Potential fishery of Nile perch Lates niloticus Linne (Pisces: Centropomidae) in Nyanza Gulf of Lake Victoria,East Africa

From bottom trawls, Lates niloticus (Lin.) in Nyanza Gulf was observed to have a gulf-wide distribution. Lates dominated the catch at Homa point. Catch rates in kg h^-1 varied from one location to another and from month to month.Its estimated production is 11 000 metric tons, netting about 22 000 000 Kenya shillings (US $2 000 000).Abundance of food items such as freshwater shrimps of the genus Caridina, and Engraulicypris argenteus, have been used as basis for estimating the growth and production of Lates.     

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.     

Changes in the diet of Nile perch,Lates niloticus (L), in the Mwanza Gulf,Lake Victoria

Samples of Nile perch (Lates niloticus L.) were collected for stomach analysis from trawl catches conducted in the Mwanza Gulf from September 1986 to September 1988. Initially haplochromine cichlids formed the main food item for the Nile perch. Despite their decline, haplochromines still formed the major part of its diet in 1986. After the virtually complete disappearance of the haplochromines in 1987 and 1988, the benthic shrimp Caridina nilotica, the pelagic cyprinid, Rastrineobola argentea and juvenile Nile perch became the main food of Nile perch. Twenty-four trawl sessions were conducted to determine the relative quantities of respective prey taken. When present, Caridina is the major prey, mainly taken by day. If absent, juvenile Nile perch constitute the main part of the diet. Seasonal differences in diet composition probably reflect seasonal fluctuations in the abundance of the main prey species.     

Egg Mimics in Haplochromine Cichlids (Pisces,Perciformes) from Lake Victoria

An ecomorphological explanation is given of the anal spots of haplochromine cichlids from Lake Victoria. Anal spots are ovoid orange-yellow spots on the anal fins of these fishes. The possible roles of sexual selection and predation as determinants of the form of these spots are discussed. Evidence is given that the anal spots are egg mimics and thus examples of intra-specific mimicry. Mimicking an egg to stimulate sexually active females presumably is an important demand on the form of the anal spots. However, no positive correlation was found between anal spot size and egg size. Light intensities of the habitat of the species investigated showed a negative correlation with anal spot size. In contrast to this, egg size did not show a negative correlation with light intensities. It is suggested that, besides mimicry, at least one more demand is involved in determining spot size: that of being conspicuous. Conspicuousness is likely to have a positive impact (benefits) on courting conspecific females, but a negative one (costs) with regard to predators who hunt visually for either adults or eggs. The actual appearance of anal spots may be a compromise. This may explain the mismatch between egg size and spot size in many species. Qualitative evidence is presented that predation pressures on adult haplochromines exerted by visually hunting predators are relatively high in lighter habitats, viz. near the rocks and near the surface. An alternative hypothesis that spots in dark surroundings cannot be seen unless they are supra-normal is discussed.     

The catfish fauna of Lake Victoria after the Nile perch upsurge

Stocks of the indigenous catfish species of Lake Victoria have decreased dramatically since the beginning of the 1980s. This decline coincided with the Nile perch boom and concomitant ecological changes in the lake. In deep water, where Nile perch densities were higher, the decline proceeded more quickly than in shallow water. In the former all catfishes eventually vanished. Of the two largest species, Clarias gariepinus and Bagrus docmak, juveniles disappeared faster than adults. This indicates that predation by Nile perch may have played an important role in their decline. Other possible impacts were the deoxygenation of deepwater areas and the decline of haplochromine cichlids which were an important food source for B. docmak, C. gariepinus and Schilbe intermedius. The various catfish species were not all affected to the same extent. The endemic Xenoclarias eupogon, which lived predominantly in deep water, may have become extinct. B. docmak currently seems to be mainly restricted to refugia in rocky habitats. Synodontis victoriae and S. afrofischeri are still present in small numbers in shallow littoral areas. Schilbe intermedius and C. gariepinus seem to be the least affected of the catfishes in littoral and sublittoral areas. This may be caused, among other reasons, by their smaller habitat overlap with Nile perch than the other species. S. intermedius is partly pelagic, and a considerable part of the C. gariepinus stock lives in bodies of water surrounding the lake. The patterns of decline of the catfishes are very similar to those observed for haplochromine cichlids in the lake. The importance of catfishes for the fisheries in the lake is currently negligible.     

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.     

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.     

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.     

