Gill lesions and mortality in common carp (Cyprinus carpio) with a dense bloom of Heterosigma-like algae in Khuzestan province

Widespread common carp (Cyprinus carpio) mortalities have been recorded in Khuzestan province fish farms in recent years. In summer of 2017 two cases of harmful algal bloom were encountered that led to massive mortality in common carp in Khuzestan, Iran. The aim of this study was to identify the possible etiologic agent of two mortalities with characteristic symptoms of gill lesions due to harmful algae. Water samples were collected and 5 moribund fishes were examined by histophatologic, scanning electron microscopic and PCR examination. In wet smear preparations, a lot of algal cells and fragments, and sloughed, necrotic epithelial cells were observed between the lamellae. In histopathologic examination of gills, hyperplasia, necrosis and algal cells surrounded by hyperplastic cells were seen in tissue sections. No inflammatory cell aggregation was noticed. In scanning electron microscopic examination the algae was found attached to the gill surface (cell diameter: 8.5 ± 4.2 µm) with 2 equal flagella. Phytoplankton analysis using direct microscopy and electron microscopy, morphologically resembling Heterosigma was identified, however, in PCR tests, Heterosigma analysis showed negative results, therefore the causative agent was called “Heterosigma-like” algae.