Diets and trophic groups

1Characterizing a fish’s diet requires a qualitative and quantitative description of prey found in its stomach. The simplest assessment method is noting the presence or absence of a type of prey in stomachs. The data is then used to calculate the percentage of occurrence which is the ratio of the number of stomachs in which one type of prey is found with the total number of stomachs studied.

2The numerical method consists of counting the number of individuals in a category of prey for the entire sample, then expressing it as a percentage of the total number of prey.

3The volumetric or gravimetric methods consist of determining the relative importance in volume or weight of a category of prey in relation to the total volume (or total weight) of all prey.

4Subjective methods consist of attributing points to each prey according to its presence or dominance in relation to all examined stomachs.

5Lauzanne (1976) proposed the use of a Feeding Index (FI) that uses both the percentage of occurrence (%OC) and the volumetric method (%V):

6This index, which varies from 0 to 100, allows researchers to identify preferences and compare the relative importance of different prey in the diet. It is useful for comparing the diets of a single species according to size, seasons, or biotope, and lends itself well to a clear graphic representation (figure 12.1).

7Following Lauzanne (1977, 1983), a few major trophic categories could be recognized in Lake Chad, and this classification can be extrapolated to most riverine tropical African fish communities.

FIGURE 12.1. Diet composition of Lates niloticus, Bagrus bajad and Synodontis schall in Lake Chad (from Lauzanne, 1976).

Dominant primary consumers

• phytoplanktivorous filter feeders consuming mainly planktonic algae, such as Sarotherodon galilaeus;
• grazers or macrophyte consumers such as Brycinus macrolepidotus;
• browsers eating the superficial layer of sediment usually composed of sedimented algae, or the periphyton growing on rocky substrates, e.g. Labeo senegalensis, Citharinus citharus, Distichodus rostratus;
• detritivores such as some tilapiines;

Dominant secondary consumers

• the zooplanctivores filtering zooplankton with their gill rakers: Alestes baremoze, Synodontis batensoda, Synodontis membranaceus;
• the benthivores, feeding mainly on benthic invertebrates: Synodontis schall, S. clarias, Hyperopisus bebe, Heterotis niloticus. A few species such as Tetraodon lineatus feed almost exclusively upon molluscs;
• surface feeders, eating predominantly terrestrial invertebrates: Brycinus macrolepidotus;

Top consumers

• strictly piscivorous fish, feeding exclusively on living fish, such as Lates niloticus and all Hydrocynus species;
• less strictly piscivore species consuming both fish, shrimps and other invertebrates: Schilbe mystus, Bagrus bajad, and young Hydrocynus.

8In practice, species feed on prey available in the ecosystem they inhabit, and diet can change from one site to another or according to the season. As a result, the above classification is only broadly valid (primary, secondary, top). Given this capacity of adaptation to available resources it is possible, for West Africa, to retain just six broad trophic categories: limnivores, micro- and macrophytophages, zooplanktophages, invertivores, omnivores, and ichtyophages (Paugy, 1994).

9The food habits of African fish have been the subject of much investigation, and the main findings are given below (table 12.I).

TABLE 12. I Selected data on the dominant food habits of adult fish from north tropical Africa.

10In the East African great lakes, fishes have developed particular trophic specializations as part of adaptive radiations. It seems that all available resources are used by Cichlidae, and even used exclusively by them. This leads us to distinguish numerous trophic categories (Witte & van Oijen, 1990; Yamaoka, 1991) (figure 12.2):

• detritivores, bottom feeders eating a mixture of planktonic and benthic components;
• phytoplanktivores;
• epilithic (rocks) and epiphytic (plants) algal grazers;
• phytophagous, eating plants;
• molluscivores, consuming shells and gastropods; including oral crushers and pharyngeal crushers;
• zooplanktivorous;
• insectivorous;
• prawn and crab eaters;
• piscivorous;
• paedophages eating eggs and embryos from other species;
• scale eaters consuming mainly scales from other species;
• external parasite eaters…

FIGURE 12.2. Trophic specializations among haplochromines in Lake Victoria. Relative number of species and relationship between diet and teeth (data from Witte & van Oijen, 1990).

