5.1.3.1 Phenotypics’ structure of Clarias gariepinus population in Asejire Lake The catch structure from spatial sampling and phenotypic structure of the obtained C. gariepinus population is discussed in this section.
(i) Fish catch structure of Asejire Lake
Biological surveys provide specimens needed for taxonomic and genetic research (McAllister et al., 1997). The spatial sampling of fish was carried out to obtain samples for genetic studies, compare catches with the earlier trends and set standards for combined monitor of catch, environment and the genotype. Fish catch from this study was of greater quantity compared to Omoike (2004). A total of 1,392 fish were caught during the sampling period compared with the 520 fish caught by Omoike (2004).
Chrysichthys nigrodigitatus was the most abundant single species totalling 30.5%
proportion in catch in Omoike (2004); its superiority in catch was also retained in the current study (49.78%). Higher number of individual fish catch during the current study may be as a result of gear selectivity and the intensity of sampling. Gura trap was used in this study and 38 sites were sampled compared with fleets of gillnet set at the back of reservoir, littoral area, middle of reservoir and industrial effluent discharge sites reported by Omoike (2004). The number of species (19) and families (12) observed in this study was similar to the respective 18 and 12 reported by Omoike (2004). However, these were lower compared to 41 species and 14 families found in Akinyemi (1987) and 23 species and 13 families observed by Elliot (1986). This showed decline in species diversity within fish families in Asejire Lake. Despite differences in gear used and intensity of sampling, there were similarities between the current finding and the reported species diversity by Omoike (2004). These indicate that this study also confirmed the reported loss of species diversity as observed by the scholar.
Claroteidae (formerly Bagriidae) family was the most abundant species in the catchment, which is also similar to the observations of Omoike (2004). However, greater proportion in catch was observed in this study (49.78 %0 compared with the 30.5 % found in Omike (2004). This could be linked with the selectivity of the gear used. The catch structure with respect to the most abundant single species, the most divergent
179
family and the trend of species richness and reduced diversity used by Omoike (2004) was similar to the observations in this study. Dominance of the Cichlidae family as the most divergent in lake system has been noted by Daddy et al. (1991) who found Cichlidae to be the dominant family in Tatabu Lake in Niger State of Nigeria. It also agreed with Olaniran’s (2000) finding on International Institute of Tropical Agriculture (I.I.T.A) Lake, in Ibadan, Nigeria.
The least composition among the species divergent families was observed in the Clariidae family. This may implies that the family is currently the most threatened divergent family in the catchment. C. gariepinus had the greatest proportion in catch among the members of the family. Omoike (2004) obtained only this specie in the family Clariidae and 0.6 % was its composition in the total catch. The obtained lower catch proportion despite more intense search, a generally higher total catch and benthic habitat focused gear suggests a gradual species decline in the catchment. This is further supported by the field observation that this specie, which is one of the most important aquaculture candidates, was becoming scarce and obtaining wild samples for research is becoming more challenging. This trend of results on the Clariidae family suggests the need for a strict management and conservation approach on the fishery at Asejire Lake.
This is achievable through legislation provided relevant data on the species and the catchment are generated. ICN (1988) asserts that among populations at risk of imminent extinction, there was diversity in population structure, selective pressures of the environment, modes of adaptation to the environment and causes for population decline.
The target species did not dominate any site though captured in low quantity at both OyS and OSS strata. This indicates that it can survive any of the strata despite their present situation but some factors that probably affect the catchment irrespective of strata affect its abundance. Lintermans (2007), Olden et al. (2007) and Jelks, et al. (2008) claim that many freshwater fish species around the world are threatened or endangered as a result of habitat degradation, altered hydrology, invasive species and disease. However, over-exploitation has also been responsible for decline (Limburg and Waldman, 2009).
Introduction of invasive species has been linked with disease and this has been discussed above with respect to the study area. Over-exploitation in the catchment has been insinuated by Omoike (2004), also reported changed values of physico-chemical parameters compared with earlier studies. However, altered hydrology due to dam water drawdown and fragmented habitat were not included. Whereas, alteration of river course in dam system could be a strong factor in species sustainability. WWF, (2010) avers that
180
Mekong giant catfish will be driven to extinction if Mekong river is dammed. Santos et al, (2011) opine that, because species morphology is somehow linked to habitat use and its performed niche, alteration in the environment, such as those from dam construction, may restrict the permanence of certain previously existing species. The depleting state of Asejire fisheries, especially the Clariidae family, could be as a result of alteration in the environment as observed in earlier studies.
The alteration especially opening of the gate of the dam, could also be instrumental to the pattern of catch that was obtained in the strata. Analysis of catches from the strata indicated that OYS had greater contribution to total catch compared to the OSS. The relatively higher catch in this zone despite its level of industrial development and watershed degradation pointed to an interruption to normal relationship between human activities and aquatic life as highlighted in the previous discussions. Moreover, catches from strata were negatively correlated during one season and positively during another. This pattern was different from that of water quality parameters, suggesting that the controlling factor for catch structure variation across strata was probably independent of the factor affecting the water quality.
It is expected that strata with higher level of human activities would have lower catch due to higher fishing pressure. However, inverse situation was observed in the studied catchment. Apart from human activity at the watershed, another factor that could alter fish distribution pattern and differentially affect strata at seasons is the opening of the dam’s gate. The opening will come erratically and have differential effect on the strata. The differential effect of water withdrawal on the catch from strata could be as a result of the depth of each stratum. This is because water depth will influence quantity of fish catch. Low depth shores would be exposed while relatively high shore have less effect. Behavioral adaptation for fish in situations of low water is to deeper areas and if well adapted to walk, they migrate to nearby swamps through different adaptive features.
Gunder, (2004) notes that, in such situation, C. gariepinus would migrate to the swamp by walking using its pectoral spine. Other organisms could colonize a small but safe micro-habitat in the catchment and inbreed.
(ii) Phenotypic structure of Clarias gariepinus population in Asejire Reservoir
Different characters were observed to possess different variability in the studied population. This results supports of the idea that each character may show a different degree of variability within a single population (Mayr, 1969). The revealed phenotypic values implicated within population heterogeneity with pectoral fin characters PECSL