| Spawning grounds are essential habitats for the early life stages of marine aquatic animals. The habitat conditions of fish spawning grounds determine the quantity and growth of eggs and larvae, which in turn have an important impact on marine fish recruitment. Historically, there was one important spawning ground of Scomber japonicus in the Yantai-Weihai offshore waters, which were considered an unstable spawning ground as it is the main breeding migration channel for most of the breeding fish stocks migrating from the Yellow Sea to the Bohai Sea in the spring, and a feeding ground when adult and juvenile fish migrate out of the Bohai Sea in summer and autumn. Therefore, the Yantai-Weihai offshore waters have not been well studied in recent years. To determine the current fish spawning ground status in this area, monthly surveys were conducted from April to September 2020. Pooled analyses of the fish eggs, larvae, and environmental data employed spatial interpolation, clustering analysis, non-metric multi-dimensional scaling, similarity analysis, and redundancy analysis (RDA). The spatio-temporal distribution patterns and monthly community turnover of the marine fish early life stages and the optimal habitat of the main species were also examined. In total, 52 fish eggs and larvae species were collected, including 337 038 eggs of 33 taxa and 2122 larvae of 28 taxa. The main spawning season was from May to June, with 21 taxa accounting for 98.32% of the total species observed during the whole year. The main spawning ground was located in northern waters from the Yantai Taozi Bay to Weihai Jiming Island. Engraulis japonicus, S. japonicus, Scomberomorus niphonius, Cleisthenes herzensteini, Seriola lalandi, Callionymus beniteguri, Sillago japonica, Acanthopagrus schlegelii, Cynoglossus joyneri, and Saurida elongata were the main spawning species. The eggs and larvae community structures presented apparent monthly variance in the Yantai-Weihai offshore waters. Apart from the end of the spawning season between August and September, the monthly species turnover rates were over 50%, with average dissimilarities between eggs and larvae communities in the adjacent months of more than 73%. In April, Pseudopleuronectes herzensteini was the dominant eggs taxon, while the dominant larvae taxon was Ammodytes personatus. In May, E. japonicus and S. japonicus were the dominant eggs species, while Sebastes schlegelii was the dominant larvae species. E. japonicus eggs and larvae were both dominant in June and July. In August, the dominant eggs taxa were C. joyneri and S. japonica, and the dominant larvae taxon was E. japonicus. There was no dominant taxon in September. From April to June, E. japonicus was the indispensable species to the community structure and contributed the most to community turnover, while S. japonicus was the second most important species. Habitat environmental factors in the Yantai-Weihai offshore waters within the survey areas from April to September 2020 varied as follows: the sea surface temperature (SST) ranged from 8.27℃ to 26.05℃, the sea bottom temperature (SBT) from 8.01℃ to 24.52℃, the sea surface salinity (SSS) from 30.87 to 33.27, and the seafloor depth from 4.4 to 82.0 m. The RDA results showed that variation in SST, SBT, SSS, and seafloor depth affected fish spawning activities significantly. The suitable spawning habitat of E. japonicus, S. japonicus, S. niphonius, S. lalandi, C. herzensteini, and C. beniteguri was the areas with low SST and SBT, and high SSS. The suitable spawning habitat of P. herzensteini and A. schlegelii was the shallow waters with low SST, while low SBT and high SSS was suitable only for P. herzensteini. The suitable spawning habitat of C. joyneri was the areas with high SST and SBT, and low SSS, and for S. elongata and Trichiurus lepturus was the medium shallow waters and areas with high SBT and low SSS, respectively. Finally, the suitable spawning habitat of S. japonica was the bottom waters with high SBT. During the peak spawning season, the species number and ecological density of fish's eggs in the Yantai-Weihai offshore waters were significantly higher than those distributed in the Liaodong Bay and Laizhou Bay, which represented the typical spawning grounds in the whole Yellow Sea and Bohai Sea. In conclusion, typical fish spawning grounds are currently distributed in the Yantai-Weihai offshore waters; therefore, research on the early life history and spawning grounds of marine fish in this area is critical. This study provides important ecological information on the early life stages of fish for conservation and formulation of spawning ground protection strategies in the Yellow Sea and Bohai Sea. |
