Investigational piscivory of some juvenile Australian freshwater fishes by the introduced Mozambique tilapia Oreochromis mossambicus

Experimental tanks were used to observe predatory effects in three different size classes of Mozambique tilapia Oreochromis mossambicus (one of the world's most widespread exotic species and generally regarded to be a herbivore or both herbivore and detritivore) when tested against 10 juvenile Australian freshwater fish species, and significant levels of predation against all were recorded. There was a general trend for larger O. mossambicus to kill more prey and this was also reflected in a separate series of experiments using juvenile barramundi Lates calcarifer over a range of size classes. Predatory effects by O. mossambicus broadly reflected the accepted models of predator–prey interactions, being that mortality (and survival) was closely related to relative body size and mouth gape limitation. Experimental evidence for piscivory in O. mossambicus was supported by field sampling that detected prey fish remains in 16% of all fish surveyed ( n = 176). The recognition of active piscivory by O. mossambicus in laboratory and field situations is the first such evidence, and suggests a need to re-evaluate the nature of their effects in introduced environments.     

The consequences of habitat occupation and habitat overlap of the introduced tilapia Oreochromis mossambicus and indigenous fish species for fishery management in a Sri Lankan reservoir

Habitat occupation and habitat overlap of the introduced tilapia Oreochromis mossambicus and co-occurring indigenous fish species were studied in a shallow lowland reservoir in Sri Lanka. Results were used to discuss the possible existence of empty habitats in reservoirs and the position of the introduced O. mossambicus in the fish community. Predictions were made of the effects of a subsidiary small-meshed gillnet fishery for indigenous minor cyprinids on the yield of tilapia. The zooplanktivorous halfbeak Hyporamphus gaimardi, an indigenous invader from brackish water estuaries and lagoons, has successfully colonized the pelagic habitat in this reservoir. The habitat of the introduced O. mossambicus overlapped significantly with those of the indigenous minor cyprinids. Only the smallest size classes of O. mossambicus (below 45 mm) arc spatially segregated from these cyprinids. Spatial distribution patterns are influenced by the rainy seasons which trigger an inshore movement of O. mossambicus and several other species around November and May. Barbus spp, (Cyprinidae) need riverine habitats for spawning, but for all other species the available habitats within the reservoir are suitable to complete their full life cycle. Exploitation of Barbus spp. with gillnets with a mesh size of 30-mm stretched mesh is predicted to have considerable detrimental effects on the existing fishery for O. mossambicus. However, a gillnet fishery with 15-mm stretched mesh to exploit Amblypharyngodon melettinus, Rasbora daniconius and H. gaimardi in the open water zone is expected to be successful without harming the existing fishery for O, mossambicus or the populations of Barbus spp.     

Haematological assessment of the effects of the anaesthetic MS 222 in natural and neutralized form in three freshwater fish species: intraspecies differences

The effects of different concentrations of natural MS 222 and neutralized MS 222 were studied on the haematology of Cyprinus carpio, Sarotherodon mossambicus and Salmo gairdneri . As judged from the results, MS 222, which is acidic in nature, produced haemo-concentration effects in all species studied, being least in the trout followed by carp and Sarotherodon mossambicus . These differences are ascribed to acid-base regulatory functions and metabolic activities of the fish species investigated. The use of neutralized MS 222 improved the haematological profiles markedly and resulted in stabilization of acid-base parameters and red blood cells sizes and numbers. Haemoconcentration effects, however, still persisted. Trout were found to be more susceptible to the stress of MS 222 anaesthesia than carp and Sarotherodon mossambicus .     

The haemoglobins of the fish Sarotherodon mossambicus (Peters): functional significance and ontogenetic changes

The haemoglobin patterns of adult and young larval stages of the fish species Sarotherodon mossambicus were determined by polyacrylamide–gel electrophoresis. Oxygen affinity determinations of the adult and larval haemolysates and of each haemoglobin component of the adult indicates that the affinity changes with the pH, temperature and osmotic pressure. The possible evolutionary significance of the multiple haemoglobins is discussed. The electrophoretic pattern of adult S. leucostictus , a species closely related to S. mossambicus was also determined and used as a comparative study.     

Food and feeding ecology of piscivorous fishes at Lake St Lucia, Zululand

The food and feeding ecology of piscivorous fish in Lake St Lucia was monitored for two years. Piscivorous fishes feed predominantly on the planktivorous Gilchristella aestuarius and Thryssa vitrirostris but a wide range of prey species was recorded. Numbers of the predominant piscivores, Argyrosomus hololepidotus and Elops machnata , in an area appear to be related to the densities of their major prey, T. vitrirostris and G. aestuarius . Large piscivorous fishes are restricted to the deeper portions of the lake, whereas small piscivores such as Johnius belengerii and Terapon jarbua feed predominantly on small fishes in the littoral zone. The highly significant correlation between the composition of prey fish species in the lake and prey fish species in the diet of piscivorous fishes, indicates that piscivores are feeding in a density dependent manner. However, factors such as habitat, fish size and swimming speed of prey species are shown to be important in prey selection. Juvenile fish of species such as Sarotherodon mossambicus, Liza macrolepis and Acanthopagrus berda remain in shallow marginal areas, thus avoiding large piscivorous fishes. However by frequenting shallow areas these species become vulnerable to bird predators, especially egrets and herons.     

