Marcusenius Macrolepidotus Research Articles

Fish as a resource in a rural river catchment in the Northern Province, South Africa

The rural population in the Mutshindudi River valley, in the north-eastern corner of the Northern Province, accept locally-caught freshwater fish as food (85%), but tinned pilchards are the most commonly eaten fish. Fishing gear was recorded in 70% of the households, but only in 36% was that gear used during that same year. Most fishers were older school-going boys using hook and line as the main fishing gear, but small gill nets, a variety of traps and small-mesh seine nets were also used. Eighteen of the 27 fish species known from the Mutshindudi River were recognised by the fishers and had their own vernacular names. Catches of fishers were dominated by small tilapias, small barb species and catfish. The average catch per fishing trip of just under three hours was 162g and the fish were small (mean 26g), which was however quite acceptable to the fishers. A few older fishers sold their catches but most were taken home to be enjoyed with the main meal. Length distribution data of fishers' catches indicated fish populations with normal size distributions, but experimental gill net catches in two pools revealed populations of fish species not normally caught by fishers, including Marcusenius macrolepidotus, Barbus marequensis and Labeo molybdinus/cylindricus. This, and a low catch per unit effort, may indicate a degree of overharvesting of angling species. Measures proposed to prevent further overexploitation include a ban on draw netting as well as restocking the river with tilapia.

Electric Organ Discharges of Mormyrid Fish as a Possible Cue for Predatory Catfish

During reproductive migration the electroreceptive African sharptooth catfish, Clarias gariepinus (Siluriformes), preys mainly on a weakly electric fish, the bulldog Marcusenius macrolepidotus (Mormyridae; Merron 1993). This is puzzling because the electric organ discharges of known Marcusenius species are pulses of a duration (<1 ms) too short for being detected by the catfishes' low-frequency electroreceptive system (optimum sensitivity, 10–30 Hz; Peters and Bretschneider 1981). On the recent discovery that M. macrolepidotus males emit discharges lasting approximately ten times longer than those of females (Kramer 1997a) we determined behavioral thresholds for discharges of both sexes, using synthetic playbacks of field-recorded discharges. C. gariepinus detected M. macrolepidotus male discharges down to a field gradient of 103 μVpeak-peak/cm and up to a distance of 1.5 m at natural field conditions. In contrast, thresholds for female discharges were not reached with our setup, and we presume the bulldogs eaten by catfish are predominantly male.

Pack‐hunting in two species of catfish, Clavias gariepinus and C. ngamensis, in the Okavango Delta, Botswana

During the annual drawdown in the Okavango Delta, Botswana, large shoals of catfish migrate upstream in the main river channels. The examination of samples of the sharptooth catfish Clarias gariepinus and blunttooth catfish C. ngamensis revealed that they were pack‐hunting and selectively preying on two species of mormyrids, the bulldog Marcusenius macrolepidotus and the churchill Petrocephalus catostoma. The predation on mormyrids may be related to prey size and abundance. In addition, catfish may sense the electrical discharge given off by the mormyrids when disturbed. The catfish feed intensively for a few months and build up body reserves and reproductive condition prior to spawning on shallow floodplains with the onset of the annual floods from Angola.

Firing Activity of the Weakly-Electric Fish Marcusenius Macrolepidotus (Mormyridae, Teleostei): Logarithmic Repartition of Inter-Pulse Intervals, and Sequential Inequality Testing

AbstractThe electric activity of mormyrid fish consists of short electric pulses, all identical, but separated by variable time intervals (inter-pulse interval or IPI). Temporal structures are here extracted from the sequence of IPIs, to show non-randomness in mormyrid electric signal. The electric activity of isolated Marcusenius macrolepidotus is comparable to that of other mormyrids. The pulse waveform is biphasic and symmetrical, it lasts 0.3 to 0.5 ms and its amplitude is about 13 volts. The pulses follow each other with extremely variable IPIs (14-500 ms) with large instantaneous variations. Two new methods of analysis have been developed here. The first is based on the fact that when IPIs are grouped in populations, longer IPI populations have a wider range than shorter ones, a lognormal rather than a normal distribution. The IPIs are therefore classified according not to absolute- but to relative differences: binwidths are in percent, not in milliseconds. The second, non-parametric method (sequential inequality testing), was first used for neurone firing activity. It considers the sign (+ or -) of the difference between successive IPIs, and compares the sequence of these signs with a theoretical random model. When M. macrolepidotus is resting, the sequence of longer and shorter IPIs is not random, as shown by redundancies of patterns of + and - signs. These redundancies and the IPI populations are typical for each individual. When the fish increases its locomotor activity (either spontaneously for a few seconds, or provoked over more than 20 s) the IPIs are shorter. The patterns of + and - are different when the fish is swimming and when resting. Behavioral and electrophysiological data suggest that receiving conspecifics may recognize these rhythm features. The analysis methods developed here may be useful to other stochastic phenomena, such as neural firing activity.

ASPECTS OF THE FISHERIES OF LAKE LIAMBEZI, CAPRIVI

SUMMARY Lake Liambezi, situated in the Linyanti Swamp, Caprivi, is inhabited by 43 fish species. Population estimates with seine nets, rotenone and explosives gave values of 74 to 157 kg ha−1. The composition of seine net catches differed markedly from gill net catches where non-Cichlids form a majority. Results of a fleet of gill nets show that best catches of 25 kg 50 m−1 net night−1 can be made with 60 mm mesh nets, catching predominantly non-Cichlids (Marcusenius macrolepidotus, Schilbe mystus, Synodontis spp.). A small commercial fishery is established on the lake, with 60 active fishermen using large mesh (100–150 mm) gill nets. Their catch consists mainly of cichlids (Sarotherodon andersoni and S. macrochir) and Clarias spp. Production figures dropped from 637 ton in 1973–74 to 115 ton in 1975–76. This drop is ascribed to reduced effort as well as a reduction in catch per unit effort. Recommendations are made to utilise the fishery potential of Lake Liambezi in a more balanced way by introducing small mesh gill nets to harvest non-Cichlid populations.