# The Survey of Fishery Resources and Spatial Distribution Using DIDSON Imaging Sonar Data

Wei Shen, Long Yang, Jin Zhang, Guangxiong Peng

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Wei Shen, Long Yang, Jin Zhang, Guangxiong Peng. The Survey of Fishery Resources and Spatial Distribution Using DIDSON Imaging Sonar Data. 6th Computer and Computing Technologies in Agriculture (CCTA), Oct 2012, Zhangjiajie, China. pp.366-375, 10.1007/978-3-642-36124-1_44 hal-01348120

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
The Survey of Fishery Resources and Spatial
Distribution Using DIDSON Imaging Sonar Data

Wei Shen ¹, long Yang ², Jin Zhang¹,*, Guangxiong Peng ³

1. College of Marine Sciences, Shanghai Ocean University, Shanghai, 201306, China;

2. The First Institute of Oceanography, SOA, Qingdao, 266061, China;

3. School of Geosciences and Info-Physics, Central South University, Changsha, 410083,
China

**Abstract:** In recent years, the DIDSON (Dual-frequency IDentification SONar), which can provide almost-video-quality images to identify objects even in turbid water has been used in enumerating fish populations, underwater structures inspection, oil/gas leakage detection and identification, underwater security, evidence searching, ship's hulls and ports safety inspection, and underwater navigation and so on.

In this paper, a designed vessel-mounted observing systems collected DIDSON data and GPS data according to designed lines in the Dishui Lake, shanghai city. Every line area was calculated according to the depth and sonar open-angle. Trained observers counted manually each fish image in every DIDSON file. Then based on the area and number of fish observed, the average density and zoning density of the Dishui lake was calculated. At last the whole fish number and distribution was calculated based on the ARCGIS software.

The practice proved the method based on DIDSON Data is very feasible, effective and accurate for fishery resources estimating.

**Keywords:** DIDSON; Survey; Fishery Resources; Spatial Distribution

<footnote>*Corresponding author, e-mail: j_zhang@shou.edu.cn</footnote>

<footnote>Supported by the Innovation Program of Shanghai Municipal Education Commission(12ZZ159), and China Postdoctoral Science Foundation (2011M501296, 2012T50832).</footnote>
# 1 Introduction

The Dishui Lake of Shanghai Lingang Town was completed on 2003. The area of whole lake is 5.56 square kilometers, the average depth is 3.7 m, and the most deep depth is 6.2 m. Since 2007, the management of the lake has put about 315 ton fry into the lake so far, the fry were mainly silver carp and bighead carp. At present, how much fish in the lake is a question. So the lakes management decides to detect the fishery resources for the further catching and management.

In the water, sonar equipment is the main observation and investigation equipment[1]. Acoustic fisheries resources survey is fast and efficient investigation of biological resources, and no damage for fisheries[1-3].

Currently, the main advanced acoustic instruments for fishery resources survey are Simrad EY-60[4], Bisonics DT-X[5], and etc.. These instruments generally utilize echo integration method[6-8] to assess the total amount of resources. When using these acoustic instruments, firstly the target strength of fish should be acquired, and then the standardized calibration should be implemented. Usually the acoustic image displayed on the screen are not dynamic, the accuracy of the fish counting is not very high. In addition, these methods also need to set the various parameters of fish, which will lead to significant differences on the identifying result[4].

This project utilized DIDSON (Dual-Frequency Identification Sonar) to detect the Dishui lake fishery resources, and acquired three days first-hand data. Based on echo counting method[9], the number of fish was counted from DIDSON image. Then the average density and zoning density of fish were calculated. At last, the total amount was calculated and the fish spatial distribution was plotted on a map based on ARCGIS software.

# 2 DIDSON

DIDSON (Dual-frequency IDentification SONar), which is invented by Washington University, can provide almost-video-quality images to identify objects even in turbid water. DIDSON has been used in fisheries management, underwater structures inspection, oil/gas leakage detection and identification, underwater security, evidence searching, ship's hulls and ports safety inspection, and underwater navigation and sonar imaging.
No other sonar in its class is able to deliver the image quality and frame rate of the DIDSON. The resulting real-time, near video-quality data is clear enough to study the behavior of darting fish, even in opaque waters.

A defacto standard for the exacting marine biology audience, this dual frequency so
nar is also ideal for construction inspection, pipe laying observation, hull and berth sw
eeps and other mission critical jobs down to 300 meters.

