The systematics of the genus Lates (Teleosti: Centropomidae) in Lake Albert, East Africa

The systematics of thc genus Lares Cuvier, 1828 in Lake Albert were examined. It is concluded that the inshore form, originally described as Lates albertianus Worthington, 1929 and subsequently as L. niloticus albertianus should be considered as a synonym of L. niloticus (Linné, 1762) but that the offshore form, Lates macrophthalmus Worthington, 1929, warrants full systematic status. It had been relegated to subspecific status, L. niloticus macrophthalmus .
The lectotype for L. macrophthalmus is selected and redescribed. A method by which the species could have evolved, based on suspected differences in partial pressures of oxygen at which the blood haemoglobins of the two species become saturated, and on desiccation of the lake in the Pleistocene period, is described.
The diagnostic value of allometry in distinguishing between the two species on a morphological basis is discussed, as is the distribution of the two species in Lake Albert.
The possible significance of interspecific differences in the shape of the caudal peduncle and caudal fin is considered.     

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.     

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 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.     

Trophic Niche Segregation in the Nilotic Ichthyofauna of Lake Albert (Uganda, Africa)

Synopsis Nile perch, Lates niloticus, and Nile tilapia, Oreochromis niloticus, were originally transplanted from Lake Albert in western Uganda to the African Great Lakes, Lake Victoria and Lake Kyoga, where they are partially implicated in reduction of the fish species diversity. Lake Albert is facing multiple environmental changes, including declining fish species diversity, hyper-eutrophication, hypoxia, and reduced fish catches. To examine the role of Nile perch and Nile tilapia in the food web in their native Lake Albert, we estimated their diets using stable nitrogen and carbon isotopes. In Lake Albert, the tilapiine congeners (closely related species), Tilapia zillii, Oreochromis leucostictus, and Sarethorodon galilaeus, and the centropomid Nile perch congener, Lates macrophthalmus, have narrower diet breath in the presence of the native O. niloticus and L. niloticus. A computerized parameter search of dietary items for five commercially important fish species (Hydrocynus forskahlii, Bagrus bayad, L. niloticus, Alestes baremose and Brycinus nurse) was completed using a static isotopic mixing model. The outcome of the simulation for most fish species compared favorably to previously published stomach contents data for the Lake Albert fishes dating back to 1928, demonstrating agreement between stable isotope values and analyses of stomach contents. While there were some indications of changes in the diets of L. niloticus and A. baremose diets over the past 20 years in parallel with other changes in the lake, for the most part, food web structure in this lake remained stable since 1928. The Lake Albert fish assemblage provides insight into the invasion success of L. niloticus and O. niloticus.     

PREDATION AND ITS EFFECTS ON MIGRATION AND SPECIATION IN AFRICAN FISHES: A COMMENT

Contrary to a recently expressed view very little factual evidence has been brought forward to support the idea that predatory fishes, and especially Lates and Hydrocynus , have had restrictive effects on speciation among other African freshwater fishes. A considerable body of data supports the view that the effects of these fishes do not differ from those of other predators and that predation has facilitated speciation. Further reasons why Lates and Hydrocynus cannot be considered as having effects different from those of other predators are given.
The situation in lakes containing these predators is reconsidered. There is no need to attempt to minimize their effects in Lake Tanganyika where their presence and effects are fully compatible with the existence of a rich endemic fauna. The situation in Lakes Albert and Rudolf is more satisfactorily explained by the brief duration of the existence of the present-day lakes and by their recent invasion by a fully differentiated Nilotic fauna, than by imagining that Lates and Hydrocynus have restricted speciation.
The suggestion that Lates and Hydrocynus were responsible for the inception of the habit of upstream migration for purposes of spawning is discredited, and what seem to be more probable reasons for the phenomenon are given.     

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.     

