The macroinvertebrate fauna of water hyacinth fringes in the Sudd swamps (River Nile,southern Sudan)

The water hyacinth now forms an almost ubiquitous fringe to open-water habitats in the permanent wetland of the Sudd. In a typical river-lake the fringe varied between 9 and 16 m in breadth with highest plant biomasses in the centre. Largest quantities of macroinvertebrates in hyacinth root-mats were obtained in drop-trap samples within 6 m of the open water. They were dominated by coleopterans, odonatans, gastropods andCaridina nilotica. Outer edges of the fringe offer easy access, good dissolved oxygen concentrations, and a variety of potential food resources for aquatic invertebrates. By contrast the less hospitable landward zone contained a reduced fauna in which coleopterans, hydracarines and gastropods were prominent. A marked reduction in the numbers of invertebrates in lift-net samples of detached hyacinth rafts was attributed to browsing by fish. Hyacinth root-mats appear to have replaced the niches formerly provided by those of the Nile cabbage for aquatic invertebrates in the Sudd.     

Zooplankton associations in the swamps of southern Sudan

Samples from the main Nile channel yield a poor zooplankton, deficient in species and heavily laden with organic debris. Samples from side arms and floodplain lakes yield a richer zooplankton, often containing 15–20 species of rotifers and ten or more species of Crustacea. When the floodplain lakes are heavily vegetated the number of species increases. For the Crustacea the index of diversity () normally varies between 0.6 and 2.4, but in a side arm of the Bahr el Zeraf it reaches 6.1 and in Lake Ambadi on the Bahr el Ghazal it reaches 8.4. The diversity of the rotifers is somewhat higher, normally ranging between 2 and 4.5, but reaching 23 in Lake Ambadi. The zooplankton of Lake Ambadi is the most distinctive and contains several species not found at other stations. The reasons for the peculiar zooplankton of Lake Ambadi are discussed.     

Fish communities in the perennial wetland of the Sudd, southern Sudan

SUMMARY. 1. Cyperus papyrus and Typha domingensis swamps were recognized as the major ecological zones in the perennial wetland of the Sudd.
2. Fish sampling, chiefly by gill-netting and electric fishing, was concentrated in the open-waters of channels and lakes together with submerged, fringing and marginal vegetation in the accessible papyrus zone. Sixty-two species were recorded including fifteen catfishes. thirteen characoids, seven cyprinids, seven mormyrids and seven cichlids.
3. The highest diversity and abundance of tishes was obtained in the shallow Sudd lakes which revealed considerable uniformity in population structures. The most numerous species in gill-nets were Alestes dentex, Synodontis frontosus and S. schall , whereas Heterotis niloticus, Mormyrus cashive and Distichodus rostratus assumed importance in terms of biomass. Micralestes avutidens and cichlids were abundant among the small fishes inhabiting fringes of Eichhornia crussipes , and air-breathing species occurred beneath the vegetation mat.
4. Flowing-water habitats exhibited a similar but restricted fauna. Catches were dominated by Synodontis frontosus in open-water and by Micralestes acutidens and Chelaethiops bibie in the margins. The latter species was shown to have a clear preference for running water.
5. Peak catches in a representative river-lake could be interpreted as the arrival of fish for breeding or their migration through the lake for reproduction elsewhere. The capture of fry or juveniles of many species in fringing or marginal vegetation indicates that the permanent floodplain of the Sudd provides suitable spawning and rearing grounds.
6. Access to the extensive Typha zone was limited but from available evidence it was inferred that air-breathing fishes, notably Heterotis and Gymnarchus , may penetrate the swamps to reach areas remote from perennial transmission channels.     

The seasonality of zooplanktivorous fish in an African reservoir (Gebel Aulia Reservoir,White Nile,Sudan)

The fish fauna of Gebel Aulia reservoir (White Nile) consists of 52 species withAlestes baremoze Joannis andBrycinus nurse Rüppell dominating in numbers and biomass. The seasonality and condition ofA. baremoze, A. dentex (L.) andB. nurse are described in relation to various limnological factors in the reservoir, close to the dam. The zooplankton assemblage is small-sized, consists mainly of small Cladocera and shows striking seasonality. The hydrological regime, temperature and the grazing effect of fish are the major factors influencing its development.The three species belong to a eurytopic but basically riverine fish guild. They show a seasonal cycle of abundance with high numbers at periods of water storage (October–April) and low numbers during riverine conditions. This seasonal pattern of occurrence, the size structure of the population and the maturity indices give strong evidence that no resident population is present at the study site, but upstream migrations of the immature fish occur at low water level (May–September). Return migrations of juvenile fish into the area at the dam proper take place from October until April.

