| The large yellow croaker(Larimichthys crocea)was once one of the four top commercial fisheries in the East China Sea,due to overfishing,the wild population of L.crocea was heavily exploited,which results in resource collapse,and various fishing grounds were no longer available for fishing seasons,and it was classified as a critically endangered species(CR)by International Union for Conservation of Nature(IUCN).Sansha Bay is an important spawning and nursery ground for the early life history of L.crocea in eastern Fujian province,which is of great significance to the recovery of population resources of L.crocea.Therefore,since the 1980s,relevant departments had carried out a certain scale of restocking programs in Sansha Bay every year,but the wild population of L.crocea had never recovered.Sansha Bay is not only the natural habitat of wild L.crocea populations,it is also an important marine aquaculture base in the southeast coast,mainly including fish cage culture(such as L.crocea)and macroalgal culture(such as Gracilaria lemaneiformis and Laminaria japonica),benefitting from the unique geographical environment and the rapid development of the marine aquaculture industry.In 2019,the production of L.crocea and macroalgal culture had reached 164,000 tons and 141,000 tons,respectively.The impact of mariculture on the ecosystem of Sansha Bay and the restoration of biological resources such as L.crocea in eastern Fujian province is still unknown.This study innovatively combined the concept of ecological connectivity,using stable isotopes,Ecopath with Ecosim,bioenergetics model,structural connectivity experiments and other methods,with the Sansha Bay integrated investigation and historical data,comprehensively analyze the impact of mariculture on the environment and the restoration of biological resources,and providing useful reference for the habitat protection and ecological restoration of Sansha Bay.The main findings are as follows:(1)Although the intensive marcoalgal culture in Sansha Bay can absorb part of nutrient input form the fish cage culture and terrestrial sources,the average concentration of dissolved inorganic nitrogen(0.52 mg/L)and soluble reactive phosphorus(0.05 mg/L)in the surface water were still at the level four of seawater standards,the eutrophication indicator(NQI)values also exceed 3 in each season.The surface water quality of inner bay was still under eutrophication.The stable isotope values of suspended particulate organic matter(SPOM)indicated that the organic matter released by the large amount of fish feed used in fish cage culture significantly affects the δ13CSPOM(-20.00 ‰)and δ15NSPOM(19.31 ‰)of SPOM,the relasing fish feed of fish cage culture was the important source of SPOM,and δ13CSPOM and δ15NSPOM can be the effectively indicator for the pollution of fish cage culture.The allelopathy caused by the intensive macroalgal culture in Sansha Bay inhibited the growth of phytoplankton,resulting in the Chl a concentration(0.58 μg/L)in the bay significantly lower than other eutrophic bays and the East China Sea,however,the inorganic nutrient was still at a high level,which appeared a non-typical high nutrient and low chlorophyll(HNLC)phenomenon.According to food source analysis based on stable isotope analysis,zooplankton was an important food source(35.8%)during juvenile period of L.crocea,but the abundance of zooplankton in Sansha Bay(12.4×103 L-1)was obviously lower than other waters in the East China Sea.and there was a trend of miniaturization.The stomach content analysis confirmed that most of the L.crocea were under starve status,therefore,the macroalgal culture and the bottom-up control together resulted in the trophic bottleneck and trophic connectivity in the early life history of L.crocea in Sansha Bay.Using macroalgal culture for ecological restoration in the bay,the quantity of macroalgal culture should be scientifically and reasonably configured to achieve ecological balance.(2)The results of the 2010 and 2019 Ecopath model showed that with the expansion of Sansha Bay’s aquaculture in the last decade,the total biomass(492.82 g/m2)and total flow(19553.75 g/m2/a)in the ecosystel of Sansha Bay in 2019 was obviously greater than 2010,and the contribution of phytoplankton to primary production had declined,accounting for only 13.5%.The detrital food chain accounted for 84.8%and 89.9%of the total food web flow in two different stages,and the total transfer efficiency was low.4.161%and 4.268%respectively.The results of the mixed trophic impact showed that the increase of phytoplankton and shellfish biomass had positive and negative effects on most other functional groups,respectively.The total primary productivity/total respiration(TPP/TR)of Sansha Bay were 1.22 and 1.59,respectively.The maturity has declined slightly,and the primary productivity of the ecosystem was still redundant.