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.     

Changes of Copepoda populations in Lake Kiret during 1969–1985

Long period analysis of Copepoda populations in Lake Kinneret indicated significant reductions in total biomass and production as well as densities of nauplii and adult stages, particularly females. The reduction of adult Mesocyclops sp. was statistically significant, but that of Thermocyclops sp. was not. Annual averages of eggs/female values and densities of copepodite stages did not show significant changes. Concentrations of adult Thermocyclops sp. became higher relative to the larger genus Mesocyclops sp. It is suggested that densities of large copepods were affected by intensification of predation pressure by particulate feeding fishes. The densities of nauplii were probably reduced by increased predation pressure of filter feeding fish.     

Filter mesh size and food particle uptake by Daphnia

Food size selection of four Daphnia, species (D. magna, D. hyalina, D. galeata, D. pulicaria) was investigated using spherical plastic beads as artificial food and with small bacteria. The size of the particles ranged from 0.1 to 35 m with special emphasis to the particle diameters between 0.1 and 1 m. In one set of experiments a mixture of differently sized particles was offered as food suspension and the selectivity of filtering was determined by comparing the size spectrum of the particles found in the gut contents with the spectrum in the food suspension. In a second series of experiments suspensions of uniformly sized particles were offered to single animals and their feeding activity was observed directly. In both types of experiments the mesh sizes of the filtering apparatus of the respective animals studied were measured after the experiments by, scanning electron microscopy. The mean sizes of the filter meshes were about 0.4–0.7 m. In all experiments the size of the particles found in the gut or those which caused high feeding activities were larger than the smallest mesh sizes of the filters. As a consequence simple mechanical sieving provides a sufficient explanation for the mechanism of particle retention of the filtering process in Daphnia. D. magna was found to feed with high efficiency on suspended freshwater bacteria, the residual species investigated showed low filtering efficiencies when bacteria were offered as food.The present study was supported by Deutscher Akademischer Austauschdients     

The use of inorganic substances to stimulate gut evacuation in Daphnia magna

Observations were done on the effect of inorganic substances on the gut evacuation process in Daphnia magna. Procedures which accelerate this process are described.     

The influx of carbon into Lake Kinneret in 1971/1972 by Jordan zooplankton

The carbon influx by Jordan zooplankton into Lake Kinneret was found to be negligible, compared to the total amount of organic carbon. The majority of zooplankton biomass (61%) is due to short pulses of Cladocera whilst 30% and 9% are due to Rotifera and Copepoda respectively.     

Predation by inland silversides on an exotic cladoceran, Daphnia lumholtzi, in Lake Texoma, U.S.A.

In the early 1990s, Lake Texoma was invaded by Daphnia lumholtzi , a large, spiny, exoticcladoceran. From April 1994 to April 1995 D. lumholtzi was most abundant in the lake in mid-summer, after native zooplankton declined in early June. Inland silversides Menidia beryllina selectively preyed on D. lumholtzi but appeared to prefer the large native zooplankton when they were present. Daphnia lumholtzi was an important prey item for silversides during summer when large native zooplankton was scarce. The invasion of D. lumholtzi into Lake Texoma may benefit some zooplanktivorous fishes by increasing foraging opportunities during a time of low prey availability.     

Diel variation in the vertical distribution of fish and plankton in Lake Kinneret: a 24-h study of ecological overlap

Diel vertical migration (DVM) behaviour is a predator avoidance mechanism observed within many zooplankton species in the presence of zooplanktivorous fish. A 24-h survey was carried out in June 1998 to investigate diel variation in the vertical distribution of fish, zooplankton and phytoplankton (chlorophyll) in Lake Kinneret, Israel. Fish revealed diel variation in vertical distribution but had no spatial overlap with zooplankton, and consequently no apparent influence on zooplankton dispersal. Zooplankton revealed some diel variation in distribution being affected by thermocline and oxycline position and movement of the internal the internal seiche wave. Cyclopoid species closely follow the movement of the seiche wave implying that, due to their greater motility, they are following conditions that are suitable to them. The Cladocera species and small rotifers only partly, which may be part of their phototaxic behaviour. Physical forces like convection, horizontal and vertical forcing probably have a role in contributing to a homogeneous distribution of the plankton by preventing stratification or interfering with the more motile zooplankton which may be attempting to migrate.     

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.     

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.