The effects of trawling on the fish stocks of Lake Victoria

Species composition for Lake Victoria can be divided into two major groups. Haplochromis includes at least four genera, the largest consisting of 120 major species, while the remaining species together number about eighteen. The fish species of this lake have been grouped into two for obvious reasons-Haplochromis forms at least 80 per cent, by weight, of the yield of the lake. When carrying out the survey, the lake was divided into thirteen sampling areas and, although there were variations, all the species were encountered throughout most of the sampling units, except that there were definite trends as one species moved as one from shallow water towards the centre of the lake. It was noted that there was a numerical decrease of species the nearer one got to the centre of the lake, and the catch rates of the species in deeper waters also tended to decrease. The species that were found most consistently throughout the whole lake included Haplochromis complex and Bagrus, and the species found to be confined almost entirely to the shallow waters was Tilapia, in particular Tilapia nilotica.

Some characteristics of the fish fauna of a dam in the Lake Victoria region of Tanzania including the effects of multispecific stocking with tilapia species

Malya dam had been stocked with several species of Tilapia. However all but the species endemic to the region, T esculentus, made only marginal contributions to the fishery. T. zilli particularly had failed to establish itself, probably because its niche had been previously filled by T. rendalii. Planktivorou Tilapia species predominate in Malya dam but in smaller dams in the region herbivoorous tilapia are relatively more abundant. Six non-cichlid species and one of Haplochromis were found in the dam. Tilapia esculenta the most economically important species, was estimated to grow within 9-10 cm first year and 16-18 cm second year. The largest, specimen caught was 30.5 cm and the smallest mature individual 19 cm. The fish of the dam grew more slowly and matured at a smaller size than those of the same species in Lake Victoria The biological pattern of T. esculenta in Malya dam was similar to that of this species in Lake Victoria.

The future prospects of the fish stocks of Lake Victoria, Uganda

Experimental trawling during the period 1981/86 and analysis of past commercial catch landings, mainly in the northern portion of Lake Victoria have indicated that the standing stocks and therefore, the estimates of sustainable yields of the most important fish species have unquestionably changed since the 1969/71 comprehensive lake-wide stock assessment survey. Lake Victoria which was originally a multi-species fishery now relies on two introduced species (Lates niloticus and Oreochromis niloticus) and one indigenous cyprinid (Rastrineobola argentea). Most of the traditional fish species, including the once dominant haplochromines, have either declined or disappeared. The catch rates in the experimental trawl catches declined from 797 kg/hr in 1969/71 to 575 kg/hr in 1981 and 166 kg/hr in 1985. The contribution of L. niloticus in the trawl catch increased from 0.9% in 1981 to 95.6% in 1985 while the contribution of the haplochromines decreased from about 91% to about 1% over the same period. The mean size of the individual fish caught (particularly the Nile perch) was also on the decline. Similar trends were also observed in the commercial fishery. However, recent observations in the Lake Kyoga commercial fishery that O. niloticus has now surpassed L.niloticus in importance may create more uncertainty regarding the future trends of the fish stocks of Lake Victoria. Inspite of the above situation, developments to increasingly exploit the fish stocks of the lake for export continue to take place. A number of fish processing and/or handling plants have been established in the Jinja, Kampala and Entebbe areas of the lake. Each of these plants is capable of handling upwards of 10 tons of fish a day, the target fish being L. niloticus and O. niloticus.

Fish exploitation and changes in the fish community of Lake George, Uganda

Unlike Lake Victoria, the fisheries of Lake George have undergone gradual changes in the size and proportion of the major commercial fish species, the Nile tilapia (Oreochromis niloticus: cichlidae) in the last 40 years (1950-1989). The size decreased from an average weight of 900g in 1950 to 430g in 1989 while percentage contribution in commercial catches during the same period declined from 92% to 36%. The over all annual commercial catches though showed a steady increase from the period 1950 when the fishery was opened to intensive and controlled exploitation, consistently high catches were observed in the 1960s and 1970s followed by a general decline in the early 1980s to amore or less stable fishery in the late 1980s. These changes are attributed to increased fishing pressure especially on the nil tilapia and to increased use of smaller gill net mesh sizes lower than the recommended 127mm mesh. The changes in gill net mesh have brought O. leucostictus, acichlid, into commercial catches confirming that the 88.9mm mesh size nets are used by the commercial fishermen to harvest smaller fish species. The commercial catches are presently dominated by the piscivorous fishes,(over 60%) whose contribution was less than 10% during initial exploitation of the virgin fishery in 1950.The piscivorous fish are mainly caught using hooks and lines. The entire fishery is believed to be exploited close to the maximum. The above trends serve to show the impact of exploitation on fish species diversity. Quantitive and qualitative changes of the major fish species on lake George are due to exploitation pressure unlike Lake Victoria where it is a combination of both exploitations and impact of fish introductions. There has been no fish introduction in Lake George.

