Abstract
Grouper fish mortality at the East Cost Libyan area of the Mediterranean Sea is one of the major problems encountered at autumn season starting from October. Sudden environmental changes associated with water pollution were recorded. Adult fish especially the grouper types were affected. This phenomenon was previously recorded in Libya at 1985. The present study was carried out to demonstrate the most prevalent isolates that may lead to this phenomenon. It was found that the gram negative oxidase positive bacterial group (Pasteurella, Vibrio and Aeromonas spp.) were the most isolated bacteria with high incidence refers especially to Pasteurella piscicida with an incidence of 64%. Black or metallic colour cysts (Microspordiosis, Glugea spp. and Plistophora spp.) representing spores or larvae of this parasite were observed on the visceral organs and abdominal cavity. Other parasites included larval stages of Contracaecum spp. and Gonapodasmius epinepheli (Didymozoid digenes). The clinical and postmortem lesions were mostly characterized by unilateral or bilateral corneal opacity and haemorrhagic spots on the skin with ulcer formation in some cases. Abdominal distension and anal prolaps were also recorded. All the internal organs were congested. The swim bladder was greatly swollen and filled with gas.