27 research outputs found

    Growth and mortality of dagaa (Rastrineobola argentea, Fam. Cyprinidae) in Lake Victoria

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    Growth and mortality parameters of the small Lake Victoria cyprinid Rastrineobola argentea were determined from length-frequency analysis, using the ELEFAN I and II programs. The results of two sampling programs, both performed during 1988, one in Uganda (mosquito seine) and the other in Tanzania (pelagic trawl), were highly corresponding, In comparison with previously published data on the growth of dagaa and some similar species, low values for L sub( infinity ) (65 mm standard length) and K (1 year super(-1)) were found. Total mortality (Z) amounted to 3.9-4.4 year super(-1). A single annual breeding peak was observed both in Uganda (October/November) and in Tanzania (February/March)

    Spatial and seasonal patterns in the feeding habits of juvenile Lates niloticus (L.) in the Mwanza Gulf of Lake Victoria

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    Flexibility in the feeding habits of juvenile Nile perch (1¿30 cm total length) was studied from September 1988 to September 1989 at four sites (depth range: 1¿25 m) in the Mwanza Gulf of Lake Victoria. During this period haplochromine cichlids were virtually absent in the area. We looked at the combined effects of predator size, season and habitat. Stomach content analysis showed that with increase in size, the diet of Nile perch shifted from zooplankton and midge larvae, to macro-invertebrates (shrimps and dragonfly nymphs) and fish. At a size of 3¿4 cm Nile perch shifted from size-selective predation on the largest cyclopoids to predation on the largest, less abundant, calanoids. Zooplanktivory ended at a size of ca. 5 cm. Although an ontogenetic shift in the diet of juvenile Nile perch was obvious at all sampling stations, the contribution of prey types appeared to be habitat related. With increasing water depth the frequency of occurrence in the diet of most prey types decreased, but that of shrimps increased. At the entrance of the gulf (20¿25 m deep) shrimps were the main food source throughout the year. Halfway the gulf (12¿16 m), Nile perch showed seasonality in their feeding behaviour. Shrimps were taken there especially during the rainy season (January to May) when their densities at this station were high, whereas cannibalism prevailed during the rest of the year. In an environment with Nile perch and dagaa as alternative prey, shrimps were taken almost exclusively. They could be regarded as a key prey for Nile perch between 5 and 30 c

    The shift to smaller zooplankton in Lake Victoria cannot be attributed to the 'sardine' Rastrineobola argentea (Cyprinidae)

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    After the population increase of introduced Nile perch (Lates niloticus) in Lake Victoria, a decrease in the relative abundance of herbivorous cladocerans and calanoids was reported for the pelagic zone. The shift from large herbivores to small-bodied predatory cyclopoids has been attributed to the increase of the zooplanktivorous cyprinid dagaa (Rastrineobola argentea), which rose concomitantly with the Nile perch. However, we observed that in the Mwanza Gulf of the lake cyclopoids dominated the zooplankton community already before the rise of dagaa. Furthermore, there are indications that dagaa takes fewer calanoids than expected from their representation in the environment, although a slight preference for cladocerans may exist. Calanoids dominated the diet of small Nile perch of about 4 cm length, but juvenile Nile perch did not occur in the study area until after the size shift in zooplankton. The lumped biomass of dagaa and small Nile perch is lower than the biomass of the original haplochromine-dominated community of zooplanktivorous fish. Thus, the decline of large zooplankters in the lake cannot be explained by intensified predation after the replacement of zooplanktivorous haplochromines by dagaa and Nile perch. Increased eutrophication, resulting in a shift in predominance from diatoms to cyanophytes, is suggested as an alternative explanation for the shift in zooplankton composition

    SYNTHEGRA 2003 042

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    SYNTHEGRA 2004 168

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    Naar aanleiding van de voorgenomen uitbreiding van een jachthaven op het adres ‘Syl 12’ te Terherne, gemeente Boarnsterhim, is ecologisch onderzoek verricht in het kader van de toetsing van de Flora- en faunawet. Het plangebied heeft een oppervlakte van circa 1 ha en ligt in het Terhornstermeer. Het meer wordt intensief gebruikt door watersporters en bevat meerdere jachthavens. Aansluitend aan de noordkant van de bestaande steigers van de jachthaven zullen in het water twee nieuwe steigers met ligplaatsen voor jachten worden aangelegd. Op basis van een op 11 oktober 2004 afgelegd veldbezoek is het plangebied aangemerkt als van vrij lage natuurwaarde. Er worden geen negatieve effecten van de voorgenomen ingreep op beschermde planten- en diersoorten verwacht. Hoewel zich In de directe omgeving van het plangebied een vogelrichtlijngebied bevindt, zal dit geen enkel effect ondervinden van de voor de uitbreiding van de jachthaven noodzakelijke werkzaamheden. De kans dat realisatie van de extra ligplaatsen tot negatieve effecten op het vogelrichtlijngebied zal leiden lijkt klein. Er hoeven geen ontheffingen in het kader van de Flora- en faunawet te worden aangevraagd

    Dietary shifts in Brycinus sadleri (Pisces : Characidae) from southern Lake Victoria

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    We studied the diet of Brycinus sadleri in Lake Victoria after extensive environmental changes during the 1980s. To check for diet expansion following these changes, as observed in some other fish species, we compared our results with data from the 1950s. Stomach contents were analysed in relation to fish size and time of the day to investigate ontogenetic and diel shifts in the diet. Comparison of diets before and after the environmental changes showed a shift to more generalist feeding. The main food sources in the 1950s were plants during daytime and surface insects at night. Hardly any plant material was eaten during the late 1980s, but chironomid larvae, chaoborid larvae and pupae, shrimps, Odonata nymphs and fish contributed to a broader daytime diet. These prey species were also commonly taken at night, though, like in the 1950s, surface insects accounted for the major part of the diet. We found dietary shifts during ontogeny. Juveniles fed exclusively on zooplankton. Chironomid larvae, relatively large food items in comparison with zooplankton, were the key prey of sub-adult fish. Their importance decreased in the diet of adults, which included even larger prey like shrimps, Odonata nymphs and fish. At night, surface insects replaced chironomid larvae in the diet of adult fish. Diet expansion may result from reduced competition after the decline in fish mass following the Nile perch boom. An alternative explanation, increasing the prey spectrum in response to deteriorated light conditions, is not likely to hold for B. sadleri. The ontogenetic shift to increasingly larger prey and the nocturnal shift to surface insects probably reflect optimal foraging behaviour and diel vertical migration patterns of the fish respectively. These dietary shifts emphasize the feeding flexibility of B. sadleri, which may have been essential in surviving the environmental changes in Lake Victoria

    The macrobenthos fraction accessible to waders may represent marginal prey

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    The relationship between relative body condition (deviation from expected mean body weight) and burying depth was investigated in five macro-zoobenthic species living in a marine intertidal habitat. Body weight increased with depth when animals of the same size were compared. The increase amounted to 50% in the clam Scrobicularia plana, ca. 40% in the worm Nereis diversi-color, 25% in the clam Macoma balthica and 20% in the cockle Cerastoderma edule and the clam Mya arenaria. Only a part of the prey was within reach of some feeding wader species. Therefore prey value may be overestimated if one does not take into account the fact that shallow and accessible prey often have a relatively poor body condition.</p
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