Fisheries Integrated Development Project
Field observations and information obtained from fishermen indicate that several fish species are implicated in attacks on rice plants in the floodplain of the central delta of the Niger. Of these, only four species attack rice for food and have been shown to gnaw through the stems of plants in the aquarium. Other species only attack rice occasionally or damage the plants during other activities. The damage caused is sometimes extensive locally but decreases upstream. Species feeding on rice seeds knocked from the panicles probably only consume between 1 and 5 percent of the total crop
Des observations sur le terrain et des informations obtenues des pêcheurs nous montrent que plusieurs espèces de poisson sont responsables d'endommager le riz dans les plaines d'inondation du delta central du Niger. Parmi celles-ci, quatre espèces utilisent le riz comme nourriture et il a été démontré qu'elles mangent les tiges des plantes en aquarium. D'autres espèces attaquent le riz qu'occasionnellement ou font des dommages aux plantes durant d'autres activités. Les dommages causés peuvent être importants localement mais baissent en amont. Les espèces se nourrissant de grains de riz consomment entre 1 et 5 pour cent de la récolte totale
Rice culture on the floodplains of African rivers is an expanding practice. Unfortunately in many areas conflict has arisen between the needs for rice and those for fisheries. These arise from two standpoints; firstly, the need of the rice farmer to apply insecticides to protect his crop from pests which may directly damage the fish stock, and secondly, the habit of some species of fish of eating floodplain grasses which result in the destruction of rice. As it is obviously in the interest of good floodplain management to ensure the coexistence of a rice and a fish crop, a study was undertaken in the Central Delta of the Niger in 1969 to investigate the problems caused by fish in the rice paddies of the area and to identify and study the behaviour of the fish which cause the damage. This report, which is based on only a short series of observations, nevertheless clarifies the extent and nature of attacks by fish on rice.
Fish specimens for study of stomach/gut contents, for aquarium experiments and for stocking the experimental field enclosure were sampled in a variety of ways:
With gillnets (1 net 150 m long × 1.5 m deep, with 51 mm bar mesh; 1 net 100 m long × 1.5 m deep, with 31 mm bar mesh; 1 net 50 m long × 1.5 m deep, with 31 mm bar mesh). These nets were always set overnight in the flooding ricefields (e.g., Mopti North Polder).
With trapnets, including 5 large wire mesh (1.6 m × 0.55 m) traps, 2 small (1.0 × 0.45 m) wire mesh traps, over which small-meshed (6.5 mm bar) netting was stretched and 1 small basket trap (1.30 m × 0.30 m). These traps were set in flood-channels, in the floodgates (between the cofferdam beams and the fish screen) and occasionally in the area immediately upstream of the floodgates.
Castnets, 1 with a 19 mm bar mesh and 1 with a 32 mm bar mesh. These were used very successfully everywhere in channels, near floodgates, open pools, etc.
Handnets of various models and small mesh sizes were used, mainly in or near the floodgates, for catching fry.
Fish were sampled with Dieldrin (insecticide) on one occasion in the Mopti South Polder (7-10-69). The area sampled covered approximately ¼ ha.
An electric fish collector was rigged up for the sampling of fry entering the floodgates and channels.
Four 100 × 50 × 25 cm aquaria wherein rice plants were set were used to determine which species attack the rice and cause damage, the nature of the damage caused and at what size they start doing this.
Fish specimens sampled were identified and the stomach/gut contents of all species caught in the rice fields and suspected of causing damage were analysed. This included nature of the material ingested, quantity and state of digestion. Size and weight of the fish were also recorded.
In order to identify fragments of rice plants, various parts of these (rootlets, stems, leaves) were mounted on slides, as well as several other aquatic grasses, for comparison. It was not usually possible to distinguish between cultivated and wild rice in the stomach contents, but identification of rice, especially in the stomachs of the Distichodus spp. proved fairly easy.
estimates of numbers of fish (fry) entering through floodgates, etc., per unit time and of their relative abundance;
direct observation of fish causing damage in the ricefields;
collecting of photographic documentation on rice-eating species, nature, intensity and extent of damage caused;
observations on the construction, maintenance, efficiency of various floodgates, screens, traditional means of protection (barriers) etc.,
incidental observation of other factors having a bearing on the problem of rice-eating fish (e.g., grazing of rice by domestic animals).
This included sampling, observations and discussions with local cultivators/fishermen in other areas besides Mopti. A questionnaire was drawn up and distributed to the agricultural assistants in order to obtain more data. Estimates were made of the extent of damage to crops in the areas visited by making transects through the ricefields in a dugout canoe and noting, over a transect length ranging from a few hundred metres to several kilometres, the amount of damage in a band ±200 m wide (about 100 m on each side). Total area planted was estimated over the same width and percentage damage occurring within this recorded. Distances (in metres) were estimated with the aid of a 200 m telezoom lens which is quite precise up to 50 m.
These are in fact the species most common in the seasonally flooded areas, whether cultivated or not. Nearly all field observations and catch records concerning the Mopti region confirmed Daget's (1954) findings; where these differ (Table I) it is usually a matter of quantity, but the abundance of a particular species is dependent on a number of variables and data over a short period certainly cannot give a true picture.
In the case of Tilapia, Alestes, Hemichromis, Citharinus, Distichodus, Hydrocynus and Lates the proportion of fry juveniles to adults is over 80 percent of the total in particular for Tilapia, where possibly as much as 99 percent of the sample was composed of young forms.
Analysis of stomach/gut contents of fish species known to, presumed to or accused of causing damage to rice crops, showed that on the basis of: (a) preference for the generally more tender rice plants, and (b) absolute abundance in the rice-growing areas (Daget 1954 and Table I) four species emerge as the most destructive: A. dentex sethente, A. baremoze (seasonally, especially on grain), D. brevipinnis and T. zillii. Of these, the last two appear by far the most detrimental because of their habit of nibbling through the succulent stalk at mid-height and at its base respectively, thus cutting down the whole plant, which floats to the surface and is then abandoned for the next stalk. These findings were confirmed by aquarium experiments which clearly showed which species attack rice, the mode of attack, the sizes at which the fish start feeding on the rice plants and the intensity of damage caused (Table III).
Some of these may, under particular circumstances, actively attack and destroy rice plants or seed.
A. nurse, for instance, has been observed (in the field) biting at the leaves of young rice plants whenever these trail in the water (not normally the case) in shallow parts (near the edges). The leaves are seized at their tips and then chewed down.
T. nilotica, when other food is scarce or epiphytic algae growing on the stems of aquatic grasses (including rice) have been scraped off, has been observed nibbling and tearing at the stalks and, in the case of young rice plants or the soft (succulent and spongy) parts of larger plants may nibble right through them. None of the other species has been observed to actually cause damage to rice plants in the field. They did, however, cause minor (incidental) damage in aquaria, at least in the case of A. leuciscus, S. nigrita, S. schall, T. galilaea and T. aurea.
This type of damage appears to be sporadic, minor and merely incidental. Except for one passage cut by Protopterus and isolated uprooted plants (not necessarily by fish), nothing worth noting was encountered in the field. Daget (1954) states that those species which make nests of aquatic weeds (i.e., Heterotis and Gymnarchus) generally breed early in the season, i.e., before the floodwaters spread into the ricefields and usually choose nesting sites in dense Echinochloa stands bordering permanent water bodies (pools, river-arms). Clarias and a large number of other siluroid fish which are benthic feeders and more or less omnivorous are known to dig and probe the mud and detritus in search of food (Daget, loc. cit.) and in so doing may occasionally uproot small plants (larger - over 70 cm rice plants were observed to be very firmly rooted).
Fish with 5 to 15 percent of rice plant remains or grain are considered to be occasional feeders on rice. This classification fits perfectly with the species cited as rice-eating by the cultivators/fishermen.
Rice grains appeared in stomach contents during the latter half of October when rice (mostly the wild rice) starts to ear. Seeds of other grasses were already present during the previous months.