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Zusammenfassung In der Fischfauna des Gebel Aulia Stausees am Weißen Nil im Sudan dominieren Arten der Gatung Alestes. Die quantitative Bedeutung und das jahreszeitliche Auftreten vonA. baremoze Joannis,A. dentex Cuvier andB. nurse Rüppell wurde im Zusammenhang mit dem saisonalen Zyklus der hydrologischen und limnologischen Bedingungen untersucht. Die Häufigkeit der Fische im Staubereich ist in der Stauphase (Oktober–April) hoch und in den Monaten, in denen kein Rückstau erfolgt (Mai–September), gering.Das saisonale Muster des Auftretens, die größenstruktur der Fischpopulation and die Maturitätsfaktoren im eigentlichen Staubereich keine seßhafte Population vorhanden ist. Die Fische unternehmen kurz vor der Geschlechtsreife anadrome Wanderungen, die Jungfische wandern katadrom.

     

Food and feeding relationships of young fish in the first weeks after the beginning of exogenous feeding in Lake Opinicon,Ontario

Synopsis Young fish of six species in an open water community all began their exogenous feeding by taking nauplii and small cyclopoids of body length (less tail) of 0.1–0.3 mm. Appearance of larvae of the different species in the system was, however, sequential, the resource being utilized by the different species in turn. The sequence was: Perca flavescens, Percina caprodes, Pomoxis nigromaculatus, Ambloplites rupestris, Lepomisgibbosus and L. macrochirus.The larvae and juveniles changes their diet rapidly as they grew. Fish 10–14 days after hatching and 8–10 mm in length i.e. close to the beginning of the juvenile period, consumed larger-bodied prey items (including several genera of Cladocera) and had more diversified diets than the 4.5–6.0 mm first-feeding larvae. These differences, and progressive dispersal of the larger young from the area, served to minimize the chances of food competition between batches of young of different ages.The composition of the fish community of larvae changed from week to week as new species entered it, increased in size and departed. Patterns of food utilization changed accordingly.Numbers of cyclopoids, their nauplii, and Bosmina longirostris, fell rapidly in May — early June, and did not increase again until August. These changes coincided with the rise and fall in numbers of the young fish in the habitat.Paper presented in the symposium lsEmbryonic and larval periods of fishes: present status and significancers, Canadian Conference for Fisheries Research (CCFFR), Fredricton, New Brunswick, January 1977.     

Fish communities in the eastern,seasonal-floodplain of the Sudd,Southern Sudan

Annual flooding of the permanent Sudd swamps innundates large areas of Oryza longistaminata grasslands. Fish migration into a southeast section of the seasonal floodplain was led by Clarias gariepinus, Polypterus senegalus and several small species which were intent on spawning. Channa obscura and juveniles of Oreochromis niloticus were prominent among later entrants. From a total of 23 species caught in seasonal habitats, 7 contributed over 97% of total estimated numerical and biomass densities. The latter ranged from 435–580 kg ha^–1. In the 1982–83 flood-season entry to the floodplain was essentially restricted to species tolerant of oxygen-deficient waters and thereby able to penetrate the papyrus and Typha swamps of the permanent wetland. Migrants became stranded in temporary pools and subject to heavy cropping by birds.     

Trichodinid ectoparasites (Ciliophora: Peritrichida) of some River Nile fish, Egypt

Four species of trichodinid ectoparasites (Ciliophora: Peritrichida) were collected from the gills of the following River Nile fish in Egypt: Hydrocynus forskalii, Mormyrus kannume, Schilbe mystus. These species are: Trichodina heterodentata Duncan, 1977, Trichodina fahaka sp. n., Trichodinella epizootica Raabe, 1950, Tripartiella dactylodentata sp. n. Photomicrographs and morphometric data are presented for each species.     

Interactions between Nile tilapia (Oreochromis niloticus) and cladocerans in ponds (Khartoum,Sudan)

Juvenile Nile tilapia (Oreochromis niloticus) are omnivorous, and the question asked in this study is how they affect on their environment? Do they mainly act as predators on the cladoceran zooplankton or do they compete with the cladocerans for phytoplankton? This problem was studied in three ponds with and three ponds without small tilapia (3–5 cm). The fish growth rate, the succession of plankton species and the changes in abiotic conditions, were monitored over a period of 67 days. The fish biomass was kept low and the mean was approximately constant (12.6 g m^?2) during the experiment. Phosphate was added to avoid phytoplankton nutrient limitation. Although the diet of Nile tilapia contained both phytoplankton and zooplankton, the fish affected the ecosystem in a similar way as zooplanktivorous fish. The fish ponds got more phytoplankton due to increase of Chlorophyta. Effects on the other phytoplankton groups Euglenophyta, Bacillariophyta, Cryptophyta and Cyanophyta could not be registered. The ponds without fish had higher densities of Daphnia lumholtzi and D. barbata. The other Cladocerans seemed less influenced by fish presence. The relative fish growth rate was most positively correlated with the density of Daphnia lumholtzi, Diaphanosmoa excisum and Bosmina longirostris. Tilapia seemes to have two feeding modes: (1) preying on large zooplankton and (2) unselective filtration of small planktonic organisms such as phytoplankton. In our experiment the first feeding mode affected the ecosystem more than the second.     

