Tidal Variations of Fish Larvae Measured Using a 15-Day Continuous Ichthyoplankton Survey in Subei Shoal: Management Implications for the Red-Crowned Crane (Grus japonensis) Population in Yancheng Nature Reserve

SIMPLE SUMMARY: The red-crowned crane Grus japonensis in the National Yancheng Rare Birds Nature Reserve needs large quantities of high-protein food sources, including juvenile fishes, to fuel their migratory journeys. Some fish species use the tide to complete inshore migration, and they gradually migrate from the open sea to the surf area to find a more suitable habitat. In this paper, we used a fixed station to assess how the number of characteristics and developmental stages of fish larvae vary with the tide and how these values are related to environmental variables. We found that the number of species and larval individuals were highest and lowest, respectively, at the highest and lowest tidal height, and they obviously increased and decreased with the rising and ebb tide, respectively. We detected a tendency for Cynoglossus joyneri, Larimichthys polyactis, and Liza haematocheila to be present at higher density during the rising tide compared with the ebb tide, indicating that they were moving to the near shore from the open sea. Our findings indicate that the variation in numbers of the larvae and juveniles depends on species and developmental stage. These results provide a better understanding of the habitat of prey species of the wild bird population. ABSTRACT: The National Yancheng Rare Birds Nature Reserve is a vitally important staging habitat for the wild population of red-crowned cranes (Grus japonensis) in China. The population relies on local high-protein food sources, such as fish juveniles, to fuel their migratory journeys. However, little is known about the ecology of the fish larvae and juveniles that migrate to the inshore area via tidal rhythm in Subei Shoal, which is adjacent to the reserve. Therefore, we used a fixed study station (32°55′1.2″ N, 121°19′58.8″ E) to conduct a continuous 15-day ichthyoplankton survey at 2 h intervals beginning at 05:00 on 25 April and ending at 03:00 on 10 May 2019. We identified the tidal variations in the number of fish larvae and juveniles and the number at various developmental stages and assessed how they were related to environmental variables such as sea surface temperature, salinity, turbidity, and tidal height in the Dafeng Sea area of Subei Shoal. We found that the number of species and larval individuals were highest and lowest, respectively, at the highest and lowest tidal height, and they obviously increased and decreased with the rising and ebb tide, respectively. Our findings indicate that the variation in numbers of the larvae and juveniles depends on species and developmental stage. The species Acanthogobius ommaturus, Pholis fangi, Cynoglossus joyneri, Liza haematocheila, and Lateolabrax japonicus and the total number of larvae were most influenced by tidal height. These results provide a better understanding of the habitat of prey species of the red-crowned crane wild population as well as scientific data that can be applied to manage the wild population in the reserve sustainably.