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Water‐choice experiments were conducted using glass eels of Anguilla japonica and A. marmorata, which coexist in subtropical East Asian rivers. The results of the present study, together with those of previous water‐choice experiments, suggest that salinity enhances the odor preferences of glass eels. Compared to A. marmorata glass eels, A. japonica glass eels were more strongly attracted to water collected from a leaf detritus‐accumulating backwater area than to normal river water under the same salinity conditions. These results suggest that interspecific differences in odor preferences may facilitate their habitat segregation. [ABSTRACT FROM AUTHOR]

Objective: Visible implant elastomer (VIE) tags are commonly used as a cost‐effective tool for marking small fish, making them valuable in mark–recapture studies. It is crucial to quantify the impact of tagging procedures on fish survival to address inferential bias in mark–recapture studies. We assessed marking‐related mortality and growth in American Eel Anguilla rostrata elvers in a 40‐day laboratory experiment, following VIE tag application. Methods: There were 500 elvers (80–149 mm) that were divided into four treatment groups and one control group. Treatment groups were tagged with two tags in three body locations (anterior, central, posterior on left bilateral side) or with two tags in all three locations, while the control group remained untagged. Eels were retained in experimental tanks, and mortality rates were compared. Result: The VIE tagging did not significantly affect survival, which was 90.9% across all treatment groups and 92% for the untagged control group; nor did it affect growth. Conclusion: The application of VIE tags on various body parts should be a safe and effective method for marking American Eel elvers. Impact statementThe effect of visible implant elastomer tags on juvenile eel survival has only been assessed for European Eel, and tagging was restricted to tail fins. By testing novel tag locations on American Eel elvers, we demonstrated that this species can be tagged multiple times with visible implant elastomer tags without affecting short‐term survival and growth. [ABSTRACT FROM AUTHOR]

Estuaries are considered as key habitats for the early life stages of fish. However, in the face of massive destruction of many estuarine intertidal areas, management and conservation measures are needed. Fish condition indicators may be used as a proxy of habitat quality and provide valuable information for management of coastal areas. In this study, the larvae of golden mullet (Chelon auratus) and European glass eels (Anguilla anguilla) were sampled in three sites of the Gironde Estuary. Different lipid classes and fatty acids were quantified: phospholipids (globally, phosphatidylethanolamine and phosphatidylcholine), triglycerides, omega‐3 (particularly docosahexaenoic and eicosapentaenoic acids), omega‐6 and C18:1. These biomarkers provide information on the nutritional status of the larvae as well as on prey availability and larvae diet between sites. One site significantly differed from the others as it seemed to offer abundant and better‐quality prey. The very high levels of omega‐3 contained in mullet larvae suggested that this site provided a high amount of diatoms. However, the mullet larvae that colonized this site also showed physiological stress that could be explained by exposure to pollutants through their prey. This work constitutes an essential baseline for developing biomarkers to assess the quality of habitats in a global change context. [ABSTRACT FROM AUTHOR]

The glass eel catch from the international River Minho, western Iberian Peninsula, forms the southernmost Atlantic series in the composite index outside the North Sea assessing European eel recruitment. Here, new experimental yield data from the lower estuary (1981–2022) and fishers´ daily records (1990–2022) were modelled to describe and compare seasonal and interannual trends. Seasonality matched river discharge climatology, possibly a more general feature in the southern range of the species distribution, with the difference between high and low season becoming less marked in recent periods of lower abundance. Glass eel yield showed a sharp decline during the 1980s, in line with the recruitment index outside the North Sea and with total Minho catch, but not with recent local estimates of catch per unit effort. This decline is corroborated by trends in fishers´ daily records, suggesting that the reduction in effort in the Minho was the consequence of a gradual adjustment to recruitment in the catchment mediated by progressively tighter local management measures. The inefficiency of fisheries regulatory measures to guarantee stock recovery alone calls for more comprehensive measures to reverse anthropogenic impacts on continental eel populations, but the task seems Herculean. [ABSTRACT FROM AUTHOR]

