The Community Structure And Particle Size Spectrum Of Fish In Daya Bay

Daya Bay is a semi-enclosed bay with subtropical characteristics in the northern part of the South China Sea.It has diverse habitats and abundant fish resources and thus,is known as the spawning ground and germplasm bank of fish resources in the northern part of South China Sea.However,with the fast economic development in the gulf region,frequent anthropogenic activities and natural disturbances have caused damages to the ecosystem of Daya Bay,and the levels of biodiversity and fish resources have been decreased significantly.On the basis of the four bottom trawling surveys conducted in Daya Bay from 2015 to 2016,the present study researched the characteristics of species components,the spatial and temporal distribution of biodiversity and resources of the fish community in Daya Bay.Compared with historical survey data,the evolution trend of fish community was analyzed,the biomass particle size spectrum of fish in Daya Bay were constructed,and the morphological characteristics of the particle size spectrum were analyzed.Based on this,the ABC curve was used to evaluate the status of the fish community in Daya Bay.This study revealed the evolutionary trends of fish community structure and the characteristics of biomass particle size spectrum of fish in Daya Bay.Thus,it provided a scientific basis for biodiversity conservation and sustainable development of fisheries in Daya Bay.The primary research achievements obtained in this study were of as follows:(1)The survey collected a total of 131 species of fish,which pertained to 84 genera in 53 families of 14 orders.Fish species of Perciformes were the most predominant ones,accounting for 54.20%,which was followed by those of Agunilliformes and Pleuronectiformes,each accounting for 9.92%.There were obsvious seasonal variations in species number,i.e.the highest in summer with 69 species whereas the lowest in spring with 32 species.The predominant species of fish communities included mainly Evynnis cardinalis,Callionymus richardsoni,Clupanodon punctatus,Trachurus japonicus,Thamnaconus hypargyreus,Leiognathus brevirostris,Apogon lineatus as well as other small fishes.(2)There were significant seasonal differences and spatial differences in fish species diversity.The seasonal variations of the diversity index(H')were in the range from 1.516 to 1.998 with the highest value being in winter and the lowest one in autumn;The seasonal variations in the evenness index(J')were in the range from 0.494 to 0.869 with the highest value being in winter and the lowest one in summer;The seasonal variations of the richness index(D')were in the range from 2.230 to 3.777 with the highest value being in summer and the lowest one in autumn.Canonical correspondence analysis(CCA)indicated that temperature,salinity and water depth were the main environmental factors affecting the fish community structure in the Daya Bay.In term of the spatial distribution,the diversity of fish in the central of the bay was higher than that that in the coastal waters.(3)There were obvious seasonal and spatial changes in the fish resource density,showing a significant downward trend.In terms of seasonal changes,the mantissa density and mass density of fish resources were the highest in summer(106574 ind/km2,632.62 kg/km2)and the lowest in winter(4615 ind/km2,115.39 kg/km2).Among them,the average weight of fish in autumn was highest(27 g),followed by those in winter(24 g)and spring(9 g),while that in summer(6 g)was lowest.In terms of spatial distribution,the mantissa density distribution(72273 ind/km2)was characterized by the highest waters in the central area of the Bay,followed by the coastal waters(50905 ind/km2),and the lowest in the Bayou waters(18075 ind/km2);and the distribution of fish resource mass density was the highest in the coastal area(673.90 kg/km2),followed by the central area(404.24 kg/km2),and the lowest in the mouth bay(170.78 kg/km2).The average body weight of fishes was 11.4 g in the coastal waters,8.1 g in the central area of the bay and 10.8 g in the mouth of the bay.(4)The biomass particle size spectrum of fish in the Daya Bay presented a "unimodal" pattern,with a fish size being in the range of 2-10 grains,and fishes were concentrated at 4-8 grain sizes;the highest peak was at 2-4 grains,and was consisted mainly of small-sized fish,such as Leiognathus brevirostris,Callionymus meridionalis,Evynnis cardinalis,Callionymus richardsoni,Trachurus japonicus etc.The curves of biomass particle size spectra in spring and winter were relatively flat with a large curvature.The summer and autumn curves were relatively steep and the curvature was small.The curvature and curvature of the curve were mainly related to a large number of small-sized Evynnis cardinalis,a small number of large-sized Harpodon nehereus,Leiognathus ruconius,and other settled fish;seasonal migratory species included Plotosus lineatus,Trichiurus haumela,and Sparus macrocephalus,Gymnothorax hepaticus,and other reef fishes.It can be seen that the number and size of supplementary groups of migratory fish,current juvenile fish,island reef fish,and perennial colony fishes have significant effects on the peak shape and curvature of the fish particle size spectrum.(5)The ABC curve showed that Daya Bay was in a serious state of disturbance throughout the year.Among them,the spring,summer and autumn were in a severe disturbance,while the winter was in a moderate disturbance;the coastal area was in a more serious disturbance,and the central area and the mouth of bay were in a state of moderate disturbance.(6)In comparison with the historical survey data,due to the disturbance of anthropogenic activities to the Daya Bay ecosystem,the fish resources in Daya Bay showed a significant downward trend during the past 30 years.The community structure of fish showed a tendency toward miniaturization and low quality.There was an obvious replacement of dominant species and the diversity level of fish resource was reduced,and the fish community structure tended to be simplified.These changes were mainly caused by the factors of over-fishing development and habitat destruction,and they also reflected some of the interferences with the adaptive responses of marine ecological environment to anthropogenic activities.