Marine artificial structures as amplifiers of Aurelia aurita s.l. blooms: a case study of a newly installed floating pier
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Increase of marine artificial structures, providing more substrate for jellyfish polyps, has been argued to increase jellyfish outbreaks, although no explicit evidence exists. We report a case study demonstrating a remarkable increase of Aurelia aurita s.l. ephyrae after the installation of a floating pier (48 × 6 m) in a fishing port on the Inland Sea of Japan. Monitoring of ephyrae from January 2010, prior to the installation of the floating pier in April 2010, to July 2011, revealed that their time-weighted average density increased 3.5 fold, from 1.1 to 3.9 ephyrae m−3, and the integrated number of ephyrae exported from the port increased 4.3 fold, from 5.7 × 106 to 25 × 106 ephyrae, after the installation. However, in a nearby port, a control site, the abundance of ephyrae decreased by ca. one third during the same period. Monitoring of polyps showed that they initially colonized the undersurface of the pier by August 2010, followed by a rapid population increase. They strobilated from December 2010 to May 2011. We computed the number of ephyrae released from the strobilae to be ca. 25 × 106, very close to the net increase of ephyrae produced and exported from the port. This study corroborates that the installation of an artificial structure provides new a substrate for polyps, which allows them to produce more ephyrae to induce medusa blooms.
KeywordsArtificial structures Ephyrae Polyps Ports Seeding grounds
We thank H. Takeoka, A. Malej and T. Kogovsek for comments on our study, and JF (Japan Fisheries Cooperatives) Kuba for supporting this survey. Our gratitude is extended to S. Nakai for his kind assistance in designing and manufacturing the L-shaped frame used in monitoring the undersurface of the pier. This study was partially supported by grants from the Agriculture, Forestry and Fisheries Research Council, Japan (project name: STOPJELLY) and from the Japan Society for the Promotion of Science (Japan-Slovenia bilateral project on jellyfish blooms).
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