There is an increase in amounts of rice material consumed by Distichodus in correlation with the season and increasing depth of the water. A corresponding decrease in the case of T. zillii is partly obscured by the fact that early in the season few adults were captured and most data apply to juveniles which eat less rice. The amount of rice plants consumed by the Alestes also decreases with time while there is a sudden increase in grain consumption as the rice matures. This was mostly wild rice, however, as the precocious cultivated varieties were only starting to mature when field work stopped.
The experiments carried out showed conclusively which species cause severe damage, as well as its nature and intensity. D. brevipinnis did the most damage over the shortest time, closely followed by T. zillii (especially on the young shoots), then D. rostratus, A. dentex sethente and the other Alestes.
Some species, e.g., D. engycephalus, were not obtained alive and hence could not be observed in the aquaria.
Species not mentioned in Table III, e.g., Labeo, other siluroids, were also studied in aquaria but were never observed to cause any damage to the rice plants.
By comparing the nature of the damage done by known species in aquaria - i.e., the characteristics of the cuts, tooth marks, lacerations, etc. - with the cut ends of stalks found in the field, it was possible to determine in most cases which species had caused the damage.
Alestes spp. (especially A. dentex sethente and A. baremoze) attack the ears (panicles) in order to obtain the seeds, frequently jumping clear out of the water according to fishermen and cultivators. A. baremoze and A. nurse have been observed jumping at Ipomaea flowers and leaves and at Echinochloa seeds hanging near the water surface. During the growing stages, the young plants may be chewed right through, obliquely, at the water line or up to 10 cm below. The floating top is then abandoned to rot but the remaining stalk can regenerate sometimes, providing water levels do not rise appreciably and the water is relatively clear, which is rarely the case. According to some reports (Risbec and Mallamaire 1949) A. dentex feed mainly on moonlit nights. Chevalier (1932a) ascribes most early damage to young plants to A. dentex sethente, but is is in fact Tilapia zillii which is the real culprit.
Distichodus spp. (especially D. brevipinnis) eat the plants and usually cut more or less straight through the stalk in a few bites at or down to 20–50 cm below the waterline. Most cut-off stems seem to be partly eaten however, and soft stems of up to 10 mm in diameter have been seen cut down by these fish.
Tilapia zillii eats the leaves, in fact stripping them off the plant, laying bare the tender stem which is then chewed through (lacerated) obliquely, at the base or, less frequently, at midwater. The floating plant is then abandoned. According to fishermen/cultivators, T. zillii no longer attacks rice after it attains a stalk height of about 1 m with a well-developed leafy head of 40–50 cm emerging. Field observations, e.g., at Mopti and Korientzé (Plate 4) confirmed this and showed that no appreciable damage by Tilapia occurs in water over 60 cm deep. Entire ricefields of several hectares have been reported as laid to waste by T. zillii.
This may be as little as 10 cm in the case of young T. zillii. When water depths are over 60 cm, A. dentex and later, Distichodus spp., start eating the rice, so that it is not until a depth of 1.5–2 m is reached that depredation ceases, although field observations showed very little fresh damage in water over 1.2 m deep. This generally coincides with stabilization of the flood and/or flowering of the rice.
This is extremely difficult to determine because of the variations in growth rate of the rice in relation to soil fertility, time of planting, time and rate of flooding, etc.
As a rule, rice plants stand about 50 cm high when flooding starts (see Viguier 1959). This year, however, plants in most fields around Mopti were only about 30 cm high, some as small as 10 cm. Flooding normally starts early in August and it is during this month and in September that most damage occurs. By the end of September most plants stand well over a metre high and damage decreases substantially. During October and the first half of November the only depredations are those caused by Distichodus (decreasingly so) and during November and December by Alestes eating the ripening panicles.
In other regions, these dates will differ somewhat, both rains and flooding starting a little earlier further upstream and later in the areas downstream from Mopti.
High density of the rice plants effectively prevents fish entering and causing extensive damage. If the rice has been densely seeded, however, and rains are deficient or late, most of the crop will be lost anyhow.
Situation of the ricefield, especially with respect to natural channels, permanent ponds, dead ends of coves, channels or old river arms, proximity to which is courting potential disaster. In some places, where dense stands of “bourgou” (Echinochloa stagnina) or other water weeds entirely surround the field, a fair degree of protection appears to be ensured.
Intense damage, whenever reported or observed always appears to be localized and concentrated and is usually blamed on T. zillii or on Distichodus spp. These appear to occur more frequently in shoals or small groups, even in the adult stage, then other rice-eating species and, since findings indicate that even quite young fish - which do occur in shoals - already feed actively on rice, this may well be exact. (See also Daget 1952, 1954, 1956, 1959.)
No damage occurs in “standing” rice paddies, where water levels are of the order of 50 cm or less, and usually only a small amount of damage occurs in the shal-lower parts of “floating” rice paddies (less than 1 m) (Wertheimer 1954). This can only be explained by the more ligneous nature of the stalks of the varieties grown in these paddies (which are “precocious” varieties, i.e., flowering earlier, and by the fact that owing to their situation on a higher level, water and fish reach them considerably later. Also, by this time the fish are less hungry and more dispersed (Daget 1954 and field/laboratory observations on condition of the fish). These conclusions were reached after examination of various rice varieties at the Ilitémé Station of the IRAT and discussions with Mr. Bidaux.
Likewise, little or no damage is ever reported from areas upstream of Ké Macina, on the Niger and Djenné, on the Bani rivers. A quick visit to San on 19 October revealed that all major rice-eating species are commonly present but that no major damage occurs.
Water flow appears to have the effect of driving fish to travel on. In Tenenkou area, no screens were placed in the floodgates whenever the flood was strong and water flowed right on through the paddies and out of the polder again at the down-stream end. No damage is reported at these times; but if the flood is weak and the flow poor or the water stagnates, severe damage can occur. Whenever such depredations start, the downstream floodgates are opened and within two days the fish move out and travel on (information from agricultural assistant and cultivators/fishermen). The potential danger inherent in this method, however effective it may be, is that it may lead to irregular flooding of the paddies, especially during years of poor floods.
In general, the amount of damage increases substantially the further downstream one goes and this is clearly reflected in the increasing sophistication of protection measures taken by the cultivators further down-river.
In aquaria (Table III) otherwise unfed fish started to attack rice plants at the size of approximately 3–4 cm S.L. for T. zillii, 4 cm for D. brevipinnis (the smallest specimen obtained), and 4–5 cm for A. baremoze and A. dentex sethente.
Alestes spp. up to 9 or 10 cm, when fed regularly, never touched the rice plants. The other two species over the sizes of 5–6 cm for T. zillii, 4–5 cm for D. brevipinnis preferred rice to the alternative diets.
Field observations and stomach analysis data showed that A. dentex and A. baremoze will eat grain from a size of about 5–6 cm upwards and rice plants from 9–10 cm S.L., i.e., fish in their second year (Daget 1952, 1954); Distichodus spp. eat rice plants from 5–6 cm; 13–14 cm and 19 cm S.L. for D. brevipinnis; D. rostratus and D. engycephalus respectively (data for the latter two species are considered insufficient, however, as regards juveniles); T. zillii from 5–6 cm S.L. upwards.
These are the sizes above which at least 25 percent of the stomachs/guts examined contained fresh (not dead) rice plant material.
Second-year (or over) specimens of these species when captured in the rice fields had well over 50 percent rice in their stomachs, clearly showing their preference for this diet within this environment.
These sizes correspond closely to those observed in the aquarium experiments (Table III) with well-fed fish.
Observation of feeding activities of young first-year fish in the field confirmed information gathered from cultivators/fishermen that they generally do not cut down rice plants (except for T. zillii in the case of undersized plants) but inflict minor damage, due to the fact that they move about quite haphazardly (from plant to plant).
This is highly variable and hard to estimate. Observable damage is often severe, concentrated and spectacular (Plates 4–10).
However, field estimates based on transects showed that estimates of overall damage in the areas visited do not exceed 10–15 percent of the crop.
Species said to cause serious harm:
Alestes dentex sethente and A. baremoze, Distichodus spp., Tilapia zillii.