尼罗尖吻鲈和鳜鱼染色体组型分析及比较

采用PHA、秋水仙碱腹腔或背部肌肉注射,活体培养法,以前肾为材料,低渗-空气干燥法制片,进行染色体观察,运用Micromeasure version 3.3染色体分析软件和Photoshop 7.0软件首次分析了尼罗尖吻鲈的染色体数目和核型,并同鳜鱼染色体数目和核型进行了分析比较,对今后拟采取的杂交尝试提供理论基础。结果显示：尼罗尖吻鲈染色体众数为2n=48,核型公式为2m+4sm+12st+30t,染色体臂数（NF）为54;鳜鱼染色体众数为2n=48,核型公式为6sm+12st+30t,染色体臂数（NF）亦为54;两种鱼染色体短臂上均无随体,单臂染色体较多。分析表明尼罗尖吻鲈与鳜鱼杂交成功的可能性较大。     

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 .     

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.     

The distribution and variation of Daphnia lumholtzi (Crustacea: Cladocera) in relation to fish predation in Lake Albert, East Africa

In Lake Albert Daphnia lumholtzi is found in two forms. One has a pointed anterior prolongation, or helmet, on the head. The other has a shorter rounded head, and was originally described as a separate species, D. monacha . The latter form dominates the zooplankton in the middle of the lake where planktivorous fish are rare or absent. The helmeted form becomes commoner near the margins of the lake and reaches its greatest abundance in Ndaiga Lagoon, where planktivorous fish are common. The possession of a helmet is associated with a reduction in the size of the carapace compared to the round headed form. The carapace with its contained eggs is the most conspicuous part of a cladoceran, so that the helmeted forms are at an advantage in the presence of planktivorous fish which locate their prey by sight. The mid-lake monacha forms are larger than specimens of the same form in Ndaiga Lagoon, where it is shown that Alestes baremose feeds selectively on the larger specimens of the monacha form.
The helmeted form produces more, but smaller eggs than the monacha form. The total brood volume (= mean egg volume x mean number eggs per female) is greatest in the midlake monacha forms. The selective advantages of variations in egg size and the possession of a helmet are discussed. It is concluded that the data from Lake Albert support the hypothesis of Brooks (1965) concerning the adaptive significance of helmet development in Daphnia .     

Recruitment control and production of market-size Oreochromis niloticus with the predator Lates niloticus L. in the Sudan

Experiments were conducted in earth ponds (242–1260m²) to evaluate the Nile perch (Lates niloticus L.) as a predator for recruitment control and the production of marketable Oreochromis niloticus L. in the Sudan. Supply of the predatory fish was maintained by induced spawning of L. niloticus brooders by raising the water level in a 1–0 ha earth pond. Lates niloticus fingerlings (7–5 cm total length) were then stocked with same size O. niloticus at the ratios 1:5, 1:10 and 1:15, Lates:Oreochromis , respectively. Over a period of 7 months Lates niloticus reduced young 50g) Oreochromis population and enhanced the production of preferred-size ( 200 g) Oreochromis. The ratio 1:5 Lates:Oreochromis was established to be the most desirable for Lates -with a total production of 0.2428 kg/m² of O. niloticus with 55.7% (by weight) at the target size averaging 287.5g. The ratios 1:10 and 1:15 produced 0.2106 and 0.2153 kg m² of O. niloticus , with 49.4% and 16.9% averaging 235.0 and 210.0 g, respectively.     

Fishes and fisheries of Lake Nubia,Sudan

Developing lake conditions in Lake Nubia were found to offer good commercial fisheries potential. This prospect initiated fishery research since 1967.In this study, 26 fish species belonging to 10 families were recorded. Compared with previous studies the total number has decreased and some of the species that were commercially important during the early years of the lake formation, are now negligible in occurrence (Distichodus, Citharinus, Bagrus). The species widely distributed in the lake are Labeo niloticus, Lates niloticus, Sarotherodon niloticus, Hydrocynus forskallii, Alestes baremose and Eutropius niloticus. Their highest production occurs during two periods: February to May and July to September. The dominant fish species feed on different items in a balanced equilibrium with the ecological and biological conditions.     

