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.     

Immunocytochemical identification of prolactin cells in the pituitary gland of tilapia larvae (Oreochromis mossambicus: Teleostei)

Summary Using an antiserum to highly purified chum salmon prolactin, prolactin cells were identified in the putative rostral pars distalis of newly hatched tilapia larvae (Oreochromis mossambicus) by the immunogold method for the electron microscope. In the putative rostral pars distalis, some cells had another kind of secretory granule which was much less numerous, much smaller in size, and without immunoreactivity to salmon prolactin antiserum. Controls incubated with salmon prolactin-preabsorbed antiserum or normal serum showed no immunoreactive cells, confirming the specificity of the antiserum. The possible role of prolactin in the osmoregulation of tilapia larvae is discussed.     

In vitro stimulation and inhibition of steroid hormone release from postovulatory follicles of the tilapia,Oreochromis mossambicus

Summary Postovulatory follicles of the tilapia, Oreochromis mossambicus, were incubated with graded doses of salmon gonadotropin to identify the steroid hormones released by this tissue. In addition, the effects of either cytochalasin B or colchicine on steroid hormone release were studied. After the incubation, the tissue was examined by electron microscopy. Postovulatory follicles released testosterone and estradiol-17B in a dose-dependent manner with gonadotropin. There was no detectable release of progesterone or 17a-OH-progesterone. When stimulated with high doses of gonadotropin, the steroidogenic cells showed an increase in smooth endoplasmic reticulum, Golgi complexes, and lipid droplets. Also, microfilaments became arranged in orderly bundles and were found close to the numerous secretory vesicles and lipid droplets. Upon incubation with gonadotropin and either colchicine or cytochalasin B, the cells still appeared steroidogenic, but the filaments were not organized nor associated with vesicles or lipid droplets. Release of steroid hormone decreased significantly. Also in these tissues, vesicles were no longer numerous in the apical region of the granulosa cells, but were located primarily near smooth endoplasmic reticulum and Golgi complexes. This suggests that disruption of the cytoskeleton results in reduced steroid hormone synthesis or release.     

The nutrition of stomachless phytoplanktivorous fish in comparison with Tilapia

The contents of alimentary tracts of stomachless phytoplanktivorous fish, including silvercarp (Hypophthalmichthys molitrix), Esomus danrica thermoicos, and Amblypharyngodon meletinus were investigated microscopically and compared with those of Tilapia (Oreochromis mossambicus, a fish with stomach).The predominance of detritus in the food and the low efficiency with which phytoplankton is utilized prove, that stomachless fish are not primarily herbivorous. The possibility of omnivorous feeding as an alternative is discussed. These results may contribute to our understanding of the role of filterfeeders in aquaculture and ecosystems.     

A statistical power analysis of the 'internal reference' technique for comparing growth and growth depensation of tilapia strains

Experiments were conducted to compare the growth and growth compensation of three strains of juvenile Oreochromis niloticus . Ten full sib families (10 replicates) per strain were split and grown under crowded and uncrowded conditions for 3 weeks (the treatment). Both treatments were then grown an additional 12 weeks under less crowded conditions (the compensation). Standard length measurements were made at the end of crowding and the end of compensation. Each replicate included a size-matched control population of a fourth (red) reference strain. ANCOVA with the reference strain used as a cofactor revealed significant strain effects on specific growth throughout the experiment. The reference strain removed most of the random among-replicate error variance as shown by an increase of r 2 from 0.06 to 0.91 when it was included in the statistical models. If the reference fish had not been used, approximately 450 replicate families would have been needed to achieve the sensitivity of the present experiment (a difference of 7% among strains significant at P =0.05). We conclude that the CLSU strain grows significantly more slowly than the Israel and NIFI strains under the experimental conditions, that the crowding effect was essentially eliminated after 12 weeks of compensation, and that the reference strain greatly improved the resolution of the strain-testing experiment.     

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.     

Otolith microstructure pattern in Oreochromis niloticus (L.)

