Int. J. Aquat. Biol. (2017) 5(1): 7-11; DOI:  

ISSN: 2322-5270; P-ISSN: 2383-0956

Journal homepage: www.ij-aquaticbiology.com 
© 2016 Iranian Society of Ichthyology 

Short Communication 
Seasonal changes in length-weight relationship and condition factor of Nile tilapia, 

Oreochromis niloticus (Linnaeus, 1758) (Cichlidae) in Lake Naivasha, Kenya 
  

James Last Keyombe*1, John O. Malala1, Edna Waithaka2, Ruth M. Lewo3, Benson Ojowi Obwanga4 
 

1Kenya Marine and Fisheries Research Institute, Turkana Research Station, P.O. Box 205-30500, Lodwar, Kenya. 
2Kenya Marine and Fisheries Research Institute, Naivasha Research Station, P.O. Box 837-20117, Naivasha, Kenya. 

3Nakuru County Fisheries Department, Naivasha Sub-County, P.O. Box 135-20117, Naivasha, Kenya. 
4Laikipia University, Department of Biological and Biomedical Sciences, P.O. Box 1100-20300, Nyahururu, Kenya. 

 

 

 

 

 

 

Article history: 
Received 13 October 2016 

Accepted 22 January 2017 

Available online 2 5 February 2017 

Keywords:  
Allometry 

Condition factor 

 Length-weight relationship 

Oreochromis niloticus 

Abstract: The study compared the length-weight relationship and condition factor (K) of Nile tilapia 
(Oreochromis niloticus) in Lake Naivasha, Kenya, between the wet and dry seasons. Fish samples 

were collected monthly by gill netting from February to December 2015. A total of 372 samples of 

O. niloticus were analysed. The b values in the length-weight relationships were observed as 3.077 

and 3.366 in the wet and dry seasons, respectively. The values of b exhibited positive allometric 

growth which were the important indication that the species was growing faster in weight than length. 

The K values of the fish ranged 1.18-4.1 during the wet season and 0.8-3.0 during the dry season.  

All mean monthly K values were found to be greater than 1, the highest being in June (2.33±0.05) 

and lowest in February (1.31±0.28), an indication of a healthy status and general well-being of the 

O. niloticus population in Lake Naivasha. The study concluded that seasonal variation has no great 

influence in the length-weight relationship and condition factor of O. niloticus in Lake Naivasha. 
 

Introduction 

The fishery of Lake Naivasha mainly depends on six 

non-native fish species: Oreochromis niloticus (Nile 
tilapia), O. leucostictus (Blue spotted tilapia), Tilapia 
zilli (Gervais), Micropterus salmoides (Black bass), 
Cyprinus carpio (Common carp) and Clarias 
gariepinus (African catfish). The riverine Barbus 
amphigramma and Procambarus clarkii (Red 
Louisiana Crayfish) have been supporting the fishery 

of Lake Naivasha significantly since 1980s (Oyugi 

et al., 2011). Oreochromis niloticus (Nile tilapia) 
was first introduced in Lake Naivasha around 1967 

but disappeared by 1971 for reasons unclear to date 

(Hickley et al., 2008). Anthropogenic disturbance of 

the littoral vegetation, particularly through clearance 

and burning could have previously reduced its 

spawning areas as reported by Hickley et al. (2008) 

resulting to its disappearance in the 1970s. The 

Government of Kenya through the Economic 

                                                           
* Corresponding author: James Last Keyombe 

E-mail address: katalitsa@yahoo.com 

Stimulus Package (ESP) reintroduced the Nile 

Tilapia between 2010 and 2013 to boost the local 

fisheries in Lake Naivasha. The fingerlings were 

sourced locally in accredited government fish 

hatcheries. Since its reintroduction, Kenya Marine 

and Fisheries Research Institute (KMFRI) data 

shows a constant increase in landings by fishermen 

in all the fish landing beaches around Lake Naivasha. 

The same has been witnessed during the monthly 

fish stock assessment exercises on the lake. 

Length-weight relationship (LWR) is a useful tool 

in a wide range of applications such as estimation of 

biomass from length data, estimation of a species 

condition factor and comparisons among life history 

and morphologic differentiations of the same species 

in other aquatic systems (Binohlan and Pauly, 2000). 