Purworejo Regency is located in the south coast of Java where rivers flow into the Indian Ocean. The Jali River in Purworejo becomes a migration route for eels where juvenile eels (glass eels) move from the ocean to freshwater passing through the estuary. During migration, glass eels will make efforts to survive, one of which is to manage their osmotic pressure (osmolarity). With regards to those, a study on the upstream migration of glass eels in Jali River was carried out. Glass eels samplings were conducted using hand-held lift nets at the new moon phase, from 20:00 pm to 4:00 am. Sampling stations included the estuary, a branch river, and below the dam. The osmolarity of both glass eels and media (river water) were measured. Furthermore, the chemical and physical parameters of the waters as the natural habitat for glass eels were also measured. Glass eel samples were then identified to species level. The results showed that glass eels entering the Jali River consisted of Anguilla bicolor bicolor and Anguilla nebulosa nebulosa with the former being the most common. Total length of glass eels ranged between 44.10 to 69.70 mm with total weights 0.08 to 0.54 g. Glass eel migration has a close relation with surface current and tide. The osmotic work level (OWL) of glass eels approaches 0.34 mOsm/l H2O which is close to iso-osmotic. This happened at salinity 0 o/oo so that the glass eels migrate to fresh water which has a lower salinity to achieve isoosmotic condition. [ABSTRACT FROM AUTHOR]

Growing demand for water requires resource managers to be innovative to minimise impacts on aquatic ecosystems. There are increasing concerns about fish damage and losses at water intakes, but few studies have examined the effectiveness of different screening material for excluding New Zealand fishes. We experimentally tested six screens (50–100 and 100–200 mm rock bunds, 3 mm woven mesh, 1.5, 2 and 3 mm wedge-wire), on two introduced and five native fish species. Bluegill bully (> 32 mm), common bully (> 30 mm), Canterbury galaxias (> 47 mm) and rainbow trout (> 40 mm) were screened effectively by 3 mm woven mesh and wedge-wire. Chinook salmon (> 43 mm) and īnanga whitebait (> 46 mm) were excluded by 2 mm wedge-wire, but shortfin elvers (< 80 mm) penetrated 3 and 2 mm wedge-wire, and glass eels (< 63 mm) penetrated 1.5 mm wedge-wire. Rock bunds were effective barriers for rainbow trout but were ineffective, and acted as habitat, for bluegill bully, Canterbury galaxias and shortfin eels. We recommend that 1.5 mm wedge-wire screens are used in the tidal zone of New Zealand rivers, 2 mm screens are used beyond this zone with an upstream transition to 3 mm screens dependent on catchment-specific fisheries values. [ABSTRACT FROM AUTHOR]

The existence of ultraviolet (UV) biofluorescence in nature has been documented in a wide range of flora and fauna. Biofluorescence is utilised throughout biological and ecological functionality, and although invisible to the human visual perception, this spectral range is known to be integral for communication across flocks, swarms, shoals and between individuals. Under UV illumination, European eel juveniles (glass eel) were found to fluoresce bright yellow. Follow‐up experiments using the UV light source on preceding eel life stages such as pigmented glass eels (elvers), 'bootlace eels' (juvenile yellow eels), older yellow eels and migrating silver eels, all found that these stages had lost the capacity for biofluorescence. The information gathered from this study suggests that the use of UV light for presence/absence glass eel arrival surveying could be a useful and effective tool for researchers in the research field. [ABSTRACT FROM AUTHOR]

