A Three–Year Comparison of Fluctuations in the Occurrence of the Giant Jellyfish (Nemopilema nomurai)

Abstract

In this study, acoustic, sighting, trawl, and marine environmental surveys were used to determine the vertical distribution and density of giant jellyfish that have been observed in Korean waters over the past 3 years. From 2020 to 2022, annual surveys were conducted in May and July in the East China Sea and waters adjacent to South Korea. The acoustic data were processed by identifying and eliminating all signals considered as noise while excluding those suspected to be jellyfish signals. Subsequently, a single target detection method was employed. Giant jellyfish are distributed mostly in the middle and low layers. In May 2021, the average population density of giant jellyfish was recorded as 11.6 (ind./ha), which was the highest density. In July 2022, this value decreased to 1.7 (ind./ha), marking the lowest density. The sighting survey, which allows for the identification of jellyfish distributed in the surface layer, exhibited a difference of approximately 0.13 times compared to the acoustic survey conducted in the middle and low layers in 2020. In 2021 and 2022, this difference was approximately 0.11 times and 0.24 times, respectively. The average of this difference was 0.16 times or greater.

1. Introduction

Zooplankton are classified according to their size. Nemopilema nomurai, which belongs to the Scyphozoa class and Rhizostomae order, is a jellyfish species classified as macroplankton of size 200 μm or larger. It is found in the waters near Korea around May. N. nomurai is one of the giant jellyfish species found in the East China Sea, South Sea, and East Sea and is known to grow up to a maximum size of 2 m [1]. In particular, since the 2000s, there has been a significant increase in the occurrence of giant jellyfish, resulting in severe adverse effects in the fishing and marine industries [2,3,4,5]. Furthermore, it is known that the increase in the population of giant jellyfish results in a decrease in the production of zooplankton [6]. The increase in giant jellyfish numbers may be attributed to various factors such as overfishing, climate change, and environmental pollution. Therefore, it is necessary to explore countermeasures to protect marine resources in order to address the problems caused by mass production.

To address these issues, it is necessary to gather various information such as the migration routes and physiological and ecological characteristics of jellyfish. The methods primarily used to determine the distribution and migration routes of jellyfish include visual inspection and trawl surveys with the use of fishing gear for collection. The advantage of these methods is that they can be used to quickly assess the abundance of jellyfish. However, their disadvantage is that they can only be used to assess a limited area; difficulties exist in determining the distribution of giant jellyfish that migrate in different layers [5,7].

The advantage of acoustic survey methods, however, is that they can be used to survey large areas relatively quickly and information regarding marine organisms distributed throughout the water column can be assessed rapidly [8]. To determine the distribution of marine organisms underwater using sound, it is necessary to obtain information regarding the target strength (TS) [5,9]. There is ongoing research comprising the analysis of the swimming behavior and ecological characteristics of various marine organisms and research on the TS for multiple frequencies is consistently being conducted.

In this study, acoustic, sighting, trawl, and marine environmental surveys were thus used to determine the vertical distribution and density of giant jellyfish that have been observed in Korean waters over the past 3 years.

2. Materials and Methods

2.1. Acoustic Data Collections

In this study, annual surveys were conducted in the East China Sea and the adjacent seas of Korea from 2020 to 2022, specifically in the months of May and July. In May, the focus was on understanding the influx of N. nomurai, which is not native to Korea, through the Kuroshio Current. In July, the investigation aimed to determine the distribution and abundance of the jellyfish that had already been introduced. The objective was to analyze acoustic data to determine the vertical distribution and density of giant jellyfish entering Korea (Figure 1,

Table 1. Survey schedule.

Year	May		July
	Survey Period	Research Vessel	Survey Period	Research Vessel
2020	15–28 May	Tamgu 8st	1–16 July	Tamgu 21st
2021	17–27 May	Tamgu 21st	29 June–11 July	Tamgu 20st
2022	28 May–5 June	Tamgu 21st	1–12 July	Tamgu 20st

). The acoustic data were collected in conjunction with the scientific research vessel “Tamgu,”, which is affiliated with the National Institute of Fisheries Science, during a precision survey conducted by the institute. The data were collected along with marine environmental data.