LWR measurement is also a useful tool that provides 

important information concerning the structure and 

function of fish populations in any aquatic systems 



8 
 

Keyombe et al./ Seasonal changes in length-weight relationship and condition factor of Nile tilapia 

(Anderson and Neumann, 1996). When the b value 
is less than 3, the fish has a negative allometric 

growth but when it is greater than 3, it has a positive 

allometric growth and when it is equal to 3, the fish 

has isometric growth (Khairenizam and Norma-

Rashid, 2002). Change of b values depend primarily 
on the shape and fatness of the fish species as well as 

physical, chemical and biological factors such as 

temperature, salinity, food, stomach fullness, sex and 

stage of maturity (Sparre and Venema, 1998; Sarkar 

et al., 2013).  
Fish condition factor is an important concept in 

fisheries management and can be used to assess the 

health and potential of any fishery to support the 

fishing pressure. The condition factor often referred 

to as K provides information on the general well-

being of a fish and health condition of a population. 

Condition factor is estimated by comparing 

individual fish weight of a given length to a standard 

weight. Fish body condition is known to vary 

seasonally depending on changes in gonadal 

development, food availability, and other 

environmental factors (Pope and Willis, 1996). It is 

usually influenced by the type of fish species, sex, 

season, maturity stage among other factors 

(Anyanwu et al., 2007). The role of the condition 

indices as stated by Stevenson and Woods (2006) is 

to quantify the health of individuals in a population 

or to tell whether a population is healthy relative to 

other populations. When fish of a given length 

exhibits higher weight it means they are in better 

condition (Anwa-Udondiah and Pepple, 2011). 

The determination of seasonal variations of b and 
K values of O. niloticus, a reintroduced fish species 
in Lake Naivasha, is therefore useful in assessing the 

well-being, growth performance and feed utilization 

of the fish in the lake. The main focus of this paper 

is to assess the seasonal variations in LWR and 

condition factor of O. niloticus in Lake Naivasha 
between February and December 2015. 

 

Materials and Methods 

The study was carried out at random sampling sites 

in Lake Naivasha, an equatorial rift valley lake with 

an annual atmospheric temperature that rarely falls 

below 20°C (Hickley, 2008). The lake is a shallow 

freshwater body, situated in the eastern rift valley of 

Kenya (0°46'S, 36°20'E) at an altitude of about 1890 

m above sea level. It covers a surface area varying 

between 120 Km2 and 160 Km2 depending on the 

wet and dry seasons respectively (Hickley et al., 

2008). The lake’s mean depth varies 4-6 m (Hickley 

et al., 2008). The lake is the major source of fish for 

the surrounding community and fresh water for the 

numerous horticultural industries in the area. Apart 

from transient streams, the lake is fed by the 

perennial Malewa and Gilgil rivers with the former 

being the main one (Kitaka et al., 2002). There are 2 

wet seasons in the months of March-May (long 

rains) and October-November (short rains) 

characterized by high lake water levels (Oyugi et al., 

2011).  

Sampling procedure comprised setting of gill nets 

of variable mesh size ranging from 25-70 mm at 

dusk, with lifting after approximately 12 hours of 

fishing. Fish caught in the nets were identified at the 

species level, length and weight of individual fish 

were measured in situ and recorded for each 
sampling occasion. The total length (TL in cm) of 

each fish was measured using a meter rule. Weight 

of each fish was measured using the SF-400 digital 

weighing balance. The length-weight relationship 

was calculated using the formula by Wooton (1990) 

as: 

W= aLb 
Where W is the body weight of fish in grams, L 

the total length in centimeters, a, the intercept and b 
the slope of the regression line. Condition factor (K) 

was estimated following Fulton (1902) and Le Cren 

(1951): 

K=W/L3x100 
Where K is the condition factor, W is the body 

weight of fish in grams, L the total length in 
centimeters. 

