The food of Bagrus docmac (Forsk.) (Pisces: Siluriformes) and its relationship with Haplochromis Hilgendorf (Pisces: Cichlidae), in Lake Victoria, East Africa

The diet of the siluriform fish Bagrus docmac (Forsk.) is described in relation to various localities in Lake Victoria, East Africa. Invertebrates which are the principal food of young fish, but generally of little significance in adults, are particularly poorly represented in fishes from Tanzanian waters and most important in those from the Kavirondo Gulf where the ontogenetic changes in diet differ from those found elsewhere in the Lake. B. docmac starts feeding piscivorously at about 10 cm standard length with the transition from a principally invertebrate to a primarily piscivorous diet occurring at 15 cm S.L. In adult B. docmac fish are found in over 95 % of individuals with food. Haplochromis is the all important food type, and Tilapia , the most valuable commercial fish in the lake, is not preyed upon at all. Bathymetric changes with respect to the invertebrate food are noted, and it is shown that piscivorous feeding occurs mainly by day and invertebrate feeding by night. Quantitative estimates of consumption of Haplochromis by B, docmac are computed from the results of a series of 24 h fishing surveys and found to exceed published data from elsewhere. The formula for the computation of a daily ration is given as Log R =2.973 Log L —3.36089 and it is calculated that the mean annual consumption of Haplochromis by B. docmac is approximately 75% of the mean standing stock of the prey fish, as compared with a commercial catch of about 5% of the mean standing stock.     

The growth of Tilapia mossambica Peters (Pisces: Cichlidae) in Lake Sibaya, South Africa

The scales of Tilapia mossambica Peters * from Lake Sibaya show clear rings formed by broken, widely-spaced circuli in the anterior field. In order to determine the rate and time of scale ring formation, the number of circuli in the marginal increment was determined. Five scales from the pectoral region of 2223 fishes caught at regular intervals over two years were examined. When plotted as histograms, the number of circuli in the marginal increment showed a sharp movement to the left in September–October and again in January, indicating that the rings were formed during these months in the majority of fish examined. Pre- and postbreeding feeding migrations, and increases in condition factor, were associated with scale ring formation. Otoliths and opercula were also examined for age determination. Annual length increments were calculated for 450 T. mossambica collected throughout the year, and growth curves plotted. T. mossambica from Sibaya reach maturity after one year at a standard length of about 8 cm in females, and after two years at 10 cm in males. The breeding population had a standard length mode of 11–12 cm in females and 17 cm in males. The maximum final size was about 24 cm.     

Ecological studies on Chaoborus (Diptera, Chaoboridae) in Lake George, Uganda

(1) Chaoborus anomalus and Chaoborus ceratopogones were found together throughout the year in Lake George, Uganda, the larvae forming 65-85% by number of the macro-benthos of the central lake area. (2) First and second instar larvae were wholly planktonic and third and fourth instar larvae mainly benthic during the day, migrating into the water column at night. A proportion of the third and fourth instar larvae were found in the water column during the day in the centre of the lake where light attenuation was greatest. C. anomalus showed a greater tendency to be benthic than C. ceratopogones. (3) Planktonic larvae were distributed in dense patches thought to be cohorts resulting from oviposition by swarms of adult females. The maximum density of planktonic larvae recorded was 31.7 × 10²/m² column. Third and fourth instar larvae were more evenly distributed in the benthos of the central lake area, the maximum density recorded was 79.9×10²/m². (4) First and second instar larvae occurred mainly in the inshore regions, very few were found in the centre of the lake. Oviposition is likely to take place inshore. The patches of planktonic larvae were not stationary. Dispersal of larvae away from the shore may be active or may result from complex oscillations of the water mass produced by storms. In C. ceratopogones the floating egg batches could he carried away from the north shore by currents produced by inflows and prevailing winds. (5) Adults of C. ceratopogones were more numerous during the rainy seasons than during the dry seasons. A similar, though less marked seasonality was found in the occurrence of C. anomalus adults. The biomass of benthic larvae of C. ceratopo-gones was estimated at 387.0 mg C/m² in the dry season, June 1970, and 130 0 mg C/m² in the rainy season, October 1970. That of C. anomalus was constant at 241.7 and 254.3 mg C/m² in June and October, 1970 respectively. (6) No difference was found in the horizontal distribution of the larvae, or in the size and weight attained by the various stages. (7) Interspecific differences were found in the mean number of eggs per adult female (309, C. anomalus and 224, C. ceratopogones); in the nature ofthe egg batches; n i the morphology of the larval mouthparts and pharyngeal sphincters, as well as i n the vertical distribution of the larvae and seasonal occurrence of the adults. On the basis of these differences the co-existing species are thought not to be competing for the same resources.     

