CN117875550A - A method for assessing fish resources in shallow water fisheries - Google Patents

Abstract

The invention discloses a method for evaluating the fish resource quantity of a water body in shallow water fishery, which relates to the technical field of fishery acoustics.

Description

A method for assessing fish resources in shallow water fisheries

Technical Field

The invention relates to the technical field of fishery acoustics, and in particular to a method for evaluating the amount of fish resources in a shallow water fishery body.

Background technique

Among common media such as sound, light, and electromagnetism, ultrasound is the only effective medium that can propagate over long distances in water. Therefore, the use of hydroacoustic methods to observe and study fish and other living organisms or water environment characteristics in water bodies is a widely used technical means. In existing applications, the cruise detection method using a hydroacoustic transducer pointing vertically to the bottom of the water is widely used in the assessment of fish resources in the ocean and deep lakes and reservoirs, and can achieve good assessment results. However, in shallow (water depth between 2 and 5 meters) lakes, reservoirs, ponds, and ditches, the vertical cruise detection method has a very small sampled water volume due to the influence of the near-field effect, and its applicability is relatively poor.

However, in shallow lakes, reservoirs, ponds, ditches and other aquaculture and fishery waters, the evaluation of fish behavior, density, size and resource quantity is of great significance for the scientific management of the aquaculture process, precise feeding, disease prevention and control, and benefit prediction, which can significantly improve production efficiency and benefits. At present, the traditional evaluation methods for fish resources in such shallow waters mainly include biological sampling and optical observation, which have problems such as damage, high cost, limited observation range and low efficiency. The hydroacoustic method has outstanding advantages such as non-destructiveness, accuracy and efficiency. Among them, the horizontal hydroacoustic method has a good application effect in shallow water systems, but its technical requirements are quite different from those of vertical hydroacoustics. There is an urgent need for an accurate evaluation method for fish resources in shallow waters.

Summary of the invention

The purpose of the present invention is to provide a method for assessing the amount of fish resources in shallow water fishery water bodies, so as to solve the problems existing in the above-mentioned prior art and enable the amount of fish resources in shallow water aquaculture fishery water bodies to be accurately assessed.

To achieve the above object, the present invention provides the following solutions:

The present invention provides a method for assessing the amount of fish resources in a shallow water fishery body, which specifically comprises the following steps:

S1, design the sampling route, determine the trajectory, number of sections and spacing of the assessment route according to the coefficient of variation of the resource assessment results that can be accepted within the estimated assessment requirements;

S2, collect acoustic data, use the horizontal acoustic detection method to conduct navigation acoustic survey sampling according to the designed sampling route;

S3, determining the density status, determining whether the fish density distribution status is in a discrete distribution state, when it is determined that the target is in a discrete distribution state, the fish target intensity observed in situ is used for fish body length conversion;

S4, analyze the approach-avoidance effect, use the theoretical model to correct the sampling bias caused by the fish approach-avoidance effect, and estimate the actual average density of fish;

S5, determine the acoustic target intensity-body length conversion equation, obtain the equation using field measurements, or directly use the existing empirical formula;

S6, estimate the size and biomass, use the empirical formula of horizontal acoustic target intensity-body length and the body length-weight relationship to calculate the average size of fish, and use the cross-section method to estimate the total biomass in the assessed waters.

Preferably, in S1, the sampling route adopts a square waveform or a triangular waveform route, and the spacing between adjacent parallel routes depends on the requirements of the assessment time and assessment accuracy; to ensure the assessment accuracy, the coefficient of variation of the resource assessment results acceptable within the assessment requirements is estimated according to the following formula, so as to determine the number and spacing of the sections of the assessment route:

Where, CV is the coefficient of variation of the resource assessment results that the assessment requires to be acceptable, that is, the ratio of the standard deviation to the mean value; Λ is the monitoring coverage;

Where D is the length of the evaluation route, in meters; A is the area of the evaluation area, in square meters. According to the requirements, CV is less than 0.25 and Λ is greater than 4, and then the length of the evaluation route is determined. Then, the number of sections and spacing of the evaluation route are calculated based on the area of the evaluation area.