Dragonflies (Insecta,Odonata) from the Red Sea Hills and the main Nile in Sudan

Twenty-one species of dragonflies are recorded from the Nile and the Red Sea Hills in the Sudan. The majority are species of africotropical origin, but wide-ranging and tolerant of strongly fluctuating environmental conditions. A small but significant fraction is restricted to the Nile Valley, and another fraction, found only in the Red Sea Hills, is composed of Eremian species of Palaearctic origin. Besides adults, the last-instar larvae of Pseudogrion niloticum and of Paragomphus pumilio are described and figured.     

Trophic niches and feeding relationships of shorebirds in southern Brazil

Niche theory predicts that sympatric species should differ in some ecological characteristic, to allow co-existence and reduce competition for key resources. Food is critical on wintering grounds and stopover areas for migratory species that need to accumulate reserves in order to complete their migration. Wetlands of the Rio Grande do Sul coastal plain, in southern Brazil, host several species of shorebirds with similar morphology, foraging methods and diet. When these species are in sympatry, some trophic niche overlap is expected. Diets and trophic niches of migratory and resident shorebirds were investigated during the austral summer on Torotama Island, Lagoa dos Patos Estuary, Brazil. Complementary methods were used to determine the trophic ecology of three shorebird species; diet was determined through analysis of feces and food samples, using stable isotopes of carbon and nitrogen. The local invertebrate community was sampled to determine potential prey and ascertain feeding preferences of birds. Coleoptera was the most abundant taxon in the feces of all shorebirds. Trophic niche overlap in the diets was high, with the widest trophic niche found for the buff-breasted sandpiper Calidris subruficollis. Isotopic mixing models indicated differences in the main food sources of shorebirds. The isotopic niche breadth was widest for the American golden-plover Pluvialis dominica. These species, as well as the resident southern lapwing Vanellus chilensis, consumed some prey in higher proportions over others, although they had generalist diets. Migratory species with generalist habits benefit from heterogeneous environments such as floodplains during the non-breeding season.     

Effect of supersaturated water on fish in the River Nidelva, southern Norway

Fish mortalities in the River Nidelva, South Norway in 1978, were suspected to be caused by Gas Bubble Disease. In June 1980, the hydrological situation occumng in the Nidelva in 1978 was reconstructed, and the effects were analysed by keeping fish in cages. Examination of the dead fish showed that most were killed by Gas Bubble Disease. The results also showed that brown trout, Salmo trutta , was the least tolerant, and eel, Anguilla anguih , was the most tolerant to dissolved gas supersaturation. Only fish kept near the surface were killed while fish kept at 3 m depth were mildly affected due to hydrostatic pressure compensation. In contrast to the situation in 1978, few of the wild fish were killed during the experiment in 1980.     

Interannual variability in the feeding of ice fish (Notothenioidei, Channichthyidae) in the southern Scotia Arc and the Antarctic Peninsula region (CCAMLR Subareas 48.1 and 48.2)

We have studied the annual variation in food intake of three sub-Antarctic ice fish species (Champsocephalus gunnari, Chaenocephalus aceratus, and Pseudochaenichthys georgianus) and three high-Antarctic ice fish species (Chionodraco rastrospinosus, Cryodraco antarcticus, and Chaenodraco wilsoni). Stomach content analyses were conducted during bottom trawl surveys around the South Shetland Islands in 1998, 2001, 2002, 2003, and 2007, the South Orkney Islands in 1999 and 2009, and off the north-western Antarctic Peninsula in 2002, 2006, and 2006/2007 in order to obtain further insight into the amount of food of Antarctic demersal fish consume during summer. Annual variation in food intake was comparatively low within an area in the krill-feeding species C. gunnari and C. wilsoni. Food intake was much more variable, by a factor of 2 or 3 among years and areas, in larger C. aceratus and C. antarcticus, which rely heavily on fish as their dietary source. Food consumption was intermediate in the two species P. georgianus and C. rastrospinosus, which rely on both krill and fish.     

