Food composition of Clarias gariepinus (= C. lazera) (Cypriniformes,Clariidae) in Lake Kinneret (Israel)

The feeding behaviour of the freshwater piscivore, Clarias gariepinus (C. lazera) (C & V 1840) was studied over two periods: 1973–1975 and 1981–1982, in Lake Kinneret (Israel). The total number of fish analysed was 264 and their sizes (SL) and weights varied between 238 and 830 mm (146 to 5728 g). More than fifty species of plants and animals from the plankton, benthos and nekton of Lake Kinneret were identified in the intestines of C. gariepinus. Preyed fish were the most abundant food component (81%) and constituted the highest biomass, with Mirogrex terraesanctae representing the majority (although other species were also found). The potential impact of piscivory in the Kinneret ecosystem is considered.     

On Lecane tanganyikae new species (Rotifera: Monogononta, Lecanidae)

A new species of rotifer, Lecane tanganyikae n.sp., is described from the littoral of Lake Tanganyika and some water bodies in the plain of the Rusizi river near Lake Tanganyika, Bujumbura Province, Burundi. The affiliation of the new species, especially in relation to L. elsa Hauer, is discussed.     

New hyalellids (Crustacea, Amphipoda, Hyalellidae) from Lake Titicaca

Two new species of hyalellid amphipods, Hyalella crawfordi and H. gauthieri, are described from Lake Titicaca; H. echinus (Faxon, 1876) is redescribed. The H. echinus group of species is newly proposed for these three species, and a group diagnosis is provided. A key to the three species in the group is provided Full article published online at http://www.senckenberg.de/odes/06-10.htm.     

Four new species of Gomphocythere (Crustacea, Ostracoda) from Lake Tanganyika, East Africa

Four new species of the ostracod genus Gomphocythere are described from Lake Tanganyika (East Africa): Gomphocythere downingi n. sp. G. coheni n. sp., G. wilsoni n. sp., and G. woutersi n. sp. All species are endemic to the lake and are found within a variety of substrates and depths. The addition of these four new species brings the total number of endemic Gomphocythere species in Lake Tanganyika to nine. Other Gomphocythere species are known from water bodies throughout East and South Africa and in the Levant. Brooding is an important, but not a unique, preadaptation for the persistence and taxic prolific speciation of this lineage in Lake Tanganyika.     

The phenology ofProcladius crassinervis (Zett),Procladius signatus (Zett) andProcladius choreus (Mg) (Diptera,Chironomidae) at Lake Hald,Denmark

In an investigation of the phenology of the chironomid species of Lake Hald, Denmark, a very late flight activity (September–November) of the univoltine speciesProcladius choreus (Mg.) was observed. Among observations of the phenological patterns of univoltine chironomids inhabiting lakes of the Baltic type in Denmark, the phenological pattern ofProcladius choreus stands alone (JÓNSSON, 1987; LARSEN, 1991). Further two species of the genusProcladius Skuse,viz. Procladius crassinervis (Zett) andProcladius signatus (Zett), were found in the investigation and the phenological pattern of the three congeneric species was clearly allochronic, withProcladius crassinervis flying in spring,Procladius signatus flying in summer andProcladius choreus flying in autumn.     

Nest-building and communal care of young byTilapia rendalli dumeril (pisces,cichlidae) in Lake Malawi

Synopsis Tilapia rendalli of Lake Malawi dig large tunnels in their nests and practise communal care of young. Both these traits are considered to be antipredator adaptations.     

Distribution and aspects of population dynamics of the zebra mussel,Dreissena polymorpha (Pallas, 1771), in the lake IJsselmeer area (The Netherlands)

Summary The distribution, settling and growth of the zebra mussel, Dreissena polymorpha Pallas, in Lake IJsselmeer and Lake Markermeer in The Netherlands were studied from 1980 to 1985. In these lakes D. polymorpha is the most important food source for wintering diving ducks. The study was part of an investigation into the carrying capacity of the lakes for these birds.     