Preferably, in S2, two transducers are symmetrically arranged on both sides of the ship's side, the transducers are immersed in water to a depth of at least 0.5m, and the horizontal directions of the transducers are opposite and perpendicular to the direction of the ship's travel; before the evaluation begins, the scientific fish finder system is acoustically calibrated using the standard sphere method with reference to the instrument operation manual, and the calibration of the split-beam scientific fish finder system includes the system transceiver signal gain and beam parameter calibration.

Preferably, in S2, during the acoustic data underway sampling process, the set sampling parameters need to remain consistent, the speed is lower than 5km/h, the pulse length is set according to the distribution characteristics and water depth of the assessment object, and the data collection and recording range is set according to the water depth of the assessment water area; the pulse length selects a short pulse less than 0.2ms, and the data collection range is greater than 10m.

Preferably, in S3, when the fish cluster distribution in the assessment water area leads to too high density, an acoustic shading effect will be generated, resulting in a nonlinear relationship between the volume scattering intensity and the fish density, which in turn leads to analysis errors. The Nv value is calculated using the following formula (3) to determine whether the target is in a discrete distribution state:

Where, c is the speed of sound, in meters per second; τ is the pulse length, in seconds; Ψ is the decibel representation of the effective beam solid angle, in decibels; R is the distance between the target and the transducer surface, in meters; ρ _v is the volume fish density, ind./m ³ , which is calculated by the following formula (4);

Where, s _v — volume scattering coefficient, unit is m ² /m ³ ; σ _bs — backscattering area, unit is square meter, S _v — volume scattering intensity, unit is decibel, obtained from the acoustic data post-processing software in S2; TS — fish target intensity, unit is decibel, obtained from the acoustic data post-processing software in S2;

When Nv<0.1, the target is in a discrete distribution state, and the in situ observed fish target intensity is used to convert the fish body length; when Nv>0.1, the target is in a non-discrete distribution state, and the relevant analysis units or areas need to be excluded from the analysis.

Preferably, during the acoustic sampling survey, due to the avoidance behavior of fish towards detection activities, the spatial distribution density of fish will deviate from the normal state. It is necessary to use a theoretical model to correct the deviation according to the fish density distribution to obtain the correct fish density value; first, the water layer is divided symmetrically on both sides with the acoustic sampling track as the center, starting from 2m away from the track, and several water layers are divided with a spacing of 1m. It is analyzed whether the fish density in each water layer is uniform or there is a density gradient. When the fish distribution in each water layer is uniform, it is confirmed that the fish avoidance effect is not obvious, and the fish biomass in the water area is directly estimated using the density in S3.

Preferably, when there is a density gradient in the distribution of fish in each water layer, the following theoretical model is used to correct the deviation caused by the sampling process and estimate the actual average density of fish:

Wherein, d is the distance from the water layer to the transducer, in meters; _δad is the acoustic density obtained in the water layer corresponding to the distance, in ind./km ² ; l is the total number of divided water layers, a positive integer; _δmax is the maximum density recorded in each water layer, in ind./km ² ;

Therefore, the actual average density of fish in the water body δe is:

δe＝δa/P (6)

Preferably, the field measurement method for determining the acoustic target intensity-fish body length conversion equation in S5 is as follows:

First, a net cage with an opening on the top is made. The net is made of polyethylene monofilament and the mesh size is 1cm-2cm. A float is installed on the top of the net cage and a sinker is installed on the bottom, so that the net cage can float on the water surface and has a regular square shape after being stretched. According to the biological characteristics of the fish to be studied, the appropriate sample specifications and quantity are determined, and samples are taken at equal intervals between 10cm and the maximum recorded body length of the fish. The body length interval is between 2cm-20cm, and the number of samples is between 10 and 30. The acoustic target intensity of fish samples of different specifications is measured horizontally from the side of the net cage, and the number of clear and identifiable single target echo signals obtained from the measurement of each sample should be more than 300. A regression equation between the horizontal acoustic target intensity of fish and the body length of the fish is established, and the standard form of the equation is: TS=20.0×log(BL)-b20, where b20 is a constant.