Accumulation of the organophosphate blackfly larvicide Abate (temephos) in Sarotherodon mossambicus, with reference to the larvicidal control of Simulium damnosum

This paper describes the uptake and fate of the organophosphate larvicide Abate (temephos) in the tropical food fish, Sarotherodon mossambicus (Peters), during exposure to concentrations (nominally 0.001–0.1 mg dm⁻³) similar to those being produced in West African rivers by the WHO Onchocerciasis Control Programme. S. mossumbicus was able to absorb Abate both directly from the surrounding water, and by contaminated food. Maximum residue concentrations were proportional to dosage and were elevated in starving fish. Internal distribution depended on tissue fat content. Some fish produced the sulphoxide metabolite, and total residues diminished slowly during recovery.     

Occurrence and distribution of Clinostomum tilapiae Ukoli, 1966 and Euclinostomum heterostomum (Rudolphi, 1809) metacercarial infections of freshwater fish in Venda and Lebowa, southern Africa

During 1980 to 1982, parasitological surveys were carried out at large impoundments in Venda and Lebowa, southern Africa, coinciding with fish harvesting programmes. Of the various fish species examined, only Oreochromis mossambicus was found to be infected by Clinostomum tilapiae prevalent in the gills, and Euclinostomum heterostomum prevalent in the muscles. Distribution records and infection statistics are presented for these two parasite species.     

Some chemical and physical characteristics of the semen of Cyprinus carpio L. and Oreochromis mossambicus (Peters)

The chemical and physical characteristics of the semen of Cyprinus carpio and Oreochromis mossambicus were investigated on a comparative and seasonal basis. Results were obtained for the volume, colour and viscosity of the semen, the percentage live spermatozoa, sperm concentrations and the degree and the duration of sperm motility under different environmental conditions. All showed distinct interspecific and less conspicuous seasonal intraspecific differences. Values have also been determined for the organic and inorganic components and the pH and osmolality of the seminal fluid. Some inorganic and organic components indicated seasonal intraspecific differences while all the components, with the exception of sodium and galactose, indicated significant interspecific differences. These observations suggest significant interspecific differences in the testicular secretions of the two fish species which add to the basic physiological differences recorded for the spermatozoa of C. carpio and O. mossambicus . The probable function of the carbohydrates, protein and enzymes present in the semen of these fish species are discussed.     

Studies on the ichthyo-fauna in Plover Cove Reservoir, Hong Kong: I. Sequence of fish population changes

Gill-net surveys were made weekly from May 1971 to April 1974 in Plover Cove Reservoir, Hong Kong and the results analysed to determine the fish species composition in the reservoir, the seasonal fluctuation of catches and the length frequency distribution of some major species.
Over 20 species were represented in the fish fauna including almost all the naturally occurring species as well as the stocked species. Of the twenty species, some were collected regularly in gill-net samples while others occurred rarely. Sarotherodon (Tilapia) mossam-bicus (Peters) was the dominant species and contributed the main bulk of the catch. This species also showed distinct age groups in the reservoir and modal changes occurred seasonally. Thus, among the fish species studied only S. mossambicus showed a steady and progressive increase in abundance in the reservoir and the full development of this fish population with its wide range of feeding habits should be beneficial.     

Uptake, metabolism and excretion of the molluscicide N-tritylmorpholine by the tropical food fish Sarotherodon mossambicus (Peters)

Uptake, metabolism and excretion of radio-labelled N-tritylmorpholine (Frescon) were studied in the tropical food fish Sarotherodon mossambicus (Peters). After 55 h exposure to sub-lethal solutions, concentration factors in whole fish were as high as 1300 (up to 40 mg/kg), at least 80 % of the absorbed activity being unchanged Frescon. Approximately 40 % of this activity was found in bone, 16% in muscle, 16% in gut and 10% in gill. The bile was found to be a route of Frescon excretion, and recovering fish excreted 50% of the absorbed activity in 55 h. The applicability of these results to the field situation is discussed.     