Recruitment of juvenile mbuna (Pisces: Cichlidae) to experimental rock shelters in Lake Malawi,Africa

Synopsis Mbuna are species of small cichlids that inhabit the rocky shoreline of Lake Malawi. They are mouthbrooders and do not guard their young. Recruitment to the community first occurs when the female deposits free-swimming young, approximately 11 mm standard length, among the rocks. Juvenile mbuna recruit to any refuge in the rock habitat and in this study, small rock shelters were monitored daily to follow the recruitment and subsequent persistence of young. Initial mortality rate was high; less than 10% of the fish survived more than 3 weeks after deposition by the female. At the two locations studied, recruitment was greater at West Thumbi Island (11 fish per day) than at Otter Point (8 fish per day). However, fewer recruits persisted as residents at Thumbi Island because mbuna young at Thumbi were capable of excluding subsequent recruits to their shelter. Similar priority effects were not observed at Otter Point. Mbuna lack pelagic stages in the life-cycle and the species composition of local adult assemblages is an important determinant of the number and type of subsequent recruits. Priority effects may help maintain stochastic influences on recruitment success and reduce the importance of deterministic processes that are directly linked to community structure.     

Description of a commercially important pelagic species of the genus Diplotaxodon (Pisces: Cichlidae) from Lake Malawi, Africa

A new species of pelagic haplochromine cichlid of the genus Diplotaxodon is described from Lake Malawi, Africa. The species, which attains a size of 14cm s. L., feeds mainly on zooplankton, shows no sexual dimorphism in size or body proportions, breeds throughout the year and lives at depths of 20 to at least 120 m. Approximately 600 tonnes of the species were caught by trawlers in 1990–1991, in the south–eastern arm of the lake alone. It appears that this species is a major component of the pelagic community, and occupies an ecological role similar to that of the clupeid Limnothrissa miodon in Lake Tanganyika.     

The angular orientation of the black eye-bar in Haplochromis burtoni (Cichlidae,Pisces) and its relevance to aggressivity

Summary The effect of the black eye-bar in Haplochromis burtoni on the attack rate of conspecific males was found, using dummies, to depend on the angular orientation of this bar with respect to the body axis of the fish (Figs. 1, 4): The increment in attack rate increases as the black eye-bar parallels more closely the profile of the forehead and decreases as the bar approaches an orientation perpendicular to the forehead. This is valid independent of the actual body posture of the dummy bearing the black bar as could be shown by presenting dummies in a horizontal as well as in a head downward position. However, the head-down posture itself was found to cause an extra increment in attack rate, which may be considered to be additively or multiplicatively superimposed upon the increment determined by the orientation of the eye-bar with respect to the body axis (Fig. 4).Mit Unterstützung der Deutschen Forschungsgemeinschaft im Rahmen des SFB 50.We wish to thank Ann and John Thorson for their most stimulating discussion of the paper.     

