Distribution Of Pelagic Fishery Resources In The Eastern Indian Ocean And Its Relationship With Climate And Environmental Change

The Indian Ocean is a traffic center connecting Asia,Africa,Oceania and Antarctica,and thus an important region along the Maritime Silk Road.Influenced by its geographical location,monsoons,ocean currents,etc.,the Indian Ocean is high in biodiversity and rich in marine living resources.Unfortunately,we have a limited knowledge of the eastern Indian Ocean compared with elsewhere in the western Indian Ocean near the eastern coast of Africa.In integrated surveys carried out in the eastern Indian Ocean,investigation and studies concerning marine organisms have been scarce,focusing on zooplankton and phytoplankton.And no systematical studies have been conducted in large scale on nektons which are closely related to living resources.The literature to date on nekton ecosystem in the eastern Indian Ocean appears to focus on new species and new record of species,development and utilization of fishery resources(mainly tuna species),impact of climate change on nektons,etc.Among these studies the ones on new species and new record of species have been mainly conducted in the near shore while the data on development and utilization of fishery resources have been derived from commercial fishing,making the study not systemic and deep enough.By far,no studies related to diversity,community structure,present resources and the like of nektons in the eastern Indian Ocean have been reported.Basing on surveys of pelagic fishery resources in the eastern Indian Ocean Block one(88°E-92°E,2°N-5°S,two cruises in autumn and spring)and Block two(78°E-82°E,2°N～5°S,one cruise in autumn),this thesis intends to understand the status of fishery resources,species composition and diversity of nektons in the eastern Indian Ocean,determine the resources distribution of main species,study the effect of environmental factors on the distribution of main economic species,and discuss the catch fluctuation of main tuna species and its responses to climate change.The main results were as follows:(1)In the three voyage investigations in the eastern Indian Ocean,t a total of 104species of nekton belonging to 12 orders,39 families and 69 genera were collected,among which,3 orders,3 families,3 genera and 4 species were of Chondrichthyes,7orders,33 families,63 genera and 97 species of Actinopterygii;and 2 orders,3 families,3 genera and 3 species of Cephalopoda.On the whole,among the three voyage investigations,“Voyage B-1”caught the largest number of species of nekton,while“Voyage A-2”caught the smallest.The Shannon-Wiener index,Margalef richness and Pielou index were the highest among the three cruises in“Voyage B-1”based on the analysis of individual numbers.Results of One-way analysis of variance(ANOVA)with the Tukey test showed that the diversity indexes of the nekton species in the eastern Indian Ocean showed highly significant differences(P<0.01)among different regions in different seasons.The dominant species in three voyage investigations were Decapterus macarellus,Cubiceps squamiceps and Symplectoteuthis oualaniensis.The analysis of the mean individual body mass of the dominant species showed that there were significant differences in the mean individual body mass of the dominant species in different areas in different seasons.The analysis of the numerical and biomass dominance curves of swimmers indicated that the swimmer community structure in the area surveyed of the eastern Indian Ocean is relatively stable,and is less disturbed.The low disturbance is mainly related to the recruitment and growth of various species in the community,in addition to the influence from environmental changes.(2)The surveyed results of pelagic fishery resources in the eastern Indian Ocean showed that D.macarellus is the main dominant species.Due to their morphological similarities as described in the Chinese literature on fish identification,D.macarellus and Decapterus macrosoma are often mistakenly identified.In this study,D.macarellus and D.macrosoma samples were collected and reidentified using morphological and DNA barcoding techniques.The major diagnostic characteristics of D.macarellus and D.macrosoma can be summarized as follows:1)the straight-line portion of the lateral line of D.macrosoma,the majority(approximately 3/4)of which is covered with scutes in the rear end,begins below rays 13～14 of the second dorsal fin,and the scutes show no particular external characteristics;in contrast,the straight-line portion of the lateral line of D.macarellus,with the rear half covered with scutes,begins below rays 12～13 of the second dorsal fin,and the highest scute is approximately half the eye diameter;2)The predorsal scales of D.macrosoma do not reach the middle axis of the eye,presenting an“m”shape,whereas the predorsal scaled area of D.macarellus reaches or extends past the middle axis of the eye,taking on a“(?)”shape;3)The posterior end of the maxilla of D.macrosoma is truncated,and the operculum has a straight posterior margin,whereas the posterior end of the maxilla of D.macarellus is convex and round,and the operculum has an oblique posterior margin.Results revealed a large number of misidentified sequences among the homologous cytochrome oxidase(COI)sequences of the two species in the NCBI database and that the genus Decapterus may include cryptic species.