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.     

The distribution and variation of Daphnia lumholtzi (Crustacea: Cladocera) in relation to fish predation in Lake Albert, East Africa

In Lake Albert Daphnia lumholtzi is found in two forms. One has a pointed anterior prolongation, or helmet, on the head. The other has a shorter rounded head, and was originally described as a separate species, D. monacha . The latter form dominates the zooplankton in the middle of the lake where planktivorous fish are rare or absent. The helmeted form becomes commoner near the margins of the lake and reaches its greatest abundance in Ndaiga Lagoon, where planktivorousfish are common. The possession of a helmet is associated with a reduction in the size of the carapace compared to the round headed form. The carapace with its contained eggs is the most conspicuous part of a cladoceran, so that the helmeted forms are at an advantage in the presence of planktivorous fish which locate their prey by sight. The mid-lake monacha forms are larger than specimens of the same form in Ndaiga Lagoon, where it is shown that Alestes baremose feeds selectively on the larger specimens of the monacha form.
The helmeted form produces more, but smaller eggs than the monacha form. The total brood volume (= mean egg volume x mean number eggs per female) is greatest in the midlake monacha forms. The selective advantages of variations in egg size and the possession of a helmet are discussed. It is concluded that the data from Lake Albert support the hypothesis of Brooks (1965) concerning the adaptive significance of helmet development in Daphnia .     

Associations of Rotifera in the zooplankton of the lake sources of the White Nile

Plankton samples for rotifers were collected by means of vertical hauls with phytoplankton nets at eight stations on Lake Albert during the course of a year. Similar hauls were taken from Lakes Kyoga, Victoria, George and Edward in October and November. The rotifers found in the samples are listed and some estimates of their abundance and seasonal occurrence in Lake Albert are given. The most important planktonic rotifers are Keratella tropica and several species of Brachionus . Other species may also become locally or temporarily abundant; Lecane bulla becomes numerous in samples taken near vegetation after disturbance by rough weather, or where blue-green algae are abundant.
The associations of rotifers at the stations on Lake Albert and in the other lakes have been compared by means of the Sorensen Index and the index of diversity. The highest diversity is found in situations with a high rate of flow, as at the mouth of the Victoria Nile, where the extra species are non-planktonic forms swept into suspension by the current. This high diversity is associated with low numbers of individuals per unit volume. In Lake Albert rotifers are most consistently present and abundant at the mouth of the River Semliki, where the rate of flow is moderated by a large reed swamp. The middle of Lake Albert is poor in rotifers, and this may be related to the sparseness of planktivorous fish coupled with competitive elimination of the rotifers by larger crustacean zooplankters.
The associations of rotifers in Lakes Kyoga and George are similar, and resemble one another more than they resemble the associations in their neighbouring deeper lakes. The association in Lake Kyoga also resembles the associations found in water of a similar depth at the northern end of Lake Albert.     

Associations of Rotifera in the zooplankton of the lake sources of the White Nile

Plankton samples for rotifers were collected by means of vertical hauls with phytoplankton nets at eight stations on Lake Albert during the course of a year. Similar hauls were taken from Lakes Kyoga, Victoria, George and Edward in October and November. The rotifers found in the samples are listed and some estimates of their abundance and seasonal occurrence in Lake Albert are given. The most important planktonic rotifers are Keratella tropica and several species of Brachionus . Other species may also become locally or temporarily abundant; Lecane bulla becomes numerous in samples taken near vegetation after disturbance by rough weather, or where blue-green algae are abundant.
The associations of rotifers at the stations on Lake Albert and in the other lakes have been compared by means of the Sorensen Index and the index of diversity. The highest diversity is found in situations with a high rate of flow, as at the mouth of the Victoria Nile, where the extra species are non-planktonic forms swept into suspension by the current. This high diversity is associated with low numbers of individuals per unit volume. In Lake Albert rotifers are most consistently present and abundant at the mouth of the River Semliki, where the rate of flow is moderated by a large reed swamp. The middle of Lake Albert is poor in rotifers, and this may be related to the sparseness of planktivorous fish coupled with competitive elimination of the rotifers by larger crustacean zooplankters.
The associations of rotifers in Lakes Kyoga and George are similar, and resemble one another more than they resemble the associations in their neighbouring deeper lakes. The association in Lake Kyoga also resembles the associations found in water of a similar depth at the northern end of Lake Albert.     

