Abstract
Histological and haematological studies were made on apparently healthy eels (Anguilla japonica) collected from eel-culture ponds in Shizuoka prefecture through late August to mid-December, 1973. No epizooty was reported in those ponds during the period of study and all fish studied had no visible external abnormalities.
Hematocrit values and plasma Cl- concentrations were within the normal ranges in all fish throughout the seasons and no histological alterations were observed among the fish collected in August and September. Histological changes similar to those observed in fish with “branchionephritis”, however, occurred in the fish of October and progressed to advanced stages with seasons.
Epitherial hyperplasia started in the gill filaments, not in the lamellae, and formed a tong-like growth of epithelial tissue between the lamellae, and finally developed into crubbing of gill filaments. Histological changes in the kidney were rather slight.
Those changes were thought to be the reactions to the coldness and/or fasting of the hibernating eels in the pond.
