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.     

Physiological refugia: swamps,hypoxia tolerance and maintenance of fish diversity in the Lake Victoria region

In Lake Nabugabo, Uganda, a satellite of Lake Victoria, approximately 50% of the indigenous fishes disappeared from the open waters subsequent to the establishment of the introduced predatory Nile perch, Lates niloticus. This pattern is similar to the faunal loss experienced in the much larger Lake Victoria. Several of these species persisted in wetland refugia (e.g. ecotonal wetlands, swamp lagoons); however, deep swamp refugia (habitats lying well within the dense interior of fringing wetlands), are available only to a subset of the basin fauna with extreme tolerance to hypoxia. Although air-breathers are common in deep swamp refugia; we also documented a surprisingly high richness and abundance of non-air-breathing fishes. We describe several mechanisms that may facilitate survival in deep swamp refugia including high hemoglobin concentration, high hematocrit, large gill surface area and a low critical oxygen tension (P(c)). In addition, swamp-dwelling fishes showed lower PO(2) thresholds for onset of aquatic surface respiration than the lake-dwelling fishes. This suggests higher tolerance to hypoxia in the swamp fishes because they are able to withstand a lower oxygen tension before approaching the surface. We suggest that physiological refugia may be important in modulating the impact of Nile perch and indigenous fishes in the Lake Nabugabo region; this highlights the need to evaluate relative tolerance of introduced predators and indigenous prey to environmental stressors.     

A colourful youth: ontogenetic colour change is habitat specific in the invasive Nile perch

External body colour is an important trait contributing to phenotypic diversity and individual fitness in fish species. In this study, we use a combination of experimental techniques and field observations to examine patterns of colour divergence in the introduced Nile perch population of Lake Nabugabo, Uganda. We aim to determine whether the proportion of black–brown body colouration of Nile perch differs over a number of size classes, across ecologically distinct wetland edge and forest edge habitats, and whether these differences are the result of rapid (physiological) or ontogenetic (morphological) colour change. We found substantial colour differences in Nile perch between habitats, but trends were not consistent across size classes. Small Nile perch (<15 cm SL) from wetland edge habitats had darker skin pigmentation than those from forest edge; however, no significant colour differences existed between medium (15–40 cm SL) and large (>40 cm SL) Nile perch. Inter-habitat differences in colour in the small size class, and shifts in colour from juvenile to adult appear to be the result of morphological colour change associated with distinct ontogenetic shifts in resource use.     

Comparative age and growth of Nile tilapia (<Emphasis Type="Italic">Oreochromis niloticus</Emphasis> L.) in lakes Nabugabo and Wamala,Uganda

Age and growth of Nile tilapia (Oreochromis niloticus) from Lake Nabugabo and Lake Wamala, Uganda, were determined using cross-sectioned sagittal otoliths. Marginal-increment and edge analyses of Nile tilapia otoliths from Lake Nabugabo indicated formation of two annuli per 12-month period. Opaque zones associated with faster growth were observed between April and June and between September and December, coincident with the two rainy seasons of the year. Within both lakes, males were larger at age than females. Nile tilapia from Lake Nabugabo, however, had faster growth rates than Nile tilapia from Lake Wamala, and fish >3 years old from Lake Nabugabo were larger at age than those from Lake Wamala. Ages ranged from 0 to 8.0 years for Nile tilapia from Lake Nabugabo, and from 0.5 to 6.5 years for tilapia from Lake Wamala. Differences in the patterns of growth in Nile tilapia between lakes may reflect, at least in part, the relatively energy-rich omnivorous diet of Nile tilapia in Lake Nabugabo versus a phytoplanktivorous diet in Lake Wamala. Diet differences of Nile tilapia between the two lakes are ascribed to trophic changes in the lakes due to the introduction of Nile perch (Lates niloticus) into Lake Nabugabo but not Lake Wamala. Alternatively, the greater exploitation of Nile tilapia in Lake Nabugabo may have resulted in increased growth rates, whereas Nile tilapia in Lake Wamala may be subject to slower, density-dependent growth. Handling editor: J. Cambray     

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.     

