Decline Mechanisms Of Fishery Resources In The Bohai Sea Under Anthropogenic Activities

Based on the integrated research on the data investigated by our laboratory and the historical data in recent decades in the Bohai Sea which was significantly affected by anthropogenic activities, systematic studies on the changes in dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) and their effects on phytoplankton over the last 30 years in the Bohai Sea were presented; using Accelerated Life Testing model, Independent Action model and the relationship between reduction in the juvenile survival rate and reduction in population growth rate, this paper estimated the long-term lethality of combined pollution of Cd, Hg, Pb and PHs in the Bohai Bay to common fishery species (such as Pagrosomus major, Sparus macrocephlus, Penaeus japonicus, Mactra quadrangularis and Scapharca subcrenata), and the corresponding reduction of population growth rate of such common fishery species; impact on the Bohai Sea ecosystem by fishing over the last 50 years was investigated quantitatively using the Ecopath model; the variation characteristics of the fishery resources of the Bohai Sea in recent decades were analyzed. Series of new results were obtained. The main conclusions were as follows:1. The increasing concentration of nitrogen and the decreasing concentration of phosphorus resulted in the decrease of dominance of Chaetoceros spp. which was prone to be limited by phosphorus and the increase of dominance of Ceratium fusus and Ceratium furca which were prone to be limited by nitrogen. The combined pollution of Cd, Hg, Pb and petroleum hydrocarbons (PHs) caused the notable reduction of population growth rates of common fishery species in the Bohai Sea. Pollution of nitrogen, phosphorus, heavy metals and PHs was one of the important factors influencing the fishery decline in the Bohai Sea.The amount of sewage disposal and use of fertilizer in the surrounding regions of the Bohai Sea, and the Yellow River runoff were found to have a significant influence on the DIN or DIP concentrations in the Bohai Sea over the last 30 years. Changes in DIN and DIP resulted in changes in the limiting nutrients of phytoplankton in the Bohai Sea from nitrogen in the early 1980s to nitrogen-phosphorus in the late 1980s, and then to phosphorus after the 1990s. The half saturation constant (Ks) was used to evaluate the effect of nitrogen and phosphorus on the phytoplankton community structure in the Bohai Sea over the last 30 years. Cell abundance percentages of dominant phytoplankton species with high Ks values for phosphorus and low Ks values for nitrogen have decreased since the 1980s, while those of dominant phytoplankton species with low Ks values for phosphorus and high Ks values for nitrogen increased during this period. Phytoplankton community is going against the fishery species with high trophic levels in the Bohai Sea.Accelerated Life Testing model showed that the annual mean concentrations of Cd, Hg and Pb in seawaters of the Bohai Bay during 1996-2005 were higher than their safe concentrations to common fishery species in the Bohai Bay. The long-term lethality of mean concentrations of Cd, Hg, Pb and PHs in seawaters of the Bohai Bay during 1996-2005 to fishes was 4.5%, 16.3%, 0.0% and 12.0%, respectively, that to crustaceans was 0.4%, 7.9%, 0.3% and 6.6%, respectively, and that to bivalves was 10.5%, 0.2%, 0.2% and 2.3%, respectively. Using Independent Action model, it was estimated that the long-term lethality of Cd, Hg, Pb and PHs to fishes, crustaceans and bivalves was 29.7%, 14.6% and 12.9%, respectively, which resulted in the reduction of population growth rate by about 6.4%, 14.6% and 12.9%, respectively. Combined pollution was an important factor affecting the fishery decline in the Bohai Bay.2. Fishing activities caused the imbalance of the Bohai Sea ecosystem especially the fishery species with higher trohpic levels by changing the key functional group and decreasing the energy transfer efficiency of the Bohai Sea ecosystem, and by reducing the negative feedback function ability and the stability of the Bohai Sea ecosystem. Fishing was the important factor influencing the fishery decline in the Bohai Sea.The Ecopath models showed that because of the over fishing, the most key functional groups of the Bohai Sea ecosystem changed from high trophic level fishes of nekton feeding habits in the late 1950s, to macro benthic crustaceans with medium trophic levels in the early 1980s and in the 1990s. The energy transfer