Length-frequency data collected from fish landings on Lake Naivasha were used to estimate the growth parameters: total mortality (Z), growth performance index (Ø’), exploitation rate and recruitment pattern in Oreochromis leucostictus. The asymptotic length (L∞) was 38 cm and K 0.48 yr -1 Z was estimated as 3.5 yr -1, M was 0.19 yr -1, F was 2.6 yr -1 and E of 0.74. Recruitment occurs throughout the year, with a peak in January to March, while entry into the fishery occurs at a mean length of 15.9 cm. Existing restriction on the maximum number of gill nets allowed per fishing licence (10 per boat) and a minimum mesh size (10 cm) in the lake are not adhered to. Poaching using illegal mesh size nets as small as 5 cm and use of more than 10 nets per boat are common in the lake

Njiru, M.

Ojuok, J.E.

Aquatic Commons

ABSTRACT
Afr. 1. Trop. Hydrobiol. Fish. (1996/97) 7, 17-21
Population parameters of Oreochromis leucostictus
from Lake Naivasha, Kenya
Where Li =the mean length of fully selected
fish computed from L'i (lower class of the first
class used in computing Li) upwards. Li is
plotted against L'i and a linear regression is
carried out to obtain the regression coefficients
a and b.
Li = a + b L'i (1)
The asymptotic (Loo) and the ratio of the total
mortality (2) to the growth constant (K) were
estimated using modified Powell-Wetherall
method:
MATERIALS AND METHODS
per unit effort and maximum sustainable yield
but not for estimation of population parameters
such as growth and mortality rates. This paper
utilises the length-frequency data to address
this issue.
Length frequency data on O. leucostictus fished
by gill nets was collected five days a week for
the whole of 1995 from the commercial fishery.
The ELEFAN I and II computer programs
(PAULY, 1987) were used to estimate
population parameters.
Present Address
M. Njiru and J.E. Ojuok
KMFRI
Box 1881, Kisumu, Kenya
Length-frequency data collected from fish landings on Lake Naivasha were used to estimate the growth parameters: total
mortality (Z), growth performance index (¢'), exploitation rate and recruitment pattern in Oreochromis leucostictus. The
asymptotic length (Loo) was 38 cm and K 0.48 yr .t. Z was estimated as 3.5 yr ", M was 0.19 yr ", F was 2.6 yr .J and E of
0.74. Recruitment occurs throughout the year, with a peak in January to March, while entry into the fishery occurs at a
mean length of 15.9 cm.
Existing restriction on the maximum number of gill nets allowed per fishing licence (10 per boat) and a minimum mesh
size (10 cm) in the lake are not adhered to. Poaching using illegal mesh size nets as small as 5 cm and use of more than
10 nets per boat are common in the lake.
M. NJIRU AND J.E. OJUOK
Kenya Marin .. and Fisheries
Research Institute (KMFRI)
P.O. Box 837, Naivasha, Kenya
· IOreochromis leucostictus (Trewavas) is the
~most commercially important fish in Lake
'Naivasha after Micropterus salmoides (Lacepe)
and Tzlapia zillii (Gervais). The overall species
composition in catches since 19fr7 was 0.
leucostictus 87.4%, T zillii 0.5% and M.
salmoides 14.8% (MUCHIRI and HICKLEY,
1991). O. leucostictus is caught by gill nets of
10 cm mesh size set from wooden or glass-fibre
canoes and landed at a single station near
Naivasha town. The species was introduced
unintentionally in 1956 with a batch of T zillii
later flourishing in the early 1960s to dominate
the lake fishery. O. leucostictus prefers lagoons
and littoral areas where accumulation of
detritus leads to concentrations of chironomid
I larvae, its major food source (MUCHIRI and
!HICKLEY, 1991).