Morphological adaptations to feeding behaviour

11If species ended up occupying equivalent ecological niches in different types of ecosystems, they would theoretically have displayed similar adaptive responses to identical selective pressures. For diet, scientists thus advanced the hypothesis that there could be a correlation between the general morphology of species and their diet. For African fish, studies on morphology-diet relationships are scarce, with most only providing descriptions of the general diet of the taxa being analysed. Nonetheless, a few studies show that there is indeed a significant relationship between diet and species morphology (Winemiller et al., 1995; Hugueny & Pouilly, 1999). More specifically, some morphological characteristics appear to be associated with the type of diet.

12These include, for instance:

• mouth size and body size in piscivorous fish;
• gut length correlated with the behaviour of detritivores;
• orientation or position of the mouth explaining both benthic and detritivore diets.

13Similar results were obtained from observations of the same family (Winemiller et al., 1995) or in a community composed of different taxons belonging to different families (Hugueny & Pouilly, 1999). They thus seem to refute the common idea that ecomorphological studies only provide valid results when they are limited to comparisons between families (Douglas & Matthews, 1992).

14Later in the chapter, we shall provide examples of close correlations between diet and different morphological characteristics.

Head and mouth morphologies

15It has been suggested that one key innovation, the pharyngeal mechanism, was one of the major reasons for the extraordinary explosive speciation undergone by the cichlid fish in the African Great Lakes (Liem, 1973). The sharpest morphological specialization concerns adaptations to the head to collect different types of prey. Adaptive changes in the jaws are particularly striking, for example in the huge difference between the elongated mandibles of the piscivorous Rhamphochromis, and the squat, almost square-cornered jaws of the scraper Labeotropheus (Fryer, 1991) (figure 12.3).

16In many species of cyprinids, there are feeding adaptations in the shape of the mouth. In the so-called Varicorhinus mouth form, the horny (or keratinized) lips are adapted for scraping algae and aufwuchs from submerged rocks or other hard substrates. The complex ventral mouths of Labeo, with homodont pharyngeal teeth which are set close together to form a single grinding surface, are specialized to feed on epibenthic algae.

Length of the gut

17There is a correlation between the diet and the length of the gut relative to body length (Kapoor et al., 1975). Carnivorous fish usually have a large stomach and a short gut (ratio less than 1). That is the case for Lates and Hydrocynus, but also for many invertebrate feeders. Conversely, in sediment browser fish such as Labeo, the stomach is often absent or small, and the gut is usually more than ten times longer than the body length (table 12.II). Generally speaking, omnivorous fish, zooplankton feeders or invertebrate feeders have fairly short intestines, less than three times the length of the body. Furthermore, it seems that there is no significant relationship between these types of diet and the length of the gut (Paugy, 1994).

FIGURE 12.3. Example of adaptive radiation (morphological adaptation of the shape of the mouth) for a few cichlids of Lake Malawi (redrawn from Fryer & Iles, 1972).

Electric organs

18Peculiar adaptations have been developed among electric fish. The catfish Malapterurus electricus is a sluggish swimmer, but it feeds almost exclusively on fish. A powerful volley of high frequency electric organ discharges emitted close to fish can paralyse its prey (see box “Electric organ of Malapterurus electricus”). This explains why M. electricus is a more effective predator of fish in general (Sagua, 1979).

TABLE 12.II. Relative length of the gut and dominant diet in various African fish species (from Paugy, 1994).

Body form and feeding types

19Behavioural strategies for foraging are constrained by morphology, including mouth shape and position as well as body form.

