Ecological Studies On The Food Web Structures And Trophic Relationships Of Northern And Central East China Sea Using Stable Carbon And Nitrogen Isotopes

The northern East China Sea represented one of typical shallow waters within continental shelf. It served as spawning and feeding grounds for many commercial fishes for the rich food resources brought by sea currents, including Taiwan warm current, coastal currents from the Yellow Sea and the East China Sea, freshwaters from the Changjiang River and Cold water mass in the Yellow Sea. Therefore the northern East China Sea had long been considered as important marine fishery waters in China. However, fishery resources in the area were over exploited in the recent years. Theoretic studies should be enhanced in the area to make efficient and scientific managements.The present research studied trophic structures of main nektonic organisms in the northern East China Sea, based on a large scaled collection in this area and additional samples from the southern Yellow Sea and the Changjiang River Estuary during summer 2009 and spring 2010 (29°-34°N;122°-127°E). The results were listed as follows.1. Analysis on the isotope content of one of the most important fishes, small yellow croaker (Pseudosciaena polyatis) showed that theδ¹³C varied from -20.67‰to -15.43‰, andδ¹⁵N from 9.18‰to 12.23‰. Both of them had clear differences among different sampling stations. Bothδ¹³C andδ¹⁵N increased in line with the body length.2. Contents ofδ¹³C in fish samples from three study areas showed no clear differences (P>0.05) during 2009 cruises, whileδ¹⁵N in materials from the southern Yellow Sea showed discrepancies to the others (P<0.001). The materials from In 2010 cruises, values ofδ¹³C in collected fish samples exhibited significant differences (P<0.05), while there existed no difference considering those ofδ¹⁵N (P>0.05).Contents ofδ¹³C andδ¹⁵N between cruises conducted in 2009 and 2010 at three study areas showed remarked differences (P=0.015, 0.002 forδ¹³C andδ¹⁵N in southern Yellow Sea, respectively; P<0.01 forδ¹³C andδ¹⁵N in Northern East China Sea), with the exception thatδ¹³C values in the Central East China Sea (P=0.9).Among eight species collected from the southern Yellow Sea from 2009, three of which were identified as feeding exclusively on planktons (including Larimichthys polyactis, Harpadon nehereus and Engraulis japonicus), four of which feeding on both plankton and benthos and the remainder only feeding on marine benthos. Fishes from the Changjiang River Estuary (13 species) could be classified into group that fed on both plankton and benthos (Pampus argenteus, Larimichthys polyactis and Champsodon capensis) and the other group fed merely on benthos (10 species in total, such as Lophius litulon, Trichiurus lepturus, Argyrosomus argentatus and Chelidonichthys kumu). Four species from the northern East China Sea (Trachurus japonicas, Acropoma japonicum, Champsodon capensis and Saurida undosquamis) utilized both plankton and benthos as food resources, while the remainder fed only on marine benthos (a total of five species, such as Psenopsis anomala, Argyrosomus argentatus, Anago anago).Proportions of organisms with different habits which were analyzed on the basis of materials from 2010 cruises were slightly different from those in 2009. Among organisms from the Southern Yellow Sea (11 species), only one species feeding on plankton (Apogon lineatus), eight species feeding on both plankton and benthos (such as Pampus argenteus, Engraulis japonicas, Trachurus japonicas and Larimichthys polyactis) and the other two species feeding merely on benthos (Johnius belengerii and Lophius litulon). In the Changjiang River Estuary, seven species feeding both on plankton and benthos (Pampus argenteus, Engraulis japonicas, Trichiurus lepturus, Champsodon capensis and etc.), while the other nine species feeding on benthos (such as Saurida undosquamis, Larimichthys polyactis, Lophius litulon, Chelidonichthys kumu, Rhynchoconger ectenurus, Dysomma anguillare). Twelve species from the northern East China Sea all exclusively utilized marine benthos (including Trichiurus lepturus, Champsodon capensis, Saurida undosquamis, Dysomma anguillare, Muraenesox cinereus, Rhynchoconger ectenurus and etc.).Both evidences from summer 2009 and spring 2010 suggested that marine planktons contributed only a small fraction of food resources for small yellow croaker. Some small sized species, such as Acropoma japonicum and Loliolus sumatrensis mainly served as food resources in 2009 and 2010 (68.8% and 56.1%, respectively). Food resources for Chelidonichthys kumu were similar with those for small yellow croaker. The largest proportion was benthos, including Trachypenaeus curvirostris, Solenocera koelbeli and etc..3. Studies on the isotope content of the main nektonic fishes (39 species) indicated thatδ¹⁵N ranging from 6.9‰to 13.4‰, andδ¹³C from -20.7‰to -14.7‰. There existed remarked differences among individuals within species. There were present significant differences among different groups consideringδ¹³C andδ¹⁵N values (P<0.01). Values ofδ¹³C had no clear relationships with body length after using linear regression (R2=0.03, P=0.56), which was in contrast to that ofδ¹⁵N (y=0.0053x+9.987, R2=0.37, P=0.029). Close correlations of body length andδ¹³C were observed in a total of six species, including Trachypenaeus curvirostris, Metapenaeopsis dalei, Crangon affinis, Larimichthys polyactis, Decapterus maruadsi and Lophius litulon. As for theδ¹⁵N values, the correlations were observed in four species, which were Crangon affinis, Larimichthys polyactis, Pampus argenteus and Argyrosomus argentatus.Trophic level for main nektonic organisms in the northern East China Sea was about the class from2.50 to 4.01, and mostly situated in the level from 3.2 to 3.9. The number of species, with which the trophic level from 2.5 to 3 accounted for the 2.56% of the total species, while those ranging from 3 to 3.5 represented the 41.03%, such as Harpadon nehereus, Champsodon capensis, Parapenaeopsis hardwickii. Species with higher trophic levels (from 3.5 to 4) in this study was about 48.72% of the total number, including Larimichthys polyactis, Trichiurus lepturus, Saurida undosquamis, Anago anago and etc.. There were also a few species with trophic level more than four in this study (represented 7.69% of total number), i.e. Lophius litulon and Uroteuthis edulis.4. Analysis on samples collected in 2009 using"δ¹³C andδ¹⁵ two dimensional polygon"model showed Crangon affinis, Lophius litulon, Uroteuthis edulis, Setipinna taty, Loliolus sumatrensis, Paraplagusia japonica, Pleuronichthys cornutus were fringer species which suggested they played important roles in the system. Analysis on material from 2010 cruises with the same method revealed that marginal species were Benthosema pterotum, Pampus argenteus, Harpadon nehereus, Johnius belengerii, Portunus trituberculatus, Charybdis japonica, Eucrate solaris and Benthosema pterotum, respectively.5. Regression analysis showed body sizes were closely related to the averageδ¹⁵N values. The ratio of predator and food resources in the study area was 983.3. The species richness index were also related to the averageδ¹⁵N values in different size groups and the ratio of predator and food resources in the study area was 0.33.