Seasonal Distribution of Fishes at Nearshore Meadows Predominated by Zostera marina and Ulva fenestrata in the Stark Strait (Peter the Great Gulf, Sea of Japan)

Abstract

The fish species diversity, fish density, and distribution in the nearshore beds of Zostera marina and Ulva fenestrata have been assessed by SCUBA visual counts in May–September and November 2021. In total, 23 fish species have been registered. The fish species richness increases from 8–13 species in May to a maximum of 17–19 species at the end of June. Then it slightly decreases in July–September and reaches its lowest values in November (2–5 species). The same changes have been noted for the fish density. Fish diversity in the Zostera beds is higher than in the Ulva ones. The similarity of the fish species composition between Zostera and Ulva is low (the Sørensen-Czekanowski index varied from 0.32 to 0.44). In general, fish density is low (from 2.4 to 112.4 ind./50 m² in Ulva (in November) and Zostera (in July) beds, respectively), although with an exception for Opisthocentrus spp. and Gymnogobius heptacanthus juveniles. An abnormal high bottom water temperature (above 26°C) in late July–early August has led to temporal avoidance of shallow water with plant beds by fishes (except for Gymnogobius heptacanthus) and their shift to a depth of 3.5–4.5 m, where the temperature was lower by 4.0–5.7°С.