Temperature and its bearing on the distribution and chemical control of freshwater snails

No Abstract

A survey of the fish fauna of Transkei estuaries Part Four: The Mntafufu and Mzamba River estuaries

The Mntafufu and Mzamba Rivers have small catchment areas (178 and 505 km2, respectively) in the forested coastal belt of Transkei and have relatively small estuaries (5 and 2 km long, respectively). Although the annual rainfall is in excess of 1000 mm in this area, the mean annual runoff was less than 70 Mm3 in both cases. Serious floods were not observed during the survey period, while turbidity and sediment loads were usually very low. Salinities were usually high (x̄ = 30,0-38,0‰) in the bottom water and temperatures in the upper reaches were usually higher than in the lower reaches. The species composition, and seasonal and spatial abundance of the fish fauna of the two estuaries were determined by means of gill nets. A total of 1043 fish (385,8 kg) representing 42 species were caught in the Mntafufu estuary. The orangemouth glassnose Thryssa vitrirostris and V. cunnesius were the most abundant fish in the estuary. Numerically 42% of the catch (46% gravimetrically) belonged to the family Mugilidae (11 species) of which Mugil cephalus, Valamugil buchanani and V. cunnesius were the most important. A total of 389 fish (241,1 kg) were caught in the Mzamba estuary. Twenty-five species were obtained with Hilsa kelee dominating numerically. Seven species of mullet contributed 38,3% of the numbers or 54,2% of the fish biomass. The diamond mullet (Liza alata) and the flathead mullet (Mugil cephalus) were the most abundant in terms of biomass. The highest catches in the two estuaries were taken in the lower reaches. Mean number and biomass of fish taken per net was 20,5 fish and 7,2 kg, and 16,2 fish and 10,1 kg in the Mntafufu and Mzamba estuaries, respectively

Investigations into the effects of concentration and duration of exposure to formalin and malachite green on the survival of the larvae and juveniles of the common carp Cyprinus carpio L. and the sharptooth catfish Clarias gariepinus (Burchell)

Prophylactic dip treatments using formalin and malachite green were applied to 4-day old larvae and 12- and 20-day old juveniles of the European common carp, Cyprinus carpio and the African sharptooth catfish, Clarias gariepinus. Treatments consisted of 100mg/l malachite green for exposure periods of 10, 30 or 90s and 200mg/l formalin, administered for 30, 60 or 90 min. Larvae and juveniles of C. gariepinus could be treated with 100mg/l malachite green for 10s, or with 200mg/l formalin for 30min, with minimum mortalities. Both chemicals affected the survival of the C. gariepinus juveniles, especially the 90 min exposure to formalin. Juveniles of both species were extremely sensitive to 100mg/l malachite green concentrations.The articles have been scanned in colour with a HP Scanjet 5590; 600dpi. Adobe Acrobat XI Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format.University of the North. Rand Afrikaans University.mn201

Treatment of Cyprinus carpio L. and Clarias gariepinus (Burchell) embryos with formalin and malachite green : effect of concentration and length of treatment on their survival

Malachite green and formalin were investigated as prophylactic dip treatments for developing embryos of Cyprinus carpio and Clarias gariepinus. Based on the findings, recommendations are made on the respective concentrations to be used and exposure periods to be followed for both formalin and malachite green for both fish species.The articles have been scanned in colour with a HP Scanjet 5590; 600dpi. Adobe Acrobat XI Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format.University of the North.mn201

A survey of the fish fauna of Transkei estuaries Part Three: The Mtata River estuary

The species composition, seasonal abundance and spatial distribution of the fish fauna of the Mtata estuary (8,5 km long) was determined by means of gill nets. Nine hundred and ninety fish (462,9 kg) comprising 26 species were caught of which M. cephalus (43,3%), A. hololepldotus (18,9%), P. commersonnii  (7,9%),  E. machnata  (7,1%) and L. amia (5,45) were most abundant in terms of biomass. Mullet species constituted 48,3% of the biomass caught, representing 25,6% of the numbers. Seasonal patterns were ascertained for M. cephalus, M. capensis, V. buchanani, A. hololepidotus, P. commersonnii, P. kaakan, E. machnata, L. amia, J. dussumieri, T. vitrirostris and L. equula. A mean of 19,4 fish with a mean mass of 9,1 kg were caught per net per station. Numerically the highest catches were made in the upper reaches; gravimetrically in the middle reaches. Temperatures ranged from 10,5-25,5°C; a salinity gradient was usually present in all reaches; oxygen levels and turbidities were high