Fish marking is an essential tool for fisheries management, especially for evaluating the stocking of endangered fish species to support conservation and sustainable use of fish stocks. Batch marking of young European eels Anguilla anguilla (L.) prior to stocking is recommended as the benefits of stocking for the spawning stock can be evaluated by recapturing marked fish over time, therefore mass marking of young eels with substances such as alizarin red S (ARS) is becoming increasingly important. To improve the marking method and reduce marking costs when immersing glass eels in an ARS solution, eight laboratory experiments under varying conditions (e.g., temperature, ARS concentration, immersion time, osmotic induction, fish density) and with ARS from different suppliers were carried out. The results show that optimal marking of glass eels can be carried out in the field or during transport by putting approximately 50 g of glass eels per liter in 150 mg L−1 ARS solution for 3 h at 10–15°C. Lower concentrations did not result in reliable marking. Water temperatures of 5°C and below can have a stunning effect on the eels and increase mortality significantly, regardless of the concentration of ARS. Glass eel densities below 50 g L−1 in the marking bath increase marking costs unnecessarily, while a higher density of 100 g L−1 resulted in significantly higher mortality and lower marking success. A somewhat more difficult but less expensive alternative is to bathe the fish in a saline solution of 1% (10 PSU) of 80 mg L−1 ARS for 3 h at 10°C. Costs can also be significantly reduced by choice of supplier for ARS, but care should be taken as the quality of the powder appears to vary (mean percentage of sufficiently marked eels ranged from 59% to 91% among suppliers in the present study) and can lead to marking failure. The optimal marking conditions can help ensure that stocked glass eels can be reliably identified in future studies to assess stocking benefits while reducing costs. [ABSTRACT FROM AUTHOR]

Spatial scales are important for examining health of exploited fishery stocks and guiding management actions. However, information about the optimal spatial scale is still unclear for assessment of transit fisheries, such as elver fisheries of the American eel Anguilla rostrata. We applied a generalized depletion model to assess catch and effort data from three nearby rivers (within 50 km) to test the hypothesis that modeling on pooled and separate data from nearby rivers would give similar estimates of abundance and exploitation rate. Overall, pooling data from rivers within 50 km did not result in large differences (<20% in relative difference) in estimates of abundance and exploitation rate with close mean abundance estimates and similar temporal trends in abundance, exploitation rate, and relative escapement. Pooling nearby river systems can greatly reduce modeling effort, at the cost of ignoring fine‐scale variability in elver recruitment and having coarser spatial scale for the management. When only an index of annual recruitment and exploitation rate are of interest, pooling data may be practical from different locations up to 50 km. [ABSTRACT FROM AUTHOR]

The only diet that has been capable of rearing eel larvae to the glass eel stage in captivity is the shark egg (SE)-based diet (SE). This study investigated the potential of alternative dietary components, namely hen egg yolk (HEY), milk proteins, and fish protein hydrolysate (FPH), on the growth and survival of eel larvae. In the first experiment, a diet containing HEY and skimmed milk powder (HS) was compared to SE. There were no significant differences in growth and survival rate between the two diets except in the early part of the experiment period. In the second experiment, HS was modified by the addition of FPH and casein (FC), and larval performance was evaluated for three dietary regimens (SE, HS, and FC). The performance of larvae fed HS and FC was found to be comparable or superior to those fed SE. It was observed that larvae fed the alternative diets to SE were able to progress to glass eels. However, more skeletal abnormalities were observed in HS in experiment 1. The results of this study indicate that a combination of HEY and milk proteins is suitable as a larval eel diet, and that it has the potential to replace SE. [ABSTRACT FROM AUTHOR]

Simple Summary: The Japanese eel, a high-valued commercial fish species in the Yangtze River ecosystem, faces a significant decline in the resource. In the present study, we reported our 11-year investigation to track the migratory patterns of glass eels of Anguilla japonica into the Yangtze estuary. We identified two distinct annual peak arrival times which coincide with cooler water temperatures, i.e., the first peak at 6–8 °C and the second at 10–12 °C, both in sync with the strong spring tides of the lunar cycle. Despite rebounded population in a few years, an auto-regressive integrated moving average model analysis reveals a concerning trend: since 2016, rising temperatures have been associated with delayed recruitment. This change poses a threat to their migratory routes and life cycle stability. Underlining the critical nature of our research, our findings call for immediate conservation efforts to counteract the impact of climate change on the eel and to safeguard the biodiversity of the river ecosystem, a valuable asset to society. The decline of Japanese eel (Anguilla japonica) populations in the Yangtze River estuary represents a critical conservation concern. Eleven-years of daily catch data during recruitment periods (i.e., January–April, 2012–2022) indicate that annual catch averaged from 153 to 1108 eels, and show a bimodal pattern in glass eel arrivals. Utilizing seasonal-trend decomposition and generalized additive models, we demonstrated a strong correlation between catch abundance, optimal water temperatures, and lunar cycles. An auto-regressive integrated moving average (ARIMA) model predicts an increase in glass eel numbers for 2023–2024 but also points to a concerning trend of delayed recruitment timing since 2016, attributable to the 0.48 °C per decade rise in sea surface temperatures. This delay correlates with a significant decrease in the average body weight of glass eels, suggesting potential energy deficits that may hinder successful upstream migration. This study not only furthers our understanding of glass eel recruitment dynamics but also underscores the urgent need for targeted conservation measures. Additionally, it highlights the importance of sustained, detailed monitoring to mitigate the detrimental effects of climate change on these eels, vital for preserving the Yangtze River's ecological integrity. [ABSTRACT FROM AUTHOR]

