Abstract:

Bottom trawls, gill nets and other sampling devices were used to study the
distribution and abundance of Nile perch (Lates niloticus (L.)) and Nile tilapia
(Oreochromis niloticus (L.)) in the Uganda portion of Lake Victoria from 1999
to 2001.
A total of 17 fish taxa (belonging to 14 genera) were encountered in the
Uganda portion of Lake Victoria in this study. This reflects a decline in fish
species diversity over the last 30 years when compared with results from
similar studies in 1969/71. Tilapia rendalli was encountered for the first time
since it was introduced in 1950s. The fish catch was dominated by Lates
niloticus (86.5% by weight), followed by Oreochromis niloticus (9.8%). L.
niloticus was found in all areas surveyed while O. niloticus was restricted to
waters <20 m deep.
The highest catches (13.7±1.1 t km-2) of L niloticus in bottom trawls were
obtained in Zone I - between Tanzania/Uganda border and Bukakata - and
Zone III - between Kiyindi and Uganda/Kenya border - had the lowest
concentrations of fish (8.3±0.6 t km'2). This trend was also reported during
the period 1993/98.
Length frequency data for L. niloticus suggest that all size groups were found
in all areas surveyed during bottom trawling. Juveniles of L. niloticus of less
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than 6 cm TL were found in all areas surveyed. This may suggest that
breeding of L. niloticus takes place throughout the lake.
O. niloticus during the bottom trawl surveys in 1999/2000 were restricted to
water <20 m deep. The bulk of (93% by weight and 40% by numbers) O.
niloticus were mature (>22 cm TL). This was partly because trawling covered
only waters 4-60 m deep leaving the waters <4 m deep, a potential area for
the small, young fish of the species.
In the monofilament gill net surveys, L. niloticus dominated the catches in the
open deep water while O. niloticus dominated in the shallow inshore areas. In
the open deep water areas the nets set at the bottom did not catch fish; this
was attributed to low oxygen levels.
More male than female L. niloticus were caught during both the bottom trawl
and gill net during the survey period.
The fish stock densities (CPUA) in the bottom trawls have continued to
niloticus in the present study in the 4-30 m deep waters. Fish stock densities
of O. niloticus were higher in 4-10 m deep water than in waters 10-20 m deep.
The biomass of 121 000 t of L niloticus and 12 000 t of O. niloticus were
estimated in the 4-40 m water depth zone. These estimates increased from
117 053 t in 1999 to 121 930 t in 2000 for L. niloticus and from 7316 t in 1999
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decline from 36.2 t km'2 for all fish taxa during 1969/1971 to 8.8 t km'2 for L.
to 17 070 t for O. niloticus. Such increases may partly have been due to the
El-Nino rains of 1998/1999, the reduction in fishing pressure resulting from the
ban of export of fish and fish products from the Uganda portion of Lake
Victoria to the European Union during 1999.
Juvenile fish measuring <50 cm TL (size at first maturity for male L. niloticus')
constituted 72.5% of the catch by weight while 27.5% were mature fish.
Accordingly, about 32 800 tonnes of the 119 491 tonnes estimated for L.
niloticus in the 4-40 m water depth zones during the survey period 1999-2000
were mature fish. The bulk of the immature L. niloticus were caught in waters
<20 m deep (63.36 % by numbers) and in Zone III where they (immature fish)
contributed 99.02% (by numbers) during 1999-2000. The abundance of the
juvenile fish of various sizes promises good future recruitment to the
population while the relatively small numbers of large mature fish raises
concern on the future of the fishery.
In this study the asymptotic lengths (L-) for L. niloticus and O. niloticus were
estimated as 256 cm and 75 cm TL, respectively, while the respective growth
rates for the two species were 0.29 yr'1 and 0.40 yr'1. The size at first maturity
for male and female L. niloticus were 50 cm and 64 cm TL, respectively, while
the size at first maturity for O. niloticus were 21.5 cm and 22.5 cm TL for male
and female, respectively. The high mortality rates for both L niloticus (Z=1.91)
and O. niloticus (Z=1.65) increased from 1999 to 2000 and varied between
zones.
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During this study, recruitment in the fisheries of L. niloticus and O. niloticus
occurred throughout the year but with two peak periods. Peak recruitment for
both L. niloticus and O. niloticus occurred during the rainy season (March-May
and August-November).
The diet of L. niloticus varied with its size. Caridina nilotica formed the most
important food item in juvenile fish up to 40 cm TL. Fish and fish remains
were observed from fish specimens as young as 6.2 cm TL. The occurrence
of fish in the diet of L. niloticus increased with increasing size of the fish
specimens, becoming dominant (67.4% occurrence) in fish of 40 cm TL and
contributing about 100% of the diet in fish of 90 cm TL.
During this study the water temperature varied in the different habitats and
seasons but generally ranged from 28.1°C at the surface to 24°C at the
Dissolved oxygen, bottom. a requirement for all living things, varied with
water depth, season and habitats. The varied distribution of the fish during
this study was considered to have been influenced by the observed
distribution of oxygen. The high catches of fish in the 20-30 m depth area in
Zone I is likely to be due to the presence of sufficient oxygen in the area. The
low abundance offish in waters >25 m deep are partly due to insufficient or no
oxygen.