There is general agreement on these species. In all areas visited upstream from Mopti as well as Karbaye, Hyperopisus bebe occidentalis (and sometimes Mormyrus rume and Labeo spp. as well) are blamed for causing considerable damage. They cut or gnaw through the stems of rice plants at mid-height, in 0.5–1.0 m water depth and eat them. It is stated that the stomachs/guts of these fishes are regularly found to contain rice plant remains. No confirmation of this could be obtained, however, either in published data or from the examination of stomach contents. Neither had any cultivators observed the fish in action as they state feeding is nocturnal (this is correct). Other cultivators (e.g., down-river from Mopti) firmly deny that these species eat rice. A possible explanation is confusion of green chironomid larvae, often found in stomachs of these Mormyrids, with fresh leaf or stalk fragments.
The Labeo eat periphyton, filamentous algae and occasional soft-leaved plants (e.g., Myriophyllum, Nymphaea) or rootlets.
Species causing occasional or minor damage (see also Table IV)
Protopterus cuts a passage through (young) rice plants, nipping them off at their base; may also dig up plants and eat some stems as well as grain occasionally. It does this at all depths and at any time.
Alestes spp. (other than those already named in (i)) eat grain everywhere, especially A. nurse which also jumps at the panicles.
Synodontis spp. hook the stems with their serrated spines which can tear or saw them, especially during the early stages of flooding, in or near the deeper parts (e.g., dead ends of entry channels).
S. batensoda, which occurs in shoals swimming upside down near the surface (also observed by expert) is said to cause considerable damage sometimes, when they force their way through the rice; S. schall and S. nigrita can cause damage, the former digging or pulling up the plants at their base, the latter biting them through in midwater - neither species eat any part of the rice.
Tilapia spp. cut and nibble through the stalks at mid-height, especially T. nilotica, in deeper water than T. zillii (0.5–1.5 m) up until the rice flowers. T. aurea and sometimes T. galilaea may do the same.
Practically no fish species was stated to cause damage through digging activities.
Nature, time and extent of damage
The Alestes (A. dentex and A. baremoze) cut the plants near the surface and partially eat the stalk and leaves. When plants reach a height of 1–1.5 m damage ceases. Large A. dentex, however, may continue until flowering starts. Depredations start as soon as water depths reach 30–50 cm and usually cease when it is over 1 m deep. Severe damage can cover several hectares and occur over 1–2 months.
At maturation, these as well as other Alestes start feeding on the seed (and flower ing panicles) jumping clear (up to 1 m) out of the water to slap against or seize the panicle, so that the seeds fall out. Wild rice and local varieties (Oryza glaberrima) have deciduous seed which will fall when the panicle is shaken.
This continues until harvesting, provided enough water is still present at that time.
Distichodus cut clean through the stalks at mid-height or 10–20 cm from the base and eat part of the stalk (and leaves). Depredations start when water depths reach 50–70 cm and continue until the plants reach a height of about 1.5 m or until flowering or stabilization of the flood which usually coincides with flowering (Viguier 1939).
Entire plots of several dozen hectares can be completely ruined and damage can occur over a period of 2–3 months.
Tilapia zillii gnaw, lacerate and cut the young plants, generally near their base (or in midwater), during the early stages of flooding, especially if the plants are small and fragile due to late rains retarding growth or too early flooding. Depredations start in very shallow water (by young Tilapia): 10–20 cm and practically cease when water depth exceeds 50–60 cm. Areas of several hectares may be totally devastated and damage usually occurs over a period of about one month after flooding starts.
In general, damage is most extensive and frequent during the first two months after flooding starts1 and the intensity is higher in the more deeply flooded parts (where depth at stabilization is over 1.5 m) and near permanent pools.
Minimum size at which fish start causing damage
Only adult Alestes (10–15 cm S.L. and over) cut down rice plants. Distichodus spp. start at about 8–10 cm S.L., but only those exceeding 15–18 cm cause serious harm, T. zillii already cut down young plants at 4-(6)-10 cm S.L.
1 Rice paddies remain flooded for 3 to 6 months, usually 4 to 5 (Viguier 1959)
Submersible dikes topped by a fencing of sticks and reeds (Vetiveria nigritana), the latter usually plaited into a sort of mat. Entry points of water into the rice paddy are few in number (usually 2–3) and fitted with a stick and reed barrier as above (sometimes only a stick barrier serving as a screen).
The dike protects the plants against damage (mainly by T. zillii) in the early stages and by the time the flood submerges it, the rice is normally tall enough to withstand attack, except from Distichodus and large Alestes, hence the vetiver barrier.
This type of protection is extensively used in the area downstream from Lake Debo, e.g. Niafounké.
Submersible dikes without reed barrier and with a barrier in the few water inlets. This barrier may consist of:
a mass of branches (Acacia) blocking the channel
a stick and plaited vetiver barrier (or sticks only)
a wire-mesh barrier mounted on sticks, 1.7 × 1.5 to 4 × 4.5 cm mesh sizes (diameter) which last on average two years.
These protective measures are used in the Lake Debo, Korientzé and Kona areas.
Where a natural levée exists between river, arm or channel and the floodplain, water inlets (natural or dug out) to the rice fields situated 10–30 m back of the stream are kept as narrow (at least the artificial ones: 1–2 m) and as few in number as possible and fitted with a simple stick or stick and reed barrier (e.g., area just south of Lake Debo, Tenenkou).
Fish barriers made of reeds or other aquatic grasses and often fitted with traps, at least during the early stages of flooding (the first month), placed in channels leading to the floodplain.
These are abandoned when fishing falls off or often break when the flood becomes too heavy. Such barriers were mostly found in the areas upstream from Mopti (e.g. Soye, Diafarabé, Tenenkou).
Siting of ricefields away from permanent waterbodies (river arms, pools) and from dead ends of natural channels where flood waters tend to stagnate and then slowly spread through dense vegetation (e.g., Kona, Soye, Kolenzé areas).
Leaving a dense stand of wild grasses surrounding the field (mainly Echinochloa stagnina or, where plots are more or less separated/surrounded by a ridge of debris (weeds) from clearing piled up over the years, of Vetiveria nigritana, or others). This is apparently effective in preventing access of fish to the rice fields and varies in width from 1 to 4 m.
Such “barriers” were observed even in areas where the cultivators were not aware of their effect, e.g., Kona, Soye. At Kolenzé, where this and siting of fields (see (v)) is practically the only means of protection this is done purposefully.
In many places an area of open water 1–3 m wide is left around the fields and Gallais (1967) states that this forms a protection since many fish will hesitate to cross open water. Neither field observations nor questioning of cultivators/fishermen confirmed this however.
Whenever floodwaters or rainstorms do not destroy the dikes or barriers, they are reported to be quite efficient (i.e., when asked, peasants report little or no damage). Unfortunately, this is the exception rather than the rule. In some areas e.g., Kolenzé-Ouromodi, no traditional means of protection seems possible other than those mentioned under (v) and (vi), the land being too low, flat and water channels innumerable.
From the data gathered, previously published information, and information obtained from various people in the field (mainly cultivators and fishermen), it appears clearly established that only the macrophytophagous fish species constitute a menace to rice crops. Of these, only four (Table V) are usually present in sufficient numbers to cause visible damage. Hence these are the species against which protection measures must be directed.
As regards the granivorous Alestes spp., the amount of rice grain eaten certainly does not exceed 5 percent of the crop and is probably less than 1 percent. Although no quantitative data could be gathered - the expert had to leave before most of the rice matured - the few field observations made did show that, even when Alestes were present in large numbers (e.g., near flood gate of Mopti South Polder) next to ripening and ripe rice plots, little damage was observable (very few Alestes were seen jumping at the panicles to dislodge the seeds).
Furthermore, the Alestes are the first fish to leave the floodplain, migration starting as soon as the water stops rising (Daget 1952a, 1954, field observations and information from fishermen). At this time most of the rice starts flowering and is not yet mature. Hence most fish will have left the rice paddies by the time the rice matures.