Phylogeography of the Percichthyidae (Pisces) in Patagonia: roles of orogeny, glaciation, and volcanism

We used molecular evidence to examine the roles that vicariance mechanisms (mountain-building and drainage changes during the Pleistocene) have played in producing phylogeographical structure within and among South American fish species of the temperate perch family Percichthyidae. The percichthyids include two South American genera, Percichthys and Percilia, each containing several species, all of which are endemic to southern Argentina and Chile (Patagonia). Maximum-likelihood phylogenies constructed using mitochondrial DNA (mtDNA) control region haplotypes and nuclear GnRH3-2 intron allele sequences support the current taxonomy at the genus level (both Percichthys and Percilia form strongly supported, monophyletic clades) but indicate that species-level designations need revision. Phylogeographical patterns at the mtDNA support the hypothesis that the Andes have been a major barrier to gene flow. Most species diversity occurs in watersheds to the west of the Andes, together with some ancient divergences among conspecific populations. In contrast, only one species (Percichthys trucha) is found east of the Andes, and little to no phylogeographical structure occurs among populations in this region. Mismatch analyses of mtDNA sequences suggest that eastern populations last went through a major bottleneck c. 188 000 bp, a date consistent with the onset of the penultimate and largest Pleistocene glaciation in Patagonia. We suggest that eastern populations have undergone repeated founder-flush events as a consequence of glacial cycles, and that the shallow phylogeny is due to mixing during recolonization periods. The area of greater diversity west of the Andes lies outside the northern limit of the glaciers. mtDNA mismatch analysis of the genus Percilia which is restricted to this area suggests a long-established population at equilibrium. We conclude that patterns of genetic diversity in these South American genera have been primarily influenced by barriers to gene flow (Andean orogeny, and to a lesser extent, isolation in river drainages), and by glacial cycles, which have resulted in population contraction, re-arrangement of some watersheds, and the temporary breakdown of dispersal barriers among eastern river systems.     

The impact of the introduction of Nile perch, Lates niloticus (L.) on the fisheries of Lake Victoria

The piscivorous Nile perch was introduced into Lake Victoria some 30 years ago, since when it has completely transformed the fishing industry and the species composition of the fish fauna of the lake. The original multispecies fishery, based mostly on cichlids (haplochromines, tilapias), cyprinids ( Barbus, Labeo, Rastrineobola ) and siluroids ( Bagrus, Clarias, Synodontis, Schilbe ), has changed dramatically to one based on three species: the introduced Nile perch, the cyprinids, Rastrineobola argenrea (Pellegrin), and the introduced Nile tilapia, Oreochromis niloticus (Linnaeus).
Within 25 years of its introduction the Nile perch became ubiquitous and now occurs in virtually every habitat with the exception of swamps and affluent rivers. It has preyed on all other species with profound effects, especially on the stocks of haplochromines. These originally comprised 80% of the total fish biomass in Lake Victoria, but have now decreased to less than 1% offish catches from the Kenyan waters of the lake. The fishermen of Lake Victoria have adjusted to this ecological crisis by using large-meshed nets to catch Nile perch, which has become the most important commercial species. For the first time in the history of Lake Victoria, fish fillets are now being exported to several overseas countries: the fillets are all from Nile perch.     

Ecomorphology of retinal structures in zooplanktivorous haplochromine cichlids (Pisces) from Lake Victoria

Synopsis Before the decline of the species flock of haplochromine cichlids of Lake Victoria due to the Nile perch upsurge, there were many co-existing haploehromine species such as the taxonomically and ecologically well-studied zooplanktivores of the Mwanza Gulf. In spite of the scarcely separated niches of some of these species, no sign of competition for space or food could be demonstrated. As is argued in this paper, optical differentiation could well be an aspect of adaptive radiation of these zooplanktivores, particularly among the highly sympatric species. Our hypothesis is based on the morphological modifications of retinal structures in nine zooplanktivorous species. Interspecific variation was observed in composition, size and density of the photoreceptors and ganglion cells. The analyses included the intraretinal variation and size dependency of some of the structural parameters. The optical functions deduced from retinal structure indicate distinct interspecific differences in sensitivity thresholds and a slight differentiation in visual resolution. These functions correlate poorly with the photic conditions of the species-specific habitats. The optical properties can, on the other hand, be connected with the more subtle differentiation in food items and feeding behaviour among these species. It is our concluding hypothesis, that the optical differentiation among the haplochromine zooplanktivores primarily served resource partitioning by different modes of visual prey detection rather than niche partitioning by habitat.     

