Investigation on the mortality of juveniles in captive stock of the Indian halibut Psettodes erumei

A study was undertaken to investigate the cause of mortality in the captive stock of Indian halibut, Psettodes erumei (Bloch & Schneider, 1801) (Psettodidae). Halibuts were severely infected with two species of parasitic copepods, Protochondracanthus alatus (Heller, 1868) and P. trilobatus (Pillai, 1964) (Poecilostomatoida, Chondracanthidae) and also with protozoan Amyloodinium sp. (Blastodinida, Oodiniaceae) on gills. Since the parasitic copepods have high host specificity on P. erumei, a large number of copepods were collected. Gill lamellae appeared pale and excess mucous secretion was observed on the body surface. Histopathological changes were mainly in the gill tissues with severe lamellar hypertrophy and hyperplasia, lamellar fusions with the presence of trophonts of Amyloodinium sp. Wild caught tilapia, and juveniles of mullet and milkfish used as live feed were suspected to be the cause of transmission to the rearing system. Infected fishes showed loss of appetite, irregular swimming, restlessness, exhaustion and gaping at the surface. Samples tested were negative for betanodavirus and systemic bacterial infection. Management intervention and multiple treatment of rearing facility using formalin @ 200 ppm controlled the infection, and half of the stock could escape from mortality.