T he Nile tilapia {Oreochromis niloticus),b\vLt tilapia, {Oreochromis aureus) and specimens showing intermediate external appearance to both species were sampled from Lake Edku and examined for differences in morphological characteristics to determine the natural hybrids of these species. The results of univariate analyses indicated highly significant differences of biotnetric characters between the hybrid groups and parental species. Multivariate analysis of meristic and morphometric values showed significant differences between all examined fish groups. The number of biometrics characters, which revealed significant differences between Nile tilapia and its hybrids, ranged from four to five out of seven meristic and twelve to fifteen out of twenty-one morphometric characters. Meanwhile, between blue tilapia and hybrids there were two to five out of seven and fourteen to sixteen out of twenty-one meristic and morphometric characters respectively. Also discriminate function analysis indicated that the biometric variables giving the most separation between the hybrid and parental species were dorsal fin spines, vertebrae number, pectoral fin length, postorbital length, head length and interorbital width. A classification to groups using the above functions derived from morphometric data yielded stronger separation than meristic counts for the examined groups. The results indicated the existence of natural hybridization between O. niloticus and 0. aureus in Lake Edku.
I n order to elucidate the food and feeding habits of Bagrus bajed fish, stomach contents of 188 specimens with total body length ranging from 22 to 71cm were examined. Samples were collected monthly during the period from July 2000 to December 2001 from the commercial catch of El-Nozha Hydrodrome. The results of relative gut index indicated that B. bajed is carnivore. The range of its food material is wide, including cichiid species (Oreochromis niloticus, Oreochromis aureus, Sarotherodon galilaeus and Tilapia zillii), mullets (Mugil cephalus and Liza ramada), Clarias garlepinus, fish eggs, amphipods (Corophium volutator), shrimps (Leander serratus) , aquatic insects (chironomid larvae and Coleoptera), vegetable matter and detritus. The maximum feeding intensity was in spring, while the lowest was in winter. Seasonal variation in the diet composition of B bajed revealed that fish prey was the main food item eaten throughout the year except in spring, where it represents the second important food item-after crustaceans. Variation in food species with fish length revealed an increase in the percentage of fish prey in the diet of this species. The diversity in diet of B. bajed, revealed low values in spring increasing in autumn and winter to reach the maximum in summer. Selectivity coefficient for fish prey group of B. bajed showed that cichiid species were negatively selected (-0.08), while the positively selected fishes were Clarias gariepinus (+0^57) and mullets (+ 0.29).
The present investigation was designed to evaluate the dangerous effects of the Pb and Hg on Clarias gariepinus inhabiting Lake Mariout and assay the possible protective effect of meso-2, 3 dimercaptosuccinic acid (DMSA). The test fish were divided into four groups kept in plastic tanks (20 fish each) containing 30L of Lake Mariout water. Group (1): Standard group (zero time); Group (2): Unexposed DMSA group, Group (3): Low dose DMSA group; Group (4): High dose DMSA group. Fish exposed to DMSA showed significant decrease in lead (P≤0.05) and mercury concentrations (P≤0.05) in kidney, liver, gills, muscle and blood than unexposed group. The present results clearly indicate a significant decrease in RBCs, Hb, Hct, and platelet counts while a significant increase in MCH, MCHC, and WBCs in the Clarias gariepinus collected from the main basin of Lake Mariout. A marked significant decrease in AST, ALT, urea, creatinine (P≤0.05) was observed in DMSA groups. A significant increase in CAT, GPX and SOD was observed after exposure to DMSA. In addition, DMSA exposure improved the histopathological alterations in fish liver and kidney.
Immigrant narrow–barred Spanish mackerel, West African Spanish mackerel and specimens with an external appearance somewhere between these putative parents were collected from Abu Qir Bay, East Alexandria, Egypt. The hybrid index results and univariate and multivariate analysis indicated a natural hybridization between these two species. The discriminant function analysis successfully classified individual fish in the data to one of the three fish groups. Squared Mahalanobis distances extracted from the groups indicated the three groups were clearly distinct from each other. Moreover, distances between the hybrid and Scomberomorus tritor were longer than those of the hybrid and S. commerson. The mean values of the condition factor for the hybri were significantly higher than those of S. commerson. Natural mortality of the hybrid was significantly lower than that of the exotic parent (S. commerson), indicating that the environmental conditions in the examined region are more suitable for the hybrid type species than for the invasive parental species.
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