Current Status of International Fishery Stocks in 2023

Japanese Eel, Anguilla japonica
Hiroshi Hakoyama1 , Leanne Faulks1 , Sakie Kodama1 , Chiaki Okamoto1 , Hiroka
Fujimori1 , and Masashi Sekino2
1

2

Nagano University
National Research Institute of Fisheries Resources
March 31, 2024

Related Organaizations
The organizations responsible for the management of Japanese eels include the Food and Agriculture
Organization of the United Nations (FAO), the International Union for Conservation of Nature (IUCN),
and the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).
In addition, cooperation among Japan, China, Korea, and Chinese Taipei is promoted through theInformal Consultation on International Cooperation for Conservation and Management of Japanese
Eel Stock and Other Relevant Eel Species (hereafter referred to as Informal Consultation), which began
in September 2012 at the invitation of Japan.

Recent Trends
Japanese eel is traded internationally in its various life stages. Adult eels are consumed directly as food,whereas glass eels and eel fries are used as seedlings for aquaculture. The decline of the Japanese eel
population continues to be of concern to fisheries and conservation managers. Promoting population
management through both domestic measures and international cooperation is crucially important for
ensuring the sustainable use of the species. In 2014, Japan, China, the Republic of Korea, and Chinese
Taipei released a Joint Statement at the 7th Informal Consultation, restricting input of eel seeds into
aquaculture ponds: the amount of input of eel seeds for the 2014–2015 input season was to be no morethan 80% of that of the 2013–2014 input season (hereafter referred to as Joint Statement).
The conservation of eel species, including Japanese eel, has also been discussed at CITES. According
to the Decisions of the Conference of the Parties at the 18th meeting held in August 2019, range statesof non-CITES eel species in international trade are encouraged to collaborate and cooperate with otherrange states to develop shared objectives, establish monitoring programs, and enhance knowledge ofthe biology of the species. As a follow up from this, the 14th Informal Consultation, held between
May and July 2021, agreed that there was a need for scientific advice, and the first Scientific Meetingon Japanese eel and other relevant eels (hereafter referred to Scientific Meeting) was held online in
April 2022. The second scientific meeting was held in person in Ueda (Nagano), Japan in May 2023.

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Current Status of International Fishery Stocks in 2023

At this meeting, scientists exchanged opinions and shared information on scientific findings related toeels, particularly Japanese eel. The scientists also agreed on a roadmap for joint research to strengthen
cooperation in research activities related to Japanese eel, including the collection and compilation of
long-term time-series and eDNA data to understand stock trends, and the exchange of informationon techniques to clarify migration routes to spawning grounds and the analysis and evaluation of
migration route data. In addition, opinions were exchanged on the proposed standard working format
for statistics on the catch, input into aquaculture ponds, aquaculture, and trade of glass eel, juvenile
eel, and adult eel.
The 16th Informal Consultation, held in Tokyo in July 2023, discussed and confirmed the status ofcompliance with the Joint Statement, reviewed the management measures taken by each country/region
since the Joint Statement, set the maximum input into aquaculture ponds volume for the next fishingseasons (November 2023 to October 2024), discussed the preparation and maintenance of statistics on
the harvest, input into aquaculture ponds volume, and trade of glass eels in each country/region, anddiscussed the results of the second Scientific Meeting.

Usage
One typical eel dish in Japan is grilled eel or Kabayaki, often served as Unaju or Unadon. There arealso some local eel dishes in Japan such as Hitsumabushi and Seiromushi. Japanese eel serum contains
poison; it should never be eaten raw (Yoshida et al., 2008). In China, Japanese eel consumption has
seen a recent increase following the development of products suitable for Chinese consumers, including
dried shredded snack eel, eel hot-pot, and eel zongzi. Japanese eel is not only used as a food source but
also as an ingredient for traditional medicine in China and on the Korean peninsula (Kuroki, 2019).

Overview of the Fishery
Farmed eels account for most of the domestic supply in Japan. However, wild yellow eels (developmental stage) are still caught by longlines or traps set in freshwater and brackish waters (Mochioka,2019). The history of eel fisheries in Japan extends back to the Edo era (1603–1868). In fact, official
data of eel catches have been available since 1894 (Hakoyama et al., 2016). The domestic eel catch
was stable at 3,000 to 4,000 tons in the early 19th century, but it decreased during World War II.
Although it recovered temporarily to the 3,000-ton range in the 1960s, the catch has been declining
since 1970(Fig.1). The domestic catch in 2022 was 59 tons, the lowest ever. The number of inland
fishers (the number of fishery management entities in lakes that mainly targeted yellow/silver eels)
during this period also decreased remarkably (Hakoyama et al., 2016). Eel farming developed in the
19th century, with domestic eel farming using elver eels (juvenile stage) having begun in 1879. The eel
farming industry became viable in the 1920s, as the methods for raising glass eels advanced. Aquaculture production exceeded the wild catch in the 1930s(Tanaka, 2019): its peak amounted to 39,704
tons in 1988, but it has been stable at around 20,000 tons since 1997; production was 19,155 tons in
2022 (Fig.2).
Glass eels, which serve as seeds for eel farming, are harvested in four regions: Japan, China, Korea,
and Chinese Taipei. In Japan, the harvest period is generally December–April. Harvesters catch eelsby scooping or using set nets in coastal estuaries and river mouths. Glass eel fisheries are managed2
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Current Status of International Fishery Stocks in 2023

