ニジマスの人工採卵に関する基礎研究-IV

Abstract

The number and weight of eggs stripped from a female, and the weight of an egg were measured for 133 rainbow trout (body length, 28.5-55.0cm; body weight, 461-3430g) reared at the Oizumi Trout Hatchery and 28 fish (body length, 36-52cm; body weight, 755-2156g) raised at the Nagano Prefectural Trout Hatchery. The relations between the values above mentioned and the size of those fish, as well as the same between values and condition factor were examined. The results obtained were as follows: 1) The number of eggs (EN) shows a curvilinear relation with the body length (to the last scale) (L, cm) and shows a linear relation with body weight (W, g)(Figs. 1 and 2). The equations are log EN=2.25483 log L-0.33071, EN=1.1042W+739.86 for the Oizumi trout and log EN=1.38970 log L+1.15993, EN=1.1248 W+1206.3 for the Nagano. The number of eggs varies greatly among fish of nearly the same size. 2) The weight of eggs (EW, g) shows a curvilinear relation with body length (Fig. 3) as shown by equations; for the Oizumi fish, log EW=3.05644 log L-2.73171, and for the Nagano, log EW=2.02782 log L-0.95194. 3) The ratio of weight of eggs to body weight (102×EW/W) and gonad index (105×EW/L3) are calculated 13.80±0.357 and 236.2±8.11 respectively on the fish from the Oizumi, and these ratios, found in wide range, do not show sign of correlations referred to body length. Whereas, the same ratios for the Nagano fish, 17.20±0.988 and 289.5±26.27 respectively, show a slight reverse correlation to the body length. 4) The weight of an egg (Ew, mg) increases curvilinearly with the body length, expressed by equations, logEw=0.68872 logL+0.79911 for the Oizumi, and logEw=0.57181 logL+0.99851 for the Nagano (Fig. 4). 5) The 2-year-old fish which experiences spawning for the first time is featured by a ighter weight of an egg, a lower value of 102×EW/W, and a higher value of condition factor as compared with those in the 3-year-old of nearly same size (Fig. 9). There is a tendency that the number of eggs of the 2-year-old fish is slightly larger than that of the 3-year-old, but not different in the weight of eggs. The difference in the number of eggs between the two year classes is lesser than the variation recognized in each class, it may be, thus, stated that the number of eggs does change greatly with the size of fish more than due to age. 6) Among the fish of nearly the same size, there is a tendency that the heavier the av-erage weight of an egg is weighed, the lower the number of total eggs are counted (Fig. 5). But no definite relationship is recognized between the average weight of an egg and the weight of total eggs.