Genetic affinities of an introduced predator: Nile perch in Lake Victoria, East Africa

Although the introduction of Nile perch, Lates niloticus , to Lake Victoria has received intense global attention, especially in relation to its impact on endemic cichlid species and on fishery yields, fundamental information on its taxonomy and population genetics is lacking. Most importantly, the introduced fish originated from two lakes (Lakes Albert and Turkana) containing three Lates species, and it has never been entirely clear which of these became established in Lake Victoria, or indeed whether the Lake Victoria population is derived from hybridization between Lates species. In addition, genetic drift caused by the relatively small founder population (≈ 400), the initially slow population increase followed by a period of explosive population growth, and selection pressures in the new environment may have resulted in substantial genetic changes. Allozyme data indicated that the introduced Nile perch of Lake Victoria were mainly L. niloticus from Lake Albert, although maximum likelihood estimates of stock contributions (GSI) suggested the presence of L. macrophthalmus. In contrast, introduced Nile perch in adjacent smaller lakes (Lakes Kyoga and Nabugabo) appeared to be entirely L. niloticus . The effect of the introductions on allozyme diversity varied among lakes and appeared to be uncorrelated to the number of fish introduced.     

Predation by Nile perch Lates niloticus (L.) on Oreochromis niloticus (L.), Cyprinus carpio (L.), Mugil sp. and its role in controlling tilapia recruitment in Egypt

Vulnerability of three fish groups (tilapia, common carp, mullet) to predation by Nile perch Lates niloticus was evaluated. The study was conducted in aquaria and in cages placed in concrete ponds. Common carp Cyprinus curpio was the more vulnerable to predation by Nile perch followed in order by mullet ( Mugil cephulus and M. cupito ) and Nile tilapia Oreochromis niloricus . Medium-size Nile perch (30–32 cm t.l. ) were able to consume tilapia up to 14 cm t.l. , while tilapia of 15 cm total length were not eaten. Ratios of 1 : 10 to 1 : 15 between Nile perch and ready-to-spawn females of O. niloticus were found to be adequate to control tilapia reproduction.     

Effects of Nile perch, Lates niloticus, on functional and specific fish diversity in Uganda's Lake Kyoga system

The introduction of Nile perch, Lates niloticus, to Lake Victoria, East Africa, interacted with eutrophication to cause a reorganization of the lake's food web and the extirpation of many endemic fishes. The Lake Kyoga satellite system lies downstream from Lake Victoria. It encompasses species‐rich lakes where Nile perch are absent or very rare, and low diversity lakes where L. niloticus is abundant. In 1999 we surveyed seven lakes in the Kyoga system using experimental monofilament gill nets (1/4–1 inches variable mesh). At Boston University we assessed δ¹⁵N signatures of epaxial muscle from subsamples of the catch (n = 361). These signatures are often highly correlated with the near‐term mean realized trophic position of an individual organism. A neural network analysis of fish length, species name, trophic level, and lake of origin fish explained 94% of the sample variance in δ¹⁵N. We analysed statistical patterns in these signatures at a number of spatial scales. The relationship between trophic level and δ¹⁵N varied greatly among lakes. Higher diversity perch‐free lakes had greater variance in δ¹⁵N values and fish lengths than lower diversity Nile perch lakes, suggesting an important relationship between species diversity and functional diversity. Against expectations, lake size was negatively correlated with δ¹⁵N. Patterns in stable isotope signatures indicated that Nile perch lakes have shorter food chains than perch‐free lakes. The results throw up two management problems for the Kyoga system. Impacted lakes need to be studied to understand and ameliorate the community‐level effects of Nile perch introduction, whereas the species‐rich nonperch lakes, which harbour a large proportion of the remaining diversity of regionally endemic taxa, are in need of conservation planning.     