Observations on the Cladocera of Lake George, Uganda

Seven species of Cladocera were recorded from the shallow, equatorial Lake George in Uganda, during the period 1967–72. Of these only three, Moina micrura Kurz, Ceriodaphnia cornuta Sars and Daphnia barbata Weltner, are found in the open water and they form a small proportion of the total zooplankton biomass, which is dominated by a cyclopoid copepod. All three species are found throughout the year and carry parthenogenetic eggs at all times. They are smaller species than those predominating in some other East African lakes and some comparisons are also made between C. cornuta and four other species of that genus studied in Great Britain. The mean body lengths and clutch sizes of ovigerous females of C. cornuta vary little through the year. This is, presumably, a reflection of the relatively constant environment in Lake George. Alona poppei is only found in the open water intermittently and is thought to be swept from the peripheral swamps during heavy rains.
The general scarcity of Cladocera in Lake George is thought to be due, either to a shortage of food particles of suitably small size in the phytoplankton, which is dominated by large blue-green algae, or to predation. Although Cladocera are certainly eaten by fish, quantitative data, with which to substantiate either of these hypotheses, is lacking.     

Systematics of the tribe Trematocarini (Perciformes: Cichlidae) from Lake Tanganyika, Africa

 Phylogenetic relationships among eight Trematocara species and a single Telotrematocara species included in the Tanganyikan cichlid tribe Trematocarini were investigated on the basis of morphological features. The monophyly of the tribe is supported by the presence of hypertrophied sensory pores on the head, tendon “c” of adductor mandibulae section 1, a single scale row between the upper lateral line and body axis, great depth of the anteriormost infraorbital (reversed in Trematocara caparti and T. stigmaticum), and the absence of a lower lateral line. Trematocara is paraphyletic unless Telotrematocara is treated as a junior synonym. Received: December 10, 2001 / Revised: March 18, 2002 / Accepted: April 4, 2002     

The evolution of the fishery of Oreochromis niloticus (Pisces: Cichlidae) in Lake Victoria

The Lake Victoria ecosystem has experienced changes associated with fishing levels, a rise in lake level in the 1960s, fish introductions, and human activities in the drainage basin. Following the fish introductions of the 1950s and early 1960s, Oreochromis niloticus has become the most abundant and commercially important species among the tilapiines, and the only species which has managed to co-exist with the Nile perch in Lakes Victoria and Kyoga. There is, however, little published information on the biology and ecology of the specie in the new habitats. It has therefore been found necessary to initiate studies on the characteristics of O. niloticus in Lake Victoria.     

The feeding habits of Alestes baremose and Hydrocynus forskali (Pisces) in Lake Albert, East Africa

The stomach contents of 1922 Alestes baremose Joannis and 1184 Hydrocynus forskali Cuvier caught in Lake Albert, East Africa were examined. The fish were caught in gill nets in three main different types of habitat; in lagoons, inshore in the main body of the lake and offshore. Zooplankton, ephemeropteran imagines and higher plant species constituted almost the whole diet of A, baremose , although the relative importance of these three items varied with habitat. In the lagoons and inshore Hydrocynus forskali was mainly piscivorous, but offshore where the largest specimens occur it was feeding mainly on Caridina nilotica. The role of Hydrocynus species as predators in Lake Albert is discussed and it is shown that in this lake predation intensity bythis genus must be considered entirely in terms of H.forskali .     

Ecological and evolutionary consequences of the trophic polymorphism in Cichlasoma citrinellum (Pisces: Cichlidae)

The neotropical cichlid fish Cichlasoma citrinellum is polymorphic in the structure of its pharyngeal jaw apparatus and external morphology. The pharyngeal jaws are either gracile and bear slender, pointed teeth (papilliform) or robust with strong, rounded teeth (molariform). Molariform morphs have a ‘benthic’, and papilliform morphs a ‘limnetic’ body form. Furthermore, this species is also polychromatic, with yellow and black morphs. The molariform morphology of the pharyngeal jaw apparatus adapts the fish for cracking and feeding on snails. Based on analysis of stomach contents, 94% of the molariform morph ate snails whereas only 19%, of the papilliform morph did so. This result suggests that the morphs occupy different ecological niches. The morphology of the pharyngeal jaw apparatus does not correlate significantly with sex, but it does with body colouration (P<0.005). Cichlasoma citrinellum mate assortatively with their own colour; therefore a mating preference for colour may lead to genetic isolation of trophic morphs. The frequency of the molariform morph differs strikingly among populations of five Nicaraguan lakes and its abundance is correlated with the abundance of snails, the fishes' principal prey item. Among populations the frequency of molariform morphs decreases in the dry season. Morphology possibly changes reversibly within particular individuals between seasons. These results suggest that phenotypic plasticity and polymorphisms may be an adaptive characteristic of cichlid fishes. Patterns of intraspecific morphological variation match patterns of interspecific morphological diversification which suggests that universal developmental mechanisms canalize the possible expressions of morphology. The ability to respond morphologically to environmental shifts, in conjunction with genetically determined trophic polymorphisms and sexual selection via mate choice, could be the basis for speciation through intermediate stages of polymorphism of the impressive adaptive radiation of cichlid fishes.     