The existing empirical formulas for determining the acoustic target intensity-body length conversion equation include: the conversion formula for the horizontal acoustic target intensity-fish body length of the common freshwater farmed fish carp is:

TS = 24.63 × log (SL) - 93.97, the operating frequency of the fish finder is 200kHz;

TS = 23.61 × log (SL) - 93.78, the working frequency of the fish finder is 430kHz,

Among them, SL is the standard body length of fish.

Preferably, according to the horizontal acoustic target intensity-fish body length conversion equation determined in step S5, the fish body length is converted from the target intensity, and then the fish body weight can be converted according to the fish body length and weight relationship:

W＝a×Lb (7)

Where, W is the weight of the fish, in grams; L is the length of the fish, in millimeters; a, b are constants, obtained through measurement and regression analysis of fish samples, or using existing empirical values for the research object;

Then, the total fish biomass in the water area is calculated based on the fish density obtained in S4 and the water volume of the assessment water area.

Preferably, if the water body is small and the fish distribution density is relatively uniform, the total resource volume can be directly estimated from the fish density and water volume obtained in step 4; if the water area is large and the fish distribution density varies greatly, the cross-section method or square area method is required to estimate the fish biomass of each sub-area, and then the total fish biomass of the entire water area is accumulated:

The cross-section method uses the cross-section observation value to represent the average value of the water area between the two half cross-sections on both sides of the cross-section. The sum of the water resources represented by each cross-section is the total resource volume within the monitoring range. Suppose the resource tail number and biomass of the assessed species i in the water area represented by a given cross-section are:

In the formula, —The average integral value of the assessment category i within the section, in square meters/square kilometer;/> —Average acoustic scattering cross section of evaluation category i within the section, in square meters; D —Section length, in kilometers; S —Section spacing, in kilometers; /> - mean body weight of assessment category i, in grams;

The square area method divides the entire monitoring area into several small square areas, and the calculation is performed with the square area as the unit. The sum of the resource quantity in each area is the total resource quantity in the monitoring area. Suppose the resource tail number and biomass of the assessed species i in the waters represented by a given square area are:

In the formula, —The average integral value of assessment category i in the area, in square meters/square kilometer;/> —The average acoustic scattering cross section of the assessment type i in the square area, in square meters; A—the water area represented by the square area, in square kilometers; /> - the mean body weight of assessment species i, in grams.

Compared with the prior art, the present invention has achieved the following technical effects:

The method for assessing the amount of fish resources in shallow water fishery water bodies of the present invention can be applied to various types of shallow water fishery waters with a water depth of 2 to 5 m, including lakes, reservoirs, ponds, ditches, etc., and can assess all fish distributed above the bottom of the water body. It is particularly suitable for rapid assessment of the amount of fish resources in shallow water aquaculture and aquaculture fishery waters, and has the outstanding advantages of being non-destructive, accurate, and efficient.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

FIG1 is a schematic diagram of a route of a method for assessing fish resources in a shallow-water fishery water body according to an embodiment of the present invention;

FIG2 is another route schematic diagram of the method for assessing fish resources in shallow water fisheries according to an embodiment of the present invention;

FIG3 is a schematic diagram showing the principle of the step of dividing water layers in the approach-avoidance effect analysis according to an embodiment of the present invention;

FIG4 is a schematic diagram showing the principle of a step of measuring the intensity of a horizontal acoustic target in an embodiment of the present invention;

Among them: 1-transducer, 2-net cage, 3-float, 4-sinker.

Detailed ways

The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

The purpose of the present invention is to provide a method for assessing the amount of fish resources in shallow water fishery water bodies, so as to solve the problems existing in the prior art and enable accurate assessment of the amount of fish resources in shallow water aquaculture fishery water bodies.