The effects of diet and digestive cycle on the gastrointestinal tract pH values in the goldfish, Carassius auratus L., Mozambique tilapia, Oreochromis mossambicus (Peters), and channel catfish, Ictalurus punctatus (Rafinesque)

The digestive tract pH values of the teleost fish Oreochromis mossambicus, Carassius auratus and Ictalurus punctatus were determined in fish that were (1) starved, (2) fed herbivorous, omnivorous and carnivorous diets and (3) fed altered pH diets. The digestive tract pH profile was determined throughout an 8 h cycle. The herbivorous or carnivorous nature of the diet did not affect the pH values in any of the species. Significant differences in gut pH values were observed when the fish ingested diets with altered pHs. There was a difference between the starved and baseline fed values in Carassius auratus which was not observed in the other species. The pH values in the digestive tracts of the three species varied during the digestive cycle.     

Diel variations in the selected serum immune parameters in Oreochromis mossambicus

Almost all metabolic processes in an organism alternate through high and low activity phases with a regular periodicity of nearly 24h. These daily/diel variations are governed by factors such as light, weather conditions, availability of food or predator activity. The immune system in fish is expected to follow the same routine based on external cues from the environment which it lives. The present study was carried out to investigate such daily/diel variations in selected immune parameters such as serum lysozyme and peroxidases activity, total serum globulin level and peripheral blood leukocyte count in Oreochromis mossambicus. The fish were maintained in semi natural condition (i.e.12L:12D). The results showed significant rise in serum peroxidases and lysozyme between 0200 h and 0600 h of the day and serum cortisol exhibited elevated level between 2200 h and 0600 h. Total serum globulin exhibited peak concentration from 1400 h to 1800 h. Thus suggesting the possibility of rhythmic functioning of immune system in O. mossambicus.     

Occurrence and distribution of Dolops ranarum Stuhlmann, ectoparasite of freshwater fish in the Transvaal, South Africa

During 1982–83 fish parasitological surveys were carried out in 15 dam reservoirs in the Transvaal. The fish ectoparasite Dolops ranarum was found on eight different fish species of which Oreochromis mossambicus. Clarias gariepinus and Barbus marequensis are regarded as main hosts. Distribution records and infestation statistics are presented. It was found that D. ranarum is restricted to the Limpopo Drainage System. An evaluation of site preference on host fish reveals that the mouth cavity, in the case of O. mossambicus , and skin and fins, in the case of C. gariepinus , were the sites most intensively infested by this parasite.     

Growth rate and tissue magnesium concentration in adult freshwater tilapia, Oreochvomis mossambicus (Peters), fed diets differing in magnesium content

The growth rate and magnesium concentration in scales, bone and muscle of freshwater tilapia, Oreochronüs mossambicus (Peters), initially weighing between 70 and 300 g, were followed during low-magnesium feeding. The growth rate decreased in fish on low-magnesium diets, but no changes were observed in the magnesium concentration in the scales, bone or muscle. No changes were observed in calcium or sodium concentrations in these tissues. We conclude that adult tilapia fed a low-magnesium diet manage, in contrast to other fish species, to maintain their magnesium balance and must therefore obtain magnesium from the water.     

Effects of osmolality and ions on the motility of stripped and testicular sperm of freshwater-and seawater-acclimated tilapia, Oreochromis mossambicus

A significantly higher concentration of testicular spermatozoa was obtained from freshwater Oreochromis mossambicus (9·9×10⁹ spermatozoa ml⁻¹) than seawater O. mossambicus (4·6×10⁹ spermatozoa ml⁻¹). The mean osmolality of the urine of freshwater fish (78·5 mOsmol kg⁻¹) was significantly different from that of seawater fish (304·8 mOsmol kg⁻¹). The mean length of the mid-piece of the spermatozoa together with the tail was more variable in freshwater O. mossambicus (8·80±0·23μm) than in seawater specimens (8·27±0·18 μm). Stripped sperm of freshwater O. mossambicus was highly contaminated by urine which was a good activator of sperm motility in O. mossambicus held in both fresh and sea water. The osmolality for initiation of motility in freshwater O. mossambicus spermatozoa was from 0 to 333 mOsmol kg⁻¹ while for seawater O. mossambicus spermatozoa it was from 0 to 1022 mOsmol kg⁻¹. The optimum osmolality for motility was from 70 to 333 mOsmol kg⁻¹ for freshwater O. mossambicus spermatozoa and from 333 to 645 mOsmol kg⁻¹ for seawater fish. In freshwater O. mossambicus spermatozoa, the presence of 20 mM CaCl₂ increased the permissive osmolality of NaCl from 184 to 645 mOsmol kg⁻¹. For seawater O. mossambicus spermatozoa, solutions of NaCl devoid of CaCl₂ were unable initiate motility, but the addition of 1·5 to 30 mM CaCl₂ to the NaCl solution (0–934 mOsmol kg₁) had a full motility initiating effect.     

