Hatching rates of naturally spawned Gnathopogon caerulescens eggs on various substrates and sites

We examined the hatching rates of naturally spawned eggs of Gnathopogon caerulescens, a small cyprinid fish indigenous to Lake Biwa, central Japan. Spawning behavior was observed near the lakeshore, in satellite lakes and a small river flowing into a satellite lake, and the main substrates for the egg deposition were reed roots and stalks, willow roots near lakeshore and satellite lakes, and gravel and moss in the river. Naturally spawned egg batches assessed as 2‒5 days at each site were collected from various substrates over two years, and incubated till hatching in the laboratory. The mean hatching rate for each year was about 50% and no clear difference during each spawning season in the hatching rates was detected in both years. The mean hatching rate in the satellite lakes tended to be lower than the other spawning sites. These results suggest that more than half of naturally spawned eggs die before hatching, and the hatching rate of the eggs in the satellite lakes may be affected by some environments and egg quality due to stress for spawning parents of the higher water temperature.