20Webb (1984), while looking for correlations between locomotor morphology and the feeding niche, distinguished three basic functional locomotory mechanisms which may be linked to foraging (figure 12.4):

• to exploit food widely distributed in space and time, fish have to move at speeds that sample the greatest volume for the least expenditure of energy. This applies both to macrophage and filter-feeders that have, generally, a streamlined anterior body and a very forked caudal fin. The typical example for African fish is Alestes baremoze that filters zooplankton in the lacustrine environment (Lauzanne, 1970) and is an excellent swimmer able to perform long-distance migrations.
• to exploit very vagile food, fish have to catch their prey very quickly before they can escape and reach shelter. Therefore, they must be able to accelerate rapidly. In Africa, the typical example is Hepsetus odoe (African pike), which has a flexible body and very secluded dorsal fin to accelerate very briefly but with unbelievable speed;
• to exploit prey sometimes hidden in holes or lying on bottom, fish have to swim in very restricted spaces. The median and paired fins allow better manoeuvrability. Many cichlids fit these characteristics as well as mormyrids, notopterids, etc.

FIGURE 12.4. Ternary diagram “functionmorphology” representing the different types of fish swimming. Angles of the triangle are three major modes of foraging; the species closer to the centre are more generalist (adapted from Webb, 1984).

21It is not possible to combine all the optimum features for the different types of swimming in one fish: the optimum design for manoeuvring excludes the elements that favour high performance in accelerating or in cruising. The three basic design are therefore mutually exclusive. Locomotor specialists represent only a small proportion of fish species and the majority of fish are locomotor generalists.

22However, locomotor adaptations for food capture are of diminished importance among locomotor generalists, and other adaptations (e. g. suction or protrusible jaws) which are vital to food selection and the extension of diet breadth, are more common in foraging.

Prey selection

23The best prey for a predator is theoretically one that, in a given context, provides it with the greatest amount of energy for the least cost of capture.

24Zooplanktivorous fish have two different feeding modes (Lazzaro, 1987):

• particle feeders attack and ingest individual prey that have been visually selected;
• filter feeders do not detect prey individually but retain plankton in their gill rakers. They thus engage in passive selection of prey, and efficient filtration depends to a large extent on gill structure and the size of the particles.

25Field workers have used “electivity indices” (E) such as that devised by Ivlev, expressed as

26the proportion of food item where r_i is in the diet, and p i the proportion of item i in the environment. This index is useful when investigating whether a prey species is or is not being eaten in proportion to its abundance in the habitat, but does not distinguish mechanisms behind any selection revealed.

27In filter feeders like Synodontis batensoda in Lake Chad, the smallest captured prey measured 80 µm, but the proportion of rotifers and nauplii retained by the gill rakers increases progressively with size, up to 260 µ m. The selection of the largest planktonic crustaceans depends essentially on their ability to avoid predators. In this respect, the large cladoceran Moina micrura, with poor mobility, is easily captured whereas the more vagile diaptomids are hardly ever caught (Gras et al., 1981). Meanwhile, large Alestes baremoze adults do not retain small nauplii or rotifers in their gill rakers, but only particles larger than 400 µm. Above 880 µm all planktonic elements are retained (Lauzanne, 1970). These two zooplanktonivorous fish species thus have different behaviours and do not consume the same type of prey in the physical system where they live in sympatry.

28In Lake Victoria, it has also been shown that the decrease in the proportion of shrimp Caridina nilotica in the stomach contents of Lates niloticus is partly a result of larger spaces between gills as the fish grows larger (Hughes, 1992). Beyond 70 cm LT, the spaces are too large to retain the shrimp.

29Benthic fishes can also exert selective pressure on prey species. Three benthivore species of Lake Chad, Synodontis clarias, S. schall, and Hyperopisus bebe, essentially consume small molluscs, as shown in the comparison of size distribution of molluscs in the benthos and in the stomach contents of predator fishes (figure 12.5) (Lauzanne, 1975a).