The tilapiine fish stock of Lake Victoria before and after the Nile perch upsurge

Since the beginning of fisheries in Lake Victoria, two native tilapiine species, Oreochromis esculentus and Oreochromis variabilis , were the main target of the local fishermen. A continuous increase in fishing pressure led initially to a declining catch per unit of effort, and a smaller average fish size; eventually, there was a reduced landing of tilapiines. To boost the fisheries, three alien tilapiine species and the Nile perch Lates niloticus were introduced. Thirty years after its introduction, Oreochromis niloticus appeared to be the most successful tilapiine species. It replaced the indigenous tilapiines almost completely before the Nile perch came to dominate the ecosystem of Lake Victoria. Reduced fishing pressure on the tilapiines in the 1980s, due to the shift of the local fishery towards the Nile perch, resulted in an increase in the stock of O. niloticus and an increase in average fish size. Subsequently, the total mass of O. niloticus landed increased. The stocks of the indigenous tilapiines did not recover but declined to extremely low levels, or vanished from the main lake. Currently, these species still occur in satellite lakes of Lake Victoria, from which O. niloticus is absent. Nile perch feed on O. niloticus; however, the limited overlap in distribution between piscivorous Nile perch and O. niloticus of consumable sizes is probably an important factor in explaining the coexistence of the two species. The main cause of the disappearance of the native tilapiine species is presumed to be competitive dominance by O. niloticus .     

Mortality rates of Lates niloticus,L. in Lake Nasser

Landings of Lates niloticus from Lake Nasser belonged to age groups I–VII, with II–III predominent. Mortality rate, as computed by two different methods, ranges around 75%. Turbid flood water did not reach Lake Nasser in 1972, and this fact may be responsible for the decline in the commercial catch from 1972 to 1973.     

The taxonomy of East African Nile Perch, Lates spp. (Perciformes, Centropomidae)

The type specimens of Lates albertianus, L. macrophthalmus, L. niloticus rudolfianus and L. niloticus longispinis are contrasted with each other, with riverine L. niloticus niloticus and with a collection of Nile Perch from Lake Victoria. Contrary to current belief L. niloticus niloticus was found to differ both from L. albertianus and from L. niloticus rudolfianus , but no significant difference was found between L. albertianus and L. niloticus rudolfianus . The Lake Victoria collection was found to differ from all other taxa, suggesting that the characters currently used in the taxonomy of Lates are inappropriate for this purpose. It is recommended that a reappraisal of Nile Perch taxonomy be made using more modern techniques and that studies are initiated to discover how characters change during development under differing environmental conditions.     

Changes in the prey ingested and the variations in the Nile perch and other fish stocks of Lake Kyoga and the northern waters of Lake Victoria (Uganda)

The fish stocks of Lakes Kyoga and Victoria have changed since Nile perch, Lates niloticus (L.), was introduced, and this is reflected in the prey ingested by the predator. Initially, haplochromine cichlids constituted the main prey of most sizes of Nile perch. As the stocks of these have declined, Caridina nilotica (Roux) and Anisopteran nymphs have become the dominant food of the juveniles, while Rastrineobola argentea (Pellegrin), juvenile Nile perch and Oreochromis niloticus (L.) have become the main food of larger Nile perch. Apart from R. argentea , most of the native fish species of these lakes have disappeared. The stocks of Nile perch in Lake Kyoga, to which it was introduced earlier than to Lake Victoria, have declined after dominating the fishery since 1965. and have been superseded by O. niloricus . an introduced herbivore. Similar changes are now occurring in Lake Victoria. The Nile perch might not maintain the high yield realized in the two lakes when haplochromines were abundant. It is therefore necessary to exercise caution with high and long-term investments aimed specifically at developing the Nile perch fishery.     

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.     

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.     

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.     

Reproductive potential of the Nile perch,Lates niloticus L. and the establishment of the species in Lakes Kyoga and Victoria (East Africa)

The size at first maturity, sex ratio and fecundity of L. niloticus in Lake Kyoga have been examined, and compared with the situation in other aquatic systems. The species has the ability to reproduce enormously. It produces up to 16 million eggs. In Lake Kyoga, fecundity (F) increases with length (L) in cm according to the equation: % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaciOraiaac2% dacaGGGaGaaiinaiaac6cacaGG0aGaai4maiaacAdacaGGGaGaaiiE% aiaacccacaGGXaGaaiimamaaCaaaleqabaGaaiylaiaacAdaaaGcca% GGmbWaaWbaaSqabeaacaGGYaGaaiOlaiaacMdacaGGYaaaaaaa!44D8!\[\operatorname{F} = 4.436 x 10^{ - 6} L^{2.92} \]Information from different habitats shows that females grow to a larger size than males but the growth rate is the same in both sexes. Males mature earlier than females at 50–65 cm total length with females maturing between 60–95 cm. There are about twice as many males as females. The rapid establishment of L. niloticus in Lake Kyoga and the Nyanza Gulf of Lake Victoria following its introduction is attributed to the high reproductive potential of the species under favourable environmental conditions.     