Sandflies (Diptera: Psychodidae) in a focus of visceral leishmaniasis in White Nile, Sudan

Visceral leishmaniasis (VL) has been known to occur since the 1980s on the western bank of the White Nile River (Central Sudan), 150 km south of Khartoum, and has resulted in high mortality. The most recent outbreak of the disease in this area began in 2006. Entomological surveys were carried out during May 2008, June 2010 and May and July 2011 in the White Nile area. Sandflies were collected using Centers for Disease Control light traps and sticky oil traps in the village of Kadaba and the nearby woodland. Phlebotomus females were dissected for the presence of Leishmania promastigotes. A total of 17,387 sandflies, including six species of Phlebotomus and 10 species of Sergentomyia, were identified. The Phlebotomus species recorded were Phlebotomus orientalis, Phlebotomus papatasi, Phlebotomus bergeroti, Phlebotomus duboscqi, Phlebotomus rodhaini and Phlebotomus saevus. P. orientalis was collected in both habitats. The relative abundance of P. orientalis in the woodland habitat was higher than that recorded in the village habitat. In the woodland habitat, there was a notable increase in the relative abundance of P. orientalis during the surveys conducted in 2008 and 2010 compared to 2011. None of the 311 P. orientalis females dissected were infected with Leishmania promastigotes, although relatively high parous rates were recorded in both habitats. Based on the distribution of P. orientalis recorded in this study, this species is the most likely vector of VL in the endemic focus in the White Nile area. Further investigation is required to elucidate the seasonal abundance and distribution of the vector, as well as the transmission season of VL in both habitats so that appropriate control strategies for the vector can be designed.     

Length–weight relationships and length at first maturity for fish species in the upper Miranda River,southern Pantanal wetland,Brazil

Length–weight relationships (LWR) were estimated for 17 species and total length at first maturity (L50) for three species of freshwater fishes from the Miranda River, southern Pantanal, Brazil. The b values were compared for some species in the Paraguay River basin with the northern (Cuiabá River) part of the basin; differences in length–weight relationships were significantly different for Pseudoplatystoma corruscans, P. reticulatum (syn. P. fasciatum). First references on L50 and LWR are presented for two and eight fish species, respectively, as well as the new maximum total length for two species.     

On the biology of female Epiplatys bifasciatus (Cyprinodontidae) from southern Sudan

The biology of female Epiplatys bifasciatus (Steindachner, 1881) from the Sudd region of southern Sudan was investigated. The population was dominated by females whose growth in body dimensions relative to total length was found to be linear. The reproductive system differs from that of oviparous teleosts in having a portion of the ovary (anterior) modified for carrying the developing brood. Gradual development of oocytes towards the nursing portion of the ovary, and low absolute fecundity of the brood, suggests a rather long breeding season (June to September).A large variety of food organisms belonging to three main groups, viz. insects, crustaceans and arachnids, were taken by Epiplatys collected from the littoral zones of two rivers, the Bahr El Ghazal and the White Nile. Little variation occurred in the food of these two populations. Chironomid larvae showed the highest occurrence frequency in the fish food for both localities, and insects in general constituted over 80% of all organisms eaten.     

Food consumption by fish populations in the Warta River,Poland, before and after impoundment

The impact of impoundment on the energy budget and gross ecological efficiency (K ₁) of fish populations in sites upstream from the backwater (AB) and downstream of the dam (CD) of a manmade reservoir (17.6 km²) located in the middle reaches of the lowland Warta River (808 km long), was investigated.Consumption (MJ ha^–1 yr^-1) in four years following the reservoir's impoundment was 34–74% lower in site AB, and 43–75% in site CD than before the impoundment. K ₁ of investigated species was higher before the impoundment and only pike exhibited an inverse efficiency trend. Significant differences were also observed in the diet of certain dominant fish species, which resulted from a change in the food supply as well as a reduction in their life span after the river was dammed.     

Length–weight relationships of 14 Indian freshwater fish species from the Betwa (Yamuna River tributary) and Gomti (Ganga River tributary) rivers

The length‐weight relationships (LWRs) were studied of 588 fish covering eight families, 13 genera and 14 species (Notopterus notopterus, Gudusia chapra, Labeo calbasu, Puntius sarana, Cirrhinus mrigala, Ompok bimaculatus, Mystus tengara, Mystus cavasius, Sperata aor, Sperata seenghala, Eutropiichthys vacha, Wallago attu, Rhinomugil corsula, and Mastacembelus armatus) captured in the Betwa River (tributary of the Yamuna River) and Gomti River (tributary of the Ganga River) from December 2007 to January 2009. The b values varied between 2.4 (M. armatus) and 3.52 (P. sarana), with the mean b = 2.96 at P < 0.001 for all species. The observations are significant for conservation and management because the Betwa River has been approved under India’s first interlinking plan with the Ken River, and no length‐weight data had thus far been reported for the Gomti River. The objective was to evaluate the LWRs of these two unstudied rivers for fisheries management.