30Still in Lake Chad, up to 50 mm LS Tetraodon lineatus consumes mainly microcrustaceans and insect larvae, then, up to 125 mm LS, larger lunged molluscs living in vegetated areas (Gabbia sp., Anisus sp., Bulinus spp). Beyond 200 mm LS, T. lineatus only consumes benthic molluscs with thicker shells (Bellamya unicolor, Melania tuberculata, Corbicula africana) (Lauzanne, 1977) (figure 12.6). Similar behaviour has been observed in the lagoon fish Trachinotus teraia (Trébaol, 1991). Up to 125 mm LS, it feeds on Clupeidae larvae and juvenile shrimp, while larger individuals consume mainly Corbula trigona bivalves in Ébrié lagoon (Côte d’Ivoire).

FIGURE 12.5. Comparison of size distribution of two species of benthic molluscs (Cleopatra bulimoides and Melania tuberculata) in the Lake Chad benthos and in the stomach contents of three malacophagous fish species (Synodontis clarias, S. schall and Hyperopisus bebe) (from Lauzanne, 1975a).

31The impact of selective predation on the dynamics of planktonic or benthic natural populations may thus be significant (see chapter Role of fish in ecosystems functioning and chapter Species introductions).

FIGURE 12.6. Ontogenic change in diet of Tetraodon lineatus in Lake Chad (from Lauzanne, 1977).

Ontogenic shifts in feeding behaviour

32Most fish species vary greatly in body size during their lifetimes and often undergo drastic changes in ecology as they grow. Their dietary requirements and their feeding behaviour also undergo changes which are usually related to the ability of growing animals to handle particular food types. Several studies on African fish have highlighted these ontogenetic changes. We have already discussed the case of Tetraodon lineatus. Examples also exist in piscivorous species. Thus, in the Chari River, juveniles of Hydrocynus forskalii are strictly zooplanktivorous up to 30 mm in length. Between 30 and 45 mm, they consume both zooplankton and insects. Over 50 mm, they become frankly piscivorous. In the Southeast archipelago of Lake Chad, the pattern is the same, but the transition from one diet to another occurs in larger sizes (figure 12.7).

FIGURE 12.7. Hydrocynus forskalii: change in the proportion of food items depending on size in the Chari and Lake Chad (Southeast archipelago) (from Lauzanne, 1975b).

33Diet also changes with individual growth in cichlids. A peculiar behavioural and morphological development has been reported for Docimodus evelynae, a cichlid species from Lake Malawi, in which small juveniles (less than 50 mm SL) are facultative cleaners, removing fungi from other cichlids. From 50 to 70-80 mm SL, they are no longer cleaners, and feed upon plankton, insects and aufwuchs. Above 80 mm SL, their dentition changes (the juvenile tricuspid teeth are replaced by adult unicuspid teeth) and adult D. evelynae alter their diet to feed upon scales, fins and skin of their fish prey. Field data has also provided evidence that as D. evelynae grow, they move into deeper water. As a result of the depth restricted distributions observed for many species in Lake Malawi, the cichlids cleaned by juvenile D. evelynae in shallow water are not the same as those preyed upon by adults in deeper water (Ribbink, 1990).

Relationship between predator size and prey size

34For many fishes, there is a prey size that minimizes cost of capture by providing the greatest amount of energy to the predator. This type of relationship has been especially studied in piscivorous fishes, which are the most eclectic from this point of view. Generally it can be observed that the relative prey size (size of the prey in relation to the predator) is greater in small and large predators. The relationship thus follows a U-shaped curve.

35The relationship between the length of Lates niloticus and the size of prey was studied by Hopson (1972). Lates at all lengths are able to eat fish up to approximately one-third of their own length. The largest prey in relative size were eaten by the smallest Lates, 30 to 40 mm TL. Relative prey size fell steeply from 38% in the smallest Lates to 6% in the 30-36 cm TL group. Between 300 and 800 mm TL there was a slight increase in the relative size of prey from 6 to 10%. A marked change was noted in Lates over 850 mm TL with prawns and small characins virtually disappearing from the diet. The result is a steep rise in both relative size and average size.