I '
:f;nata collection on L. Naivasha fishery
· •commenced in 1963 but differentiating of the
· :catches into 'basis' and 'tilapia' started only in
,1974. Separate recording of O. leucostictus and
T. zillii began in 1987 (MUCHIRI and
HICKLEY, 1991). The catch data so far
collected were suitable for calculation of catch
· 'j INTRODUCTION
f
F i g . 1 R e s t r u c t u r e d l e n g t h - f r e q u e n c y d a t a o f 0 . l e u c o s / i c / u s w i t h g r o w t h c u r v e s f i t t e d u s i n g E L E F A . t \ l I .
. · · f
N o v ' D o c
S o p ' O c t
A u g
R E S U L T S
M o r t a l i t y
F i g u r e 2 r e p r e s e n t s t h e l e n g t h - c o n v e r t e d c a t c h
c u r v e . u s e d t o e s t i m a t e Z b a s e d o n g r o w t h
p a r a m e t e r s e s t i m a t e d b y E l e f a n I a n d I I . T h e
e s t i m a t e s o f m o r t a l i t y Z , M a n d F w e r e 3 . 5 ,
0 . 1 9 a n d 2 . 5 y r -
I
r e s p e c t i v e l y .
S e l e c t i o n a n d r e c r u i t m e n t p a t t e r n
F i g u r e 3 r e p r e s e n t s t h e p r o b a b i l i t y o f c a p t u r e
( r e s u l t a n t c u r v e ) d e r i v e d f r o m t h e l e n g t h -
c o n v e r t e d c a t c h - c u r v e . T h i s s h o w s t h a t 5 0 % o f
O . l e u c o s t i c t u s e n t e r s t h e f i s h e r y a t 1 5 . 9 c m .
R e c r u i t m e n t p a t t e r n d e t e r m i n e d t h r o u g h
E L E F A N I I ( F i g u r e 4 ) s h o w s O . l e u c o s t i c t u s
r e c r u i t m e n t t o o c c u r t h r o u g h o u t t h e y e a r , w i t h
p e a k s i n F e b r u a r y a n d a t t h e e n d o f J u l y .
G r o w t h
T h e o r i g i n a l - f r e q u e n c y d a t a w e r e r e s t r u c t u r e d
u s i n g E L E F A N I s u c h t h a t p e a k s w e r e r e -
e x p r e s s e d a s p o s i t i v e p o i n t s a n d c o n v e r s e l y f o r
t h e t r o u g h s b e t w e e n p e a k s . T h e p r o g r a m t h e n
f i t t e d g r o w t h c u r v e s , t h e o n e r e t a i n e d b e i n g t h a t
w h i c h t o u c h e d p e a k s w h i l e a v o i d i n g t r o u g h s
( F i g u r e 1 ) . A s y m p t o t i c l e n g t h ( L o a ) w a s
e s t i m a t e d a s 3 8 c m a n d K w a s 0 . 4 8 y r < ' . T h e
g r o w t h p e r f o r m a n c e i n d e x :
( ¢ , = L o g 1 0 K + 2 1 0 g I O L o a ) w a s t h u s c o m p u t e d
a s 2 . 8 .
F o b ' M a r A p r M a y J u n J u t
T i m a ( m o n t h s )
. J a 1
1 9 9 5
o
~1e
. 9
3 Q
g 2 3
A p l o t f o r p } ; o b a b i l i t y o f c a p t u r e b y l e n g t h w a s
o b t a i n e d f r o m t h e b a c k w a r d e x t r a p o l a t i o n o f
t h e r i g h t d e s c e n d i n g a r m o f a c a t c h - c u r v e , a n d
c a l c u l a t i n g t h e n u m b e r o f f i s h t h a t w o u l d h a v e
b e e n c a u g h t h a d i t n o t b e e n f o r s e l e c t i o n a n d
i n c o m p l e t e r e c r u i t m e n t . R e c r u i t m e n t p a t t e r n
w a s o b t a i n e d b y p r o j e c t i o n o n t o t h e t i m e a x i s
o f t h e a v a i l a b l e l e n g t h - f r e q u e n c y d a t a .