Daytime habitat use by Japanese eel (Anguilla japonica) at the yellow eel stage (total length ≥ 15 cm) and the newly recruited glass/elver stage in spring (total length 5–7 cm) was examined in three sites of different types of streams in southwestern Japan. Although habitat use by yellow eels was examined in summer and winter, no distinct difference was found between the two seasons. Yellow eels preferred lower current velocity in all the three sites in both seasons. Cover use by yellow eels differed among the three sites having different cover-type availability. In a coarse-substrate mountain stream, the majority of yellow eels used cobble/boulders as cover, whereas in the other two fine-substrate streams, where coarse substrates were rare, large parts of yellow eels burrowed into fine sediments. In addition, undercut bank and bank vegetation were substantially used and preferred by yellow eels depending on the study site, suggesting the importance of natural bank habitat. The majority of glass/elver eels used sand substrates with current velocity < 10 cm·s−1 in all the three sites, suggesting their clear preference for such habitat. However, the preference for sand shown in this study contrasts with a general trend established by previous experimental studies that glass/elver eels prefer stony substrates over sand. Our results contrary to the general trend could be attributed to glass/elver eels displaying a stronger preference for slow currents over stony substrates, implying that current velocity has a dominant effect in habitat use by glass/elver eels under natural conditions. [ABSTRACT FROM AUTHOR]

Migration speed can have important evolutionary consequences as it can affect the timing of arrival, remaining energy reserves, and habitat choice. Environmental conditions and individual phenotypic traits can impact the migration speed of individuals. In this way, estimating migration speed is of particular importance, especially for species under strong management strategies and colonizing highly diversified habitats, as is the case for the European eel. However, estimating the migration speed of glass eels, which is the life stage when eels colonize continental habitats, presents challenges due to typically low re‐capture probabilities and difficulties in tagging individuals. Using recruitment time series at two sites, one at the sea connection and another inland, we estimated the temporal lag between the two migration peaks to compute migration speed. Because we worked on the Mediterranean coasts and in a lagoon, the weak tidal amplitudes may inhibit individuals from efficiently performing the selective tidal stream transport. We obtained migration speed values coherent with the few values available in the literature for Atlantic estuaries. The values we obtained that are lower than those obtained for Atlantic estuaries are also coherent with the weak tides along the Mediterranean coasts and lead to necessary further studies to understand the migratory behavior of glass eels in such hydro‐systems. [ABSTRACT FROM AUTHOR]

Simple Summary: Sustainable strategies for improving blue food economies are essential in order to design a new approach to transitioning towards more responsible, comprehensive, exploitable production and consumption models that have a positive impact on society and the environment. Among different fish farming systems, biofloc technology (BFT) is considered one of the most cost-effective, sustainable, and environmentally friendly farming systems due to its zero-water exchange and improvement of feed conversion ratio to the dietary contribution of bioflocs growing in the system, which contribute to the nutrition of the farmed aquatic animal. Thus, a two-month trial was conducted to evaluate the feasibility of rearing European eel (Anguilla anguilla) elvers, a species that is generally farmed in recirculation aquaculture systems, using BFT. European eel (Anguilla anguilla) elvers (initial body weight (BW) = 3 g) were raised in triplicate for 60 days in a biofloc system (BFT) at 21 °C. Data from the current first study evaluating this farming technology indicated that European eel elvers adapted well to BFT systems as data on growth performance (specific growth rate = 1.48% ± 0.13 BW/day and FCR = 1.05 ± 0.09) indicated, with production costs using BFT being lower than conventional RAS units. The most critical issues associated with this aquaculture system were the maintenance of the biofloc in tanks by the regular addition of refined sugar (46% C) to keep a relationship for C:N of 20:1, and the prevention of emergence of opportunistic pathogens like the monogenean Pseudodactylogyrus sp. The overall results of this study in terms of elvers' performance and quality and the composition of the biofloc material and its microbial composition indicated that BFT, which is considered to be one of the most cost-effective, sustainable, and environmentally friendly farming systems due to its zero water exchange and improvement of feed conversion ratio by the dietary contribution of bioflocs, may be satisfactorily used for farming European eels elvers at a density of 2 kg/m3. However, further studies are needed to test this technology with older eel stages. [ABSTRACT FROM AUTHOR]

