Effects of some toxic microalgae on larval stage of the common carp (Cyprinus carpio L.) and silver carp ( Hypophthalmicthyes molitrix val)

The present study involved isolation, identification and purification of four toxic species of Cyanobacteria from Al-Ashar and Al-Khandaq canals ( Basrah-Iraq ) namely ( Microcystis aeruginosa, Microcystis, flos-aque, Hapalosiphon Welwitschii and Calothrix parietina ) as well as green algae Chlorella vulgaris from the same habitat and halotolerant green algae Dunaliella salina that isolated from brine ponds in northern Basrah / karmat Ah .Growth rate and chemical composition of all species isolates were studied . All cyanobacterial species was appeared their ability to produced hepatotoxins ( Microcystin-LR ) and the species H. Welwitschii have highly ability to produced this toxin reached to 44.415 ng /ml among other species . common carp and silver carp were fed in the laboratory Results revealed that feeding the two fish larvae C. carpio and H molitrix on the four tox,c microalgae resulted in significant reduction in their mean larval number and survival percentage (p 0.05) . On the other hand feeding the larvae on non-toxic microalgae Chlorella vulgaris and Dunaliella salina lead to a significant increase in the mean number of the larvae and the survival rate for two type of fish larvae . Negative correlation (r) were found between mean number of algal cells used in feeding and the time of feeding for the two nontoxic species Dunaliella salina and Chlorella vulgaris and for the two fish larvae. Conversely the correlation value for the toxic species ranged between less negative to positive (non-significant) .Feeding larvae on toxic species has shown a significant reduction in mean of weight and length . Comparatively a significant increase in mean weight and lengh of the larvae was noticed as a result of feeding on non- toxic species .