The DIDSON is versatile, too, with a 1.1 MHz detection frequency capable of imag
ing targets up to 30 meters away.Stored images can be converted to AVI files and JPE
Gs, and a size and distance tool is included, making analysis accurate and presentation
easy. [9]

<figure><img src="image_2.png" /><figcaption>Fig.1. Standard DIDSON (Left: main body, Right: acoustic lens)</figcaption></figure>

3 Fishery Resources Detection and Assessment

**3.1 Area and Methods**

The three-day detection almost covered whole Dishui Lakes, each designed survey
line spacing was 50 m. Due to driving, weather conditions, underwater fishing nets
and barriers, the actual survey routes spacing was about 100 m as shown in Fig. 2.
<figure><img src="image_3.png" /><figcaption>Fig. 2. The actual detecting route in Dishui lake, shanghai, China</figcaption></figure>

The detection system(Fig.3) comprises the DIDSON transducer, a set-top control box, a data cable, control software, GPS and an associated laptop computer. The DIDSON transducer is directly connected to the set-top box, which is linked to the laptop via an ethernet connection. The acoustics image is transferred from the unit to the laptop via the control software, which displays the data as a streaming image. The DIDSON Image can then be directly viewed or saved on computer. The GPS positioning information is transferred into the navigation software and the actual survey routes are recorded (Fig.2). The practices improved the DIDSON transducer should be installed under water 0.5m and the lens angle 45°is best.
<figure><img src="image_4.png" /><figcaption>Fig.3. Measurement of vessel and equipment installation</figcaption></figure>

## 3.2 Fish counting automatically based on DIDSON image

Using DIDSON image (Fig.4) and semi-automated counting software[10,11] developed ourself, the fish number of every line was counted and the location was recorded[12,13]. The fish length was calculated based on the pixel value of identified fish in the image. At last, The number checked via manual review would be saved to table ( Table 1) for late statistical analysis.

<figure><img src="image_5.png" /><figcaption>Fig.4. Fish Identification and Tracking</figcaption></figure>

<table><caption>**Table 1.** The fish count table from DIDSON image</caption><thead><tr><td>N</td><td>E</td><td>Local X</td><td>Local Y</td><td>Num</td><td>Len(cm)</td></tr></thead><tbody><tr><td>30.90368</td><td>121.9308</td><td>-36690.4516</td><td>44345.14614</td><td>1</td><td>96</td></tr><tr><td>30.90346</td><td>121.9308</td><td>-36714.39585</td><td>44347.71281</td><td>2</td><td>60</td></tr><tr><td>30.90338</td><td>121.9308</td><td>-36723.94869</td><td>44348.47933</td><td>2</td><td>72</td></tr></tbody></table>
## 3.3 Catch statistics

By the actual measurement data from the Dishui Lake management Company, the fish species composition, body length, weight, and other relevant data were acquired. Based on caught 41 silver carp, the relationship between weight and length (Fig.5) was established using the power function regression method.

<figure><img src="image_6.png" /><figcaption>Fig.5. Relationship between full-length and weight of silver carp (R2 is fitting degree, n is the number of samples)</figcaption></figure>

The power function is

$$W = 0.00006L^{2.5865} \quad (1)$$

## 3.4 Fishery Resources Quantitative Assessment Methods

The average density method and zoning density method are used in fish resources quantitative assessment in this paper.

### 3.4.1 The Average Density Method

The whole lake fish quantity value N is defined as
$$N = \rho.S \qquad (2)$$

where $\rho$ is fish average density, S is the whole lake area. The value $\rho$ is calculated by every line fish density $\rho_i$.

$$\rho = \frac{1}{n} \sum_{i=1}^{n} \rho_i \qquad (3)$$

The $\rho_i$ is calculated by the fish number and the area of line $i$.

$$\rho_i = \frac{N_i}{S_i} \qquad (4)$$

Where the value $S_i$ is defined as

$$S_i = L_i \times \frac{R_i}{2} = \frac{1}{2} \sum_{n=1}^{n} r_n l_n \qquad (5)$$

Where the $L_i$ is length of line $i$ and the $R_i$ is the slant-range from the transducer to the water bottom. The $r_n$ is changed by the water bottom terrain from time 1 to n seconds. The $l_n$ is computed by GPS positioning data.

Based on these methods above, the length, area, fish number, fish density of each survey line are calculated.

<table><caption>**Table 2.** The detection data of each survey line</caption><thead><tr><th>Line</th><th>Length(m)</th><th>Area(m2)</th><th>fish number</th><th>fish density (num/m2)</th></tr></thead><tbody><tr><td>No1</td><td>3109</td><td></td><td></td><td></td></tr><tr><td>No2</td><td>4561</td><td>15206.1</td><td>485</td><td>0.031895095</td></tr><tr><td>No3</td><td>1871</td><td>5697.7</td><td>385</td><td>0.067571125</td></tr></tbody></table>
<table><tr><td>No4</td><td>3740</td><td>13552.6</td><td>1038</td><td>0.07659047</td></tr><tr><td>No5</td><td>3828</td><td>12001.1</td><td>1152</td><td>0.095991201</td></tr><tr><td>No6</td><td>3907</td><td></td><td></td><td></td></tr><tr><td>No7</td><td>4605</td><td>17333.9</td><td>1198</td><td>0.069113125</td></tr><tr><td>No8</td><td>6703</td><td>21239.2</td><td>482</td><td>0.022693887</td></tr><tr><td>No9</td><td>1827</td><td>3451.6</td><td>269</td><td>0.077934871</td></tr><tr><td>No10</td><td>962</td><td>3167.7</td><td>190</td><td>0.059980427</td></tr><tr><td>No11</td><td>7312</td><td>22139.6</td><td>865</td><td>0.039070263</td></tr><tr><td>No12</td><td>3901</td><td>12206.9</td><td>823</td><td>0.067420885</td></tr></table>

