الفهرس 
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Abstract
The present study deals with the relationships of seasonal variation in the physical and chemical factors of water, prevalence (%) and mean intensity of the most common monogenean gill parasites infesting the gills of the four cichlid fish which inhabit Manzala and Borollus lakes in Egypt. A total of 1517 specimens of the cichlid fish; Oreochromis niloticus, O. aureus, Sarotherodon galilaeus and Tilapia zilli were examined in four sites between winter 2009 to autumn 2010. A total of 4609 of monogenean gill parasites have been detected on the investigated host fish, whereas a number of 1063 of the monogenean Cichlidogyrus halli typicus were detected on the gills of O. niloticus, 2262 on the gills of O. aureus and 1007 on the gills of S. galilaeus. Regarding to T. zilli, a number of 277 of the monogenean Cichlidogyrus arthracanthus were detected. The seasonal variations of some physical and chemical factors as well as heavy metals of 16 water samples were estimated. The prevalence and mean intensity values of C. halli typicus in O. niloticus, O. aureus and S. galilaeus and C. arthracanthus in T. zilli display higher values in southern part of both Manzala and Borollus lakes comparing with northern part. The application of cluster analysis based on the similarity in prevalence and mean intensity values for different fish types in different seasons (16 variables) led to the recognition of four and five groups respectively. The correlation between water variables and prevalence values was indicated on the ordination diagram produced by Canonical Correspondence Analysis (CCA). Morphometric data were also given for each factor and variations were discussed.