36In Lake Chad, the mean relative prey size of Hydrocynus forskalii is about 10% for small fish (50-75 mm SL) eating mainly invertebrates, but varies between 20 and 33% for fish from 100 to 400 mm SL (Lauzanne, 1975b) (figure 12.8). A relationship between predator size and prey size has also been recorded for Ichthyborus besse, a less common predator than those cited previously. The mean relative size of prey varies from 20 to 30% for individual fish less than 100 mm SL and from 30 to 40% for I. besse above 100 mm SL. The highest relative prey length observed was 50% that of the predator (Lek & Lek, 1978b).

FIGURE 12.8. Hydrocynus forskalii: relationship between the length of the predator and the size of the prey in Lake Chad (from Lauzanne, 1975b).

Variability in diet of adults

37Different field studies have suggested that many fish may vary their diet according to the seasons or environments. It seems that fish broaden their diet to include less preferred prey as the availability of the preferred prey declines. In other words, when food is scarce, they are less demanding about its quality.

38For example, Synodontis batensoda, a detritivorous fish in the river systems of the Chari (Blache et al., 1964), the Nile (Sandon & El Tayib, 1953) and the Niger (Daget, 1954), was exclusively a filter feeder in Lake Chad before the drought period (Lauzanne, 1972; Tobor, 1972). When the food offered is composed of zooplankton and floating organisms, S. batensoda swim on their backs in order to filter the food at the water surface. If the food offered is composed of benthic detritus and mud, the fish swims in a ventral position and filters sediment. In 1974, during the drying phase of Lake Chad, when the zooplankton became less abundant, lake populations of S. batensoda exhibited both these feeding behaviours (Im, 1977).

39Alestes baremoze is strictly zooplanktivore in Lake Chad (Lauzanne, 1976) and Lake Turkana (Hopson, 1982). In rivers, where zooplankton is scarce, it becomes partly phytophagous in the Chari River as well as in the Niger River but with a significant component of emerging insects (Daget, 1952). In Côte d’Ivoire, aquatic insects and terrestrial invertebrates constitute the bulk of the diet of A. baremoze in rivers (Paugy, 1978), whereas in the Kossou man-made lake, this species is zooplanktivore (Kouassi, 1978).

40More broadly, many species adapt their diets according to the restrictions of the physical system. For samples taken from different ponds of a temporary watercourse in Mali, it was shown that for the majority of species, diet similarities are greater within a sample than between same species from different samples (Paugy, 1994). This indicates that to survive in sometimes extreme conditions, species adapt – within certain limits – their diets to the available dominant resources. We can thus show that when faced with poor food diversity which is also related to poorer quality, species use the most abundant resources, and when prey becomes more numerous, fishes feed on a larger number of prey taxa. In other words, food diversity which depends on the richness of ingested prey is apparently dependent on the prey available in the environment (figure 12.9).

41There is a large body of literature on the degree of trophic specialization of endemic Cichlidae in the East African lakes. It appears that the majority of species can consume a much wider range of food than their morphological specialization would initially suggest.

42In fact, many of the Malawi Cichlidae can use one resource or another, based on availability, and are quite opportunistic (Ribbink, 1990). Species that usually graze on the biological surface layer of rocks can shift to a planktonic diet and even consume eggs or larvae of another species. Contrary to a number of common beliefs, the trophic specialization of these Cichlidae does not limit their dietary range, but could give them a competitive advantage in using certain types of food, particularly during food shortages, given that they are able to switch to a more general or opportunistic diet at any time (Witte, 1984).

FIGURE 12.9. Fish of the Baoulé River (Upper Senegal River in Mali): relationship between the diversity (Shannon index) or the regularity of feeding (ratio between diversity of items and the maximal theoretical diversity for a same number of taxa; also called equitability) and the richness of ingested preys (from Paugy, 1994).