In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

As shown in Figures 1 to 4: This embodiment provides a method for assessing the amount of fish resources in a shallow water fishery body, which specifically includes the following steps:

S1, design the sampling route, determine the trajectory, number of sections and spacing of the assessment route according to the coefficient of variation of the resource assessment results that can be accepted within the estimated assessment requirements;

S2, collect acoustic data, use the horizontal acoustic detection method to conduct navigation acoustic survey sampling according to the designed sampling route;

S3, determining the density status, determining whether the fish density distribution status is in a discrete distribution state, when it is determined that the target is in a discrete distribution state, the fish target intensity observed in situ is used for fish body length conversion;

S4, analyze the approach-avoidance effect, use the theoretical model to correct the sampling bias caused by the fish approach-avoidance effect, and estimate the actual average density of fish;

S5, determine the acoustic target intensity-body length conversion equation, obtain the equation using field measurements, or directly use the existing empirical formula;

S6, estimate the size and biomass, use the empirical formula of horizontal acoustic target intensity-body length and the body length-weight relationship to calculate the average size of fish, and use the cross-section method to estimate the total biomass in the assessed waters.

As an optional solution, in S1 of this embodiment, the sampling route adopts a square wave or triangular wave route, and the spacing between adjacent parallel routes depends on the requirements of the assessment time and assessment accuracy; the cross-section spacing of the sampling route adopting a triangular wave route is half of the spacing between the starting and ending points of a broken line. To ensure the assessment accuracy, the coefficient of variation of the resource assessment results that can be accepted within the assessment requirements is estimated according to the following formula, so as to determine the number and spacing of the sections of the assessment route:

Where, CV is the coefficient of variation of the resource assessment results that the assessment requires to be acceptable, that is, the ratio of the standard deviation to the mean value; Λ is the monitoring coverage;

In the formula, D is the length of the assessment route, in meters; A is the area of the assessment area, in square meters; according to the requirements, CV is less than 0.25 and Λ is greater than 4, and then the length of the assessment route is determined, and then the number of sections and spacing of the assessment route are calculated from the area of the assessment area. Generally, according to the requirement that Λ is greater than 4, the length of the assessment sample line must be greater than 400m in 1 hectare of water.

As an optional solution, in S2 of this embodiment, two transducers 1 are symmetrically arranged on both sides of the ship's side, the depth of the transducer 1 immersed in water is at least 0.5m, and the horizontal directions of the transducer 1 are opposite and perpendicular to the direction of the ship's travel; before the evaluation begins, the scientific fish finder system is acoustically calibrated using the standard sphere method with reference to the instrument operation manual, and the calibration of the split-beam scientific fish finder system includes the system transceiver signal gain and beam parameter calibration.

As an optional solution, in S2 of this embodiment, during the acoustic data cruise sampling process, the set sampling parameters must remain consistent, the speed is less than 5km/h, the pulse length is set according to the distribution characteristics and water depth of the evaluation object, and the data collection and recording range is set according to the water depth of the evaluation waters; the pulse length selects a short pulse less than 0.2ms, and the data collection range is greater than 10m. The data collection range is the range of data collection and recording in the horizontal direction. After the sound wave is emitted in the horizontal direction, it will form a reflected wave when it encounters the fish body. Since the beam shape is conical like a flashlight, the data collection and recording range should not be too small, and should be appropriately larger to increase the volume of the sampled water body; but it cannot be too large, because the expansion of the sound wave beam will encounter the water surface and the bottom of the water to form a reflected wave, thereby interfering with the fish body signal. In the case of a water depth of 2-5m, it is generally recommended that the data collection range is greater than 10m. Of course, if the water body is deeper, the data collection and recording range can be appropriately larger.

As an optional solution, in S3 of this embodiment, when the fish cluster distribution in the evaluation water area leads to too high density, an acoustic shading effect will be generated, resulting in a nonlinear relationship between the volume scattering intensity and the fish density, which in turn leads to analysis errors. The following formula (3) is used to calculate the Nv value to determine whether the target is in a discrete (i.e., non-clustered) distribution state:

Wherein, c is the speed of sound, in meters per second; τ is the pulse length, in seconds; Ψ is the decibel representation of the effective beam solid angle, in decibels; R is the distance between the target and the surface of the transducer 1, in meters; ρ _v is the volume fish density, ind./m ³ , which is calculated by the following formula (4);

Where, s _v — volume scattering coefficient, unit is m ² /m ³ ; σ _bs — backscattering area, unit is square meter, S _v — volume scattering intensity, unit is decibel, obtained from the acoustic data post-processing software in S2; TS — fish target intensity, unit is decibel, obtained from the acoustic data post-processing software in S2;

When Nv<0.1, that is, the distribution density of fish clusters is not high and the targets are in a discrete distribution state, the in-situ observed fish target intensity is used to convert the fish body length; when Nv>0.1, that is, the distribution density of fish clusters is high and the targets are in a non-discrete distribution state, the relevant analysis units or areas need to be excluded from the analysis.