ZOOPLANKTON OF LAKES MUTANDA, BUNYONYI AND MULEHE

Quantitative samples of zooplankton from three lakes in the Kigezi District of Uganda have been studied. The systematics of the zooplankton are considered, and some of the identifications given by Worthington & Ricardo (1936) in a previous study of one of these lakes are revised.
Lake Mulehe is the shallowest of the three lakes and contains the largost standing crop of zooplankton. This is in agreement with chemical data which indicato that the supply of nutrient salts in Lake Mulehe is higher than in the other two lakes.
In October 1962 the zooplankton of Lake Mutanda was characterized by the relative abundance of three species of Daphnia which were not found in the samples from the other lakes, although two of these species were present in Lake Bunyonyi in 1931. Rotifers were sparse in Lako Mutands, but here dominant by Tetramastix opoliensis. Lake Bunyonyi was richer in rotifers, but here the dominant species was Keratella tropica , whilo in Lake Mulehe the dominant rotifer was Synchaeta pectinata.
The zooplankton of Lake Mutanda in October 1962 was similar in composition to that of Lake Bunyonyi in 1931, but in 1962 the zooplankton of Lake Bunyonyi was more like that of Lake Mulehe. The possible causos of this change are discussed.     

Spermatozoa morphology and ultrastructure in Nile perch,Lates niloticus (Linnaeus, 1758)

Lates niloticus is a valuable commercial fish species with good potential for aquaculture. However, there is limited information on the type and structure of the Nile perch spermatozoon, which could potentially aid in culture of this species. Here, we describe the spermatozoon ultrastructure in L. niloticus using transmission and scanning electron microscopy. The spermatozoon had a round head-shape, medio-laterally flat, no acrosome, a short midpiece located laterally to the nucleus, uniflagella with one wing. The head of the spermatozoon contained the nucleus, centriolar system, proximal part of the flagellum, and cytoplasmic channel. Centrioles were arranged at an angle of 90° to each other, forming a T-shape, parallel to the nucleus. The midpiece was cylindrical, loaded with cytoplasm, five to seven spherical mitochondria; and the flagellum’s plasma membrane extended to form one lateral wing. The spermatozoa were classified as type II spermatozoa. L. niloticus spermatozoon differed from that of its Australian congener L. calcarifer, especially in the centriole arrangement and nuclear shape, length of the midpiece and the number of mitochondria and lateral wings.     

Fatty acid composition of some coregonid species (Coregonus spp.) from different European lakes

The muscle tissue of coregonid 'lavarello' (Coregonus forma hybrida ) from Lake Bolsena (Central Italy), 'bondella' (C. macrophthalmus ) from Lake Maggiore (Northern Italy) and 'bondelle' (C. macrophthalmus ) from Lake Neuchtel (Switzerland), sampled during the whitefishes' growth season, were analysed to evaluate the fat content and the fatty acid composition. In fact, the lipid profile of the muscle is strongly subjected to environmental influence and, as a consequence, its use as a parameter allowing 'identification of origin' of the fish products may be possible. ω3 HUFA (highly unsaturated fatty acids = C 20:5 + C 22:5 + C 22:6) content of fish from Lake Bolsena was lower than in coregonid from Lake Maggiore, while the saturated fraction (SFA) was higher. Fish from Lake Neuchtel showed a lower ω3 HUFA proportion than in the two other lakes. These data demonstrate that significant differences in fatty acid composition occur in similar fish species living in different lakes. Further studies are required to identify the role of each environmental variable, with special attention to temperature and food range.