Divergence in aerobic scope and thermal tolerance is related to local thermal regime in two populations of introduced Nile perch (Lates niloticus)

We tested whether thermal tolerance and aerobic performance differed between two populations of Nile perch (Lates niloticus) originating from the same source population six decades after their introduction into two lakes in the Lake Victoria basin in East Africa. We used short-term acclimation of juvenile fish to a range of temperatures from ambient to +6°C, and performed critical thermal maximum (CT_max) and respirometry tests to measure upper thermal tolerance, resting and maximum metabolic rates, and aerobic scope (AS). Across acclimation temperatures, Nile perch from the cooler lake (Lake Nabugabo, Uganda) tended to have lower thermal tolerance (i.e., CT_max) and lower aerobic performance (i.e., AS) than Nile perch from the warmer waters of Lake Victoria (Bugonga region, Uganda). Effects of temperature acclimation were more pronounced in the Lake Victoria population, with the Lake Nabugabo fish showing less thermal plasticity in most metabolic traits. Our results suggest phenotypic divergence in thermal tolerance between these two introduced populations in a direction consistent with an adaptive response to local thermal regimes.     

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.     

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.     

Gill anomalies of perch and roach from four lakes differing in water quality

This study examined gill anomalies of two common freshwater fish species, roach Rutilus rutilus and perch Perca fluviatilis , collected over five seasons in 1989–1990 from four lakes in central Finland. The lakes differed both in water quality and in fish parasite species composition. Particular attention was paid to the differences in chloride cell proliferation. Gill anomalies were generally more common and abundant in roach than in perch. Chloride cell proliferation was the most frequent histological change in roach but was that least often found in perch. Most of the changes were systemic and light in severity. In perch an unidentified lesion of noncellular eosinophilic inclusions in an enlarged epithelium was also recorded. There was a marked pattern in the severity of histological changes in perch when comparing those from the natural Lake Peurunka to those from three 'altered' lakes. Gills of perch from Lake Peurunka possessed more alterations in all the recorded parameters. In roach, a significant differences were detected in chloride cell and epithelial proliferation: these were greater in Lake Peurunka and 'polluted' Lake Vatia. In perch, all histological changes were significantly more frequent at the end of autumn 1989 when compared to other seasons. In roach, increased frequencies of all gill changes were noted in winter and spring samples. Tissue reactions to parasites were either only localized or absent. The systemic gill alterations in freshwater fish may reflect the soft nature and low winter temperature of Finnish fresh waters, in which chemical imbalances, such as acid peaks, may induce more dramatic changes in fish tissues than in harder waters.     

Nile perch,Lates niloticus,predation on the freshwater prawn,Caridina nilotica,in the Nyanza Gulf,Lake Victoria,East Africa

Synopsis The percentage of Nile perch, Lates niloticus, containing the prawn, Caridina nilotica, declined from 60–85%, for fish between 5 and 40 cm total length, to less than 10% for fish larger than 80 cm in length. The maximum number of prawns eaten by a Nile perch increased steadily, from 10 prawns for a fish in the 5–9.9 cm length group, to 913 prawns in a fish in the 60–69.9 cm length group, and then fell sharply for larger fish. The mean number of Caridina eaten by fish that were foraging on this animal followed a similar trend. Gill raker spacing increased in direct proportion to fish length and this may prevent fish larger than 70 cm in length from being effective prawn predators.     

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.     

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.     

Respiratory characters of three species of haplochromine cichlids: Implications for use of wetland refugia

Respiratory characters of three east African haplochromine cichlid species that differ in their use of hypoxic wetlands were examined to consider the potential of dissolved oxygen as one factor affecting habitat use. All three species had a large gill surface area, ranging from the 67th ( Pseudocrenilabrus multicolor victoriae ) to 98th ( Astatotilapia velifer ) percentile of the known gill size range for freshwater fishes. Pseudocrenilabrus multicolor victoriae was the most tolerant to hypoxia exhibiting the lowest aquatic surface respiration ( Rs )thresholds and lowest critical oxygen tension of the three species. Astatotilapia velifer had the highest ASR thresholds, gill ventilation rates, and level of surface activity of the three species, indicating a relatively low tolerance to hypoxia. Prognathochromis venator was intermediate in its response to hypoxia. These findings are discussed in light of survivorship and distribution patterns of these species following Nile perch introduction into Lake Nabugabo.     

