mortality rates, and prolonged observations on several systems are needed to clarify this position to any degree.
Many functions of the biology of fish living in floodplain systems vary with the hydrological cycle. Within any one year a period of high recruitment and growth rate, intense feeding and low mortality rate during the floods alternates with a period of high mortality, low growth and negligible feeding and reproduction. Variations in the intensity and duration of the floods, and the severity of drawdown conditions during low water produce corresponding fluctuations in many biological parameters. Thus, year-classes from years of good flooding and slight drawdown show greater numerical strength, growth and survival than year-classes from years of poorer flood conditions.