Phylogenetic investigations on the haplochromine Cichlidae of Lake Kivu (East Africa), based on lepidological characters*

The phylogeny of the haplochromine cichlids of Lake Kivu was studied using lepidological characters. Only one species, Haplochromis astatodon was found to have a closer relationship to a group outside Lake Kivu, namely Haplochromis sensu stricto. The other 14 species form a monophyletic lineage that is part of a subdivision in the large Lake Victoria-Edward-Kivu flock. The internal phylogeny of this lineage was resolved to the species level.     

Feeding-site utilization in three sympatric species of Petrotilapia (Pisces, Cichlidae) from Lake Malawi

Three sympatric sibling species of Petrotilapia , with similar dietary and macrohabitat requirements, occur near Monkey Bay, Lake Malawi. Males and females of all three species fed selectively on the rocky shores. Males utilized less than 6% of the space they defended as a breeding territory for feeding and although interspecific overlap of territories was considerable, only 2.5% of the feeding space used by territorial males was shared. Males primarily utilized rich food patches which occurred within the territories of highly aggressive Pseudotropheus species, whereas females tended to feed in undefended areas. Although Petrotilapia species are not highly aggressive fishes, there was a well-defined social dominance hierarchy amongst them and it is suggested that feeding-site utilization is dependent on social rank. Inter-and intraspecific differences in feeding-site utilization probably facilitate species coexistence and thus contribute to the maintenance of the high species diversity which is a characteristic of the ichthyofauna of Lake Malawi.     

General protein patterns of muscle homogenates of some Lake Victoria haplochromines (Pisces: Cichlidae)

Cichlid fishes of the great East African lakes have been the subject of many evolutionary studies over the last decades. Over 100 haplochromine species have been described from Lake Victoria alone, many of which are endemic to that lake. Two recent studies, using enzyme electrophoresis, could not detect any genetic differentiation among the examined species. Consequently, it was concluded that the gene loci which were scored in these studies were not relevant to the genetic changes which occurred during speciation. Analyses of general protein electrophoretic patterns, however, does reveal differentiation on the supraspecific level. Three basic species groups can be discerned, each associated with ecological characteristics of these species such as diet and substratum. Implications for haplochromine evolution and taxonomy are discussed. Our data seem to support the hypothesis that trophic differentiation may have occurred within habitats, implying the possible importance of sympatric speciation in haplochromine evolution in Lake Victoria.     