Yellow perch genetic structure and habitat use among connected habitats in eastern Lake Michigan

Maintenance of genetic and phenotypic diversity is widely recognized as an important conservation priority, yet managers often lack basic information about spatial patterns of population structure and its relationship with habitat heterogeneity and species movement within it. To address this knowledge gap, we focused on the economically and ecologically prominent yellow perch (Perca flavescens). In the Lake Michigan basin, yellow perch reside in nearshore Lake Michigan, including drowned river mouths (DRMs)—protected, lake‐like habitats that link tributaries to Lake Michigan. The goal of this study was to examine the extent that population structure is associated with Great Lakes connected habitats (i.e., DRMs) in a mobile fish species using yellow perch as a model. Specifically, we tested whether DRMs and eastern Lake Michigan constitute distinct genetic stocks of yellow perch, and if so, whether those stocks migrate between the two connected habitats throughout the year. To do so, we genotyped yellow perch at 14 microsatellite loci collected from 10 DRMs in both deep and littoral habitats during spring, summer, and autumn and two nearshore sites in Lake Michigan (spring and autumn) during 2015–2016 and supplemented our sampling with fish collected in 2013. We found that yellow perch from littoral‐DRM habitats were genetically distinct from fish captured in nearshore Lake Michigan. Our data also suggested that Lake Michigan yellow perch likely use deep‐DRM habitats during autumn. Further, we found genetic structuring among DRMs. These patterns support hypotheses of fishery managers that yellow perch seasonally migrate to and from Lake Michigan, yet, interestingly, these fish do not appear to interbreed with littoral fish despite occupying the same DRM. We recommend that fisheries managers account for this complex population structure and movement when setting fishing regulations and assessing the effects of harvest in Lake Michigan.     

The invasion of an introduced predator,Nile perch (<Emphasis Type="Italic">Lates niloticus</Emphasis>, L.) in Lake Victoria (East Africa): chronology and causes

Nile perch, a large predatory fish, was introduced into Lake Victoria in 1954. The upsurge of Nile perch in Lake Victoria was first observed in the Nyanza Gulf, Kenya, in 1979. In Ugandan waters this occurred 2–3 years later and in the Tanzanian Mwanza Gulf 4–5 years later. At the beginning of the upsurge in the Mwanza Gulf in 1983/1984 only sub-adult and adult fishes were found. The first juveniles appeared in 1985, suggesting that the initial increase of Nile perch was mainly caused by migration of sub-adults and adults. Shortly after the onset of trawl fishery in the area in 1973, haplochromines in the Mwanza Gulf started to decline. The final disappearance of the haplochromines, in 1987, only occurred after the Nile perch boom, and despite the abandoning of the haplochromine fishery in 1986. We hypothesize that the decline of haplochromines decreased predation on and competition with juvenile Nile perch and then facilitated survival of these juveniles. Consequently the immigration of sub-adult and adult Nile perch in an area may have paved the way for successful recruitment. Over-exploitation of haplochromine cichlids in the 1970s in the Nyanza Gulf, where the Nile perch upsurge was first observed, may have played a similar role.     

Shifts in the food of Nile tilapia, Oreochromis niloticus (L.) in Lake Victoria, Kenya

Studies of the food of introduced Nile tilapia, Oreochromis niloticus (L.) with respect to size, habitat and season were conducted between November 1998 and October 2000 in Kenyan waters of Lake Victoria. Stomach contents of 1980 specimens collected by demersal trawl and seining were analysed. Nile tilapia originally known to be herbivorous, feeding mostly on algae has diversified its diet to include insects, fish, algae and plant materials. The major diet of fish <5 cm total length was zooplankton whereas bigger fish included a wider range of food items in their diet. There was spatial variation in diet with insects and algae dominating in the gulf and open water habitats respectively. There was no seasonal variation in the food items ingested and diel feeding regime indicated that O. niloticus is a diurnal feeder. The shift in diet could be due to ecological and environmental changes in Lake Victoria, which have been associated with changes in composition and diversity of fish and invertebrate fauna, emergence and dominance of different flora including water hyacinth Eichhornia crassipes (Mart.) Solms‐Laub., and algae communities. The feeding habit of O. niloticus is discussed in the context of changes occurring in the lake.     

