Releasing records of
Among the edible jellyfish species, Rhopilema esculentum Kishinouye, 1891, is one of the most abundant jellyfish species consumed. Therefore, this jellyfish species is an important fisheries source in China. The jellyfish fisheries in China show annually considerable fluctuations and have a very short season. In the chapter, we firstly try to review the natural ecology of R. esculentum, which includes the distribution and migration, growth model, and survival rate in the Liaodong Bay (LDB) based on the results of our field studies for more than 20 years. Secondly, we focus on reviewing the jellyfish fishery and population dynamic in the LDB. Thirdly, we emphasize the themes, including the survey methods, catch prediction, enhancement assessment, and fishery management, based on our survey results from 2005 to 2010. Finally, we present our field and experiment results of resource restoration. The high commercial value of R. esculentum enhancement in the LDB has made this a very successful enterprise.
- Rhopilema esculentum
- population dynamic
- jellyfish fishery
- enhancement and releasing
Several species of scyphozoan jellyfish with mild stings are considered to be edible jellyfish. They are also used for medicinal purposes, such as treatment of high blood pressure, bronchitis, and a multitude of other diseases. They have been caught commercially and exploited along the coasts of Indian, Northwest Pacific, and Western Central Pacific Oceans by several countries, such as Thailand, Indonesia, Malaysia, the Philippines, Japan, South Korea, and China for over a thousand years [1–5]. Among the edible species,
Despite its importance as a commodity, scientific studies in Southeast Asia have lagged behind the rapid development of exploitation . But, in China, scientists paid more attention to the biology and fishery of edible jellyfish; a series of research projects have been carried out over 20 years for the purpose of commercial development. The research results on
In addition, the technology of artificial breeding, pond culture, and stock enhancement in nature was further developed along the coastal waters of northern China [6, 7, 19, 20]. In China, the Liaodong Bay (LDB) of Bohai Sea is one of the most important jellyfish fishing grounds, and the jellyfish fisheries in the Bay is characterized by considerable fluctuations in the catch, varying from about 400 tons to 290,000 tons, and including a short fishing season. The earliest enhancement experiment was put in practice in 1984 for the purpose of stabilizing and increasing catches. From 1984 to 2004, the tentative stock enhancement has been conducted for 11 times [6, 7, 19–21], and from 2005 to 2010, the large-scale stock enhancement of jellyfish (
In this chapter, the natural ecology of
2. Life cycle and environmental adaptation of
Researches on the life cycle of
The life cycle of
The asexual reproduction methods, strobilation regulation, and mechanism of artificial control have been demonstrated. On the basis of life history in previous periods, the feeding habits (i.e. prey taxonomic group, size), feeding rate, and growth rate of medusae and scyphistomae were examined. In the meantime, effects of physical factors (i.e. temperature, salinity, light, food, pollution, fish activity) on different development stages of jellyfish
3. Natural ecological habit of
3.1. Distribution of
R. esculentum in China
3.2. Distribution and locomotion of
R. esculentum in the Liaodong Bay
The northern part of the LDB is covered by ice blocks in winter and a high proportion of polyps over winter and they carry out strobilation in the next spring. With regard to juvenile and young medusae liberated by strobilation near their native environments, there are large amounts of researches in the LDB, including on the stock structure, growth model, geographic, and seasonal distribution and population dynamics carried out by Li et al. [15, 16], Liu et al. , and Dong et al. . More information on distribution, migration, growth, and optimum fishing season of the jellyfish species should be obtained in order to select releasing sites and establish an adequate catch prediction model before the fishing season.
Horizontal distribution and habitat depth of the
Due to being a planktonic species, the moving ability of
In mid- and late July, distribution of jellyfish
When compared, ecological characteristics and migration patterns of two important large jellyfish species
3.3. Growth model of
The growth model of the jellyfish
Lt designates the arc length of the jellyfish swimming bell; t is time in 5-d units, beginning on June 20 (t0) when the strobilae of jellyfish release ephyra larvae in the field. The correlation is significant according to the F test statistic (F = 4126 > F0.005 (1, 9) = 13.61).
4. Stock enhancement history of
4.1. Experimental release of cultured jellyfish
The tentative stock enhancement efforts were conducted 11 times between 1984 and 2004 by Liaoning Ocean and Fisheries Science Research Institute with the aim of stabilization and increase of the jellyfish fisheries. During 1984–1986, 2.00 × 105, 5.0 × 105, and 2.10 × 105 ephyra larvae (bell diameter of 5.00–15.0 mm) were released into the northern Yellow Sea from June to July each year. The recapture rate was estimated to be 1.20–2.50%.
