Introduction
The set-net fishery is a passive fishing method which installs the set-net fishing gear in a certain area to block the path of fishes moving along the tide using a leading net and to induce them to enter a playground, one or two chamber nets through the slope net. The set-net fishery can be divided into three categories: large-scale set-net fishing, medium-sized set-net fishing and small-sized set-net fishing on the basis of Article 7 of the Enforcement Decree of the Fisheries Act. Also, it has diverse set-net fishing gear types such as one side pound net, two sides pound net, cone shaped pound net, octagonal shaped pound net, and bamboo weirs, which have been adapted by marine circumstances and used in many regions around South Korea (Fishing gear and fishing method, 2019). From 2001 to 2017, the fishing license for set-net in Gyeongsangnam-do took first place (38.0%) followed by Gangwon-do (19.9%) and Gyeongsangbuk-do (17.3%). In the set-net fisheries, anchovy (45.0%) was the most caught species followed by other fishes (22%), Japanese common squid (11.0%), mackerels (7.0%) and mullets (7.0%) on the basis of catch statistical data between 2001 and 2017 (Fisheries statistics, 2019). A number of fish species are caught by set-net, mainly they are migratory species, and the dominant species of set-net fishery differs depending on the season, sea area, and the type and size of the set-net fishing gear. Also, the set-net fishery has a large influence on marine environments. The behaviors of fish entering into the set-net may rely on complex factors such as habitat, water depth, water temperature, salinity, current speed and direction (Kim et al. 1995;Cha et al. 2001;Hwang et al. 2006;Lee et al. 2016). The design and deployed location of a set-net fishing gear would affect the fish behavior. Even though fish enter into the set-net, they often come out. However, its cause and extent are not clearly determined. Therefore, it is necessary to investigate the frequency and behavioral characteristics of fish entering in the set-net. Meanwhile, one of state-of-art acoustic systems is an imaging sonar, which can be used in nearly zero visibility conditions to provide dynamic and precise image. A great amount of research using imaging sonar, which is so called acoustic camera, has been conducted for counting fish and measuring their sizes, studying on fish behavior and spatio-temporal distributional characteristics, and species separation. They were targeted primarily for salmon, jellyfish and other species in freshwater environments, such as rivers, lakes, and fish ladders (Baumgartner et al. 2006;Burwen et al. 2010;Han and Uye, 2009). Many studies have been conducted in marine environments including fish farm (Baumgartner et al. 2006;Becker et al. 2011;Boswell et al. 2008;Han and Uye, 2009;Han et al. 2009;Lee et al. 2010;Makabe et al. 2012).
In this context, by using an imaging sonar, fish behavioral properties in a set-net located off Goseong, South Korea were investigated. This is a preparatory study to support fish behavioral properties in a set-net. It is hoped that more elaborated behavioral information of fishes in the set-net is beneficial for designing and deploying a set-net fishing gear, as well as general fish behavior research in the future.
Material and Methods
The field survey was conducted from at 10:11 on June 4, 2019 to 13:32 on June 6, 2019 at a set-net owned by Sanho susan, Goseong, South Korea (Fig. 1). Strictly speaking, the set-net is a small pound net, however, general term of set-net was used in this study. Acoustic data were recorded using an imaging sonar (Blueview M900, Teledyne Marine, 900 kHz). The specification of the imaging sonar is presented in Table 1. The imaging sonar had 512 beams and the total angle of 90°. The range (vertical) resolution was approximately 2.5 cm. The ping rate was set as 2 pings per second. Three lithium batteries supplied power for a laptop (data acquisition), an imaging sonar head controller (direction of sonar head), and an imaging sonar. The batteries and other instruments were put in two plastic containers. The imaging sonar head controller was attached one side of raft, and set 30° from the water surface to look at the entrance of playground. The angle of the imaging sonar head was set after testing which angle had the best image. The longest length of the playground was 57.7 m, and the length of the chamber net was 43.3 m. The distance between the set-net and land was approximately 177.1 m. The times of sunrise and sunset at Goseong from 4 to 6 June of 2019 were examined using the sunrise and sunset website (Badatime, 2019).