As an optional solution, in the acoustic sampling survey process of this embodiment, due to the avoidance behavior of fish to the detection activity, the spatial distribution density of fish will deviate from the normal state. It is necessary to use a theoretical model to correct the deviation according to the distribution of fish density to obtain the correct fish density value; first, the water layer is divided symmetrically on both sides with the acoustic sampling track as the center, starting from 2m away from the track, and several water layers are divided with a spacing of 1m. Analyze whether the fish density of each water layer is uniform or there is a density gradient. When the fish distribution in each water layer is uniform, it is confirmed that the fish avoidance effect is not obvious, and the fish biomass in the water area is directly estimated by the density in S3. The fish density is analyzed from the fishery sonar acoustic data post-processing software, that is, after the fish finder collects the data, there is a dedicated data analysis software, and the fish density can be obtained through software analysis. By variance analysis, it is judged whether there is a difference in the fish density of each water layer. If there is no difference, it is considered that the fish density is uniform, and the total biomass can be directly calculated based on this mean. On the contrary, if the variance analysis shows that the density of each water layer is different, it is necessary to use formula 5 to calibrate the model and estimate the total biomass by the fish density after the model correction. The density gradient is the density difference. Generally, the water layer close to the transducer has a lower density due to the influence of the approach-avoidance effect, and the farther the water layer is, the less interference it receives, so the density is higher.

As an optional solution, in this embodiment, when there is a density gradient in the distribution of fish in each water layer, the following theoretical model is required to correct the deviation caused by the sampling process and estimate the actual average density of fish:

Wherein, d is the distance from the water layer to the transducer 1, in meters; _δad is the acoustic density obtained in the water layer corresponding to the distance, in ind./km ² , and δα can be obtained from the d value; l is the total number of divided water layers, which is a positive integer; _δmax is the maximum density recorded in each water layer, in ind./km ² ;

Therefore, the actual average density of fish in the water body δe is:

δe＝δa/P (6)

As an optional solution, the field measurement method for determining the acoustic target intensity-fish body length conversion equation in S5 in this embodiment is as follows:

First, make a net box 2 with an opening on the top. The net is made of polyethylene monofilament with a mesh size of 1cm-2cm. A float 3 is installed on the top of the net box 2 and a sinker 4 is installed on the bottom, so that the net box 2 can float on the water surface and has a regular cube shape after stretching. The net box 2 preferably has a length×width×depth of 1.5m×1.5m×1.5m. The float and sinker can be cylinders with holes in the middle for ropes, or they can be spheres. According to the biological characteristics of the studied fish, the appropriate sample size and quantity are determined, and samples are taken at equal intervals between 10 cm and the maximum recorded body length of the fish, and the body length interval is between 2 cm and 20 cm, and the sample quantity is between 10 and 30 fish; the acoustic target intensity of fish samples of different sizes is measured horizontally from the side of the cage 2, and the number of clear and identifiable single target echo signals obtained from the measurement of each sample should be more than 300; a regression equation between the horizontal acoustic target intensity of fish and the body length of the fish is established, and the standard form of the equation is: TS = 20.0 × log (BL) - b20, where b20 is a constant;

The existing empirical formulas for determining the acoustic target intensity-body length conversion equation include: the conversion formula for the horizontal acoustic target intensity-fish body length of the common freshwater farmed fish carp is:

TS = 24.63 × log (SL) - 93.97, the operating frequency of the fish finder is 200kHz;

TS = 23.61 × log (SL) - 93.78, the working frequency of the fish finder is 430kHz,

Among them, SL is the standard body length of fish.