The whole lake fish density $\rho$ is 0.0608261349 via formula 3, and the Dishui lake area is 5.56 sq.km so the whole lake fish number $N$ is 338193.

By the site statistics data, the mean size of Dishui lake fish is 64.26cm. So the fish average weight is 2.847kg, and the whole lake fish gross is 962835kg.

### 3.4.2 The Zoning Density Method

The fish average density method ignored the number difference of different regional, and it could not express the actual distribution of the fish, and it's accuracy was affected by the survey lines distribution. So the zoning density method was designed to improve the count accuracy.

The value K1 (num/m²) was designed to express the fish density of every survey point.

$$K1 = \frac{n}{1 \cdot d} \qquad (6)$$

Where value n is the detected fish number, value l is the distance between two survey points, value d is the DIDSON scan width.

The value K2 (kg/m²) was designed to express the fish grass of every survey point.

$$K2 = \frac{nW}{1 \cdot d} = \frac{0.00006n}{1 \cdot d} \cdot L^{2.5865} \qquad (7)$$

Where value L (cm) is length of fish, W (kg) is the fish weight.
Based on formula 6 & 7, the value K1 and K2 was calculated.

Then using Inverse Distance Weighted (IDW) interpolation within ARCGIS, the
whole lake fish density were processed as shown Fig.6.

<figure><img src="image_7.png" /><figcaption>Fig. 6. Map of K1 zoning density (unit: num/m2, grid resolution 10m)</figcaption></figure>

Based on the K1 Grid data, the whole Dishui lake fish number was calculated via
ARCGIS Surface Volume tool. And the whole number is 345253.

Based on the K2 Grid data, the whole Dishui lake fish gross was calculated via
ARCGIS Surface Volume tool. And the whole fish gross is 939885kg.

3.5 Fishery Resources Spatial Distribution

In order to understand the fish spatial distribution of Dishui lake, the pie charts of
detected fish number were plotted on a map based on the counted fish number of
every points. From the map, the Dishui lake fish spatial location are expressed
visually (Fig. 7). Actually the Figure 6 expresse the fish spatial distribution also.
<figure><img src="image_8.png" /><figcaption>Fig. 7. Map of Dishui lake fish distribution</figcaption></figure>

# 4 Analysis and Discussion

## 4.1 Summary and Analysis

In this paper, two kinds of assessment methods were used to calculate the total fish amount in Dishui lake. With average density method, the Dishui lake fish number is 338193 and the fish gross is 962835kg. And the number is 345253 via zoning density method, and the fish gross is 939885kg.

The assessment value by two methods is close, and the fish fry data supplied by Dishui lake management department was analyzed to prove that the two methods are effective. Overall, the average density method is simple, easy to operate. The zoning density method needs a large amount of calculation and requires the assist by GIS software.

Practice proved that the average method is simple and effective, can be directly applied to the assessment of fishery resources. The zoning density method witch can
visually analyse fish distribution and the amount of the different regions needs lots of
equally distributed detection data to interpolate the whole area data.

In comparison with the lake depth map, the fish were mainly concentrated in two
deep-water areas and several estuaries. Because the main fish silver carp like warm
area in winter, so they lied in deep water and lakebed. In addition, the bait was rich in
estuary, so they foraged there.

## 4.2 Question and Discussion

Due to technical and capacity constraints, there are several deficiencies need for
improvement.

In this detection, the fish species relatively simple and mainly concentrated in the
bottom of the lake, so we used a flat density. But in many cases, different fish
distributed in different water depth. So the 3D fish density in order to more accurately
measure the 3D distribution will be survey in the next study.

Sonar instruments have certain blind spots, for example DIDSON has a big noise
away from the lens 1 meter, where the fish can not be quantified. So how to solve this
question is a key point for the next research.

When using the acoustic instruments to survey fishery resources underwater, due to
the movement of the boat and the noise of boat motor, part of the fish will be driven.
So how to decide the drive coefficient is very difficult but necessary, and the current
research is still very scarce.

Applying DIDSON sonar to detect and assess fishery resources in China has just
begun, but the practices proved that it could be used effectively in large-waters
fishery resources detection and assessment.

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