L o g 1 0 ( M ) = - 0 . 0 0 6 6 - 0 . 2 7 9 L o g 1 0 L o a + 0 . 6 5 4 3
L o g 1 0 K + 0 . 4 6 3 L o g 1 0 T . ( 3 )
W h e r e L o a ( T L ) i s e x p r e s s e d i n c m , T i s w a t e r
t e m p e r a t Q r e i n ° C ( t a k e n a s 2 1 ° C ) . T h e
e s t i m a t i o n o f f i s h i n g m o r t a l i t y ( F ) w a s o b t a i n e d
b y s u b t r a c t i o n o f M f r o m Z . E x p l o i t a t i o n r a t e
( E ) i s F / Z .
¢ ' = L o g 1 0 K + 2 1 0 g I O L o a ( 2 )
W h e r e K i s t h e r a t e a t w h i c h l e n g t h a p p r o a c h e s
t h e a s y m p t o t e a n d L o a i s t h e a s y m p t o t i c l e n g t h .
T o t a l m o r t a l i t y ( Z ) w a s e s t i m a t e d u s i n g a
l e n g t h - c o n v e r t e d c a t c h c u r v e . N a t u r a l m o r t a l i t y
( M ) w a s e s t i m a t e d f r o m e q u a t i o n o f P A U L Y
( 1 9 8 0 ) :
1 8 M . N J I R U A N D J . E . O J U O K
T h e g r o w t h p e r f o r m a n c e i n d e x w a s c o m p u t e d
a c c o r d i n g t o P A U L Y a n d M U N R O ( 1 9 8 4 ) :
19
1 o·•0
-
-
0<1
-z
- 4S
-
1
0
-2 0
0 2 4 6 8
'RolativQ a rs - to)
Fig. 2. Length-convened catch cuIVe for O. leucostictus from lake Naivasha (Black dots represented points used in the analysis)
.:::.:;.
.4
.3
2 ..
· ...-" :
·1 ..' :
oL.....::i.::::::;::,:;::4.:;:2S===--"":":1S=""""-!:;'=-S--L~17=-.-::::25::--"""-=1e:-.";:1S~-­
Longtl elassos (em)
1
·g.
·8
.. 1
6 ~· . ....
• 5 .~ - -:..",.
..... "
-o
e
:J
-a.8
Fig. 3. Selection pattern of O.leucostictus from Lake N3.ivasha. Where estimates ofL~; == 14.7 COl, L;o == 1;.9 cm
2 0 M . N J I R U A N D J . E . O J U O K
3 0 ·
F i g . 4 . R e c r u i t m e n t p a t t e r n o f O . l e u c o s t i c t u s f r o m L . N a i v a s h a . F i t t i n g o f t h e g r a p h w a s d o n e u s i n g E L E F A N I I .
. "
. . .
" ' ' ' , . . L V " " ' L : . : . -
~
A u g S o p
M a y J u n J u t
" '
"
"
T h e t o t a l m o r t a l i t y r a t e Z o f 3 . 5 y r -
I
i s q u i t e
h i g h . T h i s h i g h Z c o u l d b e d u e t o i n t e n s e
f i s h i n g i n L . N a i v a s h a ( E = 0 . 7 4 ) a n d u s e o f
i l l e g a l m e s h s i z e d g i l l n e t s . T h e k e y a s s u m p t i o n
o f c a t c h c u r v e a n a l y s i s , t h a t t h e r e l a t i v e
a b u n d a n c e o f t h e d i f f e r e n t s i z e g r o u p s i n t h e
c a t c h e s r e f l e c t t h e i r a b u n d a n c e i n t h e
p o p u l a t i o n , w a s p r o b a b l y n o t m e t b e c a u s e o f
t h e s a m p l i n g g e a r u s e d .
