WATER SURFACE TENSION-RELATED DEATHS IN PRELARVAL RED-SPOTTED GROUPER

K. Yamaoka, T. Nanbu, M. Miyagawa, T. Isshiki, A. Kusaka-2000

Aquaculture, 189 (1-2): 165-176
Abstract:

Three sets of experiments were conducted on environmental changes affecting the occurrence of mass surface deaths in the prelarval stage of the red-spotted grouper, Epinephelus akaara. The first experiment, which was on light intensity, showed that a light source attracted the larvae intensely and they dashed toward it. The maximum number of dead fish was at the intensity of 2000 lx. The second experiment showed that an oil film on the water surface completely prevented the occurrence of mass surface deaths. The third experiment showed that the presence of a water current decreased the number of dead fish. These results suggest that water surface tension is a key environmental factor in the occurrence of surface deaths. Mucus secreted on the body surface of prelarvae functions as a glue when larvae are attracted to the water surface by light coming from above. The oil film deprives the water surface of surface tension, which seems to be the key to prevent the occurrence of mass surface deaths in the prelarval stage of groupers. The use of an oil film must be discontinued prior to swim bladder inflation.

(Laboratory of Aquatic Ecology, Faculty of Agriculture, Kochi University, Nankoku, Kochi 783-8502, Japan)

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