| The diversity of freshwater fish in Yangtze River watershed has a place in the world freshwater fish field.At present,due to the influence of human activities,including water resources and hydropower engineering,overfishing,and the effect of channel,wharf and other projects,thus the habitat of fish in watershed shows fragmentation status;fish biodiversity drops continuously,and the fish under threat have increased in the past ten years,which need close attention and protection.In addition,in recent years some fish resource investigation results show that the fish in the Yangtze River continue to show smaller,younger-age trend;fishery resources also have decline trend,and fish species distribution has changed as well.Therefore,it is extremely essential to investigate the species,size and behavior and other living situation of fish species in the Yangtze River basin.Traditional fishing method is available to obtain the biological information of the fish samples,buy the research on the behavior of the fish needs to use the instrument for long time observation.Fishery hydroacoustics technology has advantages in the observation of fixed points in natural water bodies,and it has little interference on the observation object,wide observation scope,and it is less influenced by the environmental than that of photology and other equipment.In the process of inland water application,this method has a key factor to identify signals of different targets and accurately measure TS(Target Strength)of different target signals,and currently the hot spot and difficulty in need of solution.Subject to the harsh experimental conditions,domestic studies carry out few TS measurement on main economic fish in the Yangtze River,which can not meet the need of fishery acoustic survey.In order to accumulate working experience for freshwater fishery resources acoustic assessment in China and provide a model for other freshwater fish acoustic researches,this study will use the laboratory measurement method and the theoretical model calculation method,to measure target strength of fishes with different swimbladder chamber structures in the Yangtze River,to analyze how the target strength be influenced by the fish species,the fish body type,the internal structure of fish(number and size of swimbladder chamber),incident angle of acoustic wave,and incident frequency of acoustic wave,to obtain fish biological parameter correlativity among body length,body weight and target strength.Later,the acoustic method is adopted for fixed-point monitoring in natural water bodies,to obtain the fish quantity and distribution in different regions and different time periods.Meanwhile the measured target strength and biological parameter correlativity is available for conversion of acoustic signal and fish body length.The results are as follows:1.This research has firstly conducted ex-situ experimental method to measure the TS of freshwater fish in the Yangtze River.From June to October 2015,the Bio Sonics DT-X(199 k Hz)scientific echosounder is used in a small tank to emit horizontal acoustic wave,and has measured the target strength of 8 species up to 48 freshwater fishes with different swimbladder chamber structures,including single chambered swimbladder fish: Pelteobagrus vachelli,Siniperca chuatsi,Silurus meridionalis;dual chambered swimbladder fish : Carassius auratus,Ctenopharyngodon idellus);three chambered swimbladder fish: Megalobrama amblycephala,Erythroculter ilishaeformis;special swimbladder structure: Channa argus.The measured direction includes fish body side and dorsal-ventral(180° range),and x-ray images are taken to more detailedly understand the structure and characteristics of the fish swimbladder.The results show that fish target strength of fish with single swimbladder chamber has cos function relationship with incident angle of acoustic wave.The maximum target strength of both body lateral and dorsal-ventral aspect appears when the fish body spindle is perpendicular to the incident wave,and the minimum target strength appears when the craniocaudal direction is along the acoustic wave,which is 10 d B less than that of maximum.The average target strength of body side is 0.5 to 5d B more than that of dorsal-ventral.The target strength of fish with single chambered swimbladder is smaller than that of the fish with similar body length and dual chambered swimbladder.Four species of fish with comparatively accurate actual measurement have been selected for the linear regression of target strength and fish biological parameter,to obtain the linear regression equation under different angles.The linear regress equation for the average TS and body length of body lateral under all angle is respectively: TS_Pelteobagrus vachelli =20.4log(SL)–69.1(R2=0.90),TS_Siniperca chuatsi =35.9log(SL)–87.9(R2=0.71),TS_Carassius auratus=22.8log(SL)–66.6(R2=0.82),TS_Ctenopharyngodon idellus =20.9log(SL)–67.3(R2=0.88).The linear regress equation for the average TS and body length of dorsal-ventral completely is respectively: TS_Pelteobagrus vachelli =26.7log(SL)–76.8(R2=0.95),TS_Siniperca chuatsi =27.2log(SL)–75.8(R2=0.83),TS_Carassius auratus =20.7log(SL)–65.7(R2=0.74),TS_Ctenopharyngodon idellus =12.6log(SL)–54.8(R2=0.69).In addition,the regress equation of other angles are also listed in this paper.2.The acoustic model of fish is used to describe the TS status of actual fish body in numerical form.The samples fish after the experiment will be anesthetized and x-ray images of fish body lateral and dorsal-ventral will be filmed.Then the internal structure of fish shown by X-ray image will be measured,such as measuring swimbladder length,height,and width,which is used to establish PSMS(prolate-spheroid modal-series model).Select 23 fishes in