M o s t t r o p i c a l f i s h s t o c k r e c r u i t m e n t c o n t i n u e s
a l l y e a r r o u n d w i t h s e a s o n a l o s c i l l a t i o n s
c o r r e s p o n d i n g t o t h e r a i n y s e a s o n s ( R U F L I a n d
V A N L I S S A , 1 9 8 2 ) . T h i s s t u d y s h o w s O .
l e u c o s t i c t u s r e c r u i t s t o t h e f i s h e r y t h r o u g h o u t
t h e y e a r w i t h p e a k c o r r e s p o n d i n g t o r a i n y
s e a s o n i n t h e w a t e r s h e d a r e a ( N o v e m b e r -
D e c e m b e r a n d A p r i l - M a y ) .
r e s u l t s i n h i g h j 1 j ' . O v e r - f i s h i n g o f O .
l e u c o s t i c t u s i s c o n f i r m e d b y h i g h e x p l o i t a t i o n
r a t e ( E = 0 . 7 4 ) . I n g e s t i o n o f d e t r i t u s i s s u i t a b l e
f o r o p t i m u m g r o w t h a n d i s n u t r i t i o n a l l y
a d e q u a t e f o r t i l a p i a s ( B O W E N , 1 9 8 2 ) . O .
l e u c o s t i c t u s f e e d s o f d e t r i t u s a n d c h i r o n o m i d
l a r v a e w h i c h c o u l d b e c o n t r i b u t i n g t o t h i s h i g h
j 1 j ' .
,
,
,
,
,
,
,
. .
. . .
~
,
. , 4 1 . . . . . . . _
-
I
,
.
,
,
.
,
,
.
,
•
,
,
,
, . '
.
.
I V .
l : : : : : L . . . . . . F Q b M e r r
o N o v D o c J a n . (month~
T U l l O
-
c
( )
: E 1 0
2
u
&
-
~ 2 0
T h e g r o w t h p e r f o r m a n c e i n d e x ( j 1 j ' ) o f O .
l e u c o s t i c t u s i n L . N a i v a s h a o f 2 . 8 i s h i g h .
G e n e t i c m a k e u p , f i s h i n g r e g i m e a n d d i e t t y p e
m a y d e t e r m i n e g r o w t h p o t e n t i a l o f a s p e c i e s
( S S E N T O N G O a n d W E L C O M E , 1 9 8 5 ) . O v e r -
f i s h i n g r e s u l t s i n r e d u c t i o n o f t h e a v e r a g e s i z e
o f f i s h a n d a f a s t e r g r o w t h r a t e . I n c r e a s e i n K
D I S C U S S I O N
.,~
T h e a s y m p t o t i c l e n g t h i n t h i s s t u d y o f 3 8 c m i s
h i g h e r t h a n t h e e s t i m a t e o f 3 2 c m r e p o r t e d b y
S I D D I Q U I ( 1 9 7 9 ) . A v a i l a b l e r e c o r d s s h o w t h a t
0 . l e u c o s t i c t u s i n L . N a i v a s h a s t a r t m a t u r i n g a t
1 6 c m ( T L ) ( f e m a l e ) a n d 1 8 c m ( m a l e )
( T R E W A V A S , 1 9 8 3 ) . F r o m t h i s s t u d y t h e
l a r g e s t g r o u p o f f i s h l i e s b e t w e e n 1 7 - 2 0 c m
( T L ) . T h e g r o w t h p a r a m e t e r K o f 0 . 4 8 y r -
I
s h o w s t h a t O . l e u c o s t i c t u s i n L . N a i v a s h a g r o w s
m o d e r a t e l y . M O R E A U e t a L . , ( 1 9 8 6 ) r e c o r d s a
K o f 1 . 1 6 y r -
I
f o r O . l e u c o s t i c t u s f r o m
L . V i c t o r i a a n d 1 . 6 2 f o r O . a u r e u s f r o m L .
m a r i u t . T h i s s t u d y s h o w s t h a t O . l e u c o s t i c t u s i n
t h e l a k e i s n o w c a u g h t a t a m u c h l o n g e r l e n g t h
t h a n i n t h e 1 9 7 0 s . B u t m o s t f i s h e n t e r i n g t h e
f i s h e r y a r e n o t g i v e n a c h a n c e t o g r o w f u r t h e r
( L s D = 1 5 . 9 e m ) .