In 2020, an acoustic survey was conducted using a split beam–type echosounder (EK–60, Simrad, Kongsberg Maritime AS, Norway) with frequencies of 38 and 120 kHz installed on the Tamgu 8st (R/V, 282 G/T) vessel, as it did not have its own acoustic equipment. Additional surveys were conducted using split beam–type echosounders (EK–80, Simrad, Kongsberg Maritime AS, Norway) attached to the Tamgu 20st (R/V, 885 G/T) and Tamgu 21st (R/V, 999 G/T) vessels, belonging to the National Institute of Fisheries Science. In these surveys, acoustic data were collected at the frequencies of 18, 38, 70, 120, 200, and 333 kHz.

2.2. Collecting Giant Jellyfish Samples

The midwater trawl fishing gear was used to produce and employ a specialized net for harvesting jellyfish. The jellyfish–specific harvesting net consists of a head rope (HR) and ground rope (GR), each measuring 28.2 m. The mesh size was 30.0 mm. Furthermore, a dedicated kite measuring 2.4 m in width and 1.0 m in height was created to increase the overall power of the jellyfish harvesting net (Figure 2). This kite was used to deploy the entrance of the net (Figure 3). During the trawl survey, the vessel maintained a speed of 2–3 knots and collected samples of N. nomurai. In all the surveys conducted, except for those in May 2020, a depth sensor (SS4 Depth Sensor, Scanmar AS, Asgardstrand, Norway) and door sensor (SS4 DoorSensor, Scanmar AS, Asgardstrand, Norway) were attached to the entrance of the jellyfish harvesting net to monitor the net width and mesh condition in real time. In addition, the N. nomurai collected using the specialized jellyfish harvesting net were measured to obtain the umbrella diameter (in cm) and number of individuals.

2.3. Acoustic Data Analysis

The acoustic data collected from the field were returned to the laboratory for further analysis using the acoustic post–processing software (Echoview V 8.0, Echoview Software Pty Ltd., Hobart, Australia). When collecting acoustic data, the impulse noise that was collected in the field was eliminated to separate the acoustic signals of the jellyfish. Frequencies of 38 and 120 kHz were used to extract the acoustic signals of the jellyfish.

First, the impulse noise was eliminated by setting exclusion thresholds for the data processing of the sea surface and seabed, thereby excluding unnecessary portions in the data processing procedure of the noise reduction algorithm. The smoothing process was then performed. This process involved averaging the values within the specified vertical sample range and if the resulting value exceeded the set threshold, it was identified as a candidate for noise. Therefore, in this study, the threshold value was set as −150 dB and values identified as potential noise were discriminated as either noise or biological echo using the two–sided comparison method used in [10]. The parameters for eliminating impulse noise are listed in

Table 2. Setting parameters for eliminating impulse noise.

Parameters	Values
Exclude above	Surface
Exclude below	Bottom
Exclude below threshold (dB at 1 m)	−150
Vertical window units	Samples
Vertical window size (samples)	5
Horizontal size (pings)	5
Threshold (dB)	10
Noise sample replacement value	Mean

.

To separate the acoustic signals of jellyfish, it is necessary to understand the frequency characteristics and differences in the frequencies 38 and 120 kHz. The frequency difference is the difference in the mean volume backscatter intensity (MVBS) at multiple frequencies. On comparing the TS by frequency, we can represent the target species we want to separate by subtracting the frequency of the larger value from that of the smaller value. The large N. nomurai exhibits a higher acoustic scattering intensity in the high–frequency range rather than the low–frequency range.

When extracting the acoustic signals of giant jellyfish using the frequency difference method, we applied the previously researched acoustic scattering intensity values [11]. Furthermore, the frequency range was determined based on the size of the giant jellyfish collected in the field via the trawl survey. The frequency range for the sound investigation is listed in

Table 3. dB–difference applied to acoustic analysis.

