福井県浦底湾におけるミズクラゲの生態-IX : 1970年春期における成体型の昼夜移動

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Following previous reports, the diel vertical migration of a population of Aurelia aurita medusae was again studied at the outer-western part of Urazoko Bay for a period of 25 hours, from 10:00, May 27 to 11:00, May 28 1970. Sampling was made every two or three hours by means of simultaneous horizontal hauls of four plankton-nets (100cm in mouth diameter and 10mm in mesh size) at the surface, 3, 7 and 13 meters deep, respectively for 10 minutes at a speed of 0.5 m/sec. Meteorological and hydrographic observations were also recorded. The results obtained are: 1) Collected medusae ranged from 3 to 12 cm in bell-diameter were all immature. Although they were all distributed at the bottom layer below 10 meter depths in the day time under fine weather conditions, about 50 percent of them migrated to the surface at 17:00. Around sunset, they were distributed almost evenly from surface to bottom layer. Later on, they were captured mainly from the bottom layer when the night was dark. The next morning, when it was fine, most of them appeared to move up to the surface at 6:30. However, their main distribution layer moved gradually to the middle layer, about 6 meters in depth at 9:00 and later again down to the bottom layer at 10:30. 2) The main factor controlling the diel vertical migration of Aurelia medusae was again confirmed to be underwater illumination.
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