1National Research Institute of Fisheries Science, Fisheries Research Agency, Ueda, Nagano 386-0031, 2Department of Marine Bioscience, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, Japan
To analyze the predation effect by Japanese dace Tribolodon hakonensis on bluegill Lepomis macrochirus eggs, one male and two female bluegill were stocked in an artificial experimental pond. Two trays with gravel were laid in each pond to serve as spawning redds. In total, 12 spawnings occurred and the number of eggs that could be counted from above varied from 2,060 to 3,850. For a brood in which the number of eggs counted from above was 2,060, close examination of the egg number revealed that the real number was 4,789, indicating that only 43% of eggs were countable from above. To equalize the number of eggs between redds, the number from above was reduced to 3,000 in 11 cases with more than 3,000 eggs. Ten Japanese dace were stocked in 6 of 11 cases and no dace in 5 cases. The number of bluegill larvae that appeared four days after spawning in ponds with dace was on average 28.5 and significantly less than that (888.0) without dace. Stocking Japanese dace in ponds and lakes is potentially useful for controlling bluegill.