Long-term changes in the fishery ecosystem structure of Laizhou Bay, China

Laizhou Bay provides a critical spawning and nursery habitat for many fishery species, including commercially important spe- cies, such as Fenneropenaeus chinensis and Larimichthys polyactis. The bay is severely stressed due to high fishing pressure and environmental changes. Based on the long-term ecosystem surveys in Laizhou Bay during the main spawning period （May） of most fishery species from 1959 to 2008, the responses of the Laizhou Bay fishery ecosystem were analyzed here, including regime shifts in species composition, biomass, species diversity, zooplankton, phytoplankton, and environmental variables. The dominant species of large-size and high economic value （e.g. Trichiurus haumela, L. polyactis） have been replaced by the short-lived, low-trophic-level planktivorous pelagic species （e.g. Setipinna taty, Engraulisjaponicus） since the 1980s, and it is probable that the small-sized pelagic fishes have been recently replaced by invertebrates （e.g. Oratosquilla oratoria, Crangon affinis）. The biomass of fishery resources declined continuously from 423.6 kg haul 1 h-1 in 1959 to 164.6 kg haul-l h i in 1982, 37.7 kg haul-1 h-1 in 1993, and less than 8 kg haul 1 h-a in 1998-2008. Moreover, the biomass of zooplankton showed an increasing trend during 1959-2006, but showed a slight decline in 2008. The abundance of phytoplankton increased from 1959 through 1982, decreased substantially in 1993, and increased again until 2004. More recently, however, the phytoplankton abundance was very low. The sea surface temperature （SST） and sea bottom temperature （SBT） in May increased by 0.23~C a-~ and 0.16~C a-~, respectively, during 1982-2008. The salinity in May showed large fluctuations and reached its lowest val- ues in 2004 and 2006. The ratio of dissolved inorganic nitrogen （DIN） to dissolved inorganic phosphate （DIP） increased. However, the dissolved silicon （DSi）：DIP and DSi：DIN ratios decreased to a low level during 1959-2008. These changes seri- ously impacted primary production, and cascade effects then changed the structure and function of the fishery ecosystem. Fur- ther analysis indicated that multiple stresses caused the alterations in the structure of the Laizhou Bay fishery ecosystem. The top-down effect was identified as the main influence on the fishery species （at the top of the food chain） over the past five decades due to the increasing fishing pressure, whereas the bottom-up effect increased over the past three decades due to the strong variat