Assessment Of Ecological Carrying Capacity Of Stock Enhancement Species In Daya Bay Based On Ecopath Model

Daya Bay is a typical semi-enclosed bay in the northern part of the South China Sea.Because of its superior natural conditions,abundant bait,and a large variety of fish,it is an irreplaceable spawning ground,breeding ground and feeding ground for many economic fish.In this study we firstly analyze the changes of the Daya Bay ecosystem,and analyze the changes of the Daya Bay ecosystem over the years.Based on the data of the fishery resources and environment survey in the southern Daya Bay,we study the species composition and diversity of fish resources,and its interannual variation.Using Ecopath with Ecosim6.5(EwE)software to construct ecopath model of Daya Bay,we analyze the ecosystem structure function and energy flow process of Daya Bay,and evaluate carrying capacity of Sparus macrocephalus,Acanthopagrus latus,Siganus oramin,Penaeus monodon,Portunus trituberculatusin.It is designed to optimize the strategy of stock enhancement in Daya Bay and provide scientific evidence and guiding recommendations for the sustainable development of fisheries.The main results of this study are as follows:(1)There are abundant phytoplankton species in Daya Bay.The number of phytoplankton species in Daya Bay showed a significant downward trend from 1985 to2012.The percentage of diatom species also showed a downward trend,while the percentage of dinoflagellate species increased overall.The diversity of phytoplankton in Daya Bay decreased from 1990 to 1990 and rose again in 2012.(2)The number of species of copepods in Daya Bay is 45.15%of the total species.The replacement of dominant species is very obvious.It has the following characteristics:the dominant species that have appeared throughout the years are Calanus sinicus,Acartia erythraea,and Penilia avirostris.Brachyuran and Macrura were not dominant species in the subsequent investigations after 1985.Since 2007,Noctiluca scintillans has become the dominant species of zooplankton in Daya Bay and has a dominant position.(3)The number of benthic species in Daya Bay varied greatly over the years.The total number of species showed an upward trend.The proportion of mollusks decreased from 67.97%in 1985 to 27%in 2016.On the contrary,the number of polychaetas from10.94%to 55.9%in 2013 and 396%in 2016.Timoclea scabra has been a dominant species of benthic animals in Daya Bay since 2004 significantly.Paraprionospio pinnata are dominant species since 2013,and the dominant species of Aglaophamus lobatus and Prionospio malmgreni have emerged.The abundance and biomass of benthic animals in Daya Bay showed a downward trend as a whole.(4)The dominant species of fish in Daya Bay has shown a significant replacement from 1985 to 2015.The dominant species in the last century were mainly fish with higher body quality and higher economic value,which were replaced by lower body quality and lower economic value gradually.In the last century,the fish in Daya Bay was mainly dominated by pelagic fish.In recent years,it was dominated by near-bottom and bottom fish.(5)A total of 113 species of fish species were identified in four voyages,belonging to 10 phylums,49 families and 81 genuses,all of which belonged to the Bonefish.The largest number of species is Perciformes taking account for 56.64%and Pleuronectiformes taking account for 10.62%,The annual average fish resource density is 575.02kg/km~2,and the annual average fish resource mantissa density is45,535.39ind/km~2.The dominant species are Leiognathus brevirostris,Parargyrops edita Tanaka,Thamnaconus hypargyreus,Apogon Kiensis,Sciaena russelli,Apogon endekataenia and Parachaeturichthys polynema.The annual average fish diversity index of Daya Bay is 2.48,the average of the evenness index is 0.65,and the average body mass of fish is 7.36g/tail.(6)The Daya Bay Ecopath model is divided into 30 functional groups.The functional transformation efficiency of the functional group is between 0.09 and 0.975.Trophic level ranges from 1 to 3.95.Trophic level of Sparus macrocephalus is 3.50,with low transformation efficiency of 0.291.Trophic level of Acanthopagrus latus is 3.25,with transformation efficiency of 0.343;the trophic level of the Siganus oramin is2.38,with transformation efficiency of 0.285;the trophic level of the Penaeus monodon is 2.67,with transformation efficiency of 0.276;The trophic level of the Portunus trituberculatus is 2.72 with transformation efficiency of 0.216.Total transfer efficiency was 7.808%and the ratio of total primary production to total respiration was 2.142.Connectance index and system omnivory index were 0.310 and 0.210,respectively.The results implied that each trophic level transfer efficiency was low,and the energy was not fully taken advantage of.Energy flow to detritus of trophic level I and II was 98.08%of that to total detritus,signifying that energy transfer was hindered and that the system has a potential for continuous enhancement.The ecological capacity of Sparus macrocephalus in Daya Bay was estimated to be 0.034 t/km~2.The ecological capacity of Acanthopagrus latus,Siganus oramin,Penaeus monodon and Portunus trituberculatus were 0.084 t/km~2,0.05 t/km~2,1.48 t/km~2and 0.88t/km~2.Combined with natural death coefficient and total death coefficient and residual rate of Sparus macrocephalus,it is calculated that when it experiences both predation death and natural death at 0~1 age,it is recommended that the number of enhancement is 3.10 million.When it only experiences natural death at 0~1 age,it is recommended that the number of enhancement is 2.18 million.