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 Oreochromis et Lates niloticus

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Nombre de messages : 37
Age : 47
Localisation : Granville, Paris (F) et Porrentruy (CH)
Date d'inscription : 12/06/2006

MessageSujet: Oreochromis et Lates niloticus   Sam 24 Juin - 9:34

Some biological aspects and life history strategies of Nile
tilapia Oreochromis niloticus (L.) in Lake Victoria, Kenya
M. Njiru1,*, J. E. Ojuok1, J. B. Okeyo-Owuor2, M. Muchiri3, M. J. Ntiba4 and
I. G. Cowx5

Abstract
The life history characteristics of introduced Nile tilapia
(Oreochromis niloticus L.) in Lake Victoria, including, sex
ratio, fecundity, reproduction, weight-length relationship
and body condition were studied and compared with those
of other populations. Samples were collected by trawling
and seining in the Kenyan sector of Lake Victoria between
June 1998 and December 2000. Males predominated over
females (sex ratio 1.42 : 1 : 00). O. niloticus spawned
throughout the year but with a peak between December
and June. Length at first maturity was (mean ± SD)
30.81 ± 0.09 for females and 34.5 ± 6 0.48 for males.
There was little seasonal variation in relative condition,
which ranged from 0.92 to 1.05 in males and 0.94 to 1.07
in females. Gonadosomatic index (GSI) was low during the
postspawning period (July to October) and high during the
protracted breeding period (December and June). Fecundity
ranged from 905 to 7619 oocytes for fish of 28 to 51 cm
total length (TL) respectively. The relationships between
fecundity (F) and total length (L), weight (W) and ovary
weight (OW) were: F ¼ 8.159L1.53, F ¼ 96.269W0.4504,
F ¼ 1806 + 39.4OW. The slope b of the weight-length
relationship was 3.08–3.32 for males and 3.07–3.22 for
females. Growth was allometric in both cases and was
significantly different from the expected value of 3. The life
history strategy of O. niloticus is discussed in context of
environmental changes occurring in the lake.
Key words: condition, fecundity, introduction, overexploitation,
weight-length




Effects of Nile perch, Lates niloticus, on functional and
specific fish diversity in Uganda’s Lake Kyoga system

Jesse D. M. Schwartz1*, Marla J. Pallin1, Robert H. Michener1, Dismiss Mbabazi2 and
Les Kaufman1

Abstract
The introduction of Nile perch, Lates niloticus, to Lake
Victoria, East Africa, interacted with eutrophication to
cause a reorganization of the lake’s food web and the
extirpation of many endemic fishes. The Lake Kyoga satellite
system lies downstream from Lake Victoria. It
encompasses species-rich lakes where Nile perch are
absent or very rare, and low diversity lakes where L. niloticus
is abundant. In 1999 we surveyed seven lakes in the
Kyoga system using experimental monofilament gill nets
(1/4–1 inches variable mesh). At Boston University we
assessed d15N signatures of epaxial muscle from subsamples
of the catch (n ¼ 361). These signatures are often
highly correlated with the near-term mean realized trophic
position of an individual organism. A neural network
analysis of fish length, species name, trophic level, and lake
of origin fish explained 94% of the sample variance in
d15N. We analysed statistical patterns in these signatures
at a number of spatial scales. The relationship between
trophic level and d15N varied greatly among lakes. Higher
diversity perch-free lakes had greater variance in d15N
values and fish lengths than lower diversity Nile perch
lakes, suggesting an important relationship between species
diversity and functional diversity. Against expectations,
lake size was negatively correlated with d15N.
Patterns in stable isotope signatures indicated that Nile
perch lakes have shorter food chains than perch-free lakes.
The results throw up two management problems for the
Kyoga system. Impacted lakes need to be studied to
understand and ameliorate the community-level effects of
Nile perch introduction, whereas the species-rich nonperch
lakes, which harbour a large proportion of the remaining
diversity of regionally endemic taxa, are in need of conservation
planning.


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