As an optional solution, in this embodiment, according to the horizontal acoustic target intensity-fish body length conversion equation determined in step S5, the fish body length is converted from the target intensity, and then the fish body weight can be converted according to the fish body length and weight relationship:

W＝a×Lb (7)

Where, W is the weight of the fish, in grams; L is the length of the fish, in millimeters; a, b are constants, obtained through measurement and regression analysis of fish samples, or using existing empirical values for the research object;

Then, the total fish biomass in the water area is calculated based on the fish density obtained in S4 and the water volume of the assessment water area.

As an optional solution, in this embodiment, if the water body is small and the fish distribution density is relatively uniform, the total resource amount can be directly estimated by the fish density and water volume obtained in step 4; if the water area is large and the fish distribution density varies greatly, the cross-section method or square area method is required to estimate the fish biomass of each sub-area, and then the total fish biomass of the entire water area is accumulated:

The cross-section method uses the cross-section observation value to represent the average value of the water area between the two half cross-sections on both sides of the cross-section. The sum of the water resources represented by each cross-section is the total resource volume within the monitoring range. Suppose the resource tail number and biomass of the assessed species i in the water area represented by a given cross-section are:

In the formula, —The average integral value of the assessment category i within the section, in square meters/square kilometer;/> —Average acoustic scattering cross section of evaluation category i within the section, in square meters; D —Section length, in kilometers; S —Section spacing, in kilometers; /> - mean body weight of assessment category i, in grams;

The square area method divides the entire monitoring area into several small square areas, and the calculation is performed with the square area as the unit. The sum of the resource quantity in each area is the total resource quantity in the monitoring area. Suppose the resource tail number and biomass of the assessed species i in the waters represented by a given square area are:

In the formula, —The average integral value of assessment category i in the area, in square meters/square kilometer;/> —The average acoustic scattering cross section of the assessment type i in the square area, in square meters; A—the water area represented by the square area, in square kilometers; /> - the mean body weight of assessment species i, in grams.

Take the evaluation of fish resources in a rectangular pond (100m×50m×2m) as an example, with a surface area of 5000m ² and a volume of 10000m ³ , where the main breeding target is largemouth bass:

1) Design the cross-sectional observation sample line length to be 300m so that the monitoring coverage (Λ) is greater than 4;

2) Use horizontal acoustic detection method to conduct underway acoustic survey sampling;

3) The fish density distribution is determined to be Nv<0.1, and the target is determined to be in a discrete distribution state. The in-situ observed fish target intensity can be used to convert the fish body length;

4) The theoretical model was used to correct the sampling bias caused by the fish avoidance effect, and the actual average fish density was estimated to be 4.9×10 ⁶ ind./km ² ;

5) The average size of fish was calculated to be 987.1 g using the empirical formula of horizontal acoustic target intensity-body length and the relationship between body length and weight;

6) The total biomass in the pond was estimated to be 24,700 kg using the cross-section method.

The method for assessing the amount of fish resources in shallow fishery water bodies of this embodiment can be applied to various types of shallow fishery waters with a water depth of 2 to 5 meters, including lakes, reservoirs, ponds, ditches, etc., and can assess all fish distributed above the bottom of the water body. It is particularly suitable for rapid assessment of the amount of fish resources in shallow water aquaculture and aquaculture fishery waters, and has outstanding advantages such as being non-destructive, accurate, and efficient.

The present specification uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the method and core idea of the present invention. At the same time, for those skilled in the art, according to the idea of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims (10)

