びわ湖のホンモロコ個体群における変動要因の解析 : III.O_+年魚における体内脂肪含有量の季節変化と"越冬"について

It is suggested that the small individuals of the under-yearling fish population, less than about 6cm in body length may drop from the population during winter and these fish are inferior to the large ones in condition factor (MAKI, 1966b). NIKOL'SKY (1963) points out whether the fish succeed in "over-wintering"depends largely on how much they deposit fat in their body during the growing season. The relation between the"over-wintering"of the under-yearling fish and the fat content in their body is reported in this paper. The sample specimens of the adult and sub-adult were taken from commercial landings between August 1965 and May 1966. The specimens of young fish, less than about 4cm in body length were collected by the author with a small trawl net in July 1966. Chloro-form-methanol solution was used for the fat extraction from the fish. The mean value of the fat content in the muscular tissue per unit of gram of body weight (wet weight) attains 16.6mg in late summer, which is the maximum of the year. Although this value tends to be maintained until March, a marked decreasing of the fat content in this part occurs between March and May, when the fish are maturating and then migrating to spawning beds. In May, the mean value of the fat content of this part decreases to 8.1mg, which is the minimum of the year. The mean value of the fat content around the internal organ also attains the maximum in late summer, 28.5mg per unit of gram of body weight and decreases to 20.2mg during the wintering migration periods. A significant reduction of the value occurs throughout winter and spring months when the fish experience wintering, maturation and spawning migration in sequence. The mean value of the fat content around the internal organ in May is 3.3mg, the minimum of the year. It is observed that the larger the fish grow, the more the fat deposits in their body exponentially. The fat content both in muscular tissue and around the internal organ increases accelerately when the fish size exceeds about 4cm in body length. The mean value of the fat content of small fish, smaller than 6cm in body length decreases more rapidly than that of large ones, larger than 6cm in body length, between December and May. This result suggests that the small individuals consume more fat in the body for energy source than larger ones during the over-wintering and maturation periods. In May, the spawning periods of the fish, the small individuals are scarcely found in the population. These facts may indicate that the small fish of under-yearling population are more disadvantageous than the large ones for over-wintering and maturation and drop from the population.