Weather-driven ecology of planktonic diatoms in Lake Tovel (Trentino,Italy)

The seasonal development of planktonic diatoms in Lake Tovel has been studied from 2002 to 2004 within the SALTO research project. The objective was to investigate the role of weather conditions and lake hydrology in regulating the development of Cyclotella and Fragilaria functional species groups. The different responses of the two species to the environmental variability and their reciprocal relationships were considered. Cyclotella showed a weak relationship with the selected environmental variables and its seasonal development was difficult to interpret, also in relation to its opportunistic behaviour. On the contrary, Fragilaria complex showed a prompt response to environmental variability and its seasonal development appeared to be mainly regulated by hydrological conditions, through nitrate nitrogen and, secondarily, by silica concentrations. Water temperature and thermal stability affected only Fragilaria species, while different nutrient requirements allowed Cyclotella species to grow under conditions of limited resources for Fragilaria. Since Lake Tovel has scarce internal nutrient reserves and its hydrology is strongly affected by precipitation both in winter and summer, diatom phenology seems to be indirectly regulated by weather conditions.     

Feeding habits of larval Mirogrex terraesanctae (Steinitz, 1952) in Lake Kinneret (Israel) I. Field study

Analysis of the food composition, distribution, lengths and dry weights of larval bleak, Mirogrex terraesanctae (Steinitz, 1952) in Lake Kinneret, Israel was conducted. Indices of electivity of ingested prey organisms were also determined. Larvae were surveyed throughout the 1989–90 reproductive seasons (Dec–Mar). Small larvae (<10.28 mm SL) preferentially select prey items smaller than 180 µ. Dominant zooplankters in the larval guts included: Bosmina longirostris, Ceriodaphnia reticulata and C. rigaudi, Asplanchna priodonita, Synchaeta oblonga, S. pectinata and juvenile cyclopoid copepods. Diaphanosoma brachiurum was rarely found (instars only) and no adult copepods were observed in larval guts. Some larvae specimens were found with up to 500 undigested (intact) accidentally ingested cells of Peridinium spp. per gut.     

Predation of Arternia cysts by water-tiger larvae of the genus Anacaena (Coleoptera,Hydrophilidae)

Artemia cysts are preyed upon by the larvae of Anacaena (Coleoptera, Hydrophilidae) in the wet supralittoral of the hypersaline Solar Lake near Elat. The larvae are specialized feeders of the cysts, as shown by laboratory experiments.     

The breeding season of tilapia, Oreochromis niloticus L. in Lake Awassa (Ethiopian rift valley)

The breeding season of Oreochromis niloticus in Lake Awassa was studied from fish caught monthly from December 1987 to November 1988 in relation to a number of environmental factors. Fish at various stages of gonad development and spawning were caught in almost all months, but breeding fish were more frequent twice a year: main peak occurred during January–March and a secondary one occurred during July–September. Gonadosomatic index (GSI) values peaked twice in the study year, a major peak being during January–March and a less pronounced one during July–September. Thus, it was concluded that O. niloticus in Lake Awassa breeds intensively in the former and less intensively during the latter period, and that some breeding occurs at any time of the year. Intensive breeding activity during January–March appeared associated with increase in solar radiation and sunshine hours d^–1 whereas that during July–September appeared associated with heavy rainfall. However, intensive breeding in both periods coincided with increase in phytoplankton biomass. Previously, phytoplankton biomass in this lake is shown to increase following increase in nutrients due to mixing- and rainfall-associated changes in the hydrography and hydrology of the lake. Thus, although further studies are required, increase in phytoplankton may be one of the environmental cues to stimulate spawning in O. niloticus in Lake Awassa whereas other factors such as sunshine and rainfall may have indirect roles through their effects on the hydrology and hydrography of the lake ultimately resulting in changes in phytoplankton biomass.     

A marine calanoid copepod (Temora longicornis) in a semi-stagnant coastal lagoon (Lake Grevelingen,S.W.-Netherlands)

The marine calanoid copepodTemora longicornis was introduced in saline Lake Grevelingen (S.W.-Netherlands), when the lake was continuously flushed with North Sea water in 1979, to enhance the reduced salinity. Development of the copepod proceeded suboptimally as a consequence of an unfavourable environment in the lake: 1. The phytoplankton assemblage, mainly consisting of crytomonad flagellates, less suitable for growth and reproduction of the adult stages. 2. An acute food shortage occurring at a crucial moment in the development. 3. A. pronounced salinity-and oxygen stratification in June, preventing further development ofTemora.Communication nr. 474 of the Delta Institute for Hydrobiological Research, Yerseke.     