Fine structure of chloride cells in freshwater- and seawater-adapted Oreochromis niloticus (Linnaeus) and Oreochromis mossambicus (Peters)

Ultrastructural features of branchial chloride cells in Oreochromis niloticus (Linnaeus) and O. mossambicus (Peters) adapted to both fresh water and sea water were compdred. In freshwater adapted fish of both species chloride cells showed similar morphological features. Multicellular complexes made of a mature chloride cell and one or more accessory cells sharing a single apical crypt have been observed. Whereas high percentages of 0. mossambicus survived at maximum salinity only a few individuals of 0. niloticus showed the capacity to adapt to sea water. In the seawater-adapted individuals of 0. niloficus and 0. rnossambicus the chloride cells showed a two- and three-fold increase in sizei. respectively. Most chloride cells are organized in large multi-cellular complcxcs with apical interdigitations of accessory cells and 'leaky junctions'. These results indicate that the difference in euryhalinity of the species studied is related to functional rather than structural differences.     

Electrophoretic identification and phylogenetic relationships of indigenous tilapiine species of Zimbabwe

In order to characterize Zimbabwean tilapias 34 populations of seven species, two in the genus Tilapia and five in Oreochromis , were analysed using horizontal starch gel electrophoresis. Twenty-seven loci were polymorphic while five were monomorphic but had fixed allelic differences among some species and six were monomorphic for the same allele in all species. No fixed allelic differences were found between O. mossambicus and O.mortimeri , and only one locus distinguished O. andersonii from O. mossambicus and O. mortimeri . Pairwise comparisons of O. mossambicus or O. mortimeri populations of genetic distances frequently showed a greater degree of difference within the species than was evident in some O. mossambicus O. mortimeri pairs. This study failed to distinguish O. mortimeri as a distinct species from O. mossambicus . The Tilapia species formed distinct clusters from Oreochromis in both consensus eighbourjoining and distance Wagner topologies. Oreochromis ossambicus, O. mortimeri and O. andersonii clustered together consistently. Although some differences from the taxonomic arrangement of Trewavas are discussed, the principal relationships revealed in our analysis are consistent with most recent studies.     

Studies of herbicide toxicity in some freshwater fishes and ectoprocta

The results of studies on herbicide toxicity to fish and freshwater Ectoprocta are reported. The effects of herbicides, viz , Tafazine, Bladex, Aresin and Dalapon were studied on Sarotherodon mossambicus, Puntius ticto, Cirrhinus mrigala, Danio sps., Heteropneustes fossilis , and Channa punctatus. S. mossambicus was more susceptible to Tafazine toxicity. Danio sps., was more resistant to Tafazine than S. mossambicus , but P. ticto had the maximum tolerance. S. mossambicus had greater resistance to Bladex than C. mrigala. C. punctatus was resistant to Aresin toxicity. Herbicide toxicity to freshwater Ectoprocta was noted with reference to Planavin, Bladex, Tafazine, and 2,4-D butyl ester and mixtures of Planavin + Mercuric chloride and Planavin + Bladex. Herbicide toxicity effected the leptoblasts of Plumatella casmiana and spinoblasts of Lophopodella carteri .     

Can phenotypic plasticity facilitate the geographic expansion of the tilapia Oreochromis mossambicus?

Climate influences the distribution of organisms because of the thermal sensitivity of biochemical processes. Animals may compensate for the effects of variable temperatures, and plastic responses may facilitate radiation into different climates. The tropical fish Oreochromis mossambicus has radiated into climates that were thought to be thermally unsuitable. Here, we test the hypothesis that thermal acclimation will extend the locomotory and metabolic performance range of O. mossambicus. Juvenile fish were acclimated to 14 degrees, 17 degrees, and 22 degrees C. We measured responses to acclimation at three levels of organization: whole-animal performance (sustained swimming and resting and recovery rates of oxygen consumption), mitochondrial oxygen consumption in caudal muscle, and metabolic enzyme activities in muscle and liver at 12 degrees, 14 degrees, 17 degrees, 22 degrees, and 26 degrees C. Thermal optima of sustained swimming performance (U(crit)) changed significantly with acclimation, but acclimation had no effect on either resting or recovery oxygen consumption. Fish compensated for cold temperatures by upregulating state 3 mitochondrial oxygen consumption and increasing activity of lactate dehydrogenase in the liver. The capacity for phenotypic plasticity in O. mossambicus means that the fish would not be limited by its locomotor performance or metabolic physiology to expand its range into cooler thermal environments from its current distribution.     

Introgressive hybridization of tilapias in Zimbabwe

Three species of tilapia native to Zimbabwe, Oreochromis mossambicus, O. mortimeri and O. macrochir are not naturally sympatric, but because of transplantation they have been brought into sympatry. Allozymes of fish examined from Lakes Kariba, Kyle and Chivero showed evidence of introgressive hybridization. The data showed non-random mating populations in Lakes Kariba and Kyle. All three reservoirs displayed individuals with intermediate principal component scores and hybrid index scores indicating hybridization.