efficiency of the Bohai Sea ecosystem, especially that of organisms at the second and the third trophic levels, decreased notably with fluctuations. The total system throughput, total biomass, sum of all flows into detritus, total primary production/total respiration (TPP/TR), the Finn's mean cycling index, Finn's mean path length and Ascendency exhibited great fluctuations in rencent decades. The TPP/TR was notable larger than 1. The increasing fishing activities resulted in the decrease of the negative feedback function ability of the Bohai Sea ecosystem. At present the Bohai Sea ecosystem is in the unstable period.3. Fishery species based on the benthic food chain declined significantly in recent decades. The mean trophic level of primary fishery landings exhited a decreasing trend during 1956-2000 in the Bohai Sea; the decrease of freshwater input, supply of food organisms and Si, and the increase of N/P ratio and Pb caused such decrease directly or indirectly.The food web in the Bohai Sea ecosystem was simplified into the pelagic food chain, the benthic food chain and the detritus food chain. Fisheries based on the pelagic food chain has replaced those based on the benthic food chain and became the first primary fisheries in the Bohai Sea since the 1ate 1950s. Biomass percentage of fisheries based on the detritus food chain increased continuously and became the second primary fisheries in the Bohai Sea in recent years. Fishing activities and change of structure of secondary production were the important factors influencing the change of the structure of major fishery resources in the Bohai Sea.There was an linear relationship between the mean trophic level of primary fishery landings (Y) and the year (X) during 1956-2000 (Y=-0.008X+19.777; R=-0.710, n=41, sig.=0.000), and that the mean trophic level of primary fishery landings decreased about 0.01 per year. The decline of fishery species with higher trophic levels and the increase of relative biomass of the fishery species with lower trophic levels in the Bohai Sea ecosystem were the root causes of the decreae of the mean trophic level of primary fishery landings in the Bohai Sea. The decrease of freshwater input, supply of food organisms and Si and the increase of N/P ratio and Pb caused direactly or indirectly the decrease of mean trophic level of primary fishery landings by changing the main fishery community structure in the Bohai Sea.4. Anthopogenic pollution has caused notable effects on the species diversity of the phytoplankton community and benthos community in the Bohai Bay. The decrease of benthos community diverysity caused by environmental pollution can further cause the decline of fishes with benthos feeding habit.Diatoms were also the primary component of phytoplankton community in the Bohai Bay in April 2008, and the cell abundance percentage of dinoflagellates was relatively low during the same period. The north and south part of the Bohai Bay mouth, which were affected directly or indirectly by the runoff of the Luanhe River and the Yellow River, respectively, had relative high cell abundance percentage of dinoflagellates. Phosphate concentration was an important factor influencing the ratio of cell abundance of diatoms to that of dinoflagellates. Species diversity and richness of phytoplankton community had the similar spatial distribution in the Bohai Bay in April 2008. Concentrations of phosphate, Cr and Hg were important factors affecting directly or indirectly species diversity and richness of phytoplankton community.Using biodiversity index, lognormal distribution of species individuals and K-dominance curves of the benthos community, assessment of the marine environment quality in the Bohai Bay in April 2008 indicated that the sea areas that were polluted more heavily located in the adjacent waters of Caofeidian Harbor and Huanghua Harbor, and the Bohai Bay mouth near to the Yellow River mouth. The three sea waters were affected greatly by anthropogenic activities. The decrease of benthic community diverysity caused by anthropogenic pollution can further cause the decline of fishes with benthos feeding habit.The average productions of fishery resources decreased notably in the Bohai Bay during 1998-2006. The dominance of benthos feeding habit fishes which were of higher trophic levels had been replaced by nekton feeding habit fishes that were of lower trophic levels. The mean trophic level of fishery resources in the Bohai Bay exhibited a decreasing trend. The fishery resources exhibited a trend of continuous decline.