a study on the role of chemical and organic fertilizer on lernaeasis in a rearing cyprinids fish farm in guilan

the aim of this study was to investigate the effect of chemical fertilizer and composite of organic and chemical fertilizer on lernaeasis in cultured cyprinids was performed in two treatments and three replications in each, in 6 one-hectare ponds. in first treatment only chemical fertilizer and in the second treatment the composite of organic and chemical fertilizer were used. the other rearing and management factors such as initial fish stocking, fingerlings’ weight, volume of inlet water and etc. were equal in both treatments. at the end of the experiment, there was not a significant difference (p>0.05) between treatments in final weight of all species. in spite of this fact, the grass carp in the first treatment showed less weight (34%) than second treatment (p>0.05). study of lernaeasis, revealed that the most sensitive fish to this parasite was grass carp that had a significant difference (p<0.05) among to other fishes in number of infected fishes and the lernaea in each, however, there was not a significant difference among the other fishes (p>0.05). mean comparison for infected grass carp and the number of parasite in each in first treatment was significantly higher than second treatment (p<0.05). this phenomenon in first treatment could be explained in a rapid chemical fertilizer performance for algae bloom as food source for copepods, and also change of zooplankton’s biological balance and overwhelming of zooplanktons (predator cyclops) that predate the copepodid stage of lernaea in second treatment. therefore, by fertilizing management, lernaea density noticeably could be controlled.