∆MVBS120–38
May 2020	Minimum range	−5.84
	Maximum range	14.05
May 2021	Minimum range	−0.22
	Maximum range	13.94
May 2022	Minimum range	4.84
	Maximum range	17.44

. On excluding signals suspected to be jellyfish signals, all the remaining signals were identified as noise and eliminated. The single target detection method was then employed. This method comprises an algorithm–based approach that is applied using various algorithms to detect and identify targets that move independently for each object. The parameter values for extracting only the acoustic signals of jellyfish are listed in

Table 4. Parameters applied to single target detection.

Parameter	Value
TS threshold (dB)	−65
Pulse length determination level (dB)	6.0
Minimum normalized pulse length (ms)	7.0
Maximum normalized pulse length (ms)	1.5
Maximum beam compensation (dB)	2.0
Minor axis angles (degrees)	0.4
Major axis angles (degrees)	0.4

.

To determine the density of giant jellyfish using acoustic surveys, the distribution density of giant jellyfish detected via the echosounder was calculated using Equation (1). Furthermore, Equation (2) was used to represent the density to compare the results of the acoustic investigation and sighting survey.

$$Count of individual jellyfish ÷ Acoustic survey area \left(ind./ha\right)$$

(1)

$$Count of individual jellyfish ÷ Sighting survey area \left(ind./ha\right)$$

(2)

3. Results and Discussion

3.1. Distribution of Giant Jellyfish by Water Column

The distribution of giant jellyfish by depth is presented in Figure 4 using an echosounder. In May 2020, the giant jellyfish were distributed throughout the water column. However, a total of 1673 individuals were detected, primarily at the approximate depth of 30–50 m. Furthermore, jellyfish detected at depths of 30–50 m accounted for 70% (1169 individuals) of the total population. In July 2020, a total of 3795 giant jellyfish were detected, with 93% (3523 individuals) of the total number detected in the 20–100–m depth range. In May 2021, a total of 3458 giant jellyfish were detected, with 87% (3012 individuals) of the total population found in the 30–70 m depth range. A total of 2255 giant jellyfish were detected in July 2021. The giant jellyfish distribution was primarily observed in the depth range of 30–90 m and accounted for 85% of the total detected individuals with a count of 1927 jellyfish found at the main distribution depth. In May 2022, a total of 2473 giant jellyfish were detected, primarily distributed in the approximate range of 30–60 m. The jellyfish population detected at the main distribution depths accounted for approximately 85% (2903 individuals) of the total. In July 2022, a total of 982 giant jellyfish were detected, primarily distributed in the low layer. The number of jellyfish detected at that depth was 733, accounting for approximately 75% of the total. The giant jellyfish is distributed throughout the water column; however, in May, it is primarily found in the middle layer, while in July, its range of vertical migration expands compared to that in May, thus causing it to be distributed in both the middle and low layers.

In May 2020, the temperature and salinity of the surveyed area ranged from 14.1 to 18.0 °C and from 32.1 to 34.3 psu, respectively. Specifically, in the vicinity of the predominant depth range of 30–50 m, the temperature ranged from 14.1 to 15.2 °C and the salinity ranged from 32.6 to 33.2 psu. In the July survey area, the range of water temperature and salinity was observed to be between 15.4 and 21.5 °C and 32.1 and 34.6 psu, respectively. The water temperature and salinity range in which giant jellyfish primarily swim were observed to be 15.4–20.8 °C and 32.3–34.4 psu, respectively. The water temperature and salinity measured in May 2021 ranged from 13.9 to 18.2 °C and from 32.3 to 34.5 psu, respectively. The temperature and salinity in the water column at approximately the main floating depth of 30–70 m were observed to be 13.9–15.3 °C and 32.8–34.4 psu. The temperature and salinity ranges in the July survey area were measured to be 14.8–23.7 °C and 31.7–34.4 psu, respectively. The marine environment at the main distribution depths was measured as 14.8–18.7 °C and 33.0–34.3 psu. In May 2022, the survey area exhibited a water temperature range of 15.1–21.0 °C and a salinity range of 31.8–34.1 psu. The water temperature and salinity at the main distribution depths were 15.2–19.1 °C and 31.8–32.9 psu, respectively. In the July survey area, the ranges of water temperature and salinity were observed to be 14.2–26.3 °C and 30.1–34.4 psu, respectively. The predominant marine environment in terms of water depth exhibited a water temperature range of 14.2–15.1 °C and a salinity range of 33.6–34.2 psu (Figure 5).

