Monthly fish surveys were made from 1997-1999 in the Kenyan waters of Lake Victoria in order to estimate the magnitude of fisheries resources. Sample sites were defined using GPS while thirty minute hauls in alternate grid squares were made. Demersal fish biomass was estimated using the swept area method, while for trawling two different trawl nets were used. Collected fish was sorted into species, measured (TL) and weighed. Smaller fish were mixed on deck and sub-samples taken. Sexual maturity stages of fish were also observed. Areas with consistency high catches were located outside major urban and riverine influence where most artisanal fishermen were concentrated. Very low catches were obtained from areas that had recently been covered by water hyacinth Eichhornia crassipe

Getabu, A.

Nyaudi, J.

English

Aquatic Commons

Results from RY Utafiti research cruises
A. GETABU & J. NYAUNDI, KMFRI, P.O. Box, 1881, Kisunm, Kenya
Abstract: Bottom trawl surveys were conducted in Kenyan waters of Lake Victoria
from September 1997 to March 1998 to assess the state of the fish stocks. The mean
fish biomass for the two most important species, Nile perch, Lates nuoticus (L.), and
Nile tilapia, Oreochrornis niloticus (L.), was estimated at 61 kg h&'. Few Nile perch
greater than 80 cm TL were caught. Very low catches were made in areas that had
recently been covered by water hyacinth, Eichhornia crassipes (Mart.) Solms.
Introduction
The major uncertainty facing Lake Victoria fisheries is the limited knowledge on the
status of the stocks. There have been few lakewide surveys aimed at estimating the
quantity of fish and their spatial and temporal variation in the lake. No lakewide
survey has been conducted since the 1969-1971 bottom trawling survey
(Kudhongania and Cordone 1974), though more limited trawl surveys have been
carried out in each country. In Kenya, for example, Asila and Ogari (1987) reported
on trawl surveys conducted from 1978-1984,
Since the mid I 950s, there have been many changes in the fishery, including the
introduction of alien species, increased fishing pressure with the introduction ofmore
efficient fishing gear and motorized fishing craft, and changes in the trophic status of
the lake (Ochumba, Gophen & Pollinger 1994; Cml 1995). The introduced Nile
perch, Lates niloticus (L.), preyed on the once abundant haplochromines, reducing
their percentage contribution to the catch to less than 1% by weight. This has had an
impact on the trophic status of the lake as well as on the population dynamics of other
fish species. The indigenous species disappeared as a combination of the effects of
predation by the Nile perch and overfishing (CIFA 1982; Witte & van Densen 1995).
These impacts, together with eutrophication (Ochumba 1990; Ochumba et al. 1994),
are responsible for the present catch composition and value of the fishery.
The most recent factor that has affected the distribution of fish in the lake, particularly
the Nyanza Gulf, is the water hyacinth, Eichhornia crassipes (Mart.) Soims. The
weed has also hampered fishing activities in Kenyan waters. Industrial, urban and
agricultural developments in the lake basin have also indirectly contributed to the
change in species and catch composition due to increased nutrient inputs from them
with resultant eutrophication (Ochumba et al. 1994).
These changes and the uncertainty about the magnitude of the fish stocks made it
necessary to conduct a lakewide survey to evaluate the fish stocks. The fish surveys
in Kenya are part of a lakewide effort by the European Union in conjunction with the
three East African fisheries institutes to estimate the magnitude of Lake Victoria
fisheries resources.
84
Methods
Monthly sampling was based on 2.5 x 2.5 n.m squares of the Kenyan waters of Lake
Victoria, using the latitude and longitude readings on the chart to define the sampling
grid. Thirty minute trawl hauls were made in alternate grid squares. A codend bag of
mesh size 5 mm was used to obtain samples of Rastrineobola argentea (Pellegrin).
The sample sites were defined using OPS (Table 1). In the period 1997-1999, it was
not always possible to sample at exactly the same position in each survey due to water
hyacinth mats. Efforts were made to sample as close as possible to the blocked site in
areas free of hyacinth.