Interviews with cultivators/fishermen (many people practise both rice culture and fishing) corroborated all the above. Most of their observations regarding fish and rice are extremely accurate, but quantitative assessments are vague. The fact that, in general, the Alestes, when jumping at the panicles will only hit them1 rather than seize them, will reduce this damage to practically nil in the case of the high-yielding introduced rice varieties (O. sativa) whose seeds are not deciduous.
1 Probably an adaptation to the fact that the seeds of most aquatic grasses, including the wild rice and the local cultivated rice are deciduous (Chevalier 1932b, Viguier 1939)
The fact that beyond a certain stage of growth and possibly a certain time of the year, the rice is no longer subject to attack is of little practical value, since, in practice, rice has to be flooded when still at a vulnerable stage.
It can be stated that, under average conditions of rainfall and flooding, little or no damage will occur after the water depths reach 1.2 m (plants then standing at least 1.5 m high). This limit may lie somewhat higher - about 1.5 m - in the “deep” rice paddies and will be considerably lower in the marginal ones. The time of cessation of depredations will vary from locality to locality, being dependent on the evolution of the flood (e.g., hardly any damage was observed in the Mopti area after mid-October, whereas severe damage occurred mostly during October and was still taking place in the Korientzé area when visited on 27–28 October 1969.
Reasons for the lack of depredations in the “standing” rice paddies (situated along the edges or on the higher levels of the floodplain) are that by the time the fish reach them they are more dispersed and less hungry (Daget 1954, and field and laboratory observations on condition of the fish). This would also explain to some extent the extreme intensity of damage in certain areas (e.g., the field mentioned by Damien) which appear to be always near the river or near a channel, a dead end or permanent pool, and where damage occurs early in the season, i.e., when fish starved during the dry (low-water) season spread laterally into newly-flooded areas (see also Plate 4). Daget (1954), regarding evolution of condition of the fish, found that this was lowest towards the end of the dry period and that condition improved markedly soon after flooding and lateral migration of the fish.
Also the varieties of rice grown in these marginal areas (and to some extent in the shallower parts of “floating rice” paddies) grow faster (i.e., ripen early) so that the stem of the plant becomes hard and ligneous sooner.
Reasons for absence of damage in upstream areas and progressively increasing damage further downstream are not all clear but the following arguments certainly apply:
Upstream areas are outside the Central Delta proper and population density of the fish there is lower, especially in view of the dry-season (low-water) downstream migrations effected by many fish (Daget, see references).
The Markala dam constitutes a barrier to many fish (Daget 1950).
Water depth in the rice paddies is less and they flood earlier; consequently, rice varieties cultivated there grow faster and become resistant to attack sooner.
Even though the fish may start breeding sooner further upstream, they will be busy spawning at the time the rice is most vulnerable (especially T. zillii) whereas peak spawning will be over further down-river by the time the ricefields flood (see also Daget 1954).
Hence a combination of factors probably explains the pattern of damage to rice crops in the area. Maximum damage will be sustained under a combination of adverse factors such as late rains and early flooding. For this reason, the development of large rice-growing polders with flood-control gates seems imperative in the long run.
A promising line of research is the development of high-yield rice varieties resistant to attack by fishes. So far, “standing rice” varieties and one shallow-water “floating rice” variety (Malobadian: up to 1.20 m maximum depth), all high-yielding O. sativa varieties are known to be more resistant to fish depredations (survey information, field observations, Mr. Bidaux, pers. comm.). If a deep-water, late-flowering “floating rice” variety with a high yield could be developed this would be a major step toward the elimination of fish damage in areas where no efficient protection exists or can be devised. At present, most late-flowering, deeper-water varieties especially the high-yielding (O. sativa) ones have soft, succulent stems, even when these become large and thick, which are easily bitten through or lacerated by fish. Local varieties (O. glaberrima), some of which are fairly resistant to attack, unfortunately have much lower yields. Some of the varieties designated as “Mogo” or “Simo” are fairly late-flowering, deep-water and resistant whilst having quite a high yield and may be worthwhile investigating.
Field observations in the Mopti area showed that considerable grazing of rice occurs by sheep, goats, donkeys and by cattle as well, marginally.
Rice plants which have been clipped through grazing will generally regenerate, but undesirable consequences are also:
Plants will be too short when floodwaters arrive and hence may be drowned.
Regenerated plants will be too small and tender to withstand attack even from small fish (e.g., those which can pass through the screens).
A lot of the damage ascribed to fish in the immediate vicinity of Mopti is, probably, a direct and/or indirect consequence of this grazing. In other areas, less insular than Mopti, where animals have ample grazing space, this is probably rare. Grazing of rice by animals should, therefore, be prevented.
As regards protective measures, the evidence accumulated amply demonstrates that in developed areas, i.e., large polders, insubmersible dikes with fish screens in the floodgates and elimination of permanent pools within them undoubtedly constitute the most efficient protection.
The author is greatly indebted to the many people who collaborated with him during his assignment and who offered assistance, advice and information, especially the following:
Mr. R. Thompson, acting UNDP Resident Representative in Mali
Mr. A. Diara, Chef de Cabinet au Ministère du Développement
Mr. J.D. Keita, Director, Eaux et Forêts
His Excellency, the Governor of Mopti
Mr. O. Sow, Chef de Cabinet du Gouverneur, Mopti
Mr. A. Szabo, FAO/UNDP/TA Fish Processing/Marketing Expert, Mopti
Mr. S. Diawara, Director, Développement Rural, Mopti
Mr. J.M. Bidaux, Director, IRAT, Mopti
Mr. B. Djenapo, President, Coopérative des Pêcheurs, Mopti
Mr. R. Orsini, Project Manager, Ségou
Mr. P. Damien, Bureau d'Etudes, Institut d'Economie Rurale
Mr. Gadelle, Bureau d'Etudes, Génie Rural
Dr. J. Daget, Laboratoire ORSTOM, Paris
The “Commandants de Cercle” and “Chefs de village” in all the areas visited.
The “Chefs du Développement Rural et Agricole” and their assistants in the areas visited.
The expert's FAO colleagues, field staff and many other people not mentioned above.
Brasseur, P., 1964 Bibliographie générale du Mali. Catal. Inst. Fr. Afr. Noire, (16):461 p.
Chevalier, A., 1932 Sur un poisson de l'Afrique occidentale très nuisible aux rizières. Rev. Bot. Appl.Agric.Trop., 12(131):547–8
Chevalier, A., 1932a Les associations végétales du lit du Moyen Niger. C.R.Soc.Biogéogr., 9(78):73–7
Clay, C.H., 1961 Design of fishways and other fish facilities. Ottawa, Fisheries Department, 301 p.
Daget, J., 1950 La passe à poissons de Markala. Bull.Inst.Fr.Afr.Noire, 12(4):1166–7
Daget, J., 1952 Mémoires sur la biologie des poissons du Niger Moyen. 1. Biologie et croissance des espèces du genre Alestes. Bull.Inst.Fr.Afr.Noire, 14(1):191–225
Daget, J., 1954 Les poissons du Niger supérieur. Mêm.Inst.Fr.Afr.Noire, (36):391 p.
Daget, J., 1956 Mémoires sur la biologie des poissons du Niger Moyen. 2. Recherches sur Tilapia zilli (Gerv.). Bull.Inst.Fr.Afr.Noire (A), 18(1):165–223
Daget, J., 1959 Note sur les Distichodus (poissons Characiformes) de l'ouest africain. Bull.Inst.Fr.Afr.Noire (A), 21(4): 1275–303
FAO/UN, 1966 Aménagement rizicole de Mopti sud. Rapport au Ministère du Développement, Mali, juillet, 26 p. (mimeo)
Gallais, J., 1967 Le Delta intérieur du Niger. Etude de géographie régionale. 2 vols. Mem.Inst.Fr.Afr.Noire, (79):619 p.
Risbec, J. and A. Mallamaire, 1949 Les animaux prédateurs et les insectes parasites des riz cultivés en Afrique Occidentale. Agron.Trop., Nogent, 4(1–2):70–6
Viguier, P., 1939 La riziculture indigène au Soudan français. Paris, Larose, 134 p.