3.2. Bell Diameter Distribution of Giant Jellyfish Collected over 3 Years

The specimens of giant jellyfish were collected using a specialized jellyfish harvesting net. In May 2020, the giant jellyfish had a minimum, minimum, and average size of 3.0 cm, 60.0 cm, and 16.5 cm, respectively. In July, the giant jellyfish size ranged from a minimum of 25 cm to a maximum of 100 cm, with an average of 56.6 cm. In May 2021, the giant jellyfish size ranged from a minimum of 7.0 cm to a maximum of 59.0 cm, with an average size of 20.5 cm. In July, the giant jellyfish size ranged from a minimum of 15 cm to a maximum of 92 cm, with an average of 43.8 cm. Furthermore, in 2022, samples of giant jellyfish were collected with a minimum, maximum, and average size of 15 cm, 100 cm, and 20.5 cm, respectively. In July, samples were collected with a minimum, maximum, and average size of 33 cm, 150 cm, and 81.0 cm, respectively. In May and June, jellyfish of an average size of approximately 20 cm and 50 cm, respectively, were observed in the distribution. However, in July 2022, the size of jellyfish that were observed to be distributed was on average 80 cm, which is more than 1.5 times larger than those collected in the previous July (

Table 5. Bell diameter distribution of giant jellyfish collected over 3 years.

Period		2020		2021		2022
	Diameter (cm)	May	July	May	July	May	July
Minimum		3	25	7	15	15	33
Maximum		60	100	59	92	100	150
Average		16.5	56.6	20.5	43.8	20.5	81.0

).

3.3. Horizontal Distribution of Giant Jellyfish

The horizontal distributions of giant jellyfish and large jellyfish, as determined via acoustic and sighting surveys, are presented in Figure 6 and Figure 7, respectively. The giant jellyfish exhibited a high–density distribution primarily in the western vicinity of the survey area but exhibited a low distribution overall. The sighting survey revealed a high–density distribution in the surveyed area, excluding the western vicinity.

The distribution density results of the giant jellyfish detected using the scientific–species detection device are listed in

Table 6. Density obtained from acoustic survey results over 3 years.

Acoustic Survey
Station	2020		2021		2022
	May	July	May	July	May	July
	Density (ind./ha)	Density (ind./ha)	Density (ind./ha)	Density (ind./ha)	Density (ind./ha)	Density (ind./ha)
1	4.2	1.1	24.5	2.7	5.2	–
2	1.8	0.8	7.2	2.7	9.1	0.0
3	3.9	2.1	5.5	7.6	9.2	3.7
4	5.1	3.3	21.6	–	8.7	3.2
5	9.9	0.1	7.5	3.2	4.4	2.5
6	9.2	3.9	21.5	1.8	0.4	3.4
7	5.1	2.5	10.0	0.9	4.7	0.2
8	9.5	10.3	18.5	2.5	14.1	0.9
9	4.6	7.4	14.1	6.8	7.7	1.2
10	0.5	4.5	7.7	5.3	6.4	4.2
11	1.8	5.6	6.0	3.9	4.0	0.7
12	18.5	2.6	10.8	5.3	6.7	2.3
13	11.4	11.0	21.1	3.8	3.1	4.0
14	12.1	18.5	10.4	6.3	11.4	0.3
15	0.5	–	12.7	8.4	7.2	0.7
16	7.9	–	1.8	–	12.5	3.5
17	26.2	11.7	8.2	7.5	8.6	3.0
18	5.3	6.7	5.6	4.3	15.8	1.6
19	6.4	4.3	20.6	11.8	17.0	0.6
20	0.9	5.9	6.8	21.5	5.8	0.1
21	0.5	12.0	1.8	–	5.8	0.7
22	–	–	–	–	–	–
23	–	2.6	–	6.2	–	3.5
24	–	34.9	–	2.4	–	0.7
25	–	10.8	–	8.8	–	0.0
26	–	12.6	–	5.9	–	0.4
Avg.	6.9	7.6	11.6	5.9	8.0	1.7

. In the table, the “–” symbol indicates that no data are available because the survey was not conducted. In May 2021, the average population density of giant jellyfish was recorded as 11.6 (ind./ha), which represented the highest density. In July 2022, this value decreased to 1.7 (ind./ha), representing the lowest density. The distribution density results for giant jellyfish sightings recorded using the sighting survey are listed in

Table 7. Density obtained from sighting survey results over 3 years.