Acoustic data analysis
The data flow for analyzing the imaging sonar data is shown in Fig. 2. For data processing and analysis, ver. 10 Echoview (Echoview Software Pty. Ltd, Australia) was used. The representative echograms in the process of data analysis are described in Fig. 3. The processed data was made by selecting only data between the upper line (3 m) and the lower line (18 m), which resulted in the reduction of the number of samples (Fig. 2b and Fig. 3b). Multi-beam background removal was used for deleting static background objects such as rocks, ropes, and various artifacts. It used a window (41 pings in this study) around a ping to compute a statistic (75% percentile) representing the static background. If the ratio of the sample Sv (volume backscattering strength) value and the background statistic was less than or equal to minimum SNR value (10 dB), the background statistic was subtracted from the Sv echogram sample to remove the background noise (Fig. 2c and Fig. 3c). The median filter was used for image smoothing to enhance the image quality. It was sequenced from minimum to maximum sample values in a 3x3 window, that is 9 samples, and then the median was calculated. The median was determined in the center of the window (Fig. 2d). Multi-beam targets were detected by clustering above-threshold samples to identify and describe target cross-sections which could be individuals or small aggregations (Dunlop et al. 2018, Fig. 2e and Fig. 3d). Target conversion was used to transform the multi-beam targets echogram (range and angle of a multi-beam target) into a single-beam echogram (range and major-axis angle, Fig. 2f). In regard to the target property threshold, it allows a target property to be selected using a minimum and/or maximum threshold value. In this study, the minimum threshold of target length was set as 12 cm, thus all targets less than 12 cm were chosen. A reason for this setting was that anchovy length caught during the survey period was about 1.5-2.0 cm. A single beam angle of the imaging sonar was 0.18°. The footprint at 18 m of range from the transducer was 5.3 cm. Thus, if an individual fish was detected at least two beams at 18 m, approximate 12 cm of footprint would be necessary. As the characteristic of individual fish, each detected multi-beam target delineates various target properties such as target length, target range, target orientation and target major-axis angle. Target length is defined as the longest distance between any two samples of the target. The Target orientation is the angle between the target length derived axis and the line perpendicular to the beam axis. For example, target orientation was 90° which means that a single fish heads up or down perpendicularly. Target range is the mean range (distance from the surface of the transducer to the target) of all samples in each target. The major-axis angle is the angle of target from the beam axis along the major-axis. The positive value of the major-axis angle is to starboard (Echoview, 2019).
Statistical analysis
In order to determine the statistical differences of target (fish) properties, the statistical calculation was performed using the Statistical Package for Social Sciences, ver. 25 SPSS (IBM Corp., USA). The one-way analysis of variance (ANOVA) was applied for four properties such as target length, target range, target orientation, and major-axis angle as dependent variables among the survey dates (4, 5, and 6 June), respectively.
Results
The influx of fish by time
The influx of fish for 4, 5, and 6 June is graphed in Fig. 4. The average times of sunrise and sunset at Goseong for three days was 5:13 and 19:36, respectively, and were marked using vertical dash lines in the figure. The influx of fish was relatively low between sunrise and sunset, but that of fish was high from sunset to sunrise in each day. The influx of fish between sunrise and sunset in 4, 5, and 6 June accounted for 42.6%, 27.3%, and 31.7%, respectively. Those from sunset to sunrise in three days were 57.4%, 72.7%, and 68.3%, respectively. The average influx of fish during day time and night time for 3 days were approximately 33.9% and 66.1%, indicating the fish were entered the set-net mostly at night time.
Target behavioral properties
In 4, 5 and 6 June, the extent of the target range was between 3 m and 15 m and the average target ranges were 6.2 m, 6.3 m, and 6.1 m, respectively. While minimal outliers were seen, the 1st and 3rd quartiles of target length in three days were 11.2 m and 11.8 m, 11.2 m and 11.8 m, and 11.3 m and 11.8 m, respectively (Fig. 5).
The target orientation started 0° and ended 180°. The first quartile and median in 4, 5, and 6 June were 29.1° and 65.8°, 28.2° and 64.2°, and 30.7° and 66.2°, respectively. It seemed that twenty five percentages of targets were oriented between 30° and 60°. Another 25% was oriented from 65° to 140°. It indicated that fish were oriented not toward a certain direction. However they seemed to swim randomly from 0° to 180°. The range of major-axis angle was from -43° to 43°. The average major-axis angle in 4, 5, and 6 June was -3.9°, -4.9°, and -4.9°, respectively. Half of targets were located between -30° to 28°. The range between median and the 3rd quartile was larger than that between the 1st quartile and median, which means that targets were located slightly in right side of the beam axis, that is the right side of the playground based on the location of the imaging sonar (Fig. 6).
