Effects of Different Water Velocities on Oocyte Development in Ayu, Plecoglossus altivelis

Abstract

The effects of different water velocities (50-60, 40-50, 30-40, 20-30, 3-5 cm/sec) on oocyte development and fecundity in Ayu were investigated. Though the number of oocytes increased with body size, there were more oocytes at water velocities faster than 30 cm/sec. The number of oocytes per cm³ of the ovary became significantly between water velocities. There was also a higher proportion of small-size eggs in the case of high velocities. A physiological reduction stage was also observed in the Ayu during the development process of the ovary and, at water velocities of 30 cm/sec or greater, it is thought that there is a tendency toward a decrease in the extent of oocyte reduction. A relationship between mean oocyte weight (X mg) and oocyte number per cm³ (Y) of the landlocked form of Ayu in Lake Biwa was obtained as follows:
Y=e^-3.94X+9.37 (r=-0.992, p<0.001)
Meanwhile, in the case of the sea-run form of Ayu, which has a different life history, different calendar years and different background prior to inclusion in the experiment, a total duprication in point distribution was observed.
Though the oocyte number obtained with water velocities in the range of 20-30 cm/sec, which can be easily attained with the energy in water inflows, is less than with faster flows, this is considered to be the optimum velocity in rationalizing egg collection since the existence of many large-size eggs would lead to an improvement in the survival rate.