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.     

Ontogenetic shifts in phenotype–environment associations in Nile perch,Lates niloticus (Perciformes: Latidae) from Lake Nabugabo,Uganda

Habitat‐associated trait divergence may vary across ontogeny if there are strong size‐related shifts in selection pressures. We quantified patterns of phenotypic divergence in Nile perch (Lates niloticus) from ecologically distinct wetland edge and forest edge habitats in Lake Nabugabo, Uganda, and we compared patterns of divergence across three size classes to determine whether trends are consistent through Nile perch ontogeny. We predicted that inter‐habitat variation in biotic (e.g. vegetation structure) and abiotic (e.g. dissolved oxygen concentration) variables may create divergent selective regimes. We compared body morphology using geometric morphometrics and found substantial differences between habitats, although not all trends were consistent across size classes. The most striking aspects of divergence in small Nile perch were in mouth orientation, head size, and development of the caudal region. Medium‐sized Nile perch also showed differences in mouth orientation. Differences in large individuals were related to eye size and orientation, as well as caudal length. The observed patterns of divergence are consistent with functional morphological predictions for fish across divergent trophic regimes, high and low predation environments, and complex and simple habitats. Although this suggests adaptive divergence, the source of phenotypic variation is unknown and may reflect phenotypic plasticity and/or genetic differences. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 449–465.     

Nile perch and the transformation of Lake Victoria

The transformation of Lake Victoria that began in 1980 followed the population explosion of Nile perch Lates niloticus, causing the apparent extirpation of 500+ endemic haplochromine species and dramatic physico-chemical changes. Officially introduced in 1962–1963, but present earlier, the reasons for the long delay before its population exploded are discussed. The hypothesis that it occurred only after the haplochromine decline is evaluated, but haplochromines declined only after the Nile perch expansion began. The sudden eutrophication of the lake was attributed to Nile perch, but evidence of eutrophication from 1950 onwards led some researchers to conclude that it was the result of climatic changes. We conclude that the haplochromine destruction disrupted the complex food webs that existed prior to the upsurge of Nile perch. The depletion of fish biomass by Nile perch may have been the source of extra phosphorus responsible for the eutrophication of the lake. After the Nile perch explosion in 1980 the fish population came to be dominated by only three species, but fisheries productivity increased at least 10-fold. Fishing has caused demographic changes in Nile perch, which may have allowed some haplochromine species to recover. The condition of the lake appears to have stabilised since 2000, partly because the fish biomass has risen to at least 2 × 10⁶ t, replacing the ‘lost’ biomass and restoring some ecosystem functioning.     

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.

     

Interactions Between Nile Perch, Lates niloticus, and Other Fishes in Lake Nabugabo, Uganda

The introduction of the predatory Nile perch, Lates niloticus, into the Lake Victoria basin coincided with a dramatic decline in fish species richness and diversity. This study focused on interactions between Nile perch and indigenous fishes in Lake Nabugabo, Uganda, a small satellite lake of Lake Victoria. We evaluated how the foraging impact of juvenile Nile perch on prey fishes varied with the size of the predator. We also evaluated the role of wetland ecotones in minimizing interaction between Nile perch and indigenous fishes. Wetland ecotones in Lake Nabugabo were characterized by complex structure (e.g., dense vegetation) and lower dissolved oxygen levels than non-wetland (exposed) areas. Nile perch (8.6–42.2cm, TL) were 3.7 times more abundant in offshore exposed areas than in inshore areas near wetland ecotones, and the proportion of Nile perch using wetland and exposed areas was independent of their body size. However, species richness was higher in waters at wetland ecotones than in exposed areas. Nile perch (5–35cm, TL) exhibited a shift in diet at approximately 30cm TL from feeding primarily on invertebrates to piscivory. Although the shift to piscivory occurred at approximately the same body size for Nile perch from both wetland and exposed habitats, the shift to piscivory was less abrupt in Nile perch captured near wetland ecotones. Nile perch from wetland areas consumed a greater diversity and a larger percentage of fish prey than those from exposed sites. However, the low abundance of Nile perch in wetland ecotones suggested that interaction between predator and prey in these areas is much reduced.     