by prefectural governments under a licensing system. Prefectural governors issue permits typically
restricting the harvesting period, fishing gear, and fishing area. Domestic catch of seeds exceeded
100 tons before 1966, but they fell drastically from 1971 to less than 20 tons in 1990 (Fig.3). Figure
3 shows the total seed catch in the sea and inland waters. Seeds caught in the sea had “glass eel”
labels as their item name or had footnotes that denote they are glass eels. The eels caught in inland
waters had no clear description or labels on them, but the quantities collected in rivers and lakes wererecorded. According to the Minister’s Secretariat Statistics Department, most seeds were assumed tobe glass eels, but some elvers might have been included in the data (Personal communication from theStatistics Department of the Minister’s Secretariat, January 14, 2011). Around 1960, the developing
eel farming industries of Shizuoka, Aichi, and Mie prefectures introduced large amounts of elvers fromthe Tone River system in Ibaraki and Chiba. During the fishing season, from the middle of Marchto late October, the seeds collected from the lower Tone River included elver eels: they were 5–20 gand 15–25 cm long. Of these, 60% were supplied to Shizuoka, Aichi, and Mie prefectures as seeds
for eel farming (Matsui, 1972a, pp. 333–335). Since at least 1978, the fishing season for seeds in thenine major aquaculture prefectures has been limited to glass eels during winter (Eel Culture ResearchCouncil, 1980). This information suggests that eels caught around 1960 included more elver eels thanrecent catches. If this is the case, the rate of decrease in glass eel catch is likely to be smaller thanthat shown in Figure 3.
The global catch of Japanese eel (not including the catch in China) decreased from 3,619 tons in
1969 to 100 tons in 2021, reflecting the trend seen in wild adult eel catches in Japan, which account for
approximately 63% of the global catch (Fig.4). Regarding recent glass eel catches from 2009 to 2023,
China had the highest annual catch, followed by Japan. The catches of these two countries accounted
for most of the total. During the last several decades, the total amount of glass eels caught in thefour regions has fluctuated from year to year, ranging from 20 to 90 tons. In the 2019 fishing season,
the catches were 3.7 tons in Japan, 14.5 tons in China, 0.6 tons in Korea, and 2.75 tons in Chinese
Taipei, but the catches in the 2020 fishing season increased significantly to 17.1 tons in Japan, 50 tons
in China, 4.5 tons in Korea, and 5.2 tons in Chinese Taipei. In the 2023 fishing season, the catches
were 5.6 tons in Japan, 40.5 tons in China, 2.2 tons in Korea, and 1.9 tons in Chinese Taipei (Fig.5).

Biological Properties
Life History
Japanese eel is a migratory species (catadromous fish) that breeds in the ocean and grows in freshwater,brackish waters, and coastal habitats. The estimated age of silver eels (eels during spawning migration
when their body color is silver) based on otoliths ranges from 4 to 17 years, with a mean age of 8
years for females (Kotake et al., 2005, 2007; Han et al., 2009; Yokouchi et al., 2009), and the spawningmigration is estimated to take 6 months (Han et al., 2009; Chang et al., 2016). Thus, the generation
time of Japanese eels is estimated to be 8.5 years. The species spawning area is regarded as the WestMariana Ridge (Tsukamoto, 1992; Tsukamoto et al., 2011). The Kuroshio current transports hatchedlarvae for thousands of kilometers to freshwater and brackish water habitats on the coasts of eastern
Asian countries. The spawning season is estimated as extending from April through August. It is
likely to take place concurrently with a new moon (Tsukamoto et al., 2003; Shinoda et al., 2011).