Glass eel viability determines the success of cultivation and governs recruitment variability, as seeds in aquaculture are recruited to its population. This study aimed to examine the quality of glass eels by using biometric measurements coupled with proximate content. Glass eels were collected monthly using lift-net fishing gear in the Cimandiry Estuary from December 2020 to April 2021. Biometric aspects were examined for length, weight, eye diameter, heart, and liver, whereas proximate analysis was run on proteins, fats, carbohydrates, and ash. The results showed that length and weight steadily increased between months, although the condition factor was indifferent. Eye diameter and mouth opening confirmed the vision of diurnal predatory fish. The heart and liver have demonstrated metabolic capability, and fast-swimming fish favor migratory catadromous fish. Proximate content demonstrated insignificant fluctuations between months and did not confirm the biometric development of the glass eel. However, the glass eels in April were the largest in size, which might be the best for aquaculture and survival in the recruitment process. [ABSTRACT FROM AUTHOR]

The biomass of the Japanese eel (Anguilla japonica) is generally determined by the recruitment of glass eels into freshwater habitats, but the behavioral biology of their inshore migration remains unknown. With the aid of an ocean prediction system, we elucidated a recruitment migration scenario that can quantitatively reproduce a regional difference in biomass in Japan, which was previously estimated by an environmental DNA sampling method. For their successfully reaching shores, it is necessary to incorporate behavioral changes of glass eels within their migration on the Kuroshio Current, such as shallower depth preferences and horizontal swimming toward lower salinity water. In particular, the latter is essential for encouraging recruitment into both the Seto Inland Sea, with a relatively high ratio (20%–30%) of the total recruitment to Japan and the coasts in the central part of the Pacific side of northern Japan (i.e., the northern limit of the habitable distribution), manifesting that glass eels actively swim toward freshwater near coastal regions. In the subsurface layer, where glass eels mainly conduct diel vertical migration, there is a bifurcation path connecting the Kuroshio Current to the second and third branches of the Tsushima Warm Current, restricting the recruitment of glass eels into the Sea of Japan side of the main inland in Japan. The simulated recruitment validated that the eDNA acts as a proxy indicator for estimating the relative biomass on the regional scale. The simulation supported that the inshore migration of glass eels is determined by active horizontal swimming. [ABSTRACT FROM AUTHOR]

Understanding recruitment of glass eels in estuaries is crucial for the conservation of the European eel (Anguilla anguilla). However, basic knowledge on estuarine-specific glass eel migration, including in estuarine harbours, is mostly lacking. Therefore, we studied glass eel migration in the Dutch–German Ems estuary and the harbour at Delfzijl (The Netherlands) and tagged glass eels with Visual Implant Elastomer tags (VIE tags). We released 2000 tagged glass eels into the Ems estuary itself and 1000 tagged glass eels into the tidal harbour at Delfzijl. At three estuarine locations, i.e., Delfzijl–Duurswold, Termunterzijl, and Nieuwe Statenzijl, glass eel collectors were strategically placed, each location being progressively situated further upstream in the Ems estuary. Most glass eels (nuntagged = 97,089, ntagged = 74) were caught at Nieuwe Statenzijl, although this location is much further upstream. Lower numbers of glass eels (nuntagged = 1856, n tagged = 31) were caught at Delfzijl–Duurswold and Termunterzijl (nuntagged = 1192, ntagged = 7). Glass eels arrived approximately a week earlier at Nieuwe Statenzijl than at the other two locations, and the migration speed of tagged glass eels was highest at Nieuwe Statenzijl (>2 km/day) and lower (<1 km/day) at Delfzijl–Duurswold. Our study highlights that migration and the resulting potential recruitment of glass eels in estuaries and harbours may vary considerably both spatially and temporally. Further research on estuarine-specific factors that influence glass eel migration, such as the (anthropogenically altered) tidal action and flow, will provide valuable information on what influences glass eel migration in estuaries. [ABSTRACT FROM AUTHOR]