Shaping of the lower jaw bone during growth of Nile tilapia Oreochromis niloticus and a Lake Victoria cichlid Haplochromis chilotes: a geometric morphometric approach

East African cichlids have evolved feeding apparatus morphologies to adapt to diverse feeding environments. However, little is known about how the morphologies are formed during development. Here, we assessed the shape changes of the lower jaw bone during growth of the Nile tilapia Oreochromis niloticus and a Lake Victoria cichlid Haplochromis chilotes using geometric morphometric methods. 'Early Juvenile to Late Juvenile' and 'Late Juvenile to Adult' transitions of the shape change during growth of both O. niloticus and H. chilotes were detected. The 'Early Juvenile to Late Juvenile' transition of the shape change in H. chilotes occurred slightly earlier than in O. niloticus. We also compared the shape changes during growth of the two cichlids. Principal component analysis showed both commonalities and differences in the morphological changes between the cichlids. Our data suggest that most shape change may have a similar pattern during the growth of O. niloticus and H. chilotes, and that the differences in adult shapes may be due to differences arising early in development, not to the difference of shape change during growth. These data provide a basis for understanding the developmental mechanisms underlying this adaptive trait of East African cichlids.     

The food of the Nile perch, Lates niloticus (L.), after the disappearance of the haplochromine cichlids in the Nyanza Gulf of Lake Victoria (Kenya)

After the disappearance of the haplochromine species in the Nyanza Gulf of Lake Victoria as a result of predation by Lates niloticus , the latter has turned its attention to aquatic invertebrates and other fish. Changes in the diet of the Nile perch with increase in its size have been observed: young L. niloticus preyed mostly on invertebrates, including crustaceans and various small aquatic insects; large, immature L. niloticus supplemented the invertebrate diet with both young and small fish; adults above 80 cm total length were mainly piscivorous. L. niloticus feeds on fish prey of about one third its own length.
The tendency of L. niloticus to switch from one prey item to another, depending on availability, is reported; e.g., in the Nyanza Gulf, the prey diet has shifted from the haplochromine to Caridina nilotica and L. niloticus juveniles.     

Co-introduction of Dolicirroplectanum lacustre,a monogenean gill parasite of the invasive Nile perch Lates niloticus: intraspecific diversification and mitonuclear discordance in native versus introduced areas

The Nile perch (Lates niloticus) is a notorious invasive species. The introductions of Nile perch into several lakes and rivers in the Lake Victoria region led to the impoverishment of trophic food webs, particularly well documented in Lake Victoria. Additionally, its parasites were co-introduced, including Dolicirroplectanum lacustre (Monogenea, Diplectanidae). Dolicirroplectanum lacustre is the single monogenean gill parasite of latid fishes (Lates spp.) inhabiting several major African freshwater systems. We examined the intra-specific diversification of D. lacustre from Lates niloticus in Lake Albert, Uganda (native range) and Lake Victoria (introduced range) by assessing morphological and genetic differentiation, and microhabitat preference. We expected reduced morphological and genetic diversity for D. lacustre in Lake Victoria compared with Lake Albert, as a result of the historical introductions. We found that D. lacustre displayed high morphological variability within and between African freshwaters, with two morphotypes identified, as in former studies. The single shared morphotype between Lake Albert and Lake Victoria displayed similar levels of haplotype and nucleotide diversity between the lakes. Mitonuclear discordance within the morphotypes of D. lacustre indicates an incomplete reproductive barrier between the morphotypes. The diversification in the mitochondrial gene portion is directly linked with the morphotypes, while the nuclear gene portions indicate conspecificity. Based on our results, we reported reduced genetic and morphological diversity, potentially being a result of a founder effect in Lake Victoria.