Wertheimer, A., 1954 Rapport de tournée (4.10.54). Bamako, Service Administration des Eaux et Forêts
Fish Species commonly present in the Ricefields
(mainly Daget 1954)
|Field survey observations and comments1|
|Protopterus annectens||very common in certain areas||common but largely eliminated in developed paddies through ploughing, cocoons being destroyed|
|Polypterus senegalus||very common||very common in shallow water with dense vegetation|
|P. bichir lapradei||fairly common||fairly common|
|Microthrissa miri||-||common around Mopti, in deeper, more open waters and channels|
|Heterotis niloticus||common in weed-beds around pools (nest)||not caught in ricefields in Mopti area|
|Hyperopisus bebe occidentalis||common in floodplain (spawning)||in rather deeper waters (over 1 m) uncommon around Mopti. Nocturnal species|
|Mormyrus rume||as above||not common around Mopti (common around Soye, Tenenkou)|
|Gnathonemus senegalensis elongatus||common in pools||common in channels and in fields in deeper water (over 70 cm)|
|Gymnarchus niloticus||common in marginal weed-beds (nest)||not common in ricefields|
|Hepsetus odoe||common in pools on floodplain||young fairly common in channels|
|Hydrocynus brevis||common in pools; young in floodplain||young very common in ricefields, especially channels|
|H. forskali||common in pools; young in floodplain||as above|
|H. vittatus||as above||not seen around Mopti|
|Alestes dentex sethente||abundant; young in floodplain||common in ricefields with over 50 cm of water|
|A. baremoze||as above||abundant in water over 50 cm deep|
|A. nurse||abundant in the floodplain||abundant everywhere especially shallow waters|
|Alestes leuciscus||extremely abundant (floodplain)||very abundant; young in shallow waters|
|A. macrolepidotus||common (surface, near edges)||common in surface waters, especially channels|
|Micralestes acutidens||in floodplain (reproduction)||sporadically common e.g., Soye, (channels)|
|Distichodus brevipinnis||very common; severe damage in ricefields||very common, in waters over 50 cm|
|D. rostratus||not common||sporadically common (around Mopti and Soye)|
|C. citharus||common in floodplain, etc.||abundant; channels, shallow edges (young)|
|Labeo senegalensis||as above||very common (young)|
|L. coubie||common (in the area)||not common in the floodplain (only caught among rocks near floodgates)|
|Barbus lepidus||common in floodplain||fairly abundant in shallow waters|
|Barilius senegalensis||very common in area||very common in channels/open waters|
|Clarias anguillaris||very common in pools and floodplain||very common everywhere (young in dense growths of weeds/rice)|
|Heterobranchus bidorsalis||fairly common in floodplain||young in rocks around floodgates|
|Schilbe mystus||very common in pools||very common (young) everywhere|
|Siluranodon auritus||not uncommon||fairly common in floodplain, pools and channels|
|Chrysichthys auratus||common in pools||common in floodplain, pools and channels|
|Auchenoglanis biscutatus||as above||common in pools|
|A. occidentalis||as above||fairly common in pools/channels|
|Synodontis batensoda||as above||common in deeper water (over 1 m)|
|S. membranaceus||as above||uncommon around Mopti|
|S. clarias||as above||fairly common in channels, etc.|
|S. nigrita||as above||fairly abundant everywhere|
|S. schall||common in pools and floodplain (young)||fairly abundant everywhere|
|Hemichromis fasciatus||common in pools and floodplain||young abundant everywhere|
|Tilapia galilaea||common everywhere||young very abundant everywhere|
|T. nilotica||as above||young abundant everywhere|
|T. aurea||fairly common in the floodplain||young common in floodplain|
|T. zillii||very abundant in the area||young fairly abundant everywhere, very abundant in shallow edges|
|Ctenopoma kingsleyae||common||not uncommon in channels|
|Channa obscura||fairly common in pools||rarely seen around Mopti|
|Tetraodon fahaka strigosus||common in pools||common in flood-channels, pools and deeper waters (over 1 m)|
|Lates niloticus||young common in pools||young fairly common in channels, open and deeper waters (over 1 m) among ricefields|
1 These estimates of abundance are based on visual observations, fish counts at entry points and sampling data
Fish Species feeding predominantly on Rice
(Stomach Contents, Aquarium and Field Observations)
(mainly Daget 1954)
|Information gathered from fishermen/cultivators||Personal observations|
|Alestes dentex sethente||grain (young also)|
adults severe damage
|mainly grain (jumps out of water to grab ears)||preferential adult diet: rice plants (stalks/leaves) at present; young omnivorous|
|A. baremoze||idem||idem - plants also||adult and young largely omnivorous with large proportion of rice|
|Distichodus brevipinnis||herbivorous (stalks, leaves) in deeper waters||damage in deep waters||very severe damage even to fairly tall (1 m) plants, in water over 70 cm deep|
|D. rostratus||idem||idem||not a common species|
|Tilapia zillii||herbivorous; young microvorous||severe damage; even in shallow water||shows preference for succulent base of stalk of young (less than 1 m) plant and strips off leaves to reach it. Major damage in water less than 60 cm deep|
|T. SPE1 (“melanopleura”)||idem||-||not seen, very rare|
1 Annotation of unidentified species as used by Thüys Van den Andenaerde (1969)
Observations on Rice-eating Fish
(in Aquaria planted with young Rice Plants)
|Species||Size range studied|
|Size at which fish were first observed eating rice|
|Mode of attack||Intensity of damage||Remarks|
|A. dentex||4–8||(4 – 5)a ± 10||Gnaw and cut obliquely through stem at base of leafy head, near water surface Also grab, pull and nibble at leaves trailing in water||2 – 3 plants cut daily by 1 (adult) and 1 – 3 (young) fish||each specimen or 2 – 3 spec. of ± équal size was observed simply for 2 – 5 days|
|A. baremoze||4.5 – 15.5||(4 – 5)a ± 9||- idem -||0 – 4 plants daily by 1 – 4 fish||- idem -|
(4 specimens, once)
|A. nurse||2.7 – 13||(± 4)a ± 6||1 plant only cut, others nibbled, but leaves chewed and cut when trailing in water||0 – 1 plant daily by 1 – 2 fish||observations on 1 – 2|
specimens over 2 – 3 days
|A. leuciscus||3.5 – 8||± 5||plants gnawed at base of leafy head, mainly on leaf sheaths||no plant cut down, minor damage only||observations on 1 – 3|
specimens over 3 – 4 days
|D. brevipinnis||4 – 21||± 4||bites mouthfulls out of the stems and cuts them down 10 – 20 cm above the bottom||5 – 9 plants cut down daily by 1 fish. Small specimen of 4 cm S.L. only bit pieces out of smaller shoots||observations on single specimens over 2 – 3 days|
|D. rostratus||12 – 19||(12)||- idem -||3 – 4 plants cut down daily by 1 fish||- idem -|
|S. batensoda||5 – 17||(13.5)||scrapes periphyton off stems, leaves; also feeds on bottom||minor damage to leaves, base of leafy head and rootlets||observations on single specimen for 4 – 7 days|
|S. schall||4.5–15||(12)||feeds mainly on bottom, digs up rootlets||minor damage to rootlets||- idem -|
|S. nigrita||4 – 10||7||feeds on periphyton and on bottom||- idem -||- idem -|
|T. zillii||0.5 – 16.5||(3 – 4)a 5–6||young eat periphyton and gradually start nibbling and tearing at tender stalk base. At later stage, tear off leaf sheaths, laying bare stalk then lacerate it||young up to 3 – 4 cm only damage plants, large fish cut down 4 – 6 plants daily||observations on 1 – 5 specimens over 2 – 4 days. Gnawing and tearing of fish at rice plants is clearly audible, even in the field|
|T. nilotica||1.5 – 18||4 – 5||start nibbling and pulling at base of rice plants (rootlets) and at stem, if no otherfood available (periphyton)||minor damage||observations on 1 – 5|
specimens over 3 – 6 days
|T. aurea||1.7 – 13.5||± 4||- idem -||- idem -||- idem -|
|T. galilaea||2 – 15.5||± 5||normally feeds on phytoplankton (on the surface) and on periphyton, as do the other Tilapia, but if no other food present will nibble at stalks, etc., of rice||- idem -||- idem -|