(3)As an important fishing area,the resource utilization status of the Indian Ocean directly influences the development of China’s distant fishery.Based on three voyage investigations of the pelagic fishery resources survey and acoustic assessment data,this study estimated fishery resources in the surveyed area of the eastern Indian Ocean.The results showed that total amount of resources at three voyage investigations was 1.77×10~6 t,8.48×10~5 t,3.95×10~5 t respectively,on a declining trend.And the data for S.oualaniensis was 1.53×10~6 t,4.23×10~5 t,2.98×10~5 t,D.macarellus 1.67×10~5 t,1.94×10~5 t,4.99×10~4 t,and C.squamiceps 1.39×10~4 t,1.66×10~4t,8.67×10~3t.On the whole,trend in resources changes of D.macarellus and C.squamiceps was consistent to each other,different from that of S.oualaniensis.Meanwhile,comparison of fishing ground distribution in three seasons indicated that resources distribution in the area surveyed changed seasonally and spatially.(4)Analyzes the effects of oceanographic environment on the distribution of S.oualaniensis and D.macarellus fishing grounds.Based on sampling fish-catch data by the survey and remote sensing and measured environmental data,the study analyzed the effects of oceanographic environment on the distribution of S.oualaniensis and D.macarellus in the surveyed area by using the generalized additive model(GAM).The results showed that the most important environmental factors affecting both S.oualaniensis and D.macarellus are Sea surface height(SSH)and Sea surface height(SST).The optimal GAM model for S.oualaniensis was Log(CPUE)～s(SSH)+s(SST)+s(Chla)+Season,with the AIC value was 117.86,the deviance explained rate was 42.5%,and the R~2 was 0.408.SSH,SST and Chla were all significantly negatively correlated with the catch per unit effort(CPUE)of S.oualaniensis,indicating that the lower the SSH,SST and Chla,the higher the CPUE of S.oualaniensis.The optimal GAM model for D.macarellus was Log(CPUE)～s(SSH)+s(SST)+Season,with the AIC value was155.76,the deviance explained rate was 24.4%,and the R~2 was 0.199.With the increase of SSH,the CPUE of D.macarellus increased at first,then descended,and then increased,with the peaking at around three cm SSH;The CPUE of D.macarellus decreased at first,then increased,and then decreased with the increase of SST,with the peaking at 29°C SST.The results of this study can provide important data support for prediction of the fishing grounds of S.oualaniensis and D.macarellus.(5)Surface-layer ichthyoplankton collected from the eastern Indian Ocean Block one in autumn and spring were identified by combined morphological and molecular(DNA barcoding)techniques,and their species composition,and abundance and its correlations with environmental variables were described.Collections comprised 109larvae and 507 eggs in autumn,which were identified to 37 taxa in 7 orders,20 families,and 27 genera.Collections comprised 249 larvae and 501 eggs in spring,which were identified to 39 taxa in 6 orders,17 families,and 28 genera.Species abundances at each station and of each species were relatively low,suggesting that this area or the time of sampling are not of major importance for spawning of fishes.GAM analysis revealed that in autumn survey high abundance of ichthyoplanktonic taxa occurred in areas with low sea surface height(SSH)and high sea surface salinity(SSS),temperature(SST),and chlorophyll a(Chla)concentration.Among these factors,SSH was most correlated to ichthyoplankton abundance,illustrating that SSH has the greatest influence on the distribution of ichthyoplankton in the surveyed area.The relatively consistent ocean current in spring leads to small changes in SSH.This was reflected in the results that the most important factor affecting ichthyoplankton in spring was SST.Higher water temperature may stimulate fish spawning and accelerate hatching.The species richness and total abundance of larvae and juveniles increase with the rising of water temperature,resulting in species replacement in time series and great increase of food organisms indirectly.However,the selected environmental factors in this study did not well explain the temporal and spatial distribution characteristics of ichthyoplankton in the surveyed sea area,indicating the possible existence of other important influencing factors.(6)Analyzed the annual variation of Katsuwonus pelamis in the Indian Ocean and its response to climate change.K.pelamis is the most productive tuna species in the Indian Ocean.In this study,standardized CPUE data of K.pelamis from 1991 to 2019 were collected along with five climate indexes related to the Indian Ocean,including Indian Ocean dipole(IOD),mean water temperature anomaly in the Eastern Indian Ocean(EIO),mean water temperature anomaly in the Western Indian Ocean(WIO),Madden-Julian Oscillation at 80°E(MJO80)and Arctic Oscillation Index(AOI).The influence of each climate index lagged up to 5 years on the CPUE change of K.pelamis was analyzed using gradient forest analysis(GFA)and GAM model.The results showed that MJO80 and IOD could well explain the interdecadal variation of CPUE of K.pelamis.GFA showed that MJO80 and its lagged 1 year reflected the highest importance of K.pelamis CPUE.The optimal GAM model was obtained by stepwise regression as Log(CPUE)～s(MJO80)+s(AOI＿2)+s(WIO＿2)with the AIC value was-66.015,deviance explained was 81.1%,and a R~2 was 0.706.The results of correlation field analysis of CPUE of K.pelamis and SST showed that there was a significant negative correlation between CPUE and water temperature in a wide range of the western Indian Ocean.The results of this study explain the mechanism of stock dynamic of K.pelamis s in the Indian Ocean,and can provide a reference for prediction of long-term fluctuation of K.pelamis.