岡山県高梁川におけるシラウオの産卵場

抄録

Sand samples from the bottom of the fishing ground of Salangichthys microdon in the Takahashi estuary were collected from 33 stations for the examination of naturally spawned eggs of the fish. The eggs were found among sand grains collected from 7 stations. Eggs were most abundant, 870 eggs per300ml sand grains, at the sand bar exposed only at the low tide and is located near the estuary dam, 5.2km upstream from the river mouth.The next highest density, 238 eggs per 300ml sand grains, was observed at the station near the river mouth.Eggs at various developmental stages were observed.The eggs, even those in the sand bar, were found to be developing successfully in spite of the wide range of fluctuation of chlorinity (0.8‰, at low tides to 16.8‰, at high tides).The spawning ground is mainly composed of sand grains ranging from one-third to equal diameter of eggs (0.92-0.95 mm).
No eggs were found to be attached to the thalli of the sea-weed, Enteromorpha prolifera, which flourished on the concrete floor of the dam and on the rubble mound below it. However, in lakes such as Kasumigaura, it has long been believed that the fish lay their eggs on the stems and leaves of aquatic plants.This difference in the spawning habits may be attributed to the difference of water currents.Since there are relatively few records of the collection of naturally spawned eggs in lakes, further investigation is essential to confirm whether the fish in lakes do not lay their eggs into sand.