a Size at which otherwise unfed fish started to attack rice plants (Fish were fed on: rice paddy, filamentous algae, water lilies, Ceratophyllum, insects, fishfry)
Fish Species feeding occasionally on Rice
(Stomach Contents, Aquarium and Field Observations)
(mainly Daget 1954)
|Information gathered from fishermen/cultivators||Personal observations|
|Alestes nurse||grain, especially rice|
|grain (jumps)||mainly granivorous and herbivorous|
|A. macrolepidotus||grain and vegetable matter||grain||omnivorous, mainly carnivorous at present|
|A. leuciscus||- idem -||grain||omnivorous and granivorous, apparently never eats rice plants|
|Micralestes acutidens||grain||grain||eats some grain|
|Barbus occidentalis||plant matter||-||not seen, rare around Mopti|
|Labeo senegalensis||-||occasionally eats plants||scrapes biological covering off stems of rice plants and may so doing cause minor damage|
|L. coubie||-||- idem -||- idem -|
|S. nigrita||plant debris and insects||occasionally eats rice plants||fibrous debris of aquatic grasses fairly frequent in stomach contents but rarely fresh and usually picked up on the bottom. May cause incidental damage when scraping biological covering off stems and leaves of rice plants|
|S. schall||omnivorous, including plant debris||occasionally eats rice plants||fibrous debris of aquatic grasses fairly frequent in stomach contents but rarely fresh and usually picked up on the bottom. May cause incidental damage when scraping biological covering off stems and leaves of rice plants|
|Tilapia galilaea||phytoplankton and benthos; damage in ricefields||occasionally rice-eating||both young and adults scrape biological covering off plants and may cause incidental damage|
|T. nilotica||phytoplankton and benthos; damage in ricefields||occasionally rice-eating||both young and adults scrape biological covering off plants and may cause incidental damage, appear less microphagous than preceding species; actively attacks stems of plants (rice) at their base at times (e.g. in aquaria when otherwise unfed)|
|T. aurea||benthos and phytoplankton||- idem -||- idem -|
Fish Species which may cause occasional Damage through other Activities
(mainly Daget 1954)
|Information gathered from fishermen/cultivators||Personal observations|
|Protopterus annectens||digs and cuts plants during nesting activities||cuts rice with teeth to make passage||minor damage observed and nest usually in densely weeded areas, e.g. edges of pools|
|Heterotis niloticus||nest area denuded of vegetation (1 – 1½ m diameter)||does not out rice||nothing observed|
|Gymnarchus niloticus||floating nest of cut grasses (Echinochloa)||cuts rice plants during nesting||- idem -|
|Clarias anguillaris||digs in mud to feed||never eats rice, nor roots||may dig up plants occasionally|
|Schilbe mystus||carnivorous mainly||eats rootlets of rice plants according to some reports||no confirmation whatever|
|Auchenoglanis sp.||omnivorous (benthon)||no damage to rice||only dead fibres of grasses in stomachs, may dig up plants incidentally|
|Synodontis sp.||mud, detritus, plant debris and invertbrates||little damage to rice may cut rice plants with serrated spines when migrating in large numbers||- idem -|
|some fresh fish and rootlets in stomachs but not particularly rice|
Plate 1 Type of damage done to young rice plants by: Alestes (1), Distichodus (2) and T. zillii (3) in aquaria. Note chewed/lacerated ends of stems/leaves in Alestes and Tilapia damage, and clean-cut bites made by Distichodus
Plate 2 Rice plants cut down near Soye by the same fish as in Plate 1. Note stems cut lower down (near base) by T. zillii
Plate 3 Cut ends of rice stems (from Kona) by same species as in Plate 1. Note chewed end of stalk in No. 3 (T. zillii)
Plate 4 Extreme damage, by T. zillii to young rice plants in Korientzé polder, along main dyke, around end of lateral flood-channel (water depth between 15 and 60 cm)
Plate 5 General view of rice paddies (right, background) near Soye. Darker (green) foreground is mainly wild rice. Water depth 0.7–1.2 m
Plate 6 Local rice variety “Mogo” (O. glaberrima) fairly resistant to attack; density normal, water depth 0.8–1.0 m
Plate 7 Patchy damage by fish in local rice variety “Shimou”; water depth 0.8–1.1 m
Plate 8 Severe damage in Indochina variety (O. sativa) probably by T. zillii, several weeks earlier; water depth at time of visit 0.8–1.1 m
Plate 9 Severe damage by Distichodus and some Alestes taking place in Indochina variety; water depth 0.7–1.0 m (freshly out stems floating on surface)
Plate 10 Quasi-total destruction of 2½ ha ricefield (near Sorguéré), mainly by Distichodus, 2 weeks before visit; Indochina and HKG 88 varieties, water depth 0.9–1.2 m
Plate 11 Piled-up dead rice plants downwind from field in Plate 10, after cessation of depredations
J.B.E. Awachie, P.C.O. Ilozumba and W.I. Azugo
Hydrobiology/Fisheries Research Unit
Department of Zoology
University of Nigeria
The freshwater fishery has an important bearing on the lives of many African communities, primarily as an important source of dietary protein and secondly as a source of subsistence income. African rivers and floodplains contain about 2 000 species of fish representing several families (Khalil, 1971); however, not all are economically valuable.
Three factors broadly determine the economic importance of fish size, edibility and abundance. African freshwater fish harbour different species of parasites, most of which are helminth and crustacean parasites. Khalil (1971) stated that 215 adult helminth parasites, 38 larval forms, have been recorded from African freshwater fish. These included 88 species of Monogeneaas, 44 species of digeneans, 39 species of cestodes, 37 species of nematodes, and 7 species of acanthocephalans.
There is, however, a striking dearth of information on the fish parasite situation in many African river systems and floodplains because detailed work has not been carried out in most countries. Available information comes from the work of Khalil in Sudan, Khalil and Thurston in Uganda, Fryer in East and Central Africa, Awachie in Nigeria and Ukoli in Nigeria and Ghana.
At present, there is apparently no record of parasites constituting a serious problem in natural waters in Africa but the need for an extensive study of the ecology of the parasites and their freshwater fish hosts cannot be overstressed since the construction of many man-made lakes is likely to alter the features of the affected river systems and floodplains to the advantage of some of the parasites. Some of them, which by existing records are inconsequential, may develop into pest species in the newly created environments. Indeed, recent observations by Awachie et al. (1976) in Kainji lake, Nigeria, indicate that copepodid ecto-parasites may become an important element in the management of the fisheries, especially Citharinus fisheries, in future. It is noteworthy that these ecto-parasites were not taken in the pre-inundation studies at the Kainji area in 1965.
La pêche en eau douce tient une large place dans la vie de nombreuses communautés africaines, tout d'abord comme source importante de protéines alimentaires et ensuite comme source de revenus. L'on trouve dans les plaines d'inondation et les cours d'eau africains quelque 2 000 espèces de poissons appartenant à plusieurs familles (Khalil, 1971); toutefois, elles n'ont pas toute une valeur économique.
Dans l'ensemble, trois facteurs déterminent l'importance du poisson sous cet angle: la taille, la comestibilité et l'abondance. Les poissons d'eau douce africains servent d'hôtes à diverses espèces de parasites, pour la plupart des helminthes et des crustacés. Selon Khalil (1971), on a observé la présence de 215 formes adultes et de 38 formes larvaires de parasites helmintiques dans des poissons provenant des eaux douces africaines. Parmi celles-ci figuraient 88 espèces de trématodes monogéniques, 44 espèces de trématodes digénétiques, 39 espèces de cestodes; 37 espèces de nématodes, et 7 espèces d'acanthocéphales.
