Diel periodicity of spawning in sciaenids

Time of day was determined for spawning of several species of sciaenid fishes by examin~ng development stages of eggs collected in estuarine and near-shore plankton samples.Estuarine samples were taken at different times of day and night but newly spawned Cynoscion nebulosus and Bairdiella chrysoura eggs were taken only during a period from just before to 3 or 4 hours after sunset.Sclaenops ocellatus and Mentichirrhus sp.eggs from near-shore Gulf of Mexico samples, taken during the morning, all contained tail-bud stage embryos, indicating evening spawning in these species.It is proposed that evening spawning reduces predation on sciaenid eggs by allowing dispersal of eggs during the night when planktivores may be less active.Overnight dispersal reduced C, nebulosus egg density from 100 m-3 during evening spawning to 1 m-3 the next afternoon.Lower egg densities during the day would reduce egg mortality due to predation.Egg predation experiments showed that predation rates increased with increasing egg density but no difference was found in predation rates between trials run in light and total darkness.