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Current Status of International Fishery Stocks in 2023

No direct observations of spawning behavior in the wild have been made. However, indirect evidence
from genetic analyses of preleptocephali larvae collected near the West Mariana Ridge indicates thateels may mate more than once over several days and with different partners (Takeuchi et al., 2022).
Laboratory studies also suggest the formation of spawning aggregations (Dou et al., 2007). The eggs,
each having a diameter of approximately 1.6 mm, are laid at a depth of 150–200 m (Tsukamoto et al.,2011) in waters of 25◦ C.
The transparent, willow-like Japanese eel larva is called a leptocephalus. Leptocephali are transported westward via the North Equatorial Current (NEC) and mesoscale vortex. They drift northward
at the boundary of the Kuroshio Current as they transform into glass eels (Fig.6). Arriving in coastal
areas, the glass eels (already about 0.2 g) will then disperse to freshwater, brackish, and coastal habitats. Upon settling in their respective habitats, glass eels shift into a benthic sheltering behavior.
Their body surface gradually turns black. Eels with sufficient pigmentation on their bodies are calledelver eels. Their ventral side becomes yellowish as they grow into the yellow eel stage: the longest eel
life stage. After living in rivers, matured yellow eels show morphological changes (e.g. enlargement of
the eyes) and transform into silver eels before migrating (Matsui, 1972b).
The recruitment season of glass eels varies by country or region, but in Japan it mainly occurs during
winter, from December through April (Han, 2011). Figure 7 presents the monthly glass eel catch from2002 (2002–2003) to 2018 (2018–2019). The peak of recruitment can occur anytime during winter or
spring and varies every year. These fluctuations in migration patterns are not yet fully understood. In
2019 in the Saigo River, Fukutsu City, Fukuoka Prefecture, a survey conducted by Kyushu University
during the new moon revealed that the largest recruitment (1,086 individuals) occurred on April 4 ,followed by February 2 (240 individuals) and May 3 (106 individuals). Only three individuals werecollected during June–August; none were collected in September (Fig.8). Nevertheless, quantitativeevaluation of the amount of migration is difficult using such a small survey.
A ten-year survey of glass eels in the Pearl River, China showed that glass eel CPUE decreased
from 2011 to 2022 and was generally higher during the full moon (and corresponding large tidal range)
and when water temperature dropped below 8°C (Shuai et al., 2023). Evidence for the influence of
lunar cycle and water temperature on glass eel recruitment is mixed and may be confounded by otherenvironmental factors(Guo et al., 2017; Hwang et al., 2014; Tzeng, 1985; Yamamoto et al., 2001).
Modelling by Shuai et al. (2023) suggested that the Pearl River glass eel CPUE for 2023–2026 willremain low but stable, but the authors warn that climate change, dams and overfishing continue tothreaten the overall spawning stock.
An analysis of glass eel recruitment in Lake Hamana, Japan from 1971 to 2014 showed that watertemperature difference (an indicator of warm water intrusions from the Kuroshio current) may be agood indicator of glass eel catch in non-meander periods of the Kuroshio (Miyake et al., 2023).
A study of glass eel body length in Chinese Taipei showed that environmental condition in the
spawning area influenced the TL of glass eels (Hsiung et al., 2022). Specifically, total length was larger
during El Niño years (possibly because of delayed recruitment due to changes in circulation etc.).
These findings suggest that eel development may be affected by climate change.

Population Genetics, Phylogeny, and Demographic History
Elucidating the population genetic structure of organisms is indispensable, not only for defining
the management units of natural resources but also for accurately estimating the effective size of4
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Current Status of International Fishery Stocks in 2023