Demersal fish biomass was estimated using the swept area method (Witte and van
Densen,1995; Sparre, 1998). Two different trawl nets were used, one with a
headrope of 14.4 m and the second new one with a headrope of 24.5 rn. Depending
on the size of the catch and the time available for sorting, either the whole catch was
sorted into species and every fish measured (TL, cm below) and weighed (g) or a sub-
sampling procedure was used. In the latter case, all L. niloticus above 35 cm TL were
sorted from the catch. The smaller fish were thoroughly mixed on deck and a
subsample of three shovelfuls (about 200 fish) was taken. The subsample and the
larger fish were individually measured and weighed. The rest of the catch was
weighed to provide a raising factor for the subsample. The other species in the catch
were also measured and weighed. Sexual maturity stages were based on the
classification in Witte and van Densen (1995).
Results
The mean fish biomass for the two most important species, L. niloticus and O.
niloticus, was estimated at 61 kg ha'' (Tables 2 & 3). Fish abundance varied over
time, e.g. off Dunga, an area which normally yielded low catches, 430 kg were caught
in a 30-minute haul inNovember 1998, but only 33.6 kg were caught in the following
month. Caridina nilotica (Roux), R. argentea and Haplochromis spp. all showed the
same trend.
Areas with relatively consistent high catches extend from west of Maboko Island up
to Mbita Channel in the depth range 5-22 m. This area is outside major urban and
riverine influence and is where most of the fishing effort by artisanal fishermen is
currently concentrated.
There were few L. niloticus greater than 80 cm and O. niloticus greater than 50 cm
though these species attain maximum sizes of 205 cm and 65 cm respectively (Asila
and Ogari 1987; Getabu 1992). Oreochromis niloticus was mainly found shallower
than 5 m with some specimens being encountered down to 15 m. In earlier surveys it
was not found deeper than 10 m.
Juvenile L. niloticus were the most abundant fish in the 5-mm mesh codend lining,
followed by C. nilotica and R. argentea. Rarer fish species appeared occasionally in
the catches, including Clarias gariepinus (Burchell), Synodontis victoriae Boulenger,
85
Synodontis afro/ischeri Hilgendorf, Protopterus aethiopicus flecke!, Schuhe
intermedius (L.), Barbus spp, and Haplochrornis spp.
Very low catches were obtained from areas that had recently been covered by water
hyacinth. Water at such sites was turbid with low oxygen levels, e.g. off Oluch River
mouth in Homa Bay, oxygen concentrations ranged from 1.09 mg U1 at the bottom to
3.87 mg U' near the surface. Only 1.75 kg per 30-minute haul were caught consisting
mainly of juveniles of a wide range of species, including L, niloticus, and O. niloticus.
Juveniles of P. aethiopicus, still with intact yolk sac, were also caught. The area also
had a high diversity of indigenous fish species, Brycinus sadien (Boulenger),
Hapiochroinis spp., Synodontis spp. and R. argentea.
Discussion
The estimated biomass of 61 kg ha'1 is low compared to the estimate of 104 kg ha1
for Kenya waters of Lake Victoria by Kudhongania and Cordone (1974), when the
fishery was dominated by Haplochromis spp.
The scarcity of larger sized fish in the present survey when compared to the earlier
surveys of Asila and Ogari (1987) may reflect an increase in fishing pressure (Othina
in prep.), leading to higher fishing mortality. This aspect will be investigated further
during this project.
The increase in depths at which O. niloticus was caught suggests that the species has
extended its ecological range. Declining stocks of the predatory L. niloticus and an
increase of detritus at the lake bottom due to debris dropping from hyacinth mats may
partially explain the increased depth range.
Studies in the Nyanza Gulf by Hughes (1992) indicated that male L. niloticus matured
from 50-55 cm when 2 years old, while females matured at 80-85 cm. Asila and
Ogari (1987) reported 50% maturity at 74 cm for males and 102 cm for females.
Later studies by Asila (unpublished data) indicated that males matured at 55 cm and
females at 70 cm. These data suggest that the length at maturity of Nile perch is being
reduced over time. Whether this is due to a lowering of growth rates or to heavy
fishing pressure selecting in favour of earlier maturing individuals is not yet known.
This report presents preliminary findings of a trawl survey in the Kenyan waters of
Lake Victoria. The sampling regime has been established and all three countries are
now using the same net specifications, with towing speed standardised. A full
lakewide survey is now, therefore, in progress.
Acknowledgements
The programme is funded by the European Union Lake Victoria Fisheries Research
Project (Ref: ACP-RPR 227), coordinated by Mr M. van der Knaap. We thank Mr D.