A survey of the fish fauna of Transkei estuaries. Part 1. The Kei River estuary

During the period of survey the 8 km long Kei estuary was subjected to periodic floods which reduced salinities to 0%o and decreased light penetration to a few centimetres. Species composition, and temporal and spatial abundance were determined for fish samples collected by means of gill nets. Twenty-six species of fish were caught with Mugil cephalus, Liza tricuspidens and Argyrosomus hololepidotus predominating, in that order. The family Mugilidae comprised 67,8% of the numbers and 66,6% of the fish biomass caught. Definite seasonal patterns could not be determined for many of the fish although M. cephalus, L. richardsoni and A. hololepidotus were more abundant in summer. The effect of river flooding had unpredictable results on Mugilidae catches which either decreased or increased after floods while catches of P. commersonii and A. hololepidotus usually increased. Largest catches were made in the middle reaches although species diversity was greatest in the lower reaches of the estuary

A survey of the fish fauna of Transkei estuaries Part Two: The Mbashe estuary

The species composition, seasonal abundance and spatial distribution of the fish fauna of the Mbashe estuary (8 km long) was determined by means of gill nets. During the 37 month survey period, 2908 fish (2319,8 kg), comprising 27 species, were caught. The family Mugilidae, representing 58,8% of the numbers and 57,4% of the biomass, predominated the catches. In terms of biomass M. cephalus, A. hololepidotus and P. commersonnii were the most important species. Seasonal patterns for 10 of the 27 species were observed. M. cephalus, A. hololepidotus and L. amiawere more abundant in spring/summer, E. machnata and V. buchanani in spring, P. commersonnii and L. tricuspidens increased numerically in winter and M. capensis in autumn, Johnius dussumieri and Valamugil cunnesius were rare in winter and summer, respectively. Largest catches were made in the middle reaches of the estuary followed by the upper and lower reaches, respectively

Changes in some water quality conditions in recycling water using three types of biofiltration systems during the production of the sharptooth catfish Clarias gariepinus (Burchell): Part I: Relative efficiency in the breakdown of nitrogenous wastes by the different biofiltration units

A comparison is made of the relative efficiencies in the breakdown of nitrogenous wastes of three types of water recirculation biofiltration units used during the production of the sharptooth catfish Clarias gariepinus (Burchell). Two types of trickling filters were employed. One contained PVC shavings with a calculated surface contact area of 1 220 m2. The second is a more sophisticated biofilter unit made up with Siporax porous sintered glass cylinders with a total surface water contact area of 32 000 m2. The third filter consisted of a rotating biological contactor unit with a water contact surface area of 271.2 m2. Investigations showed that the PVC shavings filter unit was clearly the most efficient of the three by transforming more than 96% of the NH3-N into NO3-N. This was followed by the rotating biological contactor with a 93% efficiency and lastly by the Siporax filter with an almost 93% efficiency. The outcome of the section on the production of Clarias gariepinus follows in Part 2 of this series.Articl

Changes in some water quality conditions in recycling water using three types of biofiltration systems during the production of the sharptooth catfish Clarias gariepinus (Burchell): Part II: Growth and production of sharptooth catfish over a period of 78 days

An investigation into the growth and production of the sharptooth catfish Clarias gariepinus (Burchell) was conducted over a period of 75 d in recirculating water, using three types of biofiltration systems. Yields fluctuating between 195.2 kg and 242.1 kg were obtained with the mean fish mass per m3 of water ranging between 19.5 and 22.5 kg. A biofilter system using PVC shavings was not only found to be the most efficient in the transformation of nitrogenous wastes, but also produced the highest growth and production of catfish as well as the best feed conversion ratio of the three biofiltration systems used.Articl

Dimensions of change detection within the phenomenon of change blindness

The phenomenon of change blindness (CB) has recently been investigated from a number of perspectives. Basically it entails the limited ability to perceive gross changes in one’s visual environment. In a recent experiment, Simons and Levin (1998) showed that persons do not notice when a stranger asking them for directions is switched with another person when the switch is concealed briefly by two persons walking between them carrying a door. CB specifically pertains to a limited ability to perceive disparity in scenes, changes between elements (‘second-order information’) and personal visual impressions (Rensink 2000:2). The rider is that the changes must occur during a flicker, saccade, blink, similar interruption or an eye movement (Simons & Levin 1997; Rensink 2000). One popular way to investigate change blindness is by means of the so-called flicker technique (see Simons 2000). This entails showing persons a series of slides of real-world scenes. A particular aspect of the scene is then changed. The original and the changed slide are shown consecutively with a brief blank slide inserted between them. The interposition of the grey slide creates a flickering display. It was hypothesised that this brief interruption in the visual sequence makes it difficult to perceive the changes in the scenes as it disturbs our ability to pinpoint specific changes in the scenes by interposing a series of transient movement changes in the flicker cycle. A number of interesting conclusions were made on grounds of this difficulty to perceive the change in elements of a scene. Firstly, that the brain does not build up or internalise a reasonably full and rich visual representation of the environment. Secondly, CB indicates that this representation is unstable and very sketchy and that one possibly relies on the external environment as a form of a memory extension