the 4 species measured in the sink to calculate TS of body lateral and dorsal-ventral aspect.The obtained maximum TS mean reaches-36.8±5.5d B,which differs 2.9±3.9 d B from body lateral results in the sink and 1.0±3.8 d B from dorsal-ventral results.The TS mean calculated by the model is-50.1±2.6d B,which differs 11.0±3.6d B from body lateral results in the sink and 10.4±3.5d B from dorsal-ventral results.PSMS has favourable simulation effect on the TS of single swimbladder fish,and there will be extremum if beyond simulation scope.Later,it will be subsection based on X image.At the fish head,a coordinate system will be built to measure the coordinate point value on each fish part and swimbladder.The KRM(Kirchhoff-ray approximation model)will be adopted to get change status of single swimbladder fish under 38,70,200 k Hz and fish pose obliquity of-100~270 degrees(incident wave parallels the fish head),which reflect how the fish with different swimbladder chambers respond to acoustic wave scattering.The following conclusions can be seen:(1)For the fish with same swimbladder chamber,the TS on body lateral of Pelteobagrus vachelli is smaller than that of dorsal-ventral,while the TS on body lateral of other fish is greater than that of dorsal-ventral.This conclusion is consistent with the result measured in the sink,showing that the TS is influenced by the projected area of swimbladder in different directions.(2)For the fish with different body length at the same frequency,the fish body TS with bigger swimbladder is larger.(3)TS changes with the tilt angle,whose main wave crest is obvious at 38 k Hz,and the angle for dorsal-ventral maximum is similar to the included angle between swimbladder and fish body axis.The fish with dual chambered swimbladder show double peak characteristics.When the audio frequency increases,the angle characteristics of TS maximum is unconspicuous,and not sensitive to the change of dip angle,showing the characteristic of multi wave crest.(4)The maximum and mean TS increase with the increase of frequency.The measured results in the sink have been chosen to compare with model results,the difference between the measured value and the model value of body lateral is-2.50±2.35 d B,while the difference between the measured value and the model value of dorsal-ventral is-5.02±3.00 d B,so the results of the two groups have significant differences.The two groups of data results will go through linear regression,to get linear regress equation for the average TS and body length of body lateral measured in the sink TST=22.05log(SL)-67.96(r2=0.73),the linear regression equation for the average TS and body length of dorsal-ventral is TST=18.63log(SL)-63.97(r2=0.73).The regression equation for the average TS and body length of lateral calculated by the model is TSM=25.58log(SL)-70.30(r2=0.82),while the linear regress equation for the average TS and body length of dorsal-ventral is TSM=32.21log(SL)-77.04(r2=0.93).Later,the actual measurement and model calculation formula will be used for the conversion of fish signal length in fixed-point monitoring.3.The fisheries acoustic technology has more advantages in natural waters monitoring.In addition to measuring the types of sediment,water depth information and other environment information,the scientific echosounder with different frequency can not only obtain the fish resources,spatial distribution,activity patterns and other key information within the waters,but also helps to identify the fish body size,species.From 00:00 to 24:00 in April 19,2011 monitoring points are set up as a try in the part from Luzhou to Hejiang where is the rare fish gathered section in the upstream of the Yangtze River and the junction of the rare and unique fish National Nature Reserve core zone and the buffer area,in which by using scientific echosounder EY60(frequency 200 k Hz)and high resolution imaging sonar DIDSON(long distance,the working frequency 0.7,1.2MHz),sentinel surveillance in the horizontal direction is taken,and meanwhile,in Luzhou-Hejiang section fishing operations are carried out,so as to compare the acoustic detection results and net catching data at the monitoring points to make an analysis.The monitoring data of 24 hours shows that DIDSON one day gets 192 fish individual signals,which could be converted to the average length of fish for 18.7±5.6 cm,average speed 0.19±0.13m/s,while the EY60 in the same period catches 157 fish individual signals whose average TS value is-43.8±4.4 d B,and with the use of the Pelteobagrus vachelli formula of body length TS=20.4log(SL)-69.1 measured in the sink,the converted body length is 20.2±14.8cm(8.7–118.9cm)and average fish swimming speed is 0.11±0.06 m/s.After carrying out a speed analysis on the signals detected by DIDSON in a day and night every 6 hours,it is found that the signal speed between 06:00-12:00 is low,but through the single factor variance analysis between each group,there are no significant differences between the four groups of data.The catches data shows that the number in 74.7km around Luzhou-Hejiang reach is 167 in total including 11 species of fish,which mainly contains Pelteobagrus vachelli,Coreius guichenoti,Rhinogobio ventralis,whose percentage are 32.2%,30.5%,26.3%,average total length is 22.9±4.4cm(10.5–36cm),average the body length 19.1±3.9cm(8.5–31.5cm)and average weight 136.9±64.5g(13–464g).The Pelteobagrus vachelli conversion formula using the flume to make actual measurement,compared with other fish formula,has improved the conversion accuracy,but the conversion range is larger.The results demonstrate that the DIDSON conversion result is more accurate.4.The river bed material is an important physical part of the biological habitat of the river,and it is the carrier of the