populations(N e), which can be used to evaluate population viability and perenniality. In recent years,
the population genetic structure of the Japanese eel has been a subject of intense debate. Earlier population genetic studies that used limited numbers of genetic markers supported panmixia in Japaneseeel (e.g., Ishikawa et al., 2001; Sang et al., 1994), although Tseng et al. (2006) revealed the presenceof two genetically different groups (northern and southern groups). More recently, a genome-wide
analysis of single nucleotide polymorphisms (SNPs) using the whole-genome resequencing technique(Igarashi et al., 2018) suggested that an estuary population from the Kuma River (Kumamoto Prefecture) is genetically divergent from other populations. On the other hand, another study based on
SNPs detected using restriction site-associated DNA sequencing found limited temporal and spatial
genetic differentiation among populations along the coasts of Japan and China (Gong et al., 2019).
Thus, although the population genetic structure of Japanese eel has been the subject of intense debate,most studies now clearly indicate that Japanese eel exists as a single panmictic population (Yu et al.,2020; Gong et al., 2019; Han et al., 2010; Ishikawa et al., 2001) and should be managed accordingly.
Current research now focuses on estimating the long-term and current N e of Japanese eel.
The N e is different from a standard measure of population size. It makes an estimate of thenumber of individuals that are effectively contributing to the next generation (thus it is generally
smaller than the actual population size). N e estimated from genomic data is a population index
independent of fisheries catch statistics, and estimation over year provides information other than catch
data on recent stock trends. Assessing historical changes in the N e also can provide an evolutionary
perspective on current population dynamics. Whole genome level analyses using pairwise and multiple
sequentially Markovian coalescent (PSMC and MSMC) methods (Mather et al., 2020) indicate thatbetween 4 and 1 million years ago (Ma) N e of Japanese eel decreased, then from approximately 1
Ma up until around 22,000–30,000 years ago Japanese eel underwent a steady increase in N e, peaking
at approximately 80,000 individuals (Faulks et al., 2022). During the Last Glacial Maximum (LGM:
19,000–33,000 years ago) the N e decreased to around 60,000 individuals. Due to the restricted powerof the PSMC and MSMC methods to detect changes within the last 20,000 years, changes in the
population size of Japanese eel following the LGM are still unknown. However, several studies haveused alternative methods (microsatellite markers) to assess contemporary N e and found values ranging
from approximately 400–600 to 4000–6000 (Han et al., 2008; Takeuchi et al., 2022; Tseng et al., 2003).
In addition, ongoing studies using single nucleotide polymorphism data and linkage disequilibrium
analyses(Waples and Do, 2010) indicate that the current N e is around 20,000 individuals (M. Sekino,
personal communication, January 26, 2022). Overall, these results indicate that Japanese eel has
experienced several population bottlenecks which could mean that Japanese eel may be sensitive tofurther declines in population size.
The time to the most recent common ancestor of the anguillid eels traces back to approximately
20 million years ago (Minegishi et al., 2005). The Japanese eel is a component of a monophyleticclade that contains the Indo-Pacific species. The divergence of Japanese eel from the other species isthought to have occurred at the foremost stage of the speciation process within this clade (Minegishi
et al., 2005; Zhu et al., 2018).

Growth and Maturation
The growth rate of Japanese eel varies widely among individuals, ranging from 5–20 cm per year
(Yokouchi et al., 2008, 2012). Individuals usually mature at the yellow eel stage of 5–10 years, but
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Current Status of International Fishery Stocks in 2023

some have matured at age 22, which is the estimated maximum lifespan (Chino and Arai, 2009; Kotake
et al., 2007; Lin and Tzeng, 2009; Sudo et al., 2013; Yokouchi et al., 2009). Sex is determined by theenvironment an individual experiences from the glass eel to the elver phase. The effect of sex on
the growth rate is poorly understood, but females mature at a larger size and at an older age thanmales (Chino and Arai, 2009; Kotake et al., 2007; Lin and Tzeng, 2009; Sudo et al., 2013; Yokouchi
et al., 2009). Water temperatures and food availability affect the growth and maturity rate of eels
(Yokouchi et al., 2008) . Faster and higher growth rates are observed at low latitudes with high watertemperatures (Hagihara et al., 2018) and in brackish waters (Kaifu et al., 2013; Kotake et al., 2005;
Yokouchi et al., 2008, 2012).

Distribution
Japanese eel is distributed in east Asian freshwater and coastal areas such as the Japanese coast, theKorean peninsula, mainland China, and the Philippines (Han et al., 2012; Tsukamoto et al., 2003)(Fig.9). The spawning grounds are located near the Mariana Trench. The larvae are dispersed by the
NEC and the Kuroshio Current (Shinoda et al., 2011).

HabitatYellow eels that do not settle in rivers might stay in bays or estuaries or even migrate back and forthmany times between the sea and river, displaying migratory polymorphisms and habitat shifts (Araiet al., 2003). Yellow eels tend to hide in refuges such as holes, crevices, and mud burrows (Aoyama
et al., 2005; Tesch, 2003).
Freshwater
Yellow eels are distributed over a wide area from downstream to upstream rivers or lakes. Eels arefound mainly near the shore and tend to avoid the center of rivers (Aoyama et al., 2002; Itakura
et al., 2018). Movement between both shores of the river is uncommon, suggesting short home ranges(Itakura et al., 2018). Eels prefer a deep environment and they are often concentrated in areas with agentle river slope with many stones in the riverbed (Matsushige et al., 2020). Eels that are less than 25
cm long tend to use lentic habitats with smaller substrates like sand/silt to cobbles, whereas mid-sizedfish (25–55 cm) use habitats with larger substrates like pebbles and cobbles(Kumai et al., 2023).
A stable isotope analysis of eels from the Pearl River, China indicated that eels in this systemspend most of their lifetime (post elver stage) in freshwater and the authors suggested that highhuman population density at the river mouth could be a contributing factor (Shuai et al., 2023). The
authors also highlighted the importance of understanding habitat use to aid conservation management
as strictly freshwater populations may experience different threats (easier to catch, dam construction)
than flexible polymorphic populations.
Brackish Water
Japanese eel resides in coastal habitats and occupies shallow estuaries where tidal effects are present(Matsushige et al., 2020). It also resides in tidal flats, where it burrows in sandy mud areas (Aoyama
et al., 2005).

