Abstract:The ichthyoplankton assemblage structure and succession in the Bohai Sea were studied using multivariate statistical analysis of monthly investigation data collected from April 1982 to May 1983 as background. Based on a pooled analysis of the historical data of fish eggs and larvae sampled from the horizontal trawl surveys during oceanographical expedition cruises over the past 30 years combined with the field survey of current fish habitat conditions, this study aimed to investigate the seasonal and interdecadal variations in the species diversity and key species, and the changing collaboration between the dominant species and the important species in the succession process. Compared with the survey results of 1982~1983, the profiles of the species number and abundance index of fish eggs and larvae during the survey season across different survey times present different degrees of decline (except in winter), and display a tendency towards decline at the beginning and an increase later. The species number of the fish eggs was only one half in the 1980s, and its abundance index was only one tenth in the 1980s. The species number and abundance index of fish larvae were only three fourths in the 1980s, however, these have shown an obvious increasing tendency in the recent years during winter. During the investigation period, species substitution in the early life stages of fish was obvious, otherwise, the substitution rate was significantly accelerated in the recent years. The dominant species of fish eggs did not change significantly during the same season in each investigation period, however, variation in the dominant species in fish larvae was more obvious. The dominance of the continental shelf demersal fish communities with high commercial value decreased sharply, although some species disappeared in recent surveys. The biodiversity index of fish eggs and larvae was higher during the sea temperature rising season and lower during the sea temperature cooling season. The index profiles during the survey season across different survey times showed a tendency of decreasing first and then increasing. Temperature adaptation studies on the spawning stock profiles during the survey season across different survey times showed that species number in each temperature adaptation type showed a tendency of decreasing first and then increasing. Even so, the seasonal or annual percentage of each type remained stable. Habitat studies on the spawning stock showed that the species number of the main habitat type also rose after showing a declining trend first, and presented different degrees of decline. The annual percentage of the continental shelf pelagic-neritic fish increased, in conjunction with the continental shelf demersal and benthopelagic fish decrease. Under the multiple sources of exogenous interference factors, irreversible changes have been taking place in every key link of the recruitment process in the early life stages of the fish, with habitat loss or fragmentation in the Bohai Sea during the past 30 years. Stressors such as overfishing and climate change promote multidimensional niche disturbances in the fish community and structural decline in the fishery resources, while the succession and assemblage structure of the fish eggs and larvae are the embodiment of this development.