natural reproduction of the bottom spawning fish.In order to better understand the characteristics of the fish activities in the river below the Gezhouba Dam,first in the area under Gezhouba Dam where the Chinese sturgeon spawn ground,the substrate type of its bed material is monitored,and in November 2015,by using Biosonics DT-X(210k Hz)scientific echosounder and Conan G16 camera,the river substrate types from the Gezhouba Dam section to Xiaoting are surveyed and analyzed.The monitoring section should be set every 300 meters,and three sample points on the left,middle and right of each section according to the width of the river.The roughness and hardness indexes are obtained through the analysis on the first and second echo of the bed material collected by the echo system;the bed material composition and layout state are observed by underwater video data to obtain its apparent embeddedness data,so as to analyze bed material distribution.Embeddedness and hardness are in square relation which is E = 0.15H2 + 2.04 H + 5.48,R2= 0.719,the relation between roughness and grain size is in D = 0.03R2 + 2.21R+ 52.9,R2=0.574.Understood the type of bed material,the use of synchronous observation method by means of scientific monitoring of echosounder and high resolution imaging sonar,is to realize the fixed-point monitoring on the fish behavior.From November 2015 to January 2016 in the Yichang fishery quay,Biosonics DT-X(210k Hz)scientific echosounder and high resolution imaging sonar DIDSON(long range,frequency 0.7,1.2MHz)are used to monitor in the horizontal direction,and meanwhile,in Gezhouba Dam-Yiling Yangtze River bridge section fishing operation is carried out using three-layered drift nets.The acoustic detection results and net catching data at monitoring points are taken to make an analysis.Combined with daily weather,water temperature and hydrological factors,the relationship between the ecological environment,the type of river bed material and the activity and distribution of fish is analyzed.DIDSON monitoring data shows that the fish signal occurs mostly between 18:00 and 06:00 and occasionally between 13:00 and 16:00,while the signal seldom appear between 08:00 and 12:00.A total of 508 fish signals appear in the fair current,whose average length is 29.1±5.4cm(between 14.3 and 37.1cm),average swimming speed 0.16±0.05m/s,distance from the detecting device 11.2±0.76m;a total of 286 fish signals are found in the countercurrent,average length 30.5±6.9cm(13.9–39cm),average swimming speed 0.18±0.07m/s,the average distance from the instrument 11.7±1.0m.DT-X monitoring data shows that the fish signal basically appears after 18:00,and the average is TS=-36 d B,average swimming speed 1.01m/s.Combined with the above measured results and model in this paper,TS-L conversion formulas: TST=22.05log(SL)-67.96,TSM=25.58log(SL)-70.3 are respectively obtained,and through these two conversion formulas,the fish body length of-36 d B are 28 cm,22cm,which is close to average body length of the catches.As the water temperature,water level and flow varies greatly in November,the DIDSON counting data and fish by-catch data from November 11 to December 9th,2015 are selected to conduct analysis work,and the daily number of a number of Coreius heterodon,Carassius auratus,Pelteobagrus vachelli,Rhinogobio cylindricus which are the mainly component number are selected for statistics,which shows that the fish number of the upstream and downstream is negatively related to water level and flow and the correlation coefficients are-0.40,-0.39(downstream);-0.46,-0.47(upstream),and the number of two kinds of fish is not affected by water temperature.The number of Coreius heterodon is negatively related to the water level and flow,and the correlation coefficients both are-0.31;the number of Siniperca chuatsi is positively related to the three elements,and the correlation coefficients are 0.56,0.77,0.80(water temperature,water level,flow);the number of Pelteobagrus vachelli is not affected by the water flow factor,while the number of Rhinogobio cylindricus is negatively correlated with the three elements and the correlation coefficients are-0.51,-0.46,-0.47(water temperature,water level,flow).Combined with the environmental factors and fish living habits analysis,DIDSON can realize the identification of Coreius heterodon,and the length of the conversion is similar to the net capture data.Based on the analysis of the echo characteristics distinctions of fishes with different swimbladder chambered,it helps to identify different types of fish signals,which makes the acoustic method able to be targeted to obtain individual fish size and regular behavior of different waters and their spatial distribution information.This study conducts a TS measurement and calculation on the fishes of the Yangtze River in the horizontal direction by using the experimental method and model method for the first time,and this dissertation also realized the synchronized use of imaging sonar and scientific echosounder for the first time to have a horizontal fixed-point monitoring on different regions of the Yangtze River.The obtained fish TS-L formula is applied to the TS and the number of fish,activity data of the natural water horizontal fixed-point monitoring.Combined with environmental factors,this dissertation tries to analyze the kinds and the length of fish signals spotted by the horizontal monitoring to achieve the purpose of identifying species,accumulate efficient basic data for the use of acoustic methods for monitoring and evaluating the fishery resources in the Yangtze River and play an exemplary role in vigorously carrying out the acoustic survey work in the Yangtze River in the future. |
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