The Lake Victoria environment: Its fisheries and wetlands — a review

Recent ecological changes in the Lake Victoria ecosystem have been attributed to the effects of species stockings and, in particular, from predation pressure by the Nile perch. Evidence for the decline of haplochromines due to predation by the Nile perch, while overwhelming, does not necessarily account for these gross ecological changes. Ecological theory predicts that natural fluctuation would occur in a fisheries where predator and prey species could alternate in abundance. The absence of a substantial recovery of the endemic species within cyclical abundance patterns in Lake Victoria (even though they could be delayed), particularly in the pelagic and profundal zones, points to other causal factors.Ecological changes have occurred in Lake Victoria since the turn of the century when modern fishing methods and techniques were introduced. As the human population increased and the catchment became more exposed to diverse socio-economic activities, further pressure on the ecological functioning of the lake was compounded by exotic species stockings. Thus, declining fisheries, wetland degradation and eutrophication are part of gross environmental changes that are likely to become more manifest with the increasing impact of the water hyacinth. It therefore seems appropriate to consider the ecological changes at both temporal and spatial scales and to re-examine some of the paradigms for ecological change.Although cataclysmic impacts may have occurred between the 1960's and the early 1980's, the basic cause of ecological changes, at least in the inshore zone, appears to result from human activities partly associated with a degradation of the riparian wetlands that exposes the aquatic ecosystem to catchment activities. Overexploitation and a reduction in habitat quality and quantity in this zone could be major factors in the ecological transformations. However, the importance of the littoral zone and the ecological impact of the Nile tilapia, another stocked species, are hardly known. An additional hypothesis is therefore put forward to determine whether or not wetland vegetation types in Lake Victoria are important fish habitats. This hypothesis allows for testing several interrelated sub-hypotheses about the relationships between wetland vegetation and the ecology of Nile tilapia. The water hyacinth, a recent feature of the shoreline and an additional strain to the already endangered buffer zone, may have to be regarded as a dynamic, but permanent, part of most of the littoral zone.Corresponding Editor: Prof. P. Denny     

The use of integrated constructed wetlands (ICW) for the treatment of separated swine wastewaters

Haplochromine cichlids used to be the main prey of the introduced Nile perch, Lates niloticus, in Lake Victoria. After depletion of the haplochromine stocks at the end of the 1980s, Nile perch shifted to the shrimp Caridina nilotica and to a lesser degree to its own young and the cyprinid Rastrineobola argentea. In the present study, we investigated the Nile perch diet in the northern Mwanza Gulf after resurgence of some of the haplochromine species and compared it with data collected in the same area in 1988/1989. It became clear that haplochromines are again the major prey of Nile perch. The dietary shift from invertebrate feeding (shrimps) to feeding on fish (haplochromine cichlids) occurs at a smaller size than it did when Nile perch were taking primarily dagaa and juvenile Nile perch as their fish prey. The apparent preference for haplochromines as prey has reduced the degree of cannibalism considerably, which may have a positive impact on Nile perch recruitment.     

The destruction of an endemic species flock: quantitative data on the decline of the haplochromine cichlids of Lake Victoria

Synopsis The Lake Victoria fish fauna included an endemic cichlid flock of more than 300 species. To boost fisheries, Nile perch (Lates sp.) was introduced into the lake in the 1950s. In the early 1980s an explosive increase of this predator was observed. Simultaneously, catches of haplochromines decreased. This paper describes the species composition of haplochromines in a research area in the Mwanza Gulf of Lake Victoria prior to the Nile perch upsurge. The decline of the haplochromines as a group and the decline of the number of species in various habitats in the Mwanza Gulf was monitored between 1979 and 1990. Of the 123+ species originally caught at a series of sampling stations ca. 80 had disappeared from the catches after 1986. In deepwater regions and in sub-littoral regions haplochromine catches decreased to virtually zero after the Nile perch boom. Haplochromines were still caught in the littoral regions where Nile perch densities were lower. However, a considerable decrease of species occurred in these regions too. It is expected that a remnant of the original haplochromine fauna will survive in the littoral region of the lake. Extrapolation of the data of the Mwanza Gulf to the entire lake would imply that approximately 200 of the 300+ endemic haplochromine species have already disappeared, or are threatened with extinction. Although fishing had an impact on the haplochromine stocks, the main cause of their decline was predation by Nile perch. The speed of decline differed between species and appeared to depend on their abundance and size, and on the degree of habitat overlap with Nile perch. Since the Nile perch upsurge, the food web of Lake Victoria has changed considerably and the total yield of the fishery has increased three to four times. Dramatic declines of native species have also been observed in other lakes as a result of the introduction of alien predators. However, such data concern less speciose communities and, in most cases, the actual process of extinction has not been monitored.