Echiuropus bekmanae n.sp. (Crustacea, Amphipoda, Carinogammaridae) from Lake Baikal, retrieved by a new deep-water sampling device

Lake Baikal is well known for its exceptional diversity of gammarid amphipods, with 95% of endemics from its 270 described species representing about 20% of the world freshwater fauna. Echiuropus (Asprogammarus) bekmanae n.sp. was collected by an `autonomous trap system', a sampling device thoroughly described here. Although not necrophagous itself, this new species was found in two baited trap samples (at 750 and more than 1600 m) from 1995 and 1996. This rather small (13 mm) deep water species is the eighth species of the subgenus Asprogammarus, of the Baikal endemic genus Echiuropus. Diagnoses of the subgenus and the closer species are given.     

Cyclomorphosis in Keratella tropica (Apstein) of Lake Asejire,Nigeria

Of one thousand, one hundred and four (1104), Keratella tropica collected with a 55 m mesh plankton net from the surface of Lake Asejire from February 1974 to February 1976 lorica length and width, and the lengths of the posterior spines were measured.Lake Asejire K. tropica are among the smallest in Africa. Variations in the patterns of spine length development were independent of season as individuals without left posterior spines and with relatively long left and right posterior spines occurred as commonly in the rainy as in the dry season months.Correlations between K. tropica dimensions and thirty environmental factors underscore the importance of physical environmental factors and point to biological and genetic factors as determinants of the patterns of cyclomorphosis in K. tropica.     

The late-glacial/holocene Bosmina (Eubosmina) fauna of Lake Constance (Untersee) (F.R.G.): traces of introgressive hybridization

Subfossil Bosmina (Eubosmina) remains were analysed in a sediment core from the Untersee of Lake Constance which covered the Late-Glacial/Holocene period.During the Late-Glacial and the early Holocene the lake was inhabited only by Bosmina longispina Leydig. In the Late Holocene a second species, Bosmina coregoni f. kessleri Uljanin, appeared. In the uppermost sediment layers the morphological gap between the two taxa disappeared, apparently from introgressive hybridization.     

Feeding habits of larval Mirogrex terraesanctae (Steinitz, 1952) in Lake Kinneret (Israel) II. Experimental study

Interactions between the larvae of Mirogrex terraesanctae (Steinitz, 1952) in Lake Kinneret, Israel, and their zooplankton prey were studied experimentally. Prey species preference and size selectivities were measured. Larvae were hatched in the lab from eggs collected in the field, and fed different food items in various concentrations. The food items included lake zooplankton, algae, and commercial pellets. It was shown that small, first feeding larvae (7–8.5 mm SL) prefer small bodied zooplankters (< 180 µ). The effect of these food sources on larval growth was measured. It was found that larval Mirogrex grew at a higher rate when fed zooplankton prey sized from 63 µ–250 µ. Food items smaller than 63 µ, larger than 250 µ and Scenedesmus sp., produced less than optimal growth rates. The importance of Mirogrex feeding habits and their potential influence on the Kinneret ecosystem is considered.     

Decline in zoobenthos densities in the profundal sediments of Lake Mendota (Wisconsin,USA)

High densities of zoobenthos inhabited Lake Mendota's profundal zone in the early 1900s through the mid-1940s. Chaoborus punctipennis was the most abundant organism during the winter, along with moderate densities of Chironomus spp., Pisidium sp., oligochaetes, and Procladius sp. By the early 1950s, Chaoborus punctipennis densities had declined to 10% of former levels, while Chironomus increased significantly. However, by the mid-1960s, Chaoborus, Chironomus, and Pisidium densities had decreased to very low population levels. By 1987–89, Pisidium was no longer found. Zoobenthos that had not decreased from earlier surveys were oligochaetes and Procladius, although further sampling of oligochaetes is needed to confirm current densities. These organisms are the most tolerant of severe anoxia.Four possible reasons for this decline were evaluated: (a) decline in food availability, (b) increase in fish predation, (c) use of toxic insecticides in the drainage basin, and (d) changes in the profundal sediment environment. Based on literature information and long-term data for Lake Mendota, a change in the profundal sediment environment is the most likely explanation for the decline in the less-tolerant zoobenthos species. Although the duration and extent of anoxia in the hypolimnion have not changed since the early 1900s, hypolimnetic ammonia and hydrogen sulfide concentrations apparently have increased as Mendota became more eutrophic after the mid-1940s. However, further study is needed to determine if these higher concentrations or other factors were responsible for the dramatic decline in lake Mendota's profundal zoobenthos.     

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.