Sighting Survey
Station	2020		2021		2022
	May	July	May	July	May	July
	Density (ind./ha)	Density (ind./ha)	Density (ind./ha)	Density (ind./ha)	Density (ind./ha)	Density (ind./ha)
1	27.2	93.9	2.1	84.0	0.5	–
2	0.3	55.5	0.0	0.3	1.0	3.3
3	0.0	58.8	0.0	254.0	2.8	0.0
4	0.0	0.0	0.0	–	16.1	0.0
5	0.5	74.3	0.0	412.0	0.0	0.6
6	320.7	22.3	230.0	1056.0	0.0	1.7
7	0.0	31.6	0.0	624.8	47.1	49.1
8	0.0	0.6	0.0	0.0	3.6	0.0
9	14.9	16.5	0.0	3.0	0.0	0.0
10	0.0	53.5	0.4	0.0	0.0	0.0
11	0.2	777.1	30.9	205.0	0.5	2.3
12	14.0	805.6	357.5	69.0	80.5	0.0
13	3.5	3.3	0.0	2.0	10.6	0.0
14	7.2	0.0	0.0	0.0	23.9	7.5
15	0.0	–	0.6	0.0	0.0	0.0
16	1.0	–	0.0	–	5.3	0.0
17	0.6	32.3	83.5	7.0	607.2	0.3
18	0.0	0.9	10.0	31.0	0.0	0.0
19	0.7	2.4	0.4	0.0	0.0	0.3
20	0.0	1.4	0.0	0.0	0.2	0.0
21	1.4	0.0	0.0	–	0.0	0.0
22	–	–	–	–	–	–
23	–	15.7	–	0.6	–	0.0
24	–	26.1	–	0.3	–	0.0
25	–	6.0	–	0.0	–	0.0
26	–	0.0	–	0.0	–	0.2
Total/Average	18.7	90.3	34.1	125.0	38.1	2.7

. In the table, the “–” symbol indicates that no data are available because the survey was not conducted. The average population density of giant jellyfish reached the highest level in July 2021, with a value of 125.0 (ind./ha). In contrast, in July 2022, it exhibited the lowest population density of 2.7 (ind./ha). The giant jellyfish exhibited the highest distribution density among all the surveys in 2021 and a gradual decrease in the distribution density in 2022.

The distribution density of the middle and low layers relative to the surface layer over a period of 3 years is listed in

Table 8. Distribution of mid–low water column densities relative to the surface over 3 years.

Station	Acoustic Survey Density (ind./ha)	Sighting Survey Density (ind./ha)	Ratio (%)
2020	7.3	56.1	0.1
2021	8.7	80.6	0.1
2022	4.6	19.2	0.2

. The average sound survey density in 2020 was 7.3 (ind./ha), which increased to 8.7 (ind./ha) in 2021 and decreased to 4.9 (ind./ha) in 2022. The average density of sightings in the survey was 54.5 (ind./ha) in 2020, 79.5 (ind./ha) in 2021, and 20.4 (ind./ha) in 2022. The sighting survey, which allows for the identification of jellyfish distributed in the surface layer, exhibited a difference of approximately 0.13 times compared to the acoustic survey conducted in the middle and low layers in 2020. In 2021 and 2022, this difference was approximately 0.11 times and 0.24 times, respectively. The average difference was 0.16 times or greater.

The acoustic survey and sighting survey exhibited a positive correlation. This indicates a positive correlation between the increase in the acoustic investigation results and that in the sighting survey results. Furthermore, the R² value of 0.9961 indicates a strong correlation (Figure 8).