Tweddle and Dr 1.0. Cowx for advice on data collection methods and analysis and Mr
G. Passiotis for solving mechanical problems on the research vessel. We thank
KMFRI technicians and other staff for assistance in data collection.
86
References
Asila A.A. and Ogari J. (1988) Growth parameters and mortality rates of Nile perch
(Lates niloticus) estimated from length-frequency data in the Nyanza Gulf
(Lake Victoria). In: S. Venema, J. Möller-Christensen and D. Pauly (eds)
Contributions to tropical fisheries biology. FAO Fisheries Report 389, 272-
278.
CIFA (1982) Committee for Inland Fisheries of Africa (CIFA), report of the first
session of the sub-committee for the development and management of the
fisheries of Lake Victoria, 12-14 October 1981, Mwanza, Tanzania. FAO
Fisheries Report 262, 73 pp.
Crul, C.M.R. (1995) Limnology and hydrology of Lake Victoria. Comprehensive
and comparative study of great lakes. IJNESCO/IHP-IV Project M-5.1, 78 pp.
Getabu A. (1992) Growth parameters and total mortality in Oreochromis niloticus
(Linnaeus) from Nyanza Gulf, Lake VIctoria. Hydrobiologia 232, 91-97.
Graham M. (1929) The Victoria Nyanza and its fisheries. A report on the fishing
survey of Lake Victoria 1927-1928. London, Crown Agents, 41 pp.
Hughes N.F. (1992) Growth and reproduction of Nile perch, Lates niloticus, an
introduced predator, in the Nyanza Gulf, Lake Victoria, East Africa.
Environmental Biology of Fishes 33, 299-305.
Kudhongania A. W. & Cordone A.J. (1974) Bathospatial pattern and biomass
estimate of the major demersal fishes in Lake Victoria. African Journal of
Tropical Hydrobiology and Fisheries 3, 15-31.
Ochumba P.B.O. (1990) Massive fish kills within the Nyanza Gulf of Lake Victoria,
Kenya. Hydrobiologia 208, 93-99.
Ochumba P.B.O., Gophen M. & Pollinger U. (1994) Ecological changes in Lake
Victoria after the invasion by Nile perch (Lates niloticus): the catchment,
water quality and fisheries management. In: E. Okemwa, E. Wakwabi and A.
Getabu (eds) Recent Trends of Research on Lake Victoria Fisheries.
Proceedings of the second EEC regional seminar on recent trends of research
on Lake Victoria fisheries. Mombasa, Kenya, KMFRI, pp. 29-39.
Witte F. and van Densen W. (eds) (1995) Fish Stocks and Fisheries of Lake Victoria.
A Handbook for Field Observations. Cardigan, Great Britain: Samara
Publishing, 404 pp.
Further research
This report provides preliminary estimates for the biomass of the two most important
trawl species, L, niloticus and O. niloticus. The RV Utafiti has now been modified
with the installation of a new propeller and can pull with much more power. The new
standardised trawl gear is in operation. By the time of the next FIDA WOG workshop,
more detailed assessment of the state of the stocks will be made and compared with
earlier data from Kenyan trawl research.
87
Table 1. Position and average depth (m) of trawl sampling sites in the Kenyan waters
of Lake Victoria.