Freiwasserbeobachtungen an Tilapien (Pisces,Cichlidae) in Ostafrika

T. grahami ♂♂ haben in Kolonien dicht beieinander liegende Reviere; sie beziehen sie frühmorgens bei völliger Dunkelheit und verlassen sie am späten Nachmittag wieder. Einige daraufhin beobachtete ♂♂ waren immer in der gleichen Grube zu finden. Es wird angenommen, daß dies generell gilt. Kämpfe sind äußerst selten. Revier-♂♂ kämpfen durchschnittlich nur etwa 2mal pro Woche. Revierlose ♂♂ rufen öfter Kämpfe hervor, und da Kämpfe auch nachts zu bemerken sind, ist eine teilweise Unabhängigkeit des Kämpfens vom Revierverhalten zu folgern. Ersatzhandlungen fürs Kämpfen sind das Graben und das Vertreiben von Jungfischen. Es wird angenommen, daß diese Handlungen wegen der beengten Lebensbedingungen einen positiven Selektionswert haben. Aus den gleichen Gründen sind wohl Balzhandlungen reduziert, die Laichdauer auf eine Sekunde zusammengeschrumpft und Jungfische früher als in jeder anderen Cichlidenart geschlechtsreif. Diese Frühreife hat bei T. alcalica zu einem Kampfgleichgewicht zwischen sehr unterschiedlich großen ♂♂ geführt. Sonst zeigt die Art keine Unterschiede im Verhalten zu T. grahami. Bei den Geißeltilapien T. variabilis und T. rukwaensis kann die Reviergröße in sehr weiten Grenzen schwanken. In großen Revieren schwimmen die ♂♂ viel an ihren Reviergrenzen entlang, ihr Gebiet auf diese Weise gegen Reviernachbarn markierend und auf ♀♀ hin kontrollierend. In kleinen Revieren bietet sich mehr ein Bild wie bei den Sodatilapien T. grahami und T. alcalica. Aus dem Vergleich der Gruben, der Balz und des Laichens von Soda- und Geißeltilapien lassen sich verwandtschaftliche Beziehungen zwischen beiden Gruppen erschließen. Es werden also vor allem Revierverhaltensweisen in ihrer Anpassung an wechselnde Umweltbedingungen beschrieben und Balzhandlungen zur Klärung verwandtschaftlicher Beziehungen verwandt.     

Local adaptations in populations of rock-dwelling haplochromines (Pisces:Cichlidae) from southern Lake Victoria

The radiation of cichlid fishes in the African great lakes is often described as adaptive, because, at a superficial level, cichlid fishes seem adapted to the ecological niches they occupy. However, adaptedness has rarely been studied. We''ve investigated to what extent island populations of three species of the rock-dwelling genus Neochromis, endemic to Lake Victoria, are adapted anatomically to exploit locally abundant resources. Specifically, we asked whether different resource environments were reflected in differences in the feeding apparatus, both within species and between species. In populations of two specialized biters, the algae scrapers N. rufocaudalis and N. omnicaeruleus, the biting force of the lower jaw increased with increasing amount of items that require biting in the diet. N. greenwoodi is a less specialized biter; we found differences between two populations in the hyoid position and in the premaxilla that enhance suction feeding. These adaptations were related to the amount of items requiring suction. Comparing across three sympatric pairs of species, in each case different diets were reflected in differences in anatomy.     

Sexual selection,reproductive behavior,and speciation in the mbuna species flock of Lake Malawi (Pisces: Cichlidae)

Synopsis The cichlid species flocks of the African Great Lakes represent the most extreme case of adaptive radiation among vertebrates. Recently, attention has focused on the potential for sexual selection to drive or accelerate speciation in these fishes. Cichlids as a whole are social in nature and display complex behavior, particularly during courtship and spawning; however, the extent to which changes in species recognition cues may account for species diversity among haplochromine lineages has remained speculative. Our investigations have indicated that oral incubating haplochromines show a reduction in diversity and extent of courtship relative to substrate brooding cichlids, and apparently retain aspects of a primitive specific mate recognition system. Laboratory observations of courtship in the Malawian endemicPseudotropheus zebra suggest that organization of the spawning bout is loose, and lacking in any well defined stimulus-response chain. Interspecific comparisons of in situ courtship behavior among male mbuna, lithophilous haplochromines of Lake Malawi, revealed only one potential example of species-specific behavior, and indicated that mate choice occurred prior to the onset of intense courtship. Courtship display by male mbuna does not appear critical to species recognition and may represent an evolutionary relict. Alternatively, male courtship display may have an indirect effect on successful reproduction or may be maintained through pleiotropic effects. The mbuna display no evidence of behavioral innovation and show limited interspecific differentiation in behavioral expression. More generally, there is no evidence at present to suggest that epigamic sexual selection, acting on courtship behavior, has been a major mechanism in the diversification of the haplochromine species flocks.