| Food webs,which depict the complex trophic network relationships between components of organisms in an ecosystem through predation,are the earliest and most studied type of network in ecology.Understanding the structure and complexity of food webs can help to further study the function and dynamics of ecosystems.Keystone species play an irreplaceable role in ecosystems and are important in maintaining the stability of the community structure of populations and the energy transformation and transfer of ecosystems.The eastern tropical Pacific Ocean has high biodiversity and rich fish resources,and is also an important operation area for the world’s tuna fisheries.With the continuous exploitation of tuna resources,the sustainable use of tuna fishery resources and the conservation of bycatch species have attracted great attention and focus from relevant regional management and international organisation,and Ecosystem-based fisheries management(EBFM)has become a popular research.Compared to offshore ecosystems,scientific surveys and biological sampling in the pelagic zone are more difficult,so there is still a lack of understanding of the pelagic ecosystem.In recent years,in order to fulfil the Convention on the Management of Pelagic Fisheries,China has been sending scientific observers to follow tuna production fishing vessels to the pelagic zone for real-time observation and recording of catch data,which has also provided an important avenue of investigation for China to participate in the study of pelagic ecosystems.This thesis is based on six longline tuna catch sampling surveys conducted by scientific observers in the eastern tropical Pacific Ocean from June 2017 to February 2019,analysing the composition of the catch stomach contents,combining with documented information,and constructing a pelagic food web in the eastern tropical Pacific Ocean,mainly targeting economic fish species and bycatch species caught by commercial fisheries in the long term The food webs were analysed for structural indices and important bait species were screened.With the following research content and results:Morphological observations,DNA barcoding techniques and otolith morphological analysis were mainly applied as methodological supplements to identify the composition of bait species in the collected catch stomach samples and to record the biological indices.Based on the feeding relationships among 50 species and 33 taxa obtained from experimental studies and literature review,interspecific trophic interaction networks were constructed,12 common food web network analysis indices were calculated and compared with those of nine other marine areas to study the structural characteristics of pelagic food webs in the eastern tropical Pacific Ocean.The mean trophic level(MTL)of the food web was 2.4 and the highest trophic level was 4.14 for killer whales(Orcinus orca),while economically important species in the tuna fishery,such as yellowfin tuna(Thunnus albacares),bigeye tuna(Thunnus obesus),bonito(Katsuwonus pelamis),etc.,were at higher trophic levels of 3.16,3.08and 2.99 respectively.The proportions of top,intermediate and foundation species in the food web were 10.85%,83.13%and 6.02%respectively,with the highest proportion of intermediate species,indicating that most species in this food web are both predators and prey.Analysis of the omnivory index(Omn,92.77%),connectivity complexity index(SC,13.22)and characteristic path length(Ch Path,1.85)of this food web indicated that the structure of the food web is currently in a relatively stable state and can withstand external disturbances to some extent.The clustering coefficient(CC,0.242)indicated that several sub-network structures could be formed among the food web species,with the more connected species being yellowfin tuna,bigeye tuna,sailfish(Alepisaurus ferox),swordfish(Xiphias gladius)and dolphinfish(Coryphaena hippurus).The main indicators to quantify the complexity of food webs are the total number of species(S),the total number of connections(L),the number of interactions per species(L/S)and the degree of connectivity(L/S~2).The total number of species(S)in this food web is 83 and the number of connections(L)is 542.The number of interactions per species(L/S,6.53)and the connectivity(L/S~2,0.08)indicate that the complexity of this food web is at a normal level and is resilient to anthropogenic disturbances and environmental changes.The study can improve the understanding of the food web structure of the pelagic ecosystem and provide a basic reference for future in-depth studies on the function of the food web in the tropical Pacific Ocean and ecosystem-based management of pelagic fisheries.The SURF(SUpportive Role to Fishery ecosystems)index was used to screen for important bait organisms in the pelagic food webs of the tropical eastern Pacific Ocean,based on the constructed food webs of the eastern tropical Pacific Ocean.The SURFi values for each prey species were calculated from the proportion of feeding provided by each species i as a bait organism for all its predators and the total number of connections in the food web(L).The results showed that the species and populations with high SURFi values were mackerel(Scomber spp.),cephalopods(Cephalopoda),crustaceans(Crustacea),bonito(Katsuwonus pelamis),spiny triggerfish(Diodon sp.)and lizardfish,which can be considered as important bait organisms in the food web of the eastern tropical Pacific Ocean and play an important role in maintaining the stability of the food web structure and function,and their population fluctuations may have direct or indirect effects on other species in the eastern tropical Pacific Ocean.The results of the study may provide a basic scientific basis for managers to adopt appropriate fishing strategies for different species.The implementation of scientific fisheries resource management and conservation is important for maintaining the balance and stability of the pelagic ecosystem,while promoting the sustainable development of tuna fisheries and achieving harmony between humans and nature. |
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