1. A method for assessing the amount of fish resources in a shallow water fishery, characterized in that it specifically comprises the following steps: S1, design the sampling route, determine the trajectory, number of sections and spacing of the assessment route according to the coefficient of variation of the resource assessment results that can be accepted within the estimated assessment requirements; S2, collect acoustic data, use the horizontal acoustic detection method to conduct navigation acoustic survey sampling according to the designed sampling route; S3, determining the density status, determining whether the fish density distribution status is in a discrete distribution state, when it is determined that the target is in a discrete distribution state, the fish target intensity observed in situ is used for fish body length conversion; S4, analyze the approach-avoidance effect, use the theoretical model to correct the sampling bias caused by the fish approach-avoidance effect, and estimate the actual average density of fish; S5, determine the acoustic target intensity-body length conversion equation, obtain the equation using field measurements, or directly use the existing empirical formula; S6, estimate the size and biomass, use the empirical formula of horizontal acoustic target intensity-body length and the body length-weight relationship to calculate the average size of fish, and use the cross-section method to estimate the total biomass in the assessed waters. 2. The method for assessing the amount of fish resources in shallow water fisheries according to claim 1 is characterized in that: in S1, the sampling route adopts a square waveform or a triangular waveform route, and the spacing between adjacent parallel routes depends on the requirements of the assessment time and assessment accuracy; to ensure the assessment accuracy, the coefficient of variation of the resource assessment results acceptable within the assessment requirements is estimated according to the following formula, so as to determine the number of sections and spacing of the assessment route: Where, CV is the coefficient of variation of the resource assessment results that the assessment requires to be acceptable, that is, the ratio of the standard deviation to the mean value; Λ is the monitoring coverage; Where D is the length of the evaluation route, in meters; A is the area of the evaluation area, in square meters. According to the requirements, CV is less than 0.25 and Λ is greater than 4, and then the length of the evaluation route is determined. Then, the number of sections and spacing of the evaluation route are calculated based on the area of the evaluation area. 3. The method for assessing the amount of fish resources in shallow water fisheries according to claim 1 is characterized in that: in S2, two transducers are symmetrically arranged on both sides of the ship's side, the depth of the transducers immersed in water is at least 0.5m, and the horizontal directions of the transducers are opposite and perpendicular to the direction of the ship's travel; before the assessment begins, the scientific fish finder system is acoustically calibrated using the standard sphere method with reference to the instrument operation manual, and the calibration content of the split-beam scientific fish finder system includes the system transceiver signal gain and beam parameter calibration. 4. The method for assessing fish resources in shallow water fisheries according to claim 1 is characterized in that: in S2, during the acoustic data cruise sampling process, the set sampling parameters must remain consistent, the speed is lower than 5km/h, the pulse length is set according to the distribution characteristics and water depth of the assessment object, and the data collection and recording range is set according to the water depth of the assessment water area; the pulse length selects a short pulse less than 0.2ms, and the data collection range is greater than 10m. 5. The method for assessing fish resources in shallow water fisheries according to claim 1 is characterized in that: in S3, when the fish cluster distribution in the assessment water area leads to too high density, an acoustic shading effect will be generated, resulting in a nonlinear relationship between the volume scattering intensity and the fish density, which in turn leads to analysis errors, and the Nv value is calculated using the following formula (3) to determine whether the target is in a discrete distribution state: Where, c is the speed of sound, in meters per second; τ is the pulse length, in seconds; Ψ is the decibel representation of the effective beam solid angle, in decibels; R is the distance between the target and the transducer surface, in meters; ρ _v is the volume fish density, ind./m ³ , which is calculated by the following formula (4); Wherein, s _v — volume scattering coefficient, in m ² /m ³ ; σ _bs — backscattering area, in square meters; S _v — volume scattering intensity, in decibels, obtained from the acoustic data post-processing software in S2; TS — fish target intensity, in decibels, obtained from the acoustic data post-processing software in S2; When Nv<0.1, the target is in a discrete distribution state, and the in situ observed fish target intensity is used to convert the fish body length; when Nv>0.1, the target is in a non-discrete distribution state, and the relevant analysis units or areas need to be excluded from the analysis. 6. The method for assessing fish resources in shallow water fisheries according to claim 1 is characterized in that: during the acoustic sampling survey, due to the avoidance behavior of fish towards detection activities, the spatial distribution density of fish will deviate from the normal state, and it is necessary to use a theoretical model to correct the deviation according to the fish density distribution to obtain the correct fish density value; first, the water layer is divided symmetrically on both sides with the acoustic sampling track as the center, starting from 2m away from the track, and several water layers are divided with a spacing of 1m, and it is analyzed whether the fish density in each water layer is uniform or there is a density gradient. When the fish distribution in each water layer is uniform, it is confirmed that the fish avoidance effect is not obvious, and the fish biomass in the water area is directly estimated by the density in S3. 