The numbers from 4.60 × 106 to 1.73 × 107 of ephyra larvae (bell diameter of 5–10 mm) were released in the Dayang River estuary on the northern Yellow Sea during 1988–1993. The annual recapture rate ranged from 0.07 to 1.02%.
In 2002, 1.20 × 106 juveniles (bell diameter of 20 mm) were released into Jinpu Bay, the northern Yellow Sea, where the recapture rate was estimated to be 1.20%.
In 2004, 5.30 × 106 juveniles were released to the coastal waters near to the Dayang River estuary, the northern Yellow Sea. The jellyfish catch was 79.0 tons throughout the jellyfish fishing season with each individual averaging 7.00 kg wet weight. The recapture rate was about 0.20%.
4.2. Large-scale release of jellyfish culture
Based on the life history, the technology of artificial breeding was researched and developed. We concluded that abundant juvenile jellyfish can be obtained in a very short time by adjusting the physical factors (light, temperature, salinity, and food) in jellyfish culture. During 2005–2010, yearly, over 500 million juvenile jellyfish produced by polyps in the previous year were raised to more than 20 breeding centers. The total volume of artificial breeding tanks for juveniles was about 30,000–40,000 m3 in Liaoning Province. Steady and high-efficiency production capability of juvenile jellyfish establishes the foundation for large-scale stock enhancement of jellyfish.
During 2005–2010, the large-scale release and enhancement of
|Year||Released time (day/month)||Released size BD (mm)||Released amount (108 ind.)|
|2005||25–30, May; 16–25, June||≥10.0||1.57|
5. Population dynamic survey and output forecast
5.1. Survey of released jellyfish
Mixed jellyfish stock monitoring was undertaken in late May, from early to mid-June, late June, from early to mid-July, and from mid- to late July between 2005 and 2010 in order to determine the survival, growth, and recapture rates of the released jellyfish. Eighteen survey sites within the 10-m isobaths were established in the juvenile jellyfish habitat of the LDB. The sites from 1 to 11 were within the 5-m isobaths, and the others at 5–10-m isobaths. As jellyfish grew, drift nets of different mesh size were used: dense mesh net (60 m length × 8 m height, 1 cm mesh size); middle mesh net (60 m length × 7 m length, 3 cm mesh size); and giant jellyfish net (60 m length × 8 m length, 10 cm mesh size) (Figure 2).
5.2. Various sources of mortality
The numbers of released jellyfish decreased due to various mortalities over time. It is very important to estimate how to reduce mortality during the whole developmental process from the beginning of release to the end of fishing season. It is helpful for managers to decide on appropriate measures to improve the efficiency of enhancement. Four different causes of mortality are listed in Table 2: (i) Handling mortality (M1): In the period when the jellyfish were transferred into plastic bags full in oxygen, transported, and released in the natural waters, handling mortality occurred; (ii) Abrupt mortality (M2): It occurred during 2–3 days after the cultured juvenile jellyfish were released into the sea, resulting from the sudden change of physical conditions and preys and so on; (iii) Natural mortality (M): It is caused by a combination of factors, including predation, competition, disease, and changes in environmental conditions during 40–50 days from the end of June to the beginning of fishing season; And (iv) Unlawful fishing mortality (F): Jellyfish death was caused by fishing with various kinds of nets before the fishing season opened. According to the various mortality experiments of 2005 and 2006, various mortalities were estimated by Ye et al.  and Dong et al. . The average handling mortality was estimated as 6.00%, the abrupt mortality was 79.0%, and the average natural mortality was 55.0% of the jellyfish surviving after release. But the percentage of this unlawful fishing mortality is very difficult to estimate.
|t0||N0||1.0||↓ M1: Handling mortality.|
|t1||N1||1.0||↓ M2: Abrupt mortality.|
|t2||R1 = N2 + R0||3.0||↓ F: Unlawful fishing mortality.|
|t3||R2||16.0 to 49.4||↓ M: Natural mortality|
|t4||Y=Y1 + Y2|
5.3. Catch forecast
The basic method of catch forecast is to survey the relative abundance with high-efficiency fishing drift nets in late June and early July. Eighteen sites were established in the main jellyfish fishing areas. The relative abundance is the average number of jellyfish caught on 1 net in 1 3-h catch. The catch forecast is obtained based on the model of relative abundance and fishing effort. The original forecasting equation was reported by Li et al. :
where Y = the forecasting output; x1 = relative abundance; and x2 = fishing effort. This relationship is significant (R = 0.97; standard deviation S = 2898; F = 31.57 > F (4, 2) 0.005 = 26.28).
