Twenty-five traditional and thirty-four geometric morphometric comparisons were carried out on pure and reciprocal crosses of Pangasianodon hypophthalmus (Sauvage, 1878) and Clarias gariepinus (Burchell, 1822). Thirty fish samples each of the C. gariepinus (CH), P. hypophthalmus (PH), Pangapinus (♀PH × ♂CG) and the two distinct morphotypes of the Clariothalmus (♀CG × ♂PH) (Clarias-like and Panga-like) between the ages of four and six months were used for this study. Phenotypically, the Clarias-like Clariothalmus and the Pangapinus progenies were indistinguishable from their maternal parents while the Panga-like Clariothalmus was a phenotypic intermediary of the putative parents but looks more closely to the paternal parent. Hence, both univariate proportion and multivariate analysis of the collected data successfully separated the different fishes into three multivariate spaces. The analysis of the dendrogram with complete linkage and Euclidean distance further showed the close relationship of the isolated Panga-like Clariothalmus progenies to the paternal parent, however, Clarias-like Clariothalmus and the Pangapinus were completely intermingled with their maternal parents. The most important index of discrimination of these fishes into different multivariate spaces was the fin characteristic which showed 100% exclusive ranges for the individual groups in many cases.
The production of fish in captivity necessitates the use of artificial means for its propagation. This process involves the collection and mixing of both male and female gametes to form a developing embryo (Ataguba, Okomoda & Onwuka, 2013). However, this is not 100% successful hence, the need to estimate the percentage of the egg fertilized during the process of artificial spawning. This is also one of the ways of measuring gamete quality of broodstocks and their progenies (Carrillo et al., 2000). The most widely used method to estimate fertilization rate in previous studies is that described by Ella (1987). This involves monitoring a small portion of the unfertilized egg from the time of stripping until the eggs become completely opaque (dead). Then, a subsample of the fertilized egg is taken through capillary action with the aid of a glass tube (300 mm length and 2.5 mm diameter), and the total numbers of the good (live) and bad (dead) eggs are counted. The percentage of the good eggs is then estimated as the fertilization rate. This method assumes that all good eggs at this point are fertilized while the white/opaque (bad) eggs are unfertilized (Ataguba, Annune & Ogbe, 2010). However, the above method is flawed due to several reasons; it takes several hours for the referenced standard of unfertilized eggs to become opaque hence, making the estimation of fertilization a time-consuming process. Values gotten from this method are also inaccurate due to the fact that some fertilized eggs die during the process of embryogenesis and add up to the percentage of bad eggs (dead egg) which is erroneously thought to be 100% unfertilized eggs. More so, the unfertilized eggs amidst the fertilized eggs are usually hydrated with water while the reference standard of unfertilized eggs is not. Hence, the biological response of the hydrated eggs and the time taken to become opaque differ significantly compared with the standards (which is non-hydrated and unfertilized). There is, therefore, a need to develop a more suitable method which could distinguish "hydrated" and "fertilized" eggs earlier in the embryogenesis process hence, the aim of this research. The working hypothesis for this study, therefore, is to discriminate "fertilized" and "hydrated" eggs using the shape of the animal pole during different stages of embryogenic development. The stages which gave the highest possibility for easy discrimination were identified and suggested as the best stages for accurate estimation of fertilization rate.For this study, we used the breeding of Clarias gariepinus (Burchell, 1822) as a specific example. Two pairs of sexually mature broodstocks (each for male and female) of C. gariepinus (Burchell, 1822),(1 year old and weigh between 1 and 1.3 kg) were obtained from the School of Fisheries and Aquaculture Sciences hatchery of the Universiti Malaysia Terengganu, in Malaysia. They were maintained in rectangular fibreglass tanks (fed 35% Crude Protein commercial diet) until the time of breeding. Female broodstocks were given intramuscu...
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