7. The method for assessing the amount of fish resources in shallow water fisheries according to claim 6 is characterized in that: when there is a density gradient in the distribution of fish in each water layer, the following theoretical model is used to correct the deviation caused by the sampling process and estimate the actual average density of fish: Wherein, d is the distance from the water layer to the transducer, in meters; _δad is the acoustic density obtained in the water layer corresponding to the distance, in ind./km ² ; l is the total number of divided water layers, a positive integer; _δmax is the maximum density recorded in each water layer, in ind./km ² ; Therefore, the actual average density of fish in the water body δe is: δe＝δa/P (6). 8. The method for assessing fish resources in shallow water fisheries according to claim 1, characterized in that: the on-site measurement method for determining the acoustic target intensity-fish body length conversion equation in S5 is as follows: First, a net cage with an opening on the top is made. The net is made of polyethylene monofilament and the mesh size is 1cm-2cm. A float is installed on the top of the net cage and a sinker is installed on the bottom, so that the net cage can float on the water surface and has a regular square shape after being stretched. According to the biological characteristics of the fish to be studied, the appropriate sample specifications and quantity are determined, and samples are taken at equal intervals between 10cm and the maximum recorded body length of the fish. The body length interval is between 2cm-20cm, and the number of samples is between 10 and 30. The acoustic target intensity of fish samples of different specifications is measured horizontally from the side of the net cage, and the number of clear and identifiable single target echo signals obtained from the measurement of each sample should be more than 300. A regression equation between the horizontal acoustic target intensity of fish and the body length of the fish is established, and the standard form of the equation is: TS=20.0×log(BL)-b20, where b20 is a constant. The existing empirical formulas for determining the acoustic target intensity-body length conversion equation include: the conversion formula for the horizontal acoustic target intensity-fish body length of the common freshwater farmed fish carp is: TS = 24.63 × log (SL) - 93.97, the operating frequency of the fish finder is 200kHz; TS = 23.61 × log (SL) - 93.78, the working frequency of the fish finder is 430kHz, Among them, SL is the standard body length of fish. 9. The method for assessing the amount of fish resources in a shallow water fishery according to claim 1 is characterized in that: according to the horizontal acoustic target intensity-fish body length conversion equation determined in step S5, the fish body length is converted from the target intensity, and then the fish body weight can be converted according to the fish body length and weight relationship: W＝a×Lb (7) Where, W is the weight of the fish, in grams; L is the length of the fish, in millimeters; a, b are constants, obtained through measurement and regression analysis of fish samples, or using existing empirical values for the research object; Then, the total fish biomass in the water area is calculated based on the fish density obtained in S4 and the water volume of the assessment water area. 10. The method for assessing fish resources in shallow water fisheries according to claim 1, characterized in that: If the water body is small and the fish distribution density is relatively uniform, the total resource volume can be directly estimated from the fish density and water volume obtained in step 4; if the water area is large and the fish distribution density varies greatly, the cross-section method or square area method is needed to estimate the fish biomass of each sub-area, and then the total fish biomass of the entire water area is accumulated: The cross-section method uses the cross-section observation value to represent the average value of the water area between the two half cross-sections on both sides of the cross-section. The sum of the water resources represented by each cross-section is the total resource volume within the monitoring range. Suppose the resource tail number and biomass of the assessed species i in the water area represented by a given cross-section are: In the formula, —The average integral value of the assessment category i within the section, in square meters/square kilometer;/> —mean acoustic scattering cross section of the evaluation category i within the section, in square meters; D—section length, in kilometers; S—section spacing, in kilometers; /> - mean body weight of assessment category i, in grams; The square area method divides the entire monitoring area into several small square areas, and the calculation is performed with the square area as the unit. The sum of the resource quantity in each area is the total resource quantity in the monitoring area. Suppose the resource tail number and biomass of the assessed species i in the waters represented by a given square area are: In the formula, —The average integral value of assessment category i in the area, in square meters/square kilometer;/> —The average acoustic scattering cross section of the assessment type i in the square area, in square meters; A—the water area represented by the square area, in square kilometers; /> - the mean body weight of assessment species i, in grams.