Current composition, distribution and relative abundance of the fish stocks of Lake Victoria, Uganda

An experimental bottom trawl survey was carried out in the Uganda sector of Lake Victoria during the period May 1993 through May 1995 with the aim of establishing the current composition, distribution and abundance of the fish stocks. A total of 205 successful one-hour hauls were taken using the 25.4mm mesh size codend trawl net during the 19 cruises. Fourteen fish taxa (excluding the haplochromines) were recorded with Lates niloticus constituting the bulk (97 %) of the fish retained. Haplochromines and L. nilolicus were encountered in all areas sampled while Nile tilapia (Oreochromis niloticus) and other tilapiines were restricted to waters less than 20 metres deep. An average catch of 154 kg/hr was obtained in waters less than 30 metres deep. Species diversity and relative abundance varied with depth. Only two of the fifteen fish taxa (haplochromines and L.niloticus) were recorded in waters deeper than 30 metres and the bulk of the fish by weight (92 %) was obtained in waters less than 30 metres.

Effect of inter-level relaxation and cavity length on double-state lasing performance of quantum dot lasers

Double-state lasing phenomena are easily observed in self-assembled quantum dot (QD) lasers. The effect of inter-level relaxation rate and cavity length on the double-state lasing performance of QD lasers is investigated on the basis of a rate equation model. Calculated results show that, for a certain cavity length, the ground state (GS) lasing threshold current increases almost linearly with the inter-level relaxation lifetime. However, as the relaxation rate becomes slower, the ratio of excited state (ES) lasing threshold current over the GS one decreases, showing an evident exponential behavior. A relatively feasible method to estimate the inter-level relaxation lifetime, which is difficult to measure directly, is provided. In addition, fast inter-level relaxation is favorable for the GS single-mode lasing, and leads to lower wetting layer (WL) carrier occupation probability and higher QD GS capture efficiency and external differential quantum efficiency. Besides, the double-state lasing effect strongly depends on the cavity length. (c) 2007 Elsevier B.V. All rights reserved.

Formation of an fcc phase through a bcc metastable state in crystallization of charged colloidal particles

By in situ monitoring structural changes with the reflection spectrometer during the colloidal crystallization, we present direct experimental evidence of liquid-bcc-fcc phase transition in crystallization of charged colloidal particles, as a manifestation of the Ostwald's step rule. In addition, the lifetime of the bcc metastable structure in this system decreases significantly with increasing particle volume fraction, offering a possible explanation for "exceptions" to the step rule.

Studies on state and structure of noble metals in electrocatalyst made by coprecipitation method

The electrocatalysts of Pt/C, PtRu/C and Ru/C were prepared by the impregnation method. The facet characterization, the dispersion and the particle size for the catalysts were determined by means of X-ray diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy was also used to analyze the state and the valency of the noble metals. The results show that the particle size was in nanometer range and the binary metals have come into being an alloy. The platinum in the catalysts existed in zero valency. The valency of the ruthenium on the surface is different from that in the body, while the ruthenium on the surface existed in oxide-form. PtRu/C and Pt/C are of good activity to the electrooxidation of hydrogen except Ru/C. PtRu/C is more tolerant of CO than Pt/C, and CO is only adsorbed on Pt.

A HIGH-RESOLUTION SOLID-STATE NMR-STUDY OF THE MISCIBILITY, MORPHOLOGY, AND TOUGHENING MECHANISM OF POLYSTYRENE WITH POLY(2,6-DIMETHYL-1,4-PHENYLENE OXIDE) BLENDS

An extended Goldman-Shen pulse sequence was used to observe indirectly the proton spin diffusion in the blends of polystyrene (PS) with poly(2,6-dimethyl-1,4-phenylene oxides) (PPO). The results indicate that the average distance between PS and PPO is less than 5 angstrom in the intimately mixed phase, but there are heterogeneous domains on a 100-angstrom scale. The data of spin relaxation of carbons, T1(C), for homopolymers and their blends suggest that there is a strong pi-pi electron conjugation interaction between the aromatic rings of PS and those of PPO, while the aromatic rings of PPO drive the aromatic rings of PS to move cooperatively. It is the cooperative motion that markedly improves the impact strength of PS.

In situ solid state NMR observation of the methanol-to-hydrocarbons (MTH) process over nanosized and microsized HZSM-5 zeolites

The formation of surface alkoxy species on nanosized HZSM-5 and microsized HZSM-5, after exposure to methanol and subsequent conversion to olefins, has been investigated by in situ solid state NMR. Compared to microsized HZSM-5 zeolite, the nanosized HZSM-5 zeolite was found to exhibit a higher affinity for trapping methanol species. Activation of the adsorbed methanol species resulted in the formation of various surface alkoxy species with different rigid characters, including the carboxylate-like surface species, as evidenced by deconvolution of the related spectra. The present results support the existence of the so-called carbon-pool in the conversion of methanol, which serves as the reaction precursor not only for the coupling of the species to form olefins, but also for uncontrolled polymerization to give coke on the surface. The nanosized HZSM-5 shows a distinct resistance to the formation of carbonaceous deposits on the surface.