L'on manque toutefois de renseignements sur l'infestation parasitaire des poissons dans maints systèmes hydrographiques et aires d'inondation étant donné que les études détaillées à ce sujet font défaut dans la plupart des pays. Les données disponibles proviennent des travaux effectués par Khalil au Soudan, Khalil et Thurston en Ouganda, Fryer en Afrique orientale et centrale; Awachie au Nigéria, et Ukoli au Nigéria et au Ghana.
Selon la documentation actuelle, les parasites ne constituent apparemment pas un grave problème dans les eaux naturelles africaines, mais l'on ne saurait trop insister sur la nécessité de mener une étude approfondie sur leur écologie et celle de leurs hôtes les poissons d'eau douce, étant donné que la construction de nombreux lacs artificiels va sans doute apporter aux systèmes hydrographiques et aux aires d'inondation des modifications favorables à certains de ces parasites. Plusieurs d'entre eux, qui d'après les données existantes n'ont guère d'importance, pouvaient devenir des espèces dommageables dans les environnements nouveaux ainsi créés. En fait, les récentes observations faites par Awachie et al. (1976) dans le lac Kainji (Nigéria) indiquent que des ectoparasites (copépodes) pourraient devenir un élément important dans l'aménagement des pêches, notamment pour les pêcheries de Citharinus. Il y a lieu de noter que ces ectoparasites n'ont pas été pris en considération dans les études de préinondation menées dans la zone de Kainji en 1965.
River and floodplain fisheries are important aspects of the economic activities of many rural riverine communities in Africa. For most of them, the fish from these sources represent the cheapest available dietary protein as well as the main source of income. This situation has been recognized by many governments in Africa and has led to increased effort to develop the various river basins in order to enhance their fish-producing potentials among other things. Thus, dams are constructed to provide larger bodies of water with increased fishery potentials and separate authorities are created to superintend closely the activities of the various river basins. In Nigeria, for instance, two river basin authorities - the Cross River Basin Authority and the Anambra/Imo River Basin Authority - were created this year to add to the existing ones.
Availabile records put the number of fish species present in African rivers and floodplains at about 2 000 (Khalil, 1971). While not all are of economic importance, a good number are utilized for different purposes by different communities, depending on their size, abundance and edibility. Efforts to develop and manage freshwater fisheries for increased production effects will understandably be concentrated on the economically important species. In this regard, it is considered unfortunate that there is at present scanty information on the parasites of freshwater fish, especially those of economically important species in most African countries. The situation is even more grave when it is considered that the increased number of natural and man-made lakes on the flood-plains may lead to the development of new parasite levels with direct implications for the quantity and quality, and even acceptability, of the fish produced. Their importance therefore touches not only on the health of the fish but also on the welfare of the fishermen and the entire population that depends on these fish as their normal source of animal protein.
The main commercially important freshwater fish species from two areas of Africa - West Africa (Nigeria) and Southern Africa (Middle Zambezi and Lake Kariba) - are given in Table 1.
Inasmuch as the records for bacterial parasites of freshwater fish in Africa are very scanty, it is worthwhile to point out that they exist and have been reported to be associated with a number of fish diseases. One such disease is ‘dropsy’ (Hydrops or Ascites), suspected to be caused by Aeromonas punctata (Olatubosun, 1975). Another bacterium, Aeromonas liquefaciens, has been reported to cause blindness in Clarias lazera (El Bolock and El Sarnagawi, 1975). A streptococcal disease has also been reported in Tilapia mossambica from Taiwan (FAO, 1976). This finding is of great importance to Africa since T. mossambica is one of the cherished species for stocking fish ponds all over the continent.
Fish of commercial importance
|A.||List of fish of commercial importance from various rivers in Nigeria|
|B.||List of fish of commercial importance from middle Zambezi and Lake Kariba|
(after Harding, 1966)
Fig. 1 Distribution of four different genera of crustacean parasites of fish in Africa
Four species of protozoan parasites, reported to be very pathogenic to heavily infected fish, are known to be present in African freshwaters. These are Ichythyophthirius sp., Costia sp., Chiloderella sp., and Trichodina sp., (Sarig, 1975).
Myxobolus exigims causes the knot or pimple infection (Morbulus nodulosus), usually in carps. A ciliate protozoan parasite belonging to the genus Glossatella has been reported to occur abundantly in the fry of Heterotis niloticus in Bangui (Olatubosum, 1975). Other protozoans parasites of freshwater fishes in Africa include trypanosomes in the Sudanese Nile, Congo waters, Natal, French West Africa, Uganda, and Mozambique (Sarig, 1975). Cnidosporidian parasites are reported to occur commonly in the skin and gills of fish in Lake Volta (Ghana) (Sarig, 1975).
Arthropod parasites of fish are represented mainly by the crustaceans. Cunnington (1920) produced a list of the species prevalent in the Great Lakes of East Africa. His list comprised 17 species of which one was unnamed, and one probably a juvenile stage of another listed. Sars (1909), Gurney (1928), Carpart (1944), Harding (1950) and Poll (1953) made contributions to the present state of our knowledge of crustacean parasites of the African freshwater fish species. Fryer (1953, 1968 and 1972) reported the result of an extensive ecological survey of these parasitic crustacean of African freshwater fishes mainly in East and Central Africa. The findings of the above researchers are summarized in Table 2 and Fig. 1.
It is worthy of note that the parasites listed above attack the fish of economic importance and that, although specific fish hosts have been given against the parasites, most of them have a wide host range. Thus, Opistholernaea longa parasitises both Lates niloticus and Tilapia spp, while Argulus Africanus lives on Protopterus, Clarias, Schilbe, Labeo and Tilapia as suitable hosts.
The effects of these parasites on the various fishes are many. Ergasilus, when present in large numbers, interferes with respiration; L. monodi causes proliferation of gill tissues, Ergasilus, Larnaea and Argulus are reported to cause large mortalities in fish ponds (Fryer, 1968); while Sarig (1966) reported mortality caused by Lernaea in ponds in Nigeria.
In African freshwater fish, 215 adult helminth parasites and 38 larval forms have been recorded. This number is made up of 88 species of monogeneans, 44 species of digeneans, 39 species of cestodes, 37 species of nematodes and 7 species of acanthocephalans (Khalil, 1971).
The monogeneans are by far the most important helminth parasites. Their direct life histories and choice of exposed vital organs of fish, such as the gill, eyes and skins, make them worthy of special note as potential hazards in the habitats of fish. Direct development makes them prone to more rapid multiplication in favourable situations while their attach on vital organs makes them possible killers of fish when present in large numbers. Some of the known monogean parasites of freshwater fish of Africa are presented in Table 3.
