ニジマス人工採卵に関する基礎研究-V : 生殖巣の発達と初産魚の大きさ

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• The seasonal changes in the weight of gonads, liver and viscera, as well as in the chemical composition of these organs and muscle of rainbow trout reared at the Oizumi Trout Hatchery were studied. The relationship between the growth of the fish and the development of gonads was also investigated, and the status of initial spawning of 2 year fish was discussed. The weight of organs was shown by the relative values against body weight. Net body weight (body weight minus weight of gonads) was used for referring to the weight of liver and viscera, and total body weight for gonads. The viscera used for chemical analysis excluded gonads and liver, and for calculation of relative weight of gonads were removed from viscera. The results obtained were as follows: 1) The relative weight of ovary increases rapidly during the months preceding the spawning, and reaches the maximum weight just before ovulation of all ripe eggs; in testis it shows the maximum weight in pre-spawing season (October), followed by gradual decrease in weight discharging its products (Figs. 1 and 2). 2) The seasonal changes in the relative weight of viscera, which are affected mainly by the amount of fat on intestinal organs (Table 2), shows a similar tendency in both sexes in term of fluctuation and magnitude. But the relative weight of liver in both sexes changes in a positive relation to the growth of ovary and testis respectively (Figs. 1 and 3). Protein, fat and ash contents of liver increase with the increase in its total weight (Tables 1 and 2). 3) The growth rate observed in length and weight of female fish decreases continually after the onset of sexual maturity. Especially, during the months of rapid increase of ovary, females show little growth or, inversely, reduced its net body weight (Fig 4). 4) Table 3 and Fig. 5 show the relation between the size of fish and the relative weight of ovary. Among the yearling of the same age expressed in months, the larger fish has naturally a large relative weight of ovary. The average value of the relative weight of total eggs stripped from 2 year-old which spawn for the first time was nearly 11%1). If this value is wanted for fish to spawn 2 years after hatch out, it is necessary fish grow reaching over about 30cm. in body length or nearly 500g. in weight within 2 years. Analyzing the variance of the individual size scattered around empirical growth curve and cross relation between “isomaturity curve” and growth curves, an approximate percentage of fish actually spawned in the lot may be gained. For instance, when the growth curve reaches and passes over 11% iso-maturity curve (e. g. line A in Fig. 5), the majority of fish in the lot which are indicated by the growth curve is definitely expected to arrive full maturity 2 years after hatch out. 5) In order to increase the fecundity and use of younger brood fish (2 year fish) for egg taking, it is necessary the fish fed sufficiently during the months of rapid increase of gonads.

• 公益社団法人 日本水産学会の論文

著者

• 野村 稔

  東京水産大学

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