Changes in the feeding biology of the Nile perch, Lates niloticus (L.) (Pisces: Centropomidae), in Lake Victoria, East Africa since its introduction in 1960, and its impact on the native fish community of the Nyanza Gulf

Lates niloticus is not native to Lake Victoria but was introduced during or shortly before 1960. It remained relatively uncommon until 1975, when the number in the Nyanza Gulf began to increase impressively, the estimated catch rising over 100-fold between 1978 and 1982. Originally Lates was piscivorous, its diet reflecting the composition of the native fish community. The present investigation has revealed that its diet is now almost entirely comprised of Caridina nilotica , a small microphagous prawn, and juvenile Lates. Native fish species, except for the small pelagic Rastrineobola argenteus , are very rarely consumed. This change in diet is a result of the shattering impact Lates predation has had on the native fishes, which have been virtually wiped out. The original community, which was dominated by several hundred haplochromine species and the catfishes Clarias mossambicus and Bagrus docmac which preyed upon them, and included two endemic tilapiine cichlids and 38 species of non-cichlids, no longer exists. It has been replaced by a community dominated by Lates which now accounts for well over 80% of the fish biomass in the Nyanza Gulf and very nearly 100% in the study area. The only other species regularly encountered were Oreochromis niloticus , an introduced tilapiine, and Rastrineobola argenteus , a native zooplanktivore.     

Trophic shifts in predatory catfishes following the introduction of Nile perch into Lake Victoria

This study looked for evidence of trophic shifts in the diet of two predatory catfishes ( Bagrus docmac and Schilbe intermedius ) following the establishment of introduced Nile perch ( Lates niloticus ) into lakes of the Lake Victoria basin. Bagrus docmac exhibited a shift from a primarily piscivorous diet dominated by haplochromine cichlids to a broader diet that included a significant proportion of invertebrates and the cyprinid fish, Rastrineobola argentea , which became abundant following depletion of the haplochromines. Schilbe intermedius exhibited a trophic shift from a piscivorous diet dominated by haplochromines to an insectivorous diet. The flexibility in diet exhibited by these two catfishes may have permitted these species to persist, albeit in reduced numbers, subsequent to the introduction of Nile perch and may facilitate resurgence as fishing pressure reduces numbers of large Nile perch.     

Helminth parasites of Chapalichthys encaustus (Pisces: Goodeidae), an endemic freshwater fish from Lake Chapala, Jalisco, Mexico

A total of 6 helminth species were recorded during helminthological examination of 50 Chapalichthys encaustus from Lake Chapala, Jalisco, Mexico. Helminth species identified included: Salsuginus sp. (an undescribed monogenea); Posthodiplostomum minimum (metacercariae); Cyclustera ralli (metacestode); Polymorphus brevis (cystacanth); Contracaecum sp. (nematode larvae); and Rhabdochona lichtenfelsi (adult nematode). Of these, 2 (Salsuginus sp. and R. lichtenfelsi) are specialist species. The observed species richness, individual parasite abundance, and diversity were low. Data suggest that host specificity is an important factor contributing to observed community composition and richness. Host feeding habits and helminth species availability seem to determine the characteristics of these helminth assemblages.     

The genus Xenoclarias (Pisces, Siluriformes) in Lake Victoria, with a redescription of Xenoclarias eupogon and notes on the biology of the species

The acquisition of large collections of the catfish Xenoclarias Greenwood from Lake Victoria, East Africa, demonstrated a need for a reappraisal of the taxonomy of the genus. It is suggested that only one species, Xenoclarias eupogon (Norman) is found in the lake but that it is very variable in many of its morphometric characters. The species is redescribed from data obtained for 72 fishes from various localities and depths in the lake. Observations of the food of the fish show it to be almost entirely insectivorous with a preference for Chironomidae over Chaoboridae larvae. Evidence is also put forward to support the view that X. eupogon breeds on or close to rocky shores.     

Effects of Nile perch (Lates niloticus) introduction into Lake Victoria,East Africa,on the diet of Pied Kingfishers (Ceryle rudis)

In recent years the ichthyofauna of Lake Victoria, the world's largest tropical lake, has gone through dramatic changes. The population of Nile perch, a large predator which has been introduced into the lake by man, increased explosively at the expense of many haplochromine cichlid species. At the same time, numbers of a small cyprinid (dagaa) rose sharply.

Previously Pied Kingfishers on Lake Victoria fed mainly on haplochromines. Only the youngest nestlings depended on dagaa as primary food. The current diet of adult birds clearly reflects the changes which have occurred in the fish community. Pellet analysis reveals a shift towards a diet composed of almost 100% dagaa.

The change in prey species composition has increased the number of fish a kingfisher needs to catch daily in order to meet its energetic demands, because:

1. (1)

   the mean size of haplochromines is larger than that of dagaa;

2. (2)

   (2) the mean size of dagaa has decreased since the increase in Nile perch;

3. (3)

   (3) the weight of dagaa is lower than that of haplochromines of equal size;

4. (4)

   (4) mainly juvenile dagaa and adults in poor condition are accessible to kingfishers.