O.
on
)Ie
Ily
O.
Lid
gh
les
,ns
nd
O.
mt
ny
er-
ite
Ise
of
on
lve
the
:he
of
Although there are restrictions on the maximum
number of gill nets allowed per fishing licence
(10 per boat) and a minimum mesh size (lO
em), these regulations are not adhered to
(MUCHIRI and HICKLEY, 1991). Poaching
~ using illegal mesh size nets and use of more
1han 10 nets per boat is common in the lake. A
survey carried out in 1996 showed that nets
with stretched mesh down to 5 cm are
cc,>mmonly used in the lake. Continual use of
smaller mesh sizes would lead to further
reduction of O. leucostictus sizellength at first
capture and consequently the fishers would in
turn reduce their mesh sizes to target the
smaller fish. This will decrease long term
catches of the species. BEVERTON (1959) had
predicted a similar trend due to use of smaller
~. mesh gill nets to capture the dwindling stocks
, of O. esculentus, Bagrus docmac, and Barbus
Gltianalis, once traditional species in L.
Victoria.
When this problem of small mesh sizes is
compounded with poaching it could spell doom
to O. leucostictus and the entire L. Naivasha
fishery. The collapse of O. leucostictus would
not only be disastrous to L. Naivasha
,eosystem, but also to the economy of the
people depending on the lake. Relevant
IOvernment departments which are empowered
with authority should seek immediate solution.
ACKNOWLEDGEMENTS
We wish to thank Dr. E. Okwema, Director
ICMFRI for giving us financial support. We
c;annot forget the staff involved in data
compilation especially Messr. Nyolo, Achar
and Otieno.
REFERENCES
Geverton, RJ.H., 1959. Report on the state of Lake
Victoria. Mimeo. Fisheries Laboratory,
Lowesoft.
Bowen, S.H., 1982. Feeding, digestion and growth-
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proceedings 7: 141-156.
21
Muchiri, S.M. and P. Hickley., 1991. The fishery of
Lake Naivasha. In: Cowx LG. (ed). Catch
effort sampling strategies: their application
infresh water fisheries management. Fishing
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Moreau, J., c., Bambino and D. Pauly, 1986. A
comparison of four indices of overall fish
growth performance based on 100 Tilapia
population (Fam: cichlidae). ICALARM
contribution No. 292, 14pp.
Pauly, D., 1980. On the inter-relationship between
natural mortality, growth parameters, mean
environmental temperature on 175 fish
stocks. J. Cons. Int. Explor. Mer., 39 (3) 175-
192.
Pauly, D. and J.L. Munro., 1984. Once more on
comparison of growth in fish and
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Pauly, D., 1987. A review of the ELEFAN system
for analysis of length-frequency data in fish
and aquatic invertebrates, p 7-34. In: Pauly,
D. and Morgan, G.R. (eds). Length-based
Methods in fisheries research. ICLARM.
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Rutli, H. and J. Van Lissa., 1982. Age and growth of
Engraulic)pris sardella in Lake Malawi,
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ecosystem of Lake Malawi. FIJMU75.019,
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Ssentongo, G. and G.L. Welcome., 1985. Past
History and Current trends 'in the Fisheries of
Lake Victoria, FAa Fish. Rep. 335: 123-138.
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Population parameters of Oreochromis leucostictus from Lake Naivasha, Kenya

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Population parameters of Oreochromis leucostictus from Lake Naivasha, Kenya

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