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Current Status of International Fishery Stocks in 2023

Spawning Migration
Silver eels begin their spawning migration from autumn to winter (September–February) (Kotake
et al., 2007). Japanese eel spawns in the western waters of the Mariana Islands (14◦ –16◦ N, 142◦ E),
about 3,000 km away from the coast of Japan (Tsukamoto, 1992, 2006). Although the evolutionary
process of the spawning migration in eel species remains unclear, the ancestral eel species presumably
undertook a short-distance migration in tropical coastal waters then eventually adopted a long-distancemigration, similar to that of the extant temperate eels (Arai, 2014). Results of satellite and acoustic
tracking have provided only fragmentary information related to Japanese eel migration routes (Abeet al., 2023; Fukuda et al., 2022; Manabe et al., 2011), and the comprehensive picture remains unclear.
Japanese eel performs daily vertical movement during their spawning migration (Higuchi et al., 2018;
Manabe et al., 2011). It dives into deep waters (500–800 m) to which light does not penetrate during
the day. Then it moves to shallower depths of less than 300 m during the nighttime (Chow et al.,2015; Manabe et al., 2011). The swimming depth is affected by the light intensity of the environment(Higuchi et al., 2018; Watanabe et al., 2020). The higher the lunar light intensity becomes, the deeper
the nighttime swimming depth becomes (Chow et al., 2015; Higuchi et al., 2018). Anguillid adult eels
do not feed after starting the spawning migration (Chow et al., 2010). Their vertical movement ismainly undertaken to avoid visually oriented predators such as tuna and sharks (Chow et al., 2015;
Manabe et al., 2011; Watanabe et al., 2020).
Regional differences in migratory pathway and vertical movements were observed in eels tagged on
the Sea of Japan coastline and Pacific Ocean coastline of Japan (Abe et al., 2023). One tagged eel in
the Sea of Japan did not display typical diel vertical migrations, perhaps due to the different verticalstructure of water temperature of this region (Abe et al., 2023). Abe et al. 2023 also observed thateels did not enter deep waters where the water temperature was below 4◦ C. Acoustic tracking of eels
released into the Kuroshio Current near Japan indicated that the movement of eels was largely due tothe current, but there was also some active southward swimming (Fukuda et al., 2022). In contrast,
the same study showed that active swimming was the main component of movement for eels released
near the spawning grounds. The authors suggest that solar cues may be important for eel navigationand movement, but that further studies are required.

Transport of Leptocephalus
After hatching in the West Mariana Ridge, the larval eels, or leptocephali, are transported by the
NEC to the east coast of the Philippines and are carried by the Kuroshio Current to habitat areas ineast Asia such as Japan and Chinese Taipei. They must enter the north-flowing Kuroshio and not thesouth-flowing Mindanao Current to reach the habitat areas successfully. In a simulation conducted by
Zenimoto et al. (2009), more leptocephali enter the Mindanao Current during El Niño years, thereby
decreasing the probability of northward transportation by the Kuroshio. Another simulation conductedby Chang et al. (2018b) implied that weakening of the NEC is a cause of glass eel recruitment decline
over the past 20 years.
These analyses were conducted on the assumption that the northward Ekman transport caused bytrade winds, as well as the leptocephalus’ floating depth, determine the migratory patterns of Japaneseeels (Kimura et al., 1994). Leptocephali show diel vertical migration, floating in deep waters during thedaytime and in shallow waters during the nighttime. Otake et al. (1998) reported that the nighttime

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Current Status of International Fishery Stocks in 2023