Distribution of crustacean parasites in various fish hosts in Africa
|COPEPODA||Ergasilidae||Ergasilus sarsi||Tylochromis microdon|
|Levnaidae||Lamproglena monodi||Haplochromis moffati|
|Lamproglenoides vermiformis||Tilapia zillii|
|Afrolernaea longicollis||Labeo cylindricus|
|Lernaeogiraffa hetertidicola||Heterotis niloticus|
|Lernea mflate||Heterotis niloticus|
|Opistholernaea longa||Bagrus, Tilapia|
|BRANCHIURA||Argulidae||Argulus africanus||Protepterus, Clarias, Schilbe, Labeo, Tilapia|
|Dolops ranarum||Tilapia macrochir|
|Chonopeltis schoutedeni||Gnathonemus moeruensis|
|Chonopeltis spp.||Gnathonemus sp.|
|MALACOSTRACA ISOPODA||Cymothoidae||Lironeca enigmatica||Limnothrissa miodon|
Distribution of some monogenetic trematodes in fish from Africa
(after Khalil, 1971)
|FAMILY OF PARASITE||GENUS OF PARASITE||FISH HOSTS|
|CYRODACTYLIDAE||Gyrodactylus cichlidarum||Tilapia galilaea|
|Gyrodactylus ivindoensis||Barbus holotoenia|
|Macrogyrodactylus clarii||Clarias lazera|
|Macrogyrodactylus latesi||Lates niloticus|
|DACTYLOGYRIDAE||Ancyrocephalus synodontii||Synodontis spp.|
|Annulotrema curvipenis||Alestes baremose|
|Annulotrema gravis||Alestes nurse|
|Annulotrema hepseti||Hepsetus odoe|
|Characidotrema brevipenis||Alestes baremose|
|Cichlidogyrus brevicirrus||Haplochromis spp.|
|Cichlidogyrus haplochromii||Haplochromis spp.|
|Cichlidogyrus lagoonaris||Tilapia guineensis|
|Cichlidogyrus tilapiae||Tilapia spp.|
|Cleidodiscus halli||Tilapia shirana shirana|
|Cleidodiscus pikei||Hydrocynus vittatus|
|Dactylogyrus afer||Barbus spp.|
|Dactylogyrus gabonensis||Barbus spp.|
|Dactylogyrus labeous||Labeo spp.|
|Nanotrema citharini||Citharus citharus|
|Heterotesia voltae||Heterotis niloticus|
|Schilbetrema acornis||Schilbe mystus|
|Schilbetrema hexacornis||Eutropius niloticus|
|DIPLECTANIDAE||Diplectanum lacustris||Lates albertianus|
|DIPLOZOIDAE||Deplozoon ghanense||Alestes baremose|
Fig. 2 Sites of some recent man-made lakes in Africa. In addition to the lakes shown on this map, numerous smaller man-made lakes and dams are scattered over the drier parts of the continent. Valuable fisheries have developed on these lakes, especially on Kariba and Kainji
At least 12 species of these parasites have been recorded for Tilapia spp while other commercially important fish like Clarias sp, Lates niloticus, Alestes spp, and Heterotis niloticus have their quota. Most of the recorded cases of occurrence come from Ghana and Uganda. This does not imply the absence of these parasites in other countries and should be taken as a pointer to the need for work in those countries where records are not available.
The digeneans, though not considered as to constitute serious problems, should not be overlooked. Of special importance are the members of the genus Nematobothrium which inhabit the eye socket of various species of Labeo. Awachie (1972) described the possible pathological effects of heavy infection with this parasite on Labeo sp. In one fish with 14 worms, the eyes were found to be disfigured and in a poor pathological condition.
Acanthocephalans, cestodes and nematodes occur in African freshwater fishes in varying degrees. The pathogenic status of acanthocephalans in fisheries has received very little investigation but there have been reports of cestodes and nematodes having pathological effects, including parasitic castration in a number of fish (Sarig, 1975).
The other group of parasites that need mention are the leeches which attach various organs of fish including eyes and nostrils. At Nike Lake, a floodplain lake in Nigeria, leeches destroy large numbers of fry during the flood season (Awachie, 1971). Their position as hazards in fisheries is bound to increase with the construction of dams and consequent development of resevoir fisheries. Leeches inflict wounds on the affected organs, thereby paving the way for secondary infections.
Efficient fishery management should aim not only at maximizing yields but also ensuring the acceptability of the products by consumers. Various parasite species affect fisheries by decreasing the yield, spoiling the quality of fish or rendering them aesthetically unacceptable (Awachie, 1965), thus the control of parasites should be looked upon as a major aspect of the management of river and floodplain fisheries.
From the point of view of fish production, bacteria, protozoa, monogeneans and crustaceans are the most important groups of parasites which attach freshwater fish in Africa. Ecologically, these parasites thrive best in lentic or near lentic water conditions and therefore do not constitute much of a problem in natural rivers, with the possible exception of those fishes that breed in the shallow, quiet water of the banks.
A different situation, however, arises in reservoirs and ponds. When dams are constructed across rivers, as is the case in many African rivers (Fig. 2), the features of such rivers are drastically changed. Downstream, what were seasonally flooded plains may be turned into permanent dry land, due to a sharp drop in the volume of water passing through the dam. Upstream, practically lake situations are created in erstwhile riverine situations as a result of drastic reductions in the speed of water. At the same time, what were seasonally flooded plains are brought permanently under water. The resultant lentic conditions favour the spread of the four parasite groups mentioned above in two ways: (1) by providing an ideal situation for them to grow and multiply, and (2) by increasing the host/parasite contact frequency through the restriction of the environment for the fish. This situation arises and may be more acute in permanent floodplains, ponds and in some seasonal ponds when the flood has receded. A recent survey in the Kainji Lake, Nigeria, has shown the presence of copepodid parasites, which were not recorded in the pre-impoundment studies, on four species of fish, including the commercially important Citharinus spp. (Awachie, 1965, Awachie et al, 1977).
Digenetic trematodes may assume a problem status in reservoir fisheries and ponds. The increase in number of this group arises primarily from the increased number of the snail intermediate hosts which normally occur as soon as reservoirs are formed. Thus the above recent study in Lake Kainji has also shown a remarkable increase in the incidence of Nematobothrum sp. in Labeo sp. and of Breviceacum niloticum in Citharinus sp. since the creation of the lake.
For small permanent ponds a useful technique to control the effect of parasites could be to use pumps in harvesting as is currently done in Niger-Anambra floodplains in Nigeria. Where there is economically important incidence of parasites, the pond should be completely evacuated to expose the pond bottom to sunshine before the next flood. In such cases, only fast-growing fish species have to be used in restocking. Its practicability in other parts of Africa will depend on the availability of the suitable means of evacuating the water.
For reservoirs and large ponds, where the evacuation of water is out of the question, the outbreak of parasitic diseases can only be controlled by the adoption of suitable management procedures appropriate to each case/habitat. The main approaches include:
Continuous monitoring of the biota to detect the earliest signs of important parasite problems and take remedial measures. For this to succeed, trained and competent scientists should be readily available.
Control of the fish density: a suitable cropping procedure is adopted in order to reduce the population of fish and hence reduce the chances of rapid spread of disease. The target fish may be the vulnerable species or the most susceptible year class within a species. Where the life cycle and epizootiology of the parasite is known, the above management actions should be tied up with these to minimize and control outbreaks of disease.
Adoption of relevant sanitation measures: growth of vegetation in lakes, reservoirs and ponds, especially on shores, is to be minimized. This will ensure that parasites such as leeches and snail, intermediate hosts of digenean flukes, are kept under control.
From the foregoing, it can be seen that whichever parasite problem arises, the solution will very much depend on a total and comprehensive knowledge of the biology of host-parasite system within the context of the ecology of the habitat. The use of various biocedes to suppress the growth of snails and other organisms may not be ideal and may indeed by dangerous in many areas of Africa since in many cases such ponds and reservoirs represent the only available source of water for human and livestock consumption.
The present effort at developing African rivers and their floodplains with a view to increasing their fishery potential should take into account the role of fish parasites in fish production in order to achieve optimal results. It has been shown that the most important parasites are bacteria, protozoans, monogeneans and the crustaceans Digeneans and the cestodes are not generally as important, but in certain situations they may pose problems.
Fish problems are shown to be more important in standing water conditions, e.g. man-made reservoirs, as well as natural lakes, ponds and pools, especially in the tropics where the temperature regime favours the rapid cycling of both fish and parasites.
Fish mortalities and consequent loss of production are most likely to occur in cases of heavy infestations with bacteria, protozoan, monogenean and crustacean parasites, not only because these parasites attack the vital organs of fish but also the rapidity with which their incidence build up to epizootic proportions.
An effective parasite control programme, in the tropics particularly, should be incorporated in management procedures and should be based on a sound knowledge of the biology of the host parasite parameters operating in the environment.
The importance of the fish species for stocking floodplain ponds and other lentic waters, as well as the adoption of relevant cropping procedures to minimize parasite problems, is indicated. Biocides are not recommended for parasite/pest control procedures because of their potential danger to humans and livestock.
The need for more and comprehensive information on existing fish parasites in the African region is underlined by current heavy investments by African governments on the integrated development of the rivers and floodplains by the construction of multipurpose dams and the siting of irrigation channels on both the major rivers and their tributaries. These developments will affect the levels of existing bodies of water up and downstream, and consequently the patterns of incidence of those parasites with implications for fishery production.
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