88
Location Latitude Longitude Depth
Kisumu Bay 000 07' 10" S 34° 04' 10" E 3
Paga 000 10' 35" S 340 40' 54" E 3
Awach Paga 000 16' 13" S 34°41'58"E 4
Sondu Miriu 000 18' 43" S 34°38'40"E 3
Awach-Maboko 00° 16' 18" S 34° 34' 39" E 5
Ndere 000 13' 45" S 34°31'45"E 5
Asembo-Bay 000 13' 45" S 34°26'15"E 3
Bala-Lawi 000 17' 30" S 34° 27' 30"E 10
Miti-Mbili 000 18' 45" 5 34°31'15"E 4
Gudwa 000 23' 42" S 34° 22' 49" E 13
Launda-Gembe 000 26' 38" S 34°18'56"E 7
Mirunda-Bay 000 27' 05" S 34° 20' 10" E 7
Madundu 000 25' 40" S 34°18'45"E 21
Kopiata 000 21' 58" S 34°26'22"E 11
Achieng-Oneko 000 18' 46" S 34°23'46"E 4
Rusinga Channel 000 24' 57" S 34° 12' 35" E 10
Luanda Naya 000 22' 10" 5 34°12'35"E 11
Kuja 000 54' 48" S 34° 07' 48" E 8
Karunga Bay 000 51' 32" S 34° 09' 27" E 8
Muhuru Bay 000 57' 44" S 34° 00' 06" E 25
Mageta 000 06' 46" S 34°02'37"E 18
Mageta Usenge 000 06' 12" 5 34° 02' 36" E 13
Yala 00° 03' 49" 5 34°01'49"E 3
Sio River 000 10' 15" S 34°59'31"E 5
Sisenye 00° 10' 15" S 34°58'25"E 3
Gingra Rock 00° 20' 24" S 34°26'43"E 10
Kowuor 00° 21' 26" S 34° 25' 23" E 3
Awach River 000 20' 34" 5 34° 39' 18" E 4
Sukru Island 00° 27' 11" S 34° 25' 30" E 5
Kendu Bay 3
Mabako 8
Matara 00° 34' 26" S 34° 04' 10" E 12
Rusinga S 00° 27' 44" S 34° 09' 06" E 21
*Soklo Island 5
*Naya 7
*Kjsian 4
*Off Dunga 4
*Soda Paga 4
*Miti Mbili Ndere 7
*Gudwa K'Owuor 12
*Oluch River 7
Tabk 2. Lates niloticus biomass indices (kg ha1) estimated from individual trawl catches in the
Kenyan waters of Lake Victoria. The mean biornass indices and standard deviations are given
for each station and for each monthly survey.
89
Location
1997
Sep Oct Nov Dec
1998
Oct Nov Dec
1999
Jan Mean SD
Kisumu Bay 81.3 2,0 49.5 1.7 22.0 31.3 34,1
Paga 68.5 17.6 43.1 36.0
AwachPaga 15.5 31.8 23.6 11.5
SonduMiriu 23.7 24.1 2.0 5.0 13.7 11.9
Awach-Maboko 74.9 29.6 12.6 56.9 43.5 27.8
Ndere 45.1 46.0 104.2 550 33.3 56.7 27.7
Asembo Bay 13.9 6.4 42,7 24.6 36.8 15.0 23.2 14.1
Bala-Lawi 143.3 82.1 79.2 77.3 12.2 109.5 112.0 87.9 41.0
Miti-Mbili 181.2 25.0 102.5 19.5 12.5 58.5 66.5 65.4
Gudwa 300.6 71.1 292.9 213.7 67.5 28.3 0.0 55.8 128.7 121.2
Launda-Gembe 24.2 0.0 110.0 8.1 4.3 11.3 169.0 46.7 66.1
Mirunda Bay 228.4 0.0 92.8 6.8 6.3 42.0 42.2 59.8 81.0
Madundu 91.3 0.0 0.0 124.4 30.3 11.1 113.5 142.8 64.2 60.0
Kopiata 80.9 169,9 25.2 52.3 52.9 38.8 49.0 67.0 48.4
Achieng-Oneko 40.7 169.6 169.9 128.1 1.3 35.8 14.0 79.9 73.6
Rusinga Channel 68.0 165.5 89.0 7.0 51.3 103.0 80.6 53.3
LuandaNaya 107.1 0.0 53.5 75,7
Kuja 7.4 4.2 72.6 28.1 38.6
Karunga Bay 42.0 9.4 89.3 46.9 40.2
Muhuru Bay 206.1 16.3 5.0 75.8 113.0
Mageta 44.7 44.7
Mageta Usenge 111.9 111.9
Yala 32.5 45.5 46.0 90.8 53.7 25,5
SioRiver 2.3 6.7 18.8 16.5 11.1 7.8
Sisenye 80.8 32.0 7.8 40.2 37.2
GingraRock 15.7 125.2 59.5 53.8 100.0 72.5 71.1 38.1
Kowuor 41.4 29.4 16.0 7.6 29.3 24.7 13.1
Awach River 54.4 5.5 15.0 41.5 29.1 22.7
Sukru Island 39,7 397
Kendu Bay
Mabako 8.8 12.4 42.0 21.1 18.2
Matara
Rusinga S
Soklo Island 20.4 9.2 16.9 15.5 5.7
Naya 1.2 16.3 10.8 163.5 47.9 77.3
Kisian 14.7 1.8 37.0 17.8 17.8
OffDunga 42.3 215.0 16.8 91.4 107.8
SodaPaga 13.0 13.0
Miti Mbili Ndere 31.9 11.8 21.8 14.2
GudwaK'Owuor 57.0 273.0 131.0 153.7 109.8
Oluch River 30.3 30.3
Mean 84.8 43.2 91.5 85.9 30.6 45.0 38.7 75.2 56.7 61.2
SD 77.0 55.0 83.7 61.9 25.2 67.4 34.6 55.1 61.2
Table 3. Oreochromis niloticus biomass indices (kg had) estimated from individual trawl catches in
the Kenyan waters of Lake Victoria. The mean biomass indices and standard deviations are
given for each Station and for each monthly survey.
