Abstract:

Hydro-acoustics surveys have been conducted in Lake Victoria since 1999 with the aim
of determining and explaining quantities (biomass) and distribution of various fish
groups. Herein results of the 23rd survey that was conducted from 20th October to 25th
November 2020 are presented. Data were collected and analysed following standard
operating procedures (SOPs) for hydro-acoustic surveys in Lake Victoria. The current
methods (SOPs) are able to disaggregate and attribute acoustic measurements into four
(4) fish groups, namely Nile perch, dagaa, haplochromines and other fish, and the
freshwater prawn Caridina nilotica. Results of the current survey indicate that the lake
had 3.47 million tons (t) of fish, including Caridina, which was a 29% increase from the
previous year 2019 (2.68 million tons). Nile perch (30%), was the most dominant followed
by Caridina (28%), Dagaa (27%) and haplochromines and others (15%) respectively.
The following key observations were made:
• Nile perch was generally more abundant in the western part of the lake, consistent
with findings from the previous survey.
• Nile perch biomass continued to expand, increasing by 25% compared to the 2019
survey. As in the previous survey (2019), the highest increase in biomass was
observed in Uganda and Tanzania waters;
• The average size of Nile perch also improved slightly, increasing from 15.0 cm TL
in 2019 to 18.39 cm in 2020, with the highest increase (of sizes above 50 cm TL),
observed in the north-western parts of the lake (Uganda).
• The biomass of dagaa increased by 10% relative to 2019. The increase was only
observed in Uganda and Kenya. Tanzania on the other hand recorded a decrease in
dagaa biomass (36%). Unlike the previous survey, spatial differences were
observed, with more dagaa concentrated in coastal and deep transects compared
to inshore areas.
• Haplochromines and other fish, registered a 33% increase in biomass compared to
the previous year. This group registered the highest average density (8 tons/km2)
since 2017.
• Caridina registered a 48% increase in biomass compared to the previous survey.
• The lake appeared to be undergoing stratification, with more pronounced
thermally stratified waters occurring in the South Western, North Western, North
Eastern parts of the lake, with instances of prominent anoxia and less fish at the
bottom depths.
The observed stock status may be due to a combination of many factors, but mainly, the
western side being relatively shallower, experiencing more mixing, coupled with the
rising water levels, and the level of enforcement possibly being stronger in the western
side than the eastern side. The decline in biomass of dagaa in the Tanzanian side may be
not be a big concern given the high turnover rate of the species. From the findings and
experiences of the present study we conclude and recommend that:
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I. The current stocks are in good shape, and the current level of enforcement should
be maintained and upscaled. This however, should be carried within an ecosystem
approach to fisheries management (EAFM)
II. Annual hydro-acoustic surveys should be paired with stock assessment which was
lastly done in 2016 to generate explicit management recommendations to support
the current enforcement efforts.
III. There is need to re-analyze all the data from all the past surveys so as to improve
reporting on the trends for all the monitored taxa cannot be overemphasized.