Fangqun Dai

Long-Term Changes in Fish Assemblage Structure in the Yellow River Estuary Ecosystem, China

Abstract The Yellow River estuary ecosystem is an important spawning ground for many species found in the Bohai Sea and Yellow Sea and contributes substantially to the fishery resource structure and biological reproduction in the northern China Sea. Based on long-term ecosystem surveys in the Yellow River estuary during the main spawning period (May) of most fishery species from 1959 to 2011, the responses of the ecosystem, including regime shifts in species composition, biomass, diversity, and other related factors, were analyzed in this study. Since the 1980s, the dominant large-size species of high economic value (e.g., Largehead Hairtail Trichiurus lepturus) have been replaced by short-lived, low-trophic-level, planktivorous pelagic species (e.g., Scaly Hairfin Anchovy Setipinna taty and Japanese Anchovy Engraulis japonicus). Currently, traditional commercially targeted fishes, such as the Largehead Hairtail, Red Seabream Pagrus major, and Pacific Herring Clupea pallasii, are locally extinct. There has been a rapid shift of dominant species from highly valued, high-trophic-level, large-sized demersal species with complicated age structures to low-value, low-trophic-level, small-sized pelagic species with simple age structures; this shift has resulted in major changes to the ecological cycle and restoration of fishery resources. The fish catch declined from 421.66 kg/h in 1959 to 0.25 kg/h in 2008 and then increased to 3.62 kg/h in 2011. Diversity and evenness indices showed a continuously increasing trend during 1959–2011. The Yellow River estuary may be significantly compromised by overfishing, climate change, dam construction, and pollution, resulting in the decline of traditional fishing industries and reduced biodiversity in this ecosystem.

Compound-specific isotopes of fatty acids as indicators of trophic interactions in the East China Sea ecosystem

The composition and compound-specific isotopes of fatty acids were studied within food webs in the East China Sea. Lipid-normalized stable carbon isotopes of total organic carbon had a good correlation with trophic level. Variations in fatty acid compositions among diff erent species were observed but were unclear. Diff erent dietary structures could be traced from molecular isotopes of selected fatty acids in the Shiba shrimp (Matapenaeus joyneri), the coastal mud shrimp (Solenocera crassicornis) and the northern Maoxia shrimp (Acetes chinensis). Both M. joyneri and S. crassicornis are mainly benthos feeders, while A. chinensis is a pelagic species, although they have a similar fatty acid composition. There was a good correlation for isotopes of arachidonic acid (C20:4n6; ARA) and docosahexaenoic acid (C22:6n3; DHA) among pelagic species from higher trophic levels. The isotopic compositions of DHA in benthic species were more negative than those of pelagic species at the same trophic level. The fact that the diet of benthic species contains more degraded items, the carbon isotopes of which are derived from a large biochemical fraction, may be the reason for this variation. A comparative study of benthic and pelagic species demonstrated the diff erent carbon sources in potential food items and the presence of a more complex system at the water-sediment interface.

A comparative study of spatial interpolation methods for determining fishery resources density in the Yellow Sea

Spatial interpolation is a common tool used in the study of fishery ecology, especially for the construction of ecosystem models. To develop an appropriate interpolation method of determining fishery resources density in the Yellow Sea, we tested four frequently used methods, including inverse distance weighted interpolation (IDW), global polynomial interpolation (GPI), local polynomial interpolation (LPI) and ordinary kriging (OK). A cross-validation diagnostic was used to analyze the efficacy of interpolation, and a visual examination was conducted to evaluate the spatial performance of the different methods. The results showed that the original data were not normally distributed. A log transformation was then used to make the data fit a normal distribution. During four survey periods, an exponential model was shown to be the best semivariogram model in August and October 2014, while data from January and May 2015 exhibited the pure nugget effect. Using a paired-samples t test, no significant differences (P>0.05) between predicted and observed data were found in all four of the interpolation methods during the four survey periods. Results of the cross-validation diagnostic demonstrated that OK performed the best in August 2014, while IDW performed better during the other three survey periods. The GPI and LPI methods had relatively poor interpolation results compared to IDW and OK. With respect to the spatial distribution, OK was balanced and was not as disconnected as IDW nor as overly smooth as GPI and LPI, although OK still produced a few “bull’s-eye” patterns in some areas. However, the degree of autocorrelation sometimes limits the application of OK. Thus, OK is highly recommended if data are spatially autocorrelated. With respect to feasibility and accuracy, we recommend IDW to be used as a routine interpolation method. IDW is more accurate than GPI and LPI and has a combination of desirable properties, such as easy accessibility and rapid processing.

