Pathological Studies on the Brood Rainbow Trout Died after Egg-Stripping—I

Abstract

In trout hatcheries, most of loss of brood stock had been caused after egg-stripping. If the mortality of brood stock is lowered more economical and stable egg-supply could be expected. Under these circumstances, pathological studies have been conducted to clarify the cause of loss after eggstripping.In the present study mortality and symptoms of brood rainbow trout after egg-stripping were studied from 1972 to 1977 at Koide Branch, Niigata Prefectural Freshwater Fisheries Experimental Station. The mortality during 7 months from September to March, 1972-1976, ranged between 15.9 and 25.0% for 2-year-old and between 13.0 and 52.7% for 3-year-old trout. The mortality of 3-year-old was higher than that of 2-year-old trout except 1974. The mortality increased one week after egg-stripping and ceased after about 3 weeks.Macroscopical examinations of the fish died after egg-stripping were carried out during 2 seasons, from December 1975 to January 1976 (Exp.1) and from November 1976 to December 1976 (Exp. 2). The remarkable symptoms were fungus affection on skin and gills, inflammation of intestine, redness of pyloric coeca, and atrophy of spleen. Boils due to furunculosis were observed in some instances.Incidence of infestation of external parasites were examined for the samples obtained in Exp. 2. Trichodina were observed on gill of 25.5% of 2-year-old fish and 30.0% of 3-year-old.