From 2005 to 2010, the number of fishing vessels has been constant and had little effect on catch. The forecast catch was based on the relative resource only briefly :
where Y = the forecasting output; x = relative abundance; and statistical test results were significantly correlated at 0.05 level (R = 0.85).
6. Jellyfish fisheries and management
6.1. Fishing ground and fishing methods
In Figure 2, the survey areas show the main fishing ground of about 5000 km2, and it is located in the northern part of 40°30’N with 5–10-m depth. The actual fishing areas are smaller than the real area, and there are about 10,000 fishing boats in the narrow area in blooming year. At the jellyfish fishing season, there are two fishing boats per square km. In the years of low yield or no-releasing jellyfish year, fewer fishing vessels engaged in fishing production.
The fishery is characterized by large fluctuations of the annual catch and a short fishing season that has lasted only 2–3 days in recent years. From 2005 to 2010, edible jellyfish enhancement was carried out in the LDB, and the government participated in the management of jellyfish resources. The production of jellyfish in the LDB was maintained at the level of 15.7–91.0 thousand tons, and the output value was 173–546 million Yuan. In the years without releasing jellyfish, 2010–2017, the output of jellyfish in LDB dropped to less than 2000 tons (Table 3).
|Year||Output (103 ton)||Number of fishing boats||Price (Yuan/kg)||Value (106 Yuan)|
|Releasing jellyfish year||2005||91.0||10000*||6||546|
|No releasing jellyfish year||2011||1.0||2061||16.6||17|
The jellyfish fishing grounds where great numbers of edible jellyfish occur are characterized by having a large tidal range, shallow depth, semi-enclosed waters, fresh water inflow through river systems, low salinity, and abundant foods. Fishing gear used includes various trawls, set nets, drift nets, push nets, and hand nets. The most efficient drift nets that were set at a depth of 2–10 m are placed across the current flow with a system of floats and sinkers. The length of each net is 30–50 m, with height of 8–12 m. A vessel loads 10–30 nets, which depends on the power of the vessel. The optimal jellyfish fishing period is 10–20 August. Because fishermen were eager for jellyfish fishing, and the fishing season generally advances at the end of July to early August.
7. Evaluation of releasing effect
From 2005 to 2010, a total of 1.648 billion jellyfish were released by proliferating in the LDB. The average recapture rate of releasing is 1.77%, and 25.24 million jellyfish were recaptured. The individual weight was ca. 1.50–2.50 kg and recapture output was 48,500 tons which accounted for 22.86﹪of total edible jellyfish harvest during 2005 and 2010. Higher economic benefits were created, as much as 334.37 million Yuan (Table 4).
|Year||Recapture amount (104ind.)||Recapture rate (%)||Recapture output* (104 t ind.)||Recapture value (104 Yuan)|
8. Habitat protection area in Liaodong Bay
According to the survey data for the jellyfish over the years, the concentrated distribution area of juvenile
Jellyfish were distributed in the 10-m isobaths in the northern part of the LDB; hence, the water area in the 10-m isobaths may be the habitat of jellyfish which can be delimited as the habitat protection area. In the protection areas, juvenile
9. Discussion on the reasons for the decline of quantity
9.1. Effect of marine engineering and trawl net
In recent years, the marine engineering construction and trawl net caused damage for habitat of
9.2. Effect of runoff on the resources of jellyfish in LDB estuary
Ye et al. , Jiang et al. , and Dong et al.  have used the variable runoff, the occurrences of wind and temperature, and the relative abundance to study the effects of these factors on the number of jellyfish in the LDB. The results showed that runoff was the most important factor that affected the number of jellyfish in the LDB.
9.3. Effect of the first catching time for jellyfish
The most juvenile
The authors would like to thank two teachers/researchers, Changchen Ye and Peijun Li, for their contributions to the chapter. We mourn for the bitter losses of Professor Jie-kang Chen and Professor Geng-wu Ding who made very important contributions in edible jellyfish researches during their lives. We are grateful to research team members and the crew of fishing vessel Liaoyingyu No.15228 for their efforts of more than 10 years toward ‘Natural ecology and stock enhancement on the edible jellyfish in the LDB’. The portion of the chapter was taken from our previous work (Dong et al., 2009, 2013). This research was supported by National Natural Science Foundation of China (31770458) and National Key R&D Program of China (2017YFC1404400).
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