270
13. Nile Perch Population Dynamics in Lake Victori a:
below). Growth of Nile perch is bounded by the two growth curves in
Table 13.2 from Lake Victoria in the 1980s (fast growth), and Lake Chad
(slower growth ) (Hughes 1992; Ogutu-Ohwayo 1990; Ogutu-Ohwayo 1994;
Ogutu -Ohwa yo 1999). When feeding up on haplochromines, fish grow relatively quickly, reaching a weight of 280 grams in their first year, and 1500
grams around year two. When feeding on other prey, growth is considerably slower, with fish reaching 1500 grams around year 2.5. In our model ,
growth is intermediate of these two as a function of percent diet, so that a
fish feeding on 30% hapl ochromines will experience 30% fast growth and
70% slower growth . Growth from cannibalism is calculated as slow growth
(Hughes 1986; Hughe s 1992; Ogari and Dadzie 1988; Ogutu-Ohwayo
1999). The specific values for respiration, consumption , egestion, and excretion are calculated in the same way.
We modified a mass balance model for Nile perch (Kitchell et al. 1997) in
order to calculate the energetic requirements and partitionings required to
achieve the growth scenarios described in (Hughes 1992). Similar models
have been used to address fish bioenergetics with considerable success for
other species (Bocci 1999; Ciannelli et al. 1998; Heikinheimo and Korhonen
1996; Hewett and Johnson 1992; Kitchell et al. 1974; Kitchell et al. 1977;
Penczak et al. 1999; Trudel and Boisclair 1994; Worischka and Mehner
1998). All these models are based on the premise that
c= Eg+Ex+SDA + (Gs + Gg) + (Rs + Ra) ,
(3)
so that the absolute or specific value (in joules or calories) of consumption
by any fish during a specified time will equal joules egested (Eg), plus those
excreted (Ex) , plus those used in the process of digestion ( SDA) , plus tho se
T ABLE 13.2. Von Bertalanffy growth equations and length-weight conversions for
Nile perch used in the model (For all functions, I =standard length in centimeters, w
= weight in grams, and t =time in decimal years. Lake Victoria data and formulas
from (Hughes 1992), Lake Chad data and formulas from (Hopson 1972).)
Size class
Lake Victoria (19805):
up to 5 yrs
>5yrs
7-15.9cm
16-45.9cm
46-120cm
Parameter
Function
growth curve
growth curve
I-w conve rsion
I-w conversion
I-w conve rsion
1,=1.35'93.1(l_e-o· 272 ('.O.046J)
1,=1 .35(31.96+7.68lt)
w=O.0234'12.74
w=O.0151'12.94
LakeCbad:
up to 5 yrs
>5yrs
7-15.9cm
16-45.9cm
46-120cm
growth curve
growth curve
!-w co nvers ion
l-w convers ion
l-w co nversio n
w=O.0023'13.44
1,=1.35'181(l -e-0 09(,-o·046))
w=O.0125'!3
13. Nile Perch Population Dynamics in Lake Victori a:
below). Growth of Nile perch is bounded by the two growth curves in
Table 13.2 from Lake Victoria in the 1980s (fast growth), and Lake Chad
(slower growth ) (Hughes 1992; Ogutu-Ohwayo 1990; Ogutu-Ohwayo 1994;
Ogutu -Ohwa yo 1999). When feeding up on haplochromines, fish grow relatively quickly, reaching a weight of 280 grams in their first year, and 1500
grams around year two. When feeding on other prey, growth is considerably slower, with fish reaching 1500 grams around year 2.5. In our model ,
growth is intermediate of these two as a function of percent diet, so that a
fish feeding on 30% hapl ochromines will experience 30% fast growth and
70% slower growth . Growth from cannibalism is calculated as slow growth
(Hughes 1986; Hughe s 1992; Ogari and Dadzie 1988; Ogutu-Ohwayo
1999). The specific values for respiration, consumption , egestion, and excretion are calculated in the same way.
We modified a mass balance model for Nile perch (Kitchell et al. 1997) in
order to calculate the energetic requirements and partitionings required to
achieve the growth scenarios described in (Hughes 1992). Similar models
have been used to address fish bioenergetics with considerable success for
other species (Bocci 1999; Ciannelli et al. 1998; Heikinheimo and Korhonen
1996; Hewett and Johnson 1992; Kitchell et al. 1974; Kitchell et al. 1977;
Penczak et al. 1999; Trudel and Boisclair 1994; Worischka and Mehner
1998). All these models are based on the premise that
c= Eg+Ex+SDA + (Gs + Gg) + (Rs + Ra) ,
(3)
so that the absolute or specific value (in joules or calories) of consumption
by any fish during a specified time will equal joules egested (Eg), plus those
excreted (Ex) , plus those used in the process of digestion ( SDA) , plus tho se
T ABLE 13.2. Von Bertalanffy growth equations and length-weight conversions for
Nile perch used in the model (For all functions, I =standard length in centimeters, w
= weight in grams, and t =time in decimal years. Lake Victoria data and formulas
from (Hughes 1992), Lake Chad data and formulas from (Hopson 1972).)
Size class
Lake Victoria (19805):
up to 5 yrs
>5yrs
7-15.9cm
16-45.9cm
46-120cm
Parameter
Function
growth curve
growth curve
I-w conve rsion
I-w conversion
I-w conve rsion
1,=1.35'93.1(l_e-o· 272 ('.O.046J)
1,=1 .35(31.96+7.68lt)
w=O.0234'12.74
w=O.0151'12.94
LakeCbad:
up to 5 yrs
>5yrs
7-15.9cm
16-45.9cm
46-120cm
growth curve
growth curve
!-w co nvers ion
l-w convers ion
l-w co nversio n
w=O.0023'13.44
1,=1.35'181(l -e-0 09(,-o·046))
w=O.0125'!3