In this study, the mature N. nomurai individuals were primarily observed swimming in the middle and low layers. The giant jellyfish that inhabit the East China Sea were estimated to swim primarily at depths of 10–40 m, according to a previous study [12]. Similarly, the findings of another study [3] indicated that they swim at depths of approximately 10–50 m, which aligns with the results of this study. A previous study [13] comprising the use of pop–up archival transmitting tags and ultrasonic pingers for tracking the vertical movement of 12 large individuals of N. nomurai showed that they can swim up to a maximum depth of 176 m. On average, they were observed to swim up to approximately 40 m. It was also observed that mature N. nomurai can be found even below the thermocline layer [14].

According to [7,15], from 2020 to 2021, a two–year study was conducted using acoustic surveys and sighting surveys to determine the distribution of N. nomurai in the East China Sea and the adjacent waters of Korea. The results of the surveys consistently showed a high density of N. nomurai near the Yangtze River and the southern coast of Korea. In a previous study [16], similar findings were reported in May and June 2013 near the Yangtze River estuary, where Ephyra, Metaphyra, and juvenile medusa of N. nomurai were discovered. It was reported that Ephyra and Metaphyra accounted for 67% of the individuals found.

In addition, global warming has a significant impact on the growth of jellyfish, as reported in a previous study [17]. It was found that the reproduction of N. nomurai, a type of jellyfish, increased at temperatures greater than 18.0 °C during the summer season; the rate of increase was higher than that of ghost jellyfish. Furthermore, research results indicated that high water temperatures rapidly induce strobilation and promote grape cyst production [18]. The production of grape cysts increases as the temperature increases, while it has been found to affect the strobilation and ephyra at low temperatures [19]. In a similar study [20], it was estimated that the release of N. nomurai’s larvae occurs around late April when the water temperature increases to approximately 12–18 °C. The surface temperature and salinity ranges of the areas where Ephyra and Metaphyra are mainly found were 15.0–18.0 °C and 13.0–31.4 psu, respectively. The low–layer water temperature and salinity ranges were 12.7–17.0 °C and 29.5–33.1 psu, respectively. However, the polyps of N. nomurai, in their early stage, can survive in temperature ranges of 0–27.5 °C and can endure winter without being fed. However, it has been found that grape cyst formation does not occur at temperatures below 10 °C and research has also shown that a temperature increase affects cell growth and grape cyst formation [20].

N. nomurai, which has evolved from a medusa, has been found to have a wide temperature and salinity distribution range of 7.7–26.1 °C and 29.8–34.2 psu, with little influence on the water temperature and salinity [21]. However, it has been observed that these factors have an impact on the feeding rate of N. nomurai. When the salinity was fixed at 33 psu and the temperature was adjusted to 23 °C and 26 °C, it was observed that the temperature and ingestion rate were inversely proportional. Furthermore, when the temperature was fixed at 23 °C and experiments were conducted at salinity values of 24, 27, 30, and 33 psu, it was found that the ingestion rate was lowest at 24psu, while the ingestion rate increased as the salinity concentration increased [22]. In this study, it was observed that N. nomurai predominantly inhabits the range of 32–33 psu, which indicates a correlation between their feeding rate and the marine environment of the main distribution area.

4. Conclusions

In this study, the vertical distribution and density of the large N. nomurai jellyfish in the East China Sea and Yellow Sea from May to July was investigated, beginning from 2020 to 2022, based on the marine environment. The majority of N. nomurai are primarily distributed in the middle and low layers and they have been found to inhabit a wide range of water temperatures and salinity levels, even below the thermocline layer. Furthermore, on comparing acoustic and sighting surveys, it was found that there is a positive correlation between the increase in the acoustic survey results and that in the sighting survey results. The findings of this study will be used as foundational data for future research while comparing the distribution and density of large jellyfish across different layers, as it provides an understanding of the information of the entire water column, which is in contrast to methods that can only be used to investigate limited areas. Future works should comprise a comparison of the distribution of N. nomurai across water columns and other marine environments while excluding water temperature and salinity, to better understand its peristaltic movement patterns.