As part of research-work on the usefulness of the bluegill sunfish ， Lepomis macrochirus Rafinesque， as an experimental standard animal in fisheries research， we have tried to find out a way of obtaining its fingerlings throughout the year-curriculum. As the first stage for this study， observations have been conducted on the breeding habits of this species， using specimens of full three years old in rearing tanks - 3 × 1.5 m, 0.3-0.5 m (depth) and 1 × 0.5m, 0.7 m (depth). The results so far obtained are as follows:
1) The male fish is slightly larger than the female. The tip of the snout is somewhat rounded indorsal view in case of the male, while slightly pointed in the female. No definite difference is found in other external characters between the two sexes, except that in the breeding season the color of the breast is brighter in the male than in the female.
2) In the present observations, the breeding season lasted from June until middle September. During this period, water temperature measured at 10 a. m. varied between 20,C and 28,C.
3) The male fish forms the nest by excavating through flipping and fanning out the sand and gravel with his tail. The nest is a shallow bowl-like depression of 30-70cm in diameter and 2.5-10cm in depth.
4) When the bottom of the tank is divided into three even parts respectively covered with a layer (about 15 cm thick) of gravel (mainly 2-9 mm in diameter); gravelly sand (mainly 0.4-4,8 mm); and coarse sand (mainly 0.25-2.0 mm), the nest is made exclusively at the portion covered with gravel. However, a similar nest is made on the gravelly sand when a part of the bottom of the tank is covered with this material and the other part is left nake. The nest is formed anew at each spawning in the prior case, while it is repeatedly used again in the latter.
5) The male continues to prepare the nest until the spawning begins. Meanwhile, he follows the female, kisses her near her genital pore and compells her to come to the nest.
6) The spawning act took place in daytime, often beginning at about 9 a.m or 5 p.m. The usual span of spawning takes about 2 hours. The male swims in circles around the nest side by side with the female, always on the outside of the female. Eggs are laid when the female reclines on her side and vibrates her body, while the male continues the upright posture.
7) The eggs and newly hatched larvae in the nest are guarded by the male. When other fish or animals approach the nest, he vigourously drives the invadors away from it.
8) A pair of the bluegill spawn 4 or 5 times in one breeding season. The number of eggs in one spawning varies from 21,000 to 36,000. The number of spawned eggs progressively tends to decrease by repeated spawning.