(3)Based on the constructed Ecopath model in 2019,the Ecosim module was used to simulate macroalgal culture removing,bivalves culture removing,and removing both bivalves and macroalgal culture.The results showed that when the macroalgal culture was removed,the crab biomass decreased by 80%,while the phytoplankton biomass increased by more than two times,the total system flow decreased to 9866.96 g/m2/a,and the TPP/TR and TPP/TB decreased to 0.83 and 6.78,respectively.The total transfer efficiency of the system increased to 6.53%,after removing bivalves culture,the biomass of phytoplankton and zooplankton increased by more than 2 times,and the biomass of juvenile and adult L.crocea increased by about 2 times,the total flow of the system dropped to 12068.55 g/m2/a,TPP/TR and TPP/TB increased to 2.66 and 14.29,respectively,and the system transfer efficiency increased to 6.44%;when the macroalgal and bivalves culture were removed together,the biomass of phytoplankton increased by 2.5 times,and most of the remaining functional groups’ biomass variations were within 0.5 times.The total system flow dropped to 9518.66 g/m2/a,and the TPP/TR and TPP/TB dropped to 0.98 and 7.28,respectively.The transfer efficiency increased to 6.68%.Although macroalgal culture had obvious control effects on phytoplankton biomass and eutrophication,bivalve culture had a strong control effect on phytoplankton biomass,before carrying out IMTA,each culture species should be scientifically evaluated,in order to reduce the redundancy of the ecosystem,improve the maturity of the system,and make the aquaculture ecosystem more resilient and antidisturbance.(4)Using the stable isotope method,analyzing the stable isotope characteristic values of various groups in the Sansha Bay aquaculture ecosystem and the trophic niche from individual to population.The results showed that nekton biodiversity had temporal and spatial variations.The Shannon-Wiener and Simpson diversity indices were higher in the inner bay than those outside bay in different seasons.Non-metric multidimensional scaling analysis(nMDS)and similarity analysis(ANOSIM)indicated that there were significant differences in community structure(P=0.001)between inside and outside bay.The stable nitrogen isotope of potential food sources(such as SPOM)in the food web of Sansha Bay was higher than that of other bays.Based on the kernel utilization density method(KUD),fish occupied the largest niche area in spring at 51.16,while the niche of shrimp was the smallest,only 6.03 in summer,carnivorous fish occupy the largest niche area in the fish community,which reached 52.59 in summer,L.crocea with a body length of 100～150 mm was an important part of the functional connectivity of the population and the coupling of the pelagic-bethic food web.Isotope niche(trophic niche)was an effective tool to quantify the trophic interaction and community structure.Protecting the habitat diversity and functional connectivity of key habitats was an important factor in the restoration of biological resources(5)According to the survey of L.crocea resources in Sansha Bay and adjacent waters,the average abundance of L.crocea in summer were 490.61 ind./km2 and 1333.23 ind./km2 in inner and outside bay,respectively,while in spring,the abundance and biomass of L.crocea were the smallest in both inner and outer bay,1.05 kg/km2/11.11 ind./km2 and 1.95 kg/km2/33.33 ind./km2,respectively,showing obvious temporal and spatial variation,and related with naturally migration and restocking activities of L.crocea,and L.croceas were distributed around different culture areas(macroalgal and fish cage net culture)in inner bay,and there was no obvious spatial difference.The structural connectivity experiments of L.crocea showed that the fixed nets under different conditions had different degrees of influence on the moving process of L.crocea,and under the condition of a flow velocity of 0.09 m/s,the retention rate of L.crocea was significantly higher in the cage net(21.90%)and gill net(32.72%)than under the condition of a flow velocity of 0.05 m/s(P<0.05).Based on the biogenetics model,a growth curve of L.crocea was built,and combined with the biological data of recapture after restocking of L.crocea,it showed that the existing population of L.crocea in Sansha Bay was mainly from restocking groups.The structural connectivity of the restocking populations of L.crocea in eastern Fujian was affected by many aspects such as water flow,artificial barriers,and mariculture facilities.The migration path and habitat distribution of L.crocea had changed to a certain extent.Before the protection and restoration of the key habitats of L.crocea,we should first understand the changes in its spawning grounds,nursery grounds and migration channels under the influence of human activities and climate change,and secondly using the concept of ecological connectivity and related methods to comprehensively analyze the trophic interaction in the food web,ecosystem structure and function,finally,proposing the protection and restoration strategies of key habitat.(6)Based on the previous fundamentaliy research,the protection and restoration strategies are proposed as followed:we should control the input of nutrients and organic matter from terrestrial and aquaculture activities in Sansha Bay,scientifically quantify the bioremediation of macroalgal culture through model simulation or experiments,in order to balance bio-productivity and nutrient concentration;the quantity of different mariculture species should be scientifically and reasonably quantified before carrying out IMTA for habitat restoration,and the marine investigation should be carried out regularly during the implementation to assess the impact of the IMTA on the ecosystem;Removing part of the barriers on the migration path of L.crocea,set pre-quantitatively evaluated macroalgal culture as a corridor or new habitat,to improve and protect the structural connectivity and functional connectivity of key habitats,and optimized and adjusted the mariculture pattern in Sansha Bay,reducing the quantity of cage cultures in areas with relatively poor hydrodynamic conditions,which can be replaced by macroalgal culture,and in the coastal areas with relatively good hydrodynamic conditions,cage cultures and shellfish cultures are main components,which can dilute the discharged pollutants through tide. |
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