depth gets shallower as leptocephali grow: 40 mm long eels were found at around 50 m depth. Because
the Ekman layer is shallower than 70 m, leptocephali are affected by Ekman transport after they growto a certain length, probably more than 20 mm. Weakened trade winds during El Niño seasons, which
entail a northward shift of the bifurcation point of the NEC into the Kuroshio current and Mindanao
current, might account for the decreased rate of northward larval transportation by the Kuroshio
current during this event. Another mechanism that may contribute to lower recruitment during El
Niño events is the southward shift of the salinity front. The spawning location is estimated as being
in the southwest area of the salinity front and the seamount chain cross (Tsukamoto et al., 2011).
Therefore, the latitude of the salinity front also influences leptocephalus transport. The salinity front
is usually located at about 15◦ north latitude, but it occasionally moves south beyond 5◦ north latitude.
Because the NEC south of 10◦ north latitude connects to the Mindanao Current, the southward shiftof the salinity front can prevent leptocephali from migrating northward (Kimura et al., 2001).
These models, which assume that the ocean environment prescribes the recruitment of the Japanese
eel into the habitat area, are supported by buoy tracking (Kimura and Tsukamoto, 2006) and correlations with catch per unit effort (CPUE) data from Tanegashima Island (Zenimoto et al., 2009).
However, statistical analyses using long-term glass eel catch data obtained during 1967–2008 in Chinese Taipei did not provide statistically significant support for a correlation between catch and thebifurcation point of the NEC, or the occurrence of El Niño events (Tzeng et al., 2012). Mature eels are
expected to spawn more or less concurrently with new moon periods during April–August (Shinoda
et al., 2011). Han et al. (2016) reported that recruitment in Chinese Taipei exhibits batch-like arrival waves of different cohorts at intervals of one month, which are presumably derived from different
spawning dates. In addition, leptocephali trapped in mesoscale eddies might have some influence on
recruitment patterns (Chang et al., 2018a). The large meander of the Kuroshio might not prohibit
recruitment to Japan (Chang et al., 2019).
Recent studies using eDNA, modelling and simulations have provided new insights into leptocephalus and glass eel transport(Kasai et al., 2021; Karaki et al., 2023). Kasai et al. (2021) showedthat Japanese eel eDNA concentrations in rivers all along the coastline of Japan were closely associatedwith oceanic current forecast models. In those simulations the depth of the glass eel migration was
important. Karaki et al. (2023) sought to further refine the simulations and to explain the relatively
higher concentration/biomass of eels in areas like the Seto Inland Sea and the northern Pacific coastline. They showed that to explain patterns in regional recruitment of glass eels it is important toinclude behavioral changes such as active horizontal swimming towards areas of lower salinity and apreference for shallower depths when approaching the coast.
There remains much to be uncovered about leptocephalus and glass eel transport patterns. An
understanding of these patterns might be useful for improving Japanese eel resource management in
the future.

Prey
Japanese eel starts feeding after becoming leptocephali. Although leptocephalus food preferences
have not been clarified, their hypothesized consumption of marine snow has become widely supported(Miller et al., 2013; Mochioka and Iwamizu, 1996; Otake et al., 1993). The feeding ecology of glass
eels and elver eels is also remains uncertain. Yellow eels in coastal areas and rivers feed on small fish,
crustaceans, polychaetes, shellfishes, aquatic insects, etc. (Itakura et al., 2015; Kaifu et al., 2013; Kan
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Current Status of International Fishery Stocks in 2023

et al., 2016). However, diet preferences vary depending on the growth stage, environment, and season(Kaifu et al., 2013) as well as individual differences.

Predators
Knowledge about Japanese eel predators remains limited, although eels are presumed to be preyed
upon by various animals throughout their life history. A recent study conducted at the Tone Riverrevealed that channel catfish and blackfin sea bass prey on glass eels (Miyake et al., 2018). During
the yellow eel phase, some reports have described predation of New Zealand long-fin eel by catfish,brown trout, cormorants, ducks, kingfishers, and water rats (Jellyman, 1977), and have even revealed
predation of American eel by larger conspecifics (Barker, 1997). Results suggest that Japanese eel at
the yellow eel stage is caught by carnivorous fish, birds, and mammals. In a satellite tracking study
using pop-up tags, tunas and sharks were observed to prey on silver eels during the spawning migration(Béguer-Pon et al., 2012; Manabe et al., 2011).

Population Status
The Ministry of the Environment and IUCN listed Japanese eel as an endangered species (categorized
respectively as endangered IB in 2013 and 2014). Although it is difficult to identify the causes of the
population decrease of this species, changes in the marine environment, overfishing, and deterioration of
habitats have been regarded as important factors. Particularly, reinforcing riverbanks with concrete,
in which eels cannot hide, constructing barriers impeding eel migration in rivers, and interrupting
connections between rivers and rice paddies might strongly exacerbate damage to eel habitats. To
understand the population trends, data of glass eel and yellow eel catches in Japanese habitats areimportant. Japan’s yellow eel catch exceeded 600 tons in the early 2000s, 500 tons after 2005, but lessthan 100 tons after 2015. The figure plummeted to 59 tons in 2022: the lowest figure ever (Fig.1). The
remarkable decrease in inland fishermen during this period must also have contributed to this trend,
but the degree of such contribution cannot be estimated due to the lack of necessary data. A survey
conducted in Okayama Prefecture revealed that the CPUE of yellow eel decreased by one-third inboth longline fisheries and small set net fisheries during 2003–2016 (Kaifu et al., 2018). Glass eel catch
data show a decrease to less than 10 tons in 2010–2013, a slight increase to 15 tons in 2014–2017, areversion to less 10 tons during the 2018–2019 fishing season, and then just 3.7 tons in the 2019 fishingseason (Fig.5). However, the catch increased significantly in the 2020 fishing season to 17.1 tons. The
catch for the 2023 fishing season was 5.6 tons. Although the catch of glass eels shows fluctuations,these data for Japan indicate that the population has been experiencing a long-term decrease and now
remains low. The population assessment of the species is challenging mainly because of its complex
life cycle and numerous uncertainties related to its ecology. No assessment exists except for Tanaka(2014). In 2019, the Fisheries Agency of Japan launched a research project with the goal of developing
a comprehensive assessment of Japanese eel populations.

