Improving light attraction technologies for enhanced harvest of Mukene (silver fish) on Lake Victoria

(This Project is funded by Competitive Grants Scheme - NARO)

Close to 95% of light currently used on most lakes to attract and concentrate Mukene is from pressure lamps powered by kerosene. However in 2006 industrial fishing rigs from Lake Kariba in Zimbabwe were introduced by Arrow Aquaculture Africa (AAA) firm operating at Kiyindi landing site in Buikwe district. The fishing rigs use light from electric bulbs supplied by a generator to attract and concentrate fish. The catches range from 280 – 4500 kg per rig per day according to season compared to 40 – 70 kg harvested by fishers using pressure lamps. For fishers using 1 - 7 pressure lamps per group, preliminary studies have indicated a correlation between the number of lamps and Mukene catch. However, the optimal usage of lamps for sustainable harvest of these fishes in terms of the appropriate number lamps (light intensity), light colour and appropriate light source are still unknown. On the other hand, while the use of industrial rigs is very productive in comparison to the pressure lamps, the present rigs owned by AAA are ten times more expensive than the ordinary kerosene lamp units. This makes it essential to develop cost-effective technologies that will enable increased but sustainable exploitation of Mukene. The proposed project aims to increase sustainable fisheries production of Mukene by improving light attraction technologies through the determination of the most appropriate colour, intensity, and source of light for attraction and concentration of these small fishes. The enhancement in the use of the improved technologies to attract and concentrate these fishes will increase production to at least 50 – 60%, relieve pressure on other large fish stocks, and improve local revenues and the nutritional status of the poor Mukene fishing population.

The overall objective is to increase sustainable fisheries production of Mukene from Lake Victoria (Uganda) through increased and efficient light fishery production technologies.

To investigate the appropriate light colour to attract and concentrate Mukene on Lake Victoria

Experiment I

    Effect of light colour on Mukene fishing

    The current light fishing on Lake Victoria involves use of white light by artisanal fishers (using kerosene lamps) and industrial rigs (AAA) using generator powered bulbs. There are however, up to five colours of probable fishing lights that will be investigated (White, Blue, Green, Yellow and Red). The effect of colour will be studied through variation of light colours. Using both kerosene lamps and electric bulbs, performance of different colours will be evaluated. While for electric bulbs manufactured bulbs of different colours will be procured, the clear (colourless) glasses for kerosene lamps will be painted with the appropriate colours (white light source painted with a different colour emits the light with the colour painted on glass). Some of the white bulbs will also be painted to change their colours and their performance will be compared with manufactured coloured ones to check the effect of painting as used on kerosene lamps. For each colour setup, the time taken (minutes) for Mukene to concentrate around the lamp will be recorded and the quantity of fish hauled (retained by the net) will be weighed in kilograms. The five different coloured lamps or bulbs will be set in the same area giving each setup ample space and will be timed accordingly. The hauling will be done using available fishing methods (encircling or lampara net used by artisanal fishers and lift net as used on the AAA rigs). Fish yield from the different lamp settings and the time taken will be statistically modelled to evaluate effectiveness of each colour setting.

    Expected output

  • The most appropriate colour of light for attraction, concentration and fishing Mukene on Lake Victoria determined.
To determine the appropriate source (lamps or bulbs) of light for sustainable harvest of Mukene

Experiment II

Appropriate source of light for Mukene fishing

    Three primary types of light sources will be investigated (floating kerosene powered lamps, overhead electric bulbs powered by generators and floating electric lights powered by batteries). These light sources will be set up in similar design as in Experiment 1 above and their performance parameters (time taken to concentrate fish and quantity of Mukene harvested) evaluated as in the experiment above. Using the most appropriate light colour evaluated in Experiment 1 above, and the white light (if it not the former), the appropriate light sources will be determined. The white light is being emphasised since it is the one in current use. Battery-powered, 12-volt, LED and fluorescent light models that are available on the market will be evaluated, with appropriate light colours and intensities determined from Experiment 1 above. Trials into possibilities of cost effective and less hazardous sources such as solar batteries will be explored.