43Other groups have also developed this capacity to share resources by “adapting” their morphology. An example is the Barbus (Cyprinidae) species flock in Lake Tana where each of the 14 species studied has developed a specific food strategy that avoids interspecies competition. The morphological study carried out on these 14 species also led to the creation of a predictive food model (FFM: Food-Fish Model) whose results are consistent with the biological reality observed in the stomach contents of different Barbus (Sibbing & Nagelkerke, 2001).

Feeding activity patterns

44Change in fish activity during a 24-hour period is a well-known phenomenon. It is generally linked to feeding habits. There are species that are mainly active during the day, or at night or during periods of rapidly changing light intensity (dawn and dusk) (table 12.III).

45Major foraging activities include movements between resting sites and foraging areas as well as movements to follow, if necessary, daily activity patterns of prey. For example, vertical migrations of Lake Victoria haplochromines are assumed to follow the migration of the prey Chaoborus larvae which stay in the muddy bottom during the day and migrate to the water surface during the night to feed on the zooplankton (Witte, 1984).

46Similarly, in Lake Kariba, field observations and echo-sounding surveys demonstrated that the clupeid Limnothrissa miodon, which feeds principally on Bosmina longirostris, shows a pattern of diurnal vertical migration similar to that of its prey (Begg, 1976).

47In species that detect prey visually, lunar phases may have an influence on circadian dietary rhythms. For example, in Brycinus leuciscus which usually feeds during the day, feeding activity tends to be limited during the full moon, even though feeding has no nocturnal importance. In fact, it appears that diurnal feeding decreases owing to gregariousness that occurs in preparation for the migratory phase that follows the full moon (Ghazaï et al., 1991).

Food webs

48Food webs are diagrams depicting which species in a community interact in feeding. “Food webs are thus caricatures of nature” (Pimm, 1982) but they give a picture of the processes at work in ecosystems. According to Cohen (1989) “If an ecological community is like a city, a web is like a street map of the city: it shows where road traffic can and does go.”

49A food chain describes the energy transfer through different trophic levels, from producers (vegetation for example) to top consumers. The energy transfers are subject to the laws of thermodynamics: at each link, considerable energy is dissipated from the system in the form of heat. Chain lengths are usually short and include four to five links.

50In aquatic environments, there are basically two major types of food chains depending of the main energy source represented by primary producers or detritus:

• grazing food chain: in lakes, the food webs of the pelagic zone are fundamentally based on the primary production by phytoplankton;
• detritus food chain: in contrast, the food webs in the benthic zone are usually based on organic material, including living and dead organisms, imported from pelagic communities, or of allochthonous origin.

51In small streams and rivers, the food webs are often heavily dependent on allochthonous organic material, falling into the water or washed in by rains (leaves and terrestrial insects for example). In contrast, in large rivers with a slow flow, the primary production of indigenous origin is more important.

Phytoplanktivore communities

52The shortest food chain could be illustrated by simple fish communities feeding on phytoplankton. However, Bowen (1988) pointed out that the majority of African fish do not feed directly on living plant material but depend either on zooplankton or benthic invertebrates for food. True primary consumers only belong to some genera (Labeo, Citharinus, Distichodus, Tilapia, Oreochromis and Sarotherodon), that is to say fewer than 7% of the total species.

53The cichlid Alcolapia grahami is endemic to Lake Magadi where it feeds on benthic filamentous cyanophytes, insect larvae and copepods. It was introduced in the 1950s into Lake Nakuru to control mosquito breeding. There it is the only fish species, and quickly became one of the major filter feeders exploiting the high standing crops of the cyanobacterium Spirulina platensis which provide 95% of its diet.

54Phytoplankton, mostly blue-green algae, is very abundant in Lake George. It was mainly consumed by two species, Oreochromis niloticus and Haplochromis nigripinnis, which largely dominated the fish biomass (Gwahaba, 1975).