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Current Status of International Fishery Stocks in 2023

Fisheries Management
As described above, the Japanese eel population size has been decreasing. A lack of progress has occurred in developing mathematical methods to predict population dynamics and management strategies
because it is difficult to understand the species’ biology completely and to identify causes of the population decline. Overfishing, pollution and changes in the marine environment, habitat degradation
due to shore protection works and river fragmentation, parasitic diseases, and increased predators
have been cited as factors contributing to the decline, but understanding of each factor and theirinteractions is still in its infancy (Knights, 2003; Friedland et al., 2007; Bonhommeau et al., 2008;
Arai, 2014) . It is necessary to mitigate risks leading to severe and irreversible effects (precautionary approach) (Gardiner, 2006). Consequently, Japan has taken comprehensive measures including
population management and habitat restoration.
For example, Japan called upon the People’s Republic of China and Chinese Taipei to engage in
an international discussion, the Informal Consultation, in September 2012. The Republic of Korea
joined from the fourth meeting in September 2013. In 2014, Japan, China, Korea, and Chinese Taipei
released a Joint Statement at the seventh meeting, restricting the input of eel seeds into aquaculture
ponds: the amount of input of eel seeds for the 2014–2015 input season will be no more than 80% ofthat of the 2013–2014 input season. It follows that the upper limit of input into aquaculture ponds inJapan was set at 21.7 tons. Thereafter, the upper limit of input in the next fishing season has been
discussed every year through Informal Consultation. The upper limit has remained the same because
no scientific evidence has been provided to change it. In addition, based on the Joint Statement,
the Alliance for Sustainable Eel Aquaculture (ASEA) was established as an international group ofeel management organizations from each country and region to discuss eel resource management on a
private sector basis. As of October 2019 three meetings have been held under ASEA.
To implement the upper limit, Japan introduced a licensing system for eel aquaculture underthe Inland Water Fishery Promotion Act in June 2015. The amount of initial input of eel seeds is
restricted by eel species and is allocated for each farmer under this Act. Farmers are required toreport their input amount and production amount to the central government every month. The catchof glass eels is subject to licenses issued by prefectural governments. The fishing season duration islimited. Catches of adult eels using certain fishing gear are also subject to licenses issued by prefectural
governments. Each prefecture is implementing various additional measures such as gear restriction,
upper limits of harvest for individuals, and time closure for catches of both glass and adult eels,
considering the unique situation in each prefecture. Recently, the prohibition of catching silver eels
descending to spawn has been introduced in almost all prefectures where wild adult eels are distributed.
To prevent poaching of glass eels, glass eels were designated as “specified aquatic animals and plants”
from December 2023, and penalties were strengthened. As a result, the former special catch permit forglass eels was transferred to the governor-licensed fishery in December 2023. In addition, the Act on
Ensuring the Proper Domestic Distribution and Importation of Specified Aquatic Animals and Plants
was enacted in December 2020 and came into effect in December 2022. The purpose of this Act isto prevent the illegal trade of harvested aquatic animals and plants, to ensure the proper domestic
distribution and proper import and export of specified aquatic animals and plants, prevent illegal
fishing, and contribute to the sustainable use of fishery resources. This will regulate exports and makeit obligatory for businesses that harvest and handle glass eel to report their business information,

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Current Status of International Fishery Stocks in 2023

communicate their catch numbers, and create transaction records and maintain them. In April 2022,
juvenile eels (i.e., eels less than 13 centimeters in length) were designated as Specified Class I aquatic
animals and plants subject to regulation under the Act, and the regulation will apply to juvenile eelsfrom December 2025.
In addition to fisheries management, continuous efforts have been made towards the creation andconservation of favorable riverine environments. The concept of “nature-oriented river works” hasbeen adopted in river management to conserve and create the habitat that rivers intrinsically have.A
study from Lake Shinji suggests that the use of neonicotinoid pesticides since 1993 has caused declines
in Japanese eel and Wakasagi populations by altering food web structure and dynamics (Yamamuro
et al., 2019), hence the management of pesticides and other chemicals is also an important issue ininland habitats of Japanese eel.
For future work, it will be important to ascertain population trends and to work towards thesustainable harvest of Japanese eel. To achieve this, it will be necessary to improve the accuracy
and timeliness of temporal and spatial data, expand our knowledge regarding the population genetic
structure of the species, and use the best available data for the development of a mathematical model
for population management. Also, enhancing scientific cooperation and communication with otherrange states and areas will be important, for example holding regular scientific meetings.