    Expected output

  • The most appropriate and cost effective source of light for attraction and fishing Mukene on Lake Victoria identified.
To compare the performance of surface and submerged light sources in attraction and concentration of Mukene

Experiment III

Effect of surface and submerged light

    A significant fraction of the light shining on the surface of the water is lost by reflection and, thus, will not be available to attract fish and other aquatic organisms in the food chain (which in turn attract fish to feed on them). Surface floating lights have been the standard of night Mukene fishers for many years, but in addition to losing substantial quantities of light to surface scattering, the users of these lights often have to contend with annoying swarms of insects drawn to the lights along with fish. For this and other reasons, submersible lights that use fluorescent bulbs that slide beneath the surface and light up the water depths were developed. It is common knowledge that floating and submersible lights are used to attract fish and can be used separately or in combination. Experiments will be set up to compare floating and submersible lights separately through a similar experimental setup as in Experiment 1 above. Similar bulbs and same colours will be used in each comparison. For example a setting of white floating light bulb compared with a white submersible bulb. The measured parameters will be the time taken to concentrate fish and quantity hauled as described in the experiment above. In addition, trials such as combinations of a pair of floating lights positioned above submersible lights capable of lighting multiple levels of the water column to attract fish, while also providing more lighting above the water for fishers operations will be explored. In this experiment setup possibilities of fabrication of less cost rigs using locally available materials will be investigated.

    Expected output

  • Appropriate setting of light (surface floating or submerged) determined.
To determine the appropriate light intensity (i.e. number of light sources (lamps, bulbs)) for Mukene harvest

Experiment IV

Appropriate number of light sources (Light intensity)

    After selection of the appropriate light colour and source the appropriate number of light sources will be determined. Currently on Lake Victoria different numbers of kerosene lamps are in use by different fishers in various areas of the lake. The number ranges from 2 – 9 lamps per team and mostly operating singly (one stand alone lamp per fishing operation). The impact of using single lamps as in the preceding case and possibilities of having two or three lamps together (an index of intensity) will be investigated. Light intensity will be studied by varying the number of lamps and the amount of light emitted measured as relative light intensity (RLI). RLI will be measured using simple light meters locally available on the market. Light intensity will be measured at the setting of the lamp and at the beginning of fishing (hauling). The rigs of AAA use overhead lights hanging above the water surface. The intensity of light emitted from such setting and quantity that actually illuminates the water layers will be evaluated through measurement of intensity at the different layers. To evaluate the effectiveness of the number of lamps used to harvest Mukene, the time taken to concentrate fish per setting (setup) will be recorded and the quantity of fish hauled weighed.

    Expected output

  • Ecologically safe sources of light such as Electric from generators and batteries including solar batteries and their appropriate setting technologies determined and recommended for uptake pathways.
To investigate the effect of light source, colour, and intensity on incidental harvest of non-target species (by-catch) during light fishing

Experiment V

Effect of light source and intensity on harvest of non-target species (by-catch)

    Fish and some members of their food chain have colour receptors in their eyes optimized for the light of their “space”. The implication is that knowing the appropriate colour for fishing a particular species (Mukene in this case) can help reduce capture of other non-target species. During the set up in in the above experiments, sub-samples (plastic cup weighing 500 g) of hauled fish will be taken from each setting for analysis of proportions of non-target species. The samples will be immediately fixed in 5% formaldehyde solution and carefully sealed in plastic bags. The samples will be sorted in the laboratory by taxa and proportions of taxa obtained through counting and weighing. A few samples of pure Mukene (after removal of by catch) will be measured (standard length in mm) and heaped into 1 mm length intervals. 10 specimens from each group will be examined for maturity to estimate the proportion of immature Mukene being harvested by the different setups. The proportion of by catch (non-target species and immature Mukene) harvested by a given set up will be evaluated as a fraction of the total sample.

    Expected output

  • Minimize by-catch (non-target species) by using the most appropriate light and intensity

Mukene contains phosphorous which aids brain and bone development. It has also got nutritional values such as vitamin A and E and fatty oils that prevent poor vision and promote good skin texture. Studies have shown that diets rich in Omega-3 can result in increased learning ability, problem-solving skills, focus, memory. (