The microstructure of otoliths from young and old Oreochromis niloticus (L.) were studied. Otoliths were prepared histologically except for those from newly hatched fish. Hatching results in the formation of a check in the otoliths, which appeared 1 day later. Other checks are rare in juvenile otoliths but common in adult otoliths. Faint and non-daily increments were observed within the hatching check. After hatching, increments were deposited daily. Sub-daily increments were faint and narrow, they were present in the area along the dorso-ventral axis of the otolith but did not continue into the lateral region. Discontinuous zones in the medial area appeared different from those in the lateral area. New growth centres were not only found in the juvenile fish otoliths, but also in adult fish otoliths.     

Effects of the organophosphate insecticide,monocrotophos, on acetylcholinesterase activity in the nile tilapia fish (Oreochromis niloticus) brain

The neurotoxic effects of monocrotophos on the brain of the nile tilapia fish (Oreochromis niloticus) were examined, using a static bioassay under laboratory conditions. By probit analysis the 96 h LC₅₀ value of monocrotophos was 4.9 mg/l. After 96 h exposure to acute levels of monocrotophos, the brain acetylcholinesterase (AChE) activity decreased progressively as the concentration of monocrotophos increased. In addition, four weeks following transfer to toxicant-free water after exposure to 1 mg monocrotophos, nile tilapia fish brain regained 95% of control AChE activity. The results indicate that inhibition of AChE activity in fish exposed to monocrotophos may serve as an indicator of hazard due to application of this chemical in the natural environment.Special issue dedicated to Dr. Robert Balazs.     

Microcystin-LR equivalent concentrations in fish tissue during a postbloom Microcystis exposure in Loskop Dam,South Africa

The effects of a decomposing cyanobacteria bloom on water quality and the accumulation of microcystin-LR equivalent toxin in fish at Loskop Dam were studied in May 2012. Enzyme-linked immunosorbent assay [ELISA] was used to confirm the presence of microcystin-LR equivalent in the water and to determine the microcystin (MCYST) concentration in the liver and muscle of fish. The lowest concentration of extracellular MCYST-LR equivalent was recorded in the lacustrine zone, where no cyanobacterial cells were observed, while the highest concentration (3.25 µg l^?1), 3.25 higher than World Health Organization standard, was observed in the riverine zone. Extremely high MCYST-LR equivalent concentrations of 1.72 µg MCYST-LReq kg^?1 in the liver and 0.19 µg kg^?1 in muscles of Labeo rosae, and 2.14 µg MCYST-LReq kg^?1 in the liver and 0.17 µg kg^?1 in muscles of Oreochromis mossambicus, indicate that the consumption of sufficient fish biomass might cause severe adverse effects in humans. Microscopic analyses of the stomach content of both fish species revealed low numbers of cyanobacterial Microcystis aeruginosa cells in comparison to other phytoplankton. The extracellular MCYST-LR equivalent of the decomposing bloom may have played a major role in the high levels observed in the livers of the two fish species. These findings are important for all downstream water users.     

The effect of copper and zinc at neutral and acidic pH on the general haematology and osmoregulation of Oreochromis mossambicus

The objective of this study was to determine the effect of sublethal copper and zinc concentrations at a neutral and an acidic pH, on selected haematological parameters as well as on the total osmolality and electrolyte concentrations of Oreochromis mossambicus. In general, at neutral pH copper and zinc caused blood acidosis, increases in circulating white blood cell numbers, causing stimulation of the immune system, and a rapid release of red blood cells from haemopoietic tissue, as reflected in the decreases in mean corpuscular volume. Increases in red blood cells are attributed to an increase in the oxygen-carrying haemoglobin as an adaptation to altered respiratory homeostasis caused by copper and zinc. These increases are therefore a secondary reaction to the metals and not the result of direct stimulation of the haemopoietic tissue. In contrast, at an acidic pH copper and zinc concentrations usually caused blood alkalosis and decreases in white blood cell numbers, due to the bioconcentration of metals which blocks and suppresses the leucopoietic tissue. Decreases were also recorded in red blood cell counts, haemoglobin, haematocrit and mean corpuscular volume, which can be ascribed to anaemic and hypoxic conditions, gill damage and impaired osmoregulation.