90
Location
1997
Sep Oct Nov Dec
1998
Oct Nov Dec
1999
Jan Mean SD
Kisumu Bay 7.8 2.7 0.0 4.2 0.8 3.1 3.1
Paga 4.9 2.5 3.7 1.7
Awach Paga 4.9 1.5 3.2 2.4
SonduMiriu 5.5 2.4 0.0 23.9 7.9 10.8
Awach-Maboko 2.4 1.9 1.2 30.0 8.9 14.1
Ndere 5.4 0.3 0.0 0.0 7.5 2.6 3.5
Asembo Bay 1.7 2.8 7.6 0.7 2.0 14.7 4.9 5.4
Bala-Lawi 2.0 2.1 0.0 0.0 1.1 1.2 2.9 1.3 1.1
Miti-Mbili 0.1 0.0 0.1 0.3 0.2 14.5 2.5 5.9
Gudwa 0.0 0.0 0.0 0.0 0.0 2.5 0.0 15.3 2.2 5.3
Launda-Gembe 10.4 0.0 0.0 1.3 1.5 5.0 10.0 4.0 4.5
Mirunda Bay 6.8 0.0 0.0 1.1 8.0 4.3 2.6 3.3 3.2
Madundu 0.0 0.0 0.0 0.0 0.0 0.0 9.0 11.8 2.6 4.9
Kopiata 1.6 0.0 0.0 3.4 0.0 0.0 0.0 0.7 1.3
Achieng-Oneko 4.1 0.0 0.0 2.5 0.0 0.0 27.0 4.8 9.9
Rusinga Channel 1.4 0.0 0.0 0.3 0.0 0.1 0.3 0.6
Luanda Naya 2.4 8.4 5.4 4.2
Kuja 0.0 0.0 0.0
Karunga Bay 0.0 0.0 0.0 0.0 0.0
Muhuru Bay 0.0 0.0 3.3 1.1 1.9
Mageta 0.0 0.0 0.0
Mageta Usenge 0.0 0.0
Yala 2.0 0.0 0.4 0.0 0.6 1.0
Sio River 27.3 0.0 12.6 15.6 12.0 13.5 9.8
Sisenye 0.0 0.6 0.0 0.2
GingraRock 0.0 0.0 0.0 1.6 2.3 7.5 1.9 2.9
Kowuor 1.9 0.0 0.0 1.8 6.5 2.0 2.7
Awach River 0.4 0.0 10,1 47.3 14.4 22.4
Sukru Island 10.7 10.7
Kendu Bay
Mabako 1.1 1.8 6.3 3.1 2.8
Matara
Rusinga S
Soklo Island 0.0 14.0 0.5 4.8 7.9
Naya 11.7 4.0 11.8 18.0 11.4 5.7
Kisian 1.4 0.0 23.0 8.1 12.9
Off Dunga 0.0 8,2 9.6 5.9 5.2
Soda Paga 2.6 2.6
MitiMbiliNdere 1.4 4.9 3.1 2.4
Gudwa K'Owuor 0.0 0.0 26.5 8.8
Oluch River 5.0 84.0 44.5 55.9
Mean 3.8 1.0 1.2 1.9 1.6 6.0 8.3 12.3 4.5 9.5
SO 5.8 1.2 3.0 3.3 4.6 16.7 7.9 13.2 9.5


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