Statistical results of target behavioral properties
One-way ANOVA was used to determine statistical difference between three survey dates for the target length, target range, target orientation, and major-axis angle. Since the variances in groups were not met with respect to the Test of Homogeneity of Variances, the Welch's F test was used. The result indicated Welch’s F (2, 54897.8)=12.62 (P < 0.05) for target length, Welch’s F (2, 54742.0)=39.71 (P < 0.05) for target range, a Welch’s F (2, 54521.4)=10.34 (P < 0.05) for target orientation, and F (2, 55022.8)=3.21 (P < 0.05) for angle major-axis, which means that at least one group was different from other groups. According to the Tukey HSD’s post hoc test, the target length at 4 and 6 June and 5 and 6 June differed significantly (P < 0.05), but not for 4 and 5 June (P > 0.05). The target range among all survey dates was significantly different (P < 0.05). In case of target orientation, the significant difference was found only at 5 and 6 June (P < 0.05). Major-axis angle was not significantly different at 4 and 5 June (P > 0.05), yet it was different in other dates (P < 0.05). In addition, the dominant species more than 96% was confirmed as anchovy from Sanho susan.
Discussion
Up to date, study on set-net fishery in South Korea can be categorized into three groups such as fishing gear, fisheries resources, and fish behavior. On the basis of fishing gear aspect, there were a number researches such as the model experiment on holding power of the anchors of a set-net (Yoon et al. 2001), the shape and tension test of the set-net model based on current (Kim et al. 2004), and the movement and volume variation of the set-net using pingers and wireless underwater position system (Hwang and Shin, 2003). In terms of fisheries resources, researches using set-net catch result were done for examining the fishery fluctuation with environmental factors, the composition and diversification of caught species (Cha et al. 2001;Hwang et al. 2006). With regard to fish behavior aspect, several researches were done. For example a scanning sonar was used to examine the behavior of fish school in a set-net (Kim et al. 1995), a pinger was attached to a 30 cm long amber fish to measure the swimming track and speed in a set-net (Shin and Lee, 1999), and the entering and escaping of fish in a set-net with consideration of current speed was investigated using a multi-beam imaging sonar (Lee et al. 2016). In the study using the multi-beam imaging sonar (Lee et al. 2016), the fish seemed to have been aggregated although individual fish inside the fish aggregation was separated using single target detection algorithm and fish tracking technique. This study did not show any fish aggregation which can be assumed that overall fish abundance entering the set-net was relatively low compared to the other study. Yet both studies agreed that entering fish into a set-net at day time was much lower than that in night time. In fact, it was a limitation of this acoustic equipment that the target length looked larger than the actual fish size due to beam spreading effect. When targeting small fish, a higher resolution imaging sonar would be required.
In the present study, the imaging sonar was installed to conduct the continuous acoustic imaging in order to monitor and investigate the flux patterns of the anchovy as well as the fish behavioral properties within the set-net in Goseong. The high frequency for imagery production enables effectively continuous monitoring to track fast-moving individual fish in turbid condition (Premke et al. 2003;Rose et al. 2005). The wide field of view (90°) with the optimum range of 2-60 m allows the system to monitor the wide area around set-net. A reason for a lower influx of fish during day time than night time in all experimental days may be that fish might tend to avoid the net due to clear visibility during day time. Regarding the statistical difference of fish behaviors between dates, the detection range of the imaging sonar was relatively small yet the variety of fish range, orientation and major-axis angle were observed. They seemed to be distributed in the playground quite randomly. At this stage it was difficult to know which elements caused the variety. Diverse environmental data would be required which is one of future tasks.
Various studies on set-net has been conducted, however lack research has been conducted to comprehensively integrate data in relation to set-net and to quantitatively interpret and visualize the relationship. In addition, despite the reliability research of fish behavior on set-net needs to be enhanced, it is necessary to conduct regular surveys over a longer period of time to understand seasonal behavior pattern on fish around set-net. It is a preparatory study on fish behavioral properties in a set-net, however it is hoped that more research in consideration with the above-mentioned issues would be performed.
