| Bohai Bay is an important spawning and nursery habitat for a variety of economically important fish inhabiting the Yellow and Bohai seas. Based on a meta-analysis of historical data spanning 40 years, combined with a field survey of current fish habitat conditions conducted in Bohai Bay, a long-term dataset containing early life resource surveys (including 53 voyages, from 1982 to 2021) of marine Osteichthyes was built. Based on statistical analyses, long-term trends in ichthyoplankton assemblage structure, biodiversity, and synchrony were interpreted. The results indicate that ichthyoplankton assemblage structure and the center of gravity for spawning and nursery habitats in Bohai Bay are currently experiencing continuous replacement. Seasonal variations in ichthyoplankton assemblage structure, abundance index, predominant taxa, and species diversity levels are evident. Spawning and nursery habitats are concentrated in the central and inner part of Bohai Bay west of 118º30'E. Compared with survey results from the 1980s, the taxonomic composition and abundance of the ichthyoplankton assemblage structure have changed considerably. The function of Bohai Bay as a spawning and nursery habitat for traditionally economically important fish has declined significantly. The abundance index and taxon number of ichthyoplankton in Bohai Bay fell to a historic low in the early 2010s, then rebounded significantly. The taxa number of ichthyoplankton decreased from 39 in the 1980s to 31 in the early 1990s, 34 in the late 1990s, and 22 in the 2000s to pre-2010s, then further decreased to 21 in the early 2010s. From the middle 2010s, it recovered to a certain degree, increasing to 40 species just prior to 2020. The current number of pelagic egg taxa is 22, 80% of the number in the 1980s, and 88% of the abundance in the 1980s. The current (2020—2021) number of larval fish taxa is 26, almost identical to that in the 1980s, with an abundance 1.33 times greater than that in the 1980s. Interannual and interdecadal variations in fish egg and larvae species diversity fluctuated drastically. Interannual taxon substitution was noticeable. However, the substitution rate has increased significantly in recent years. Spawning, habitat, and temperature adaptation studies of breeding stock indicate that the number of taxa first decreased, then increased. The annual proportion of the number of taxa to pelagic eggs decreased, while the proportion of the number of taxa to adhesive eggs and eggs with egg membrane filaments increased. The annual proportion of continental shelf pelagic-neritic fish taxa increased, while the continental shelf demersal and benthopelagic taxa decreased. The ecological density of numbers in the early life history (EDN-ELH) of Konosirus punctatus, Sardinella zunasi, Larimichthys polyactis, Trichiurus japonicus, Lateolabrax maculatus, and Cynoglossus semilaevis decreased significantly, whereas the EDN-ELH of Engraulis japonicus, Scomberomorus niphonius, Thryssa kammalensis, and Sillago japonica increased. Ichthyoplankton abundance in Bohai Bay mainly depends on environmental conditions in the spawning habitat, fishing intensity, and the degree of damage to the early life stages of fish resources. The temporal variation and succession of the ichthyoplankton assemblage structure in Bohai Bay were the specific manifestations of the multidimensional niche disturbance and structural performance deterioration of the fishery resources under the dual disturbance of global warming and overfishing. |