Predictability of glass eel recruitment of Japanese eel (Anguilla japonica) was examined with a simple indicator constructed using coastal water temperature under the influence of the western boundary current, the Kuroshio. The 1971–2014 glass eel catch from an estuary, Lake Hamana, Japan, was compared with the mean water temperature difference (WTD) between two coastal sites, Shirahama and Lake Hamana, during the glass eel fishing season. The directions of interannual change (i.e., increase/decrease) in the glass eel catch and WTD fully synchronized for 12 consecutive years (1995–2006), and these variables were positively correlated (r = 0.68, p = 0.0150). The WTD was correlated with the southernmost latitude of the Kuroshio path (r = 0.40, p = 0.0068) in the fishing season of the study period (1971–2014); therefore, the WTD appeared to reflect the path fluctuations of the Kuroshio and the warm water intrusions from this western boundary current. The proportion of matching directions between the interannual changes in the glass eel catch and WTD was 0.71 for the non-meander (nearshore) path years (significantly higher than 0.5, the expected proportion of matching signs due to chance; χ 2 = 2.88 , p = 0.0448), whereas it was 0.46 for the meander (offshore) path years ( χ 2 = 0.15 , p = 0.6526). Thus, the WTD tends to interannually increase/decrease with the estuarine recruitment of A. japonica glass eels when the Kuroshio flows with non-meander paths. This environment-recruitment relationship (western boundary current-recruitment link) in the Japanese eel demonstrates the importance of continuous environmental monitoring of coastal waters. [ABSTRACT FROM AUTHOR]

The microplastic loads in elvers of the critically endangered European eel Anguilla anguilla, sampled in the lower reaches of three English rivers, were very low (incidence: 3.3%, mean ± s.d.: 0.03 ± 0.18 particles) and did not vary with body length or between rivers. Particles were mostly black, polyolefins, fibres and fragments of size 101–200 μm. Current levels indicate a low contamination pressure locally and, consequently, management efforts might prioritise mitigating the effects of other stressors affecting the species. [ABSTRACT FROM AUTHOR]

Traditional eel farmers face the problem of poor quality and high price of commercial glass eel diet, this feed has produced poor fish performance, as well. The lack of essential amino acids may be the cause. Molecular methods are used to determine the rapid response of glass eels to a formulated diet. The purpose of this study was to determine how the effect of adding lysine to commercial feed affects the expression of genes related to growth hormone GH. Pasta feed was given to glass eels (1600 heads) which were cultured for 60 days. The cylindrical plastic ponds were framed with steel and covered with HDPE plastic (300μm) in size {(3.14cm) x (85cmx85cm) x (60cm)} filled with water 907,460 L ≈ 0.91 m3. Real time RTPCR results in GH suggest that the level of lysine (2%) added to the diet has strong effect on GH gene expression during eight weeks cultivation. [ABSTRACT FROM AUTHOR]

Being able to systematically detect parasitic infection, even when no visual signs of infection are present, is crucial to the establishment of accurate conservation policies. The nematode Anguillicola crassus infects the swimbladder of anguillid species and is a potential threat for eel populations. In North America, naïve hosts such as the American eel Anguilla rostrata are affected by this infection. The accidental introduction of A. crassus following restocking programs may contribute to the actual decline of the American eel in Canada. We present a quantitative real time PCR-based method to detect A. crassus infection in final and intermediate hosts. We tested two protocols on samples from different geographical origins in Canada: 1) a general detection of A. crassus DNA in pools of young final hosts (glass eels) or crustacean intermediate hosts 2) a detection at the individual