Water surface tension-related deaths in prelarval red-spotted grouper

Three sets of experiments were conducted on environmental changes affecting the occurrence of mass surface deaths in the prelarval stage of the red-sp otted grouper, Epinephelus akaara. The first experiment, which was on light intensity, showed that a light source attracted the larvae intensely and t hey dashed toward it. The maximum number of dead fish was at the intensity of 2000 1x. The second experiment showed that an oil film on the water surf ace completely prevented the occurrence of mass surface deaths. The third e xperiment showed that the presence of a water current decreased the number of dead fish. These results suggest that water surface tension is a key env ironmental factor in the occurrence of surface deaths. Mucus secreted on th e body surface of prelarvae functions as a glue when larvae are attracted t o the water surface by light coming from above. The oil film deprives the w ater surface of surface tension, which seems to be the key to prevent the o ccurrence of mass surface deaths in the prelarval stage of groupers. The us e of an oil film must be discontinued prior to swim bladder inflation. (C) 2000 Elsevier Science B.V. All rights reserved.