Relatively simple trophic webs: pelagic fish communities of East African Great Lakes

55The trophic hierarchy among the pelagic fish of East African Great Lakes is relatively simple because few fish species are adapted to the pelagic zone of inland waters. For example, in Lake Tanganyika the trophic web is mainly organized from phytoplankton which is consumed by zooplankton (Coulter, 1991). The latter is the main food of two clupeid species, Stolothrissa tanganicae and Limnothrissa miodon, which appear from their diet and life history to be narrowly specialized for pelagic life and are key members of the pelagic food chain, linking planktonic and piscivorous trophic levels (figure 12.10). Juveniles of Limnothrissa are zooplanktivores, but adults feed regularly on juvenile and adult Stolothrissa. The two clupeids are the prime food for predators, which are essentially two species of Lates. An alternative is in a phytoplanktonshrimp-fish food web (figure 12.10).

FIGURE 12.10. Food web of the pelagic fish community of Lake Tanganyika (from Coulter, 1991). Heavy arrows for major food preference, grey arrow for secondary food preference.

56It is interesting to highlight that the pelagic food web of Lake Tanganyika is marine in nature: the primary grazer is a diaptomid copepod just as in many productive marine systems, and the primary planktivores (Clupeidae) are also of marine origin, as are the main predators, Lates, which also belong to a family of marine origin (Hecky, 1991).

57In Lake Kivu, the pelagic food web is relatively simple including the introduced zooplanktophage Limnothrissa miodon (from Lake Tanganyika), the microphytophage Haplochromis kamiranzovu, the micropredator Raiamas moorei (feeding on insects but also on juvenile fish), and the piscivore Haplochromis vittatus (Ulyel, 1991).

58Trophic webs can sometimes change without necessarily becoming more complicated. A striking example can be seen in Lake Victoria. In some zones, after the introduction of Lates niloticus, the stock of Haplochromis spp. which served as the essential food for predators has almost totally disappeared (Ligtvœt & Witte, 1991). Since then, although certain species still exist, they are no longer the major prey, and the predator, which now often exhibits cannibalistic behaviour, feeds almost exclusively on a single Cyprinidae species (see chapter Fish communities in East African rift lakes).

More complex food webs of shallow lakes

59In Lake Chad, before the drought of the early 1970s, trophic webs – including in free waters – were much more complex than in the pelagic zones of the great lakes owing to the shallowness of the waters. Lauzanne (1976 and 1983) distinguishes two closely intertwined chains in the open waters of Lake Chad. The first one, planktonic, starts with phytoplankton, and the second one, benthic, is partly of detritic origin. The planktonic network leads to planktivorous Alestidae (Micralestes acutidens, Brycinus nurse, Alestes baremoze), Mochokidae (Synodontis membranaceus, Synodontis batensoda), and Mormyridae (Pollimyrus isidori). In turn, these fishes are prey to several predator species (figure 12.11) that consume relatively few of the species from the detritivore and benthic chain (see also box “Ecopath”). Therefore, as in Lake Tanganyika, zooplanktivorous fishes serve as the main food source of Lates and Hydrocynus. Similar results were obtained in Lake Turkana (Hopson & Ferguson, 1982).

60It should be noted that terrestrial invertebrates that fall on the open waters of Lake Chad, sometimes tens of kilometres from the coast, play an important role in the diets of a few species such as Schilbe niloticus, which on several occasions has been observed to have stomachs full of crickets. This observation has also been made, to a lesser degree, in Schilbe uranoscopus and Bagrus bajad. Similar observations on the diet of Schilbe niloticus have been made in Lake Albert (Worthington, 1932). S. niloticus plays an important role in the diet of other predators in Lake Chad, and in a way represents the main link of a third trophic chain, one that connects external sources to top predators.

FIGURE 12.11. Food webs in the open waters of Lake Chad (modified from Lauzanne, 1983).

61Prior to the introduction of L. niloticus, certain areas on the edge of Lake Victoria provided an example of a relatively complex trophic web.

FIGURE 12.12. Ecopath II: model of Lake Chad during the “normal” period (from Palomares et al., 1993).