 

Results and Discussion 

A total of 372 specimens of O. niloticus (191 during 
wet and 181 during dry season) were used in this 



9 
 

Int. J. Aquat. Biol. (2017) 5(1): 7-11 

 

study. Allometry coefficient values b obtained were 
3.077 and 3.366 in the wet and dry seasons, 

respectively (Figs. 1, 2).  

The relative condition factor of O. niloticus in 
Lake Naivasha ranged from 1.18 to 4.1 during the 

wet season and 0.8 to 3.0 during the dry season. The 
wet and dry seasons had fish with mean K values of 

1.92±0.09 and 1.89±0.49, respectively. The highest 

and lowest K values recorded during the sampling 

period were 2.33±0.05 and 1.31±0.28 in June and 

February 2015 respectively, both falling within the 

dry season (Fig. 3).  

The results showed that O. niloticus had positive 
allometric growth patterns (b>3) in both the dry and 

wet seasons indicating that the fish is growing faster 

in weight than length. This could be linked to the 

high productivity of Lake Naivasha in both the wet 

and dry seasons (Kitaka et al., 2002). However, these 
results differed with a study by Outa et al. (2014) 

done between November 2013 and January 2014 in 

the same lake who recorded the b value as 2.3. 
Differences in b values can be ascribed to one or a 
combination of factors including differences in the 

number of specimens examined, location and season 

effects and distinctions in the observed length ranges 

of the specimens caught and the duration of sample 

collection (Moutopoulos and Stregiou, 2002). The 

marked difference in the b value obtained by Outa et 
al. (2014) could also be attributed to the short period 

spent by the fish in Lake Naivasha since its 

introduction in 2011. The current study showed no 

marked differences in a and b values between 
O. niloticus in Lake Naivasha and populations 
elsewhere. This is supported by a study of 

O. niloticus in Lake Baringo, a Rift Valley lake in 
Kenya, by Kembenya et al. (2014) where b values 
were 3.08 and 3.04 in the wet and dry seasons, 

respectively.  

Based on these results, O. niloticus in Lake 
Naivasha had mean monthly K values of greater than 

1 in all the months sampled an indication of general 

well-being and stable physiological status of the fish 

in the lake. However, the condition factors were 

relatively higher than the values (1.97-2.63) 

documented by Outa et al. (2014) in an earlier study 

of the same fish species in Lake Naivasha. The 

present higher K values might be attributed to 

adaptation of feeding habits of O. niloticus in Lake 
Naivasha to the different trophic levels in the lake 

enabling it to comfortably obtain food all year round. 

According to Canonico and Arthington (2005), 

O. niloticus is known to adapt readily to a range of 

Figure 1. Length-Weight relationship of Oreochromis niloticus 
in Lake Naivasha during the wet season in 2015. 

Figure 2. Length-Weight relationship of Oreochromis niloticus 
in Lake Naivasha during the dry season in 2015. 

Figure 3. Mean monthly condition factor of Oreochromis 
niloticus in Lake Naivasha in both wet and dry seasons of 2015. 



10 
 

Keyombe et al./ Seasonal changes in length-weight relationship and condition factor of Nile tilapia 

environmental factors such as salinity, low oxygen 

levels and can feed at different tropic levels when 

need arises. Deekae et al. (2010) noted that several 

factors affect the condition factor of fishes. These 

range from feeding, spawning, food nutrient 

composition and fat accumulation. The variations of 

condition factor (K) in fish according to King (1995) 

may be due to food abundance, adaptation to the 

environment and gonadal development. The mean 

condition factor in the wet and dry seasons in this 

study varied slightly with those obtained by 

Kembenya et al. (2014) in Lake Baringo, where 
O. niloticus were observed to have K values ranging 
from 0.59 to 1.89 during the dry season and 0.80 to 

3.70 during the wet season.  

 

Conclusions 

This study revealed that seasonal variation has no 

great influence in the length-weight relationship and 

condition factor of O. niloticus in Lake Naivasha. 
The faster growth of O. niloticus in weight than 
length favours the fishermen and fish traders around 

Lake Naivasha as it enhances its profitability. The K 

values obtained for the fish showed that the 

population was in good condition, an indication of 

the healthy status of the population. It is equally an 

indication of the ability of Lake Naivasha to sustain 

a healthy population of O. niloticus. 
 

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