Attraction of an artificial reef: a migratory demersal flounder remains in shallow water under high temperature conditions in summer

Offshore manmade structures, termed artificial reefs (e.g., wind farms, oil platforms, and manmade islands), affect the movement and migration of wild migratory marine animals. Certain types of migratory pelagic and benthic fish are known to utilize artificial reefs as feeding areas, for spatial reference (landmarks for their navigation), or as meeting points. However, little is known about the impact of artificial reefs on the migratory behavior of demersal flatfish. The marbled flounder (Pseudopleuronectes yokohamae), an important commercial flatfish species, exhibits seasonal migration between shallow water for spawning in winter and deeper water to avoid high water temperatures in summer. In this study, 39 tagged marbled flounders were released in the shallow water of the inner Osaka Bay area in June 2016, and then actively tracked, predominantly in deeper waters (100–140 m), from a fishing boat every week until mid-September 2016. Our active tracking detected no tagged fish in the deeper waters, whereas 36% of the fish were detected in the shallow water (10–15 m) around an artificial reef (offshore manmade island) at a high water temperature (26 °C). We suggest that a portion of the marbled flounder population did not show seasonal migration but remained in the shallow water around the artificial reef, even though the water temperature was high in summer. We suggest that this is related to the abundant food supply associated with the artificial reef.