ABSTRACT
The African catfish Clarias gariepinus is a non-native fish species to Agulu lake. The timing and method of its introduction into the Lake is not clearly known, but it is thought that it probably came through the selected rivers. The biology and ecology of catfish in Lake Nigeria is poorly known. This lack of knowledge makes it difficult to estimate the trophic level, reproductive capacity and ecological role of the species in the Lake. Therefore, this study aimed to assess the feeding habit, fecundity and biology of C. gariepinus. The feeding habit was assessed by analysing the stomach contents of 400 fish using the point method. The major food items consumed were zooplankton 24.6%, fish 23.1% and insects 21.4%. Ontogenic shift in the diet was evident where juvenile fish fed on high proportion of phytoplankton, insects and detritus while adults fed more on cray fish and zooplankton. Food selection using Strauss linear index revealed positive prey selectivity for Bacillariophyceae, Chlorophyceae and Cyanophyceae and negative selection for Desmidiaceae, Dinophyceae and Euglenophyceae. The sex ratio was significantly different from the hypothetical ratio of 1:1 (χ2=77.28, df = 5, P < 0.05). Length-Weight relationship of males and females of C. gariepinus among all sampling stations was not significantly different (t=0.373, df = 6, P > 0.05) from the isometric exponent value of 3. Relative condition factor showed that C. gariepinus in Agulu lakeare in a good condition with a value of 1 except at Crescent Island station with a value of 0.95. Length at first maturity (Lm50) of females and males was 18.9 cm and 42 cm respectively. Fecundity ranged from 1,260 to 354,361 eggs. There was significant variation in fecundity between different size classes (one way ANOVA, F = 9.13, df = 2, P < 0.05). The relationship between fecundity with total length and total weight were curvilinear while the relationship of fecundity with ovary weight was linear. A significant correlation was observed between fish condition factor and pH (r = 0.83, P < 0.05) and dissolved oxygen with fish length-weight relationship (r = 0.87, P < 0.05). The relationships between temperature and conductivity with condition factor and length-weight relationship were not significant (P > 0.05). This study found out that C. gariepinus is a generalist and has an opportunistic feeding behaviour ingesting a variety of food organisms ranging from phytoplankton to fish. In addition, the biology of C. gariepinus is affected by key water quality parameters such as pH and dissolved oxygen in Agulu lake.
CHAPTER ONE
INTRODUCTION
1.1 Background of the study
The African Catfish Clarias gariepinus (Burchell, 1822) is distributed widely in Africa ranging from the Nile River Basin to West Africa and Algeria in the North (Solak and Akyurt, 2001). They inhabit freshwater lakes, rivers, swamps and lagoons as well as man-made habitats (Nyamweya et al., 2010). African Catfish is known for its hardiness and adaptability to various adverse environmental conditions. They are omnivorous and feed on a variety of organisms including fish and plant materials. Catfishes have a suprabranchial air breathing organ which function as a lung enabling them to breath and survive in low oxygen concentrations and survive long periods out of water (Abidjan and Ivoire, 2014). Clarias gariepinus is an important food fish and one of the most important commercial freshwater species (Dadebo, 2000; Nyamweya et al., 2010). Studies have shown that, the African Catfish is also generally considered to be one of the most successful fish species in tropical aquaculture (Nyamweya et al., 2010). Kwak et al., (2011) showed that the ecology of C. gariepinus is relatively well studied in some African Lakes including Lakes Victoria and Agulu lake Nigeria. However, nothing is known about the species in Agulu lake apart from recent studies on its length-weight relationships and condition factor in the lake (Keyombe et al., 2015).
Earlier studies indicate that apart from providing food, the fingerlings of C. gariepinus are used as baitfish for the Nile Perch fishery in lake Victoria due to their resistance to extreme conditions and can wriggle on the hook for over a week (Ponzoni and Nguyen, 2008). According to Lung’ayia, (1994), C. gariepinus is euryphagous in its feeding habit and breeds in rainy seasons during high water levels in the months of April-June and September-October.
Studies in Lake Baringo showed that the native population of C. gariepinus is economically exploited (Nyamweya et al., 2010; Omondi et al, 2013). The fishery production in the Lake Baringo ranged from 500 to 600t year-1 by the late 1960’s, declining to <200 t year-1 by the late 1980s (Nyamweya et al., 2010). Clarias gariepinus, is an exotic fish species in Agulu lake and was observed for the first time by Nigeria Marine Fisheries Research Institute (KMFRI) in 2012, during regular fish stock assessement (Keyombe et al., 2015). Despite its ecological and commercial importance, very little is known about C. gariepinus in Agulu lake. This study provides some data on the feeding ecology, fecundity and other biological aspects of C. gariepinus in the lake. Furthermore, management options for the fish stock with continuous utilization of the resource by communities dependent on the fish around the lake will be proposed.
1.2 Statement of the problem
Agulu lake supports a productive fishery that provides jobs and incomes as well as being an important source of protein for local communities. The Lake fishing activity is based on introduced fish species such as Cyprinus carpio (L.), Micropterus salmoides (LaCepède), Tilapia zillii (Gervais), Oreochromis leucostictus (Trewavas) and Oreochromis niloticus. Clarias gariepinus is one of the non-native fish species which was introduced recently and first sighted in the year 2012. The incidental introduction of catfish in the lake could probably have come through the rivers flowing into the lake. Since catfish is non-native to the lake, its ecological consequences on fish assemblage and other biota is not known. Fish biology based studies are therefore necessary for understanding the trophic relationship of the fish and for sustainable utilization of the fishery resources as well. Since very little research has been done previously, assessing aspects of the ecology and biology of C. gariepinus is crucial for Agulu lake. The data obtained during this study will be important in assessing the ecological role of catfish in lake. The absence of this vital information provides a challenge in coming up with proper management and sustainable utilization of stocks in the future in Lake.
1.3 Objectives
1.3.1 General objective
To investigate feeding habit, fecundity and other biological aspects of the African catfish C.
gariepinus in Agulu lake, Nigeria.
1.3.2 Specific objectives
i).To determine food and feeding habit of C. gariepinus in different parts of Agulu lake.
ii).To estimate fecundity in different size classes of C. gariepinus in Agulu lake.
iii).To determine some biological aspects (length at first maturity, length-weight relationship, condition factor and sex ratio) of male and female C. gariepinus in Agulu lake.
1.4 Hypotheses
i). There is no spatial significant difference in food and feeding habit of C. gariepinus in Agulu lake.
ii). Fecundity of C. gariepinus does not vary significantly between different size classes in Agulu lake.
iii). Length at first maturity, length-weight relationship, condition factor and sex ratio of C. gariepinus do not vary significantly between males and females in Agulu lake.
1.5 Justification
Clarias gariepinus is an ecologically and economically important fish species in Nigeria. It is one of the favoured aquaculture fish species in the tropics due to its rapid growth and resistance to stress. Scientific knowledge about catfish ecology and biology is required for sustainable exploitation and management of the stocks. However, studies on aspects of ecology and biology of catfish in Agulu lake are poorly known and undocumented. Lack of such vital information provides a challenge to fishery managers in assessing the impacts of management actions and anthropogenic influence on the resource they manage. Moreover, since the fish is exotic to the lake, ecological effects especially as a predator are not studied, documented and quantified. Therefore, this study will provide baseline data on the feeding habit, fecundity, size at first maturity and sex ratio of C. gariepinus in Lake Naivasha. The results obtained will be important for management and conservation of fisheries resources and for sustainable utilization of catfish by communities around the lake