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Current Status of International Fishery Stocks in 2023

Summary Table
World catch∗ 1 (in the past 5 years)

100–126 tons,100 ton in 2021 (recent),
and 117 ton average (2017–2021)
59–78 tons,59 ton in 2022 (recent),
and 66.2 ton average (2018–2022)
Yellow and glass eel catches from annual coastal
and freshwater fishery statistics and the trendof yellow eel CPUE in Okayama Prefecture wereused for evaluation.
Yellow eel catch (59 tons, 2022)
Glass eel catch (4 tons, 2018)
Japanese eel abundance is low and the trend is
stationary (the long-term trend of the above indices since the 1960s is decreasing, but the indices are flat over a 5-year period).
Under review
Management of cultured seedlings in ponds
Prohibition of catching larvae (length restrictions based on fishery regulation)
From December 2025, juvenile fish will be regulated as ”Regulations on Class I Specified
Aquatic Animals and Plants” under the ”Act on
Ensuring the Proper Domestic Distribution andImportation of Specified Aquatic Animals andPlants”.
Prohibition of catching parental eels descending
to spawn (Setting of non-fishing periods based on
inland fishery management committee instructions, etc.)
FAO,IUCN,CITES
Under review
Under review

Japanese catch∗ 2 (in the past 5 years)
Resource assessment methods

Resource status

Management goals
Control measures

Management organizations
Up-to-date year of the resource evaluation
Next year of the resource evaluation∗
1 FAO (2023): not including the catch in China∗
2 Fisheries statistics of the Government of Japan: The Annual Report of Catch Statistics on Fisheryand Aquaculture

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Current Status of International Fishery Stocks in 2023

Citation: Hiroshi Hakoyama, Leanne Faulks, Sakie Kodama, Chiaki Okamoto, Hiroka Fujimori, andMasashi Sekino, (2023) Japanese Eel (Anguilla japonica). In Fisheries Agency of Japan, and Japan
Fisheries Research and Education Agency (eds.), Current Status of International Fishery Stocks in
2023. http://kokushi.fra.go.jp/R05/R05_82_ELJ_English.pdf

The Japanese Eel

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Current Status of International Fishery Stocks in 2023

Figure 1: Catches of yellow eels in Japan.
Data are based on the fisheries statistics of the Government of Japan (Hakoyama et al., 2016, The
Annual Report of Catch Statistics on Fishery and Aquaculture in 2022).

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Current Status of International Fishery Stocks in 2023

Figure 2: Aquacultural production of eels in Japan.
Data are based on the fisheries statistics of the Government of Japan: The Statistical Yearbookof Ministry of Agriculture and Forestry and the Annual Report of Catch Statistics on Fishery and
Aquaculture.

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Current Status of International Fishery Stocks in 2023

Figure 3: Total seed catch (including glass eels and elver eels) in the sea and inland watersin Japan.
Data from fisheries statistics of the Government of Japan: The Annual Report of Catch Statistics on
Fishery and Aquaculture (Hakoyama et al., 2016, The Annual Report of Catch Statistics on Fisheryand Aquaculture in 2018).

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Current Status of International Fishery Stocks in 2023

Figure 4: Global catches of wild Japanese eel (including all stages).
These data are based on FAO statistics (FAO, 2023) (not including the catch in China).

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Current Status of International Fishery Stocks in 2023

Figure 5: Global harvest of Japanese glass eels.
These data are based on the Joint Press Release of the Informal Consultation (Fisheries Agency, 2017,2023).

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Current Status of International Fishery Stocks in 2023

Figure 6: Glass eels. Photo by Chiaki Okamoto.

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Current Status of International Fishery Stocks in 2023

Figure 7: Monthly glass eel harvest in Japan from the 2002–2003 fishery season throughto the 2018–2019 season.
These data are based on the harvest reports subject to the special catch permission of the eel seedlings(glass eels) contingent on the prefectural fishery regulations.

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Current Status of International Fishery Stocks in 2023

Figure 8: Numbers of individuals by survey date obtained from Saigo River, Fukutsu City,and Fukuoka Prefecture (2019).

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Current Status of International Fishery Stocks in 2023

40N

30N

20N

10N
120E

130E

140E

150E

Figure 9: Distribution map of the Japanese eels (modified from Han et al., 2012; Tsukamoto
et al., 2003).

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Current Status of International Fishery Stocks in 2023

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