Changes in fish diversity and community structure in the central and southern Yellow Sea from 2003 to 2015

The central and southern Yellow Sea is an important overwintering ground for many fish species in the Bohai Sea and Yellow Sea. For better understanding the status of the fish community after years of heavy exploitation, variations in fish community structure and diversity were analyzed using data from bottom trawls during 2003–2015. Five fish assemblage indices all showed fluctuations without clear trends from 2003 to 2015, yet there were strong positive and significant correlations ( P < 0.05) among them. The top-five dominant species accounted for a high weight percentage (49.7%–82.1%) in the annual fish catch. Multivariate analysis showed that two year groups could be pooled for the fish community: Group I consisted of the years 2006, 2007, 2008 and 2015, while Group II consisted of the years 2003, 2004, 2005, 2009, 2010 and 2014; the groups aggregated with 63.71% similarity, indicating a high level of similarity among all years. The multivariate dispersion values were 1.455 and 0.818 for Groups I and II, respectively, indicating greater variances in fish assemblage structure in Group I than that in Group II. Similarity of percentage analysis demonstrated that the average similarities for Group I and Group II were 71.58% and 67.51%, respectively. Size-spectra analysis revealed no consistent trend in the intercept and slope ( P > 0.05); there were also no significant differences between the slope of the size-spectra and fishing effort. The catch per unit effort and mean individual weight analyses of the whole fish assemblage both showed a significantly decreasing trend over time. Overall, the results showed that the fish community structure in the central and southern Yellow Sea was relatively stable from 2003 to 2015 and the study could be used as a reference for supporting ecosystem-based fishery management.

Biological responses of small yellow croaker ( Larimichthys polyactis ) to multiple stressors: a case study in the Yellow Sea, China

Temporal changes in biological characteristics of small yellow croaker Larimichthys polyactis in the Yellow Sea were examined for the period of 1960–2008. The body size and age of small yellow croaker decreased substantially, in particular, average length of fish in 2008 was reduced by ~85% than those occurring in 1985, and at that time ~93% of the total catch was dominated by one-year-old individuals. Correspondingly, growth parameters also varied significantly over the years, i.e., k (growth coefficient) and t0 (zero-length age) gradually increased from 0.26 and–0.58 year in 1960 to 0.56 and–0.25 year in 2008, respectively. Although, L∞ (body length) sharply decreased from 34.21 cm in 1960 to 24.06 cm in 2008, and tr (inflexion age) decreased from 3.78 year in 1960 to 1.61 year in 2008. There was a great increase both in natural mortality coefficient and fishing mortality coefficient. However, according to the gray correlation analysis, changes in the biological characteristics of small yellow croaker were induced by different stressors ranked as: fishing vessel power>feeding grade>sea surface temperature. This study suggests that the active fishery management measures for biological characters of fish populations should be considered.

Population Dynamics of Fish Species in a Marine Ecosystem: A Case Study in the Bohai Sea, China

Abstract There were rapid shifts of the dominant species in the Bohai Sea from the 1950s to the 1990s, with large-sized, high-valued species (e.g., Small Yellow Croaker Larimichthys polyactis and Largehead Hairtail Trichiurus lepturus) being replaced by small-sized, low-valued species (e.g., Japanese Anchovy Engraulis japonicus and Hairfin Anchovy Setipinna taty). From the 1990s to the present, the Small Yellow Croaker and some of the small-sized species (Hairfin Anchovy and Dotted Gizzard Shad Konosirus punctatus) have become the dominant species. The food web is now simple, with species from relatively low trophic levels controlling the energy flow within the fishery ecosystem. Along with the shift in community structure, the abundance of dominant species changed, the diversity of fish species and species number density decreased, and interannual and seasonal variations in species number density were found. Fish abundance had a decreasing trend and fish interannual and seasonal distribution greatly changed. Variation in the sizes of the ecological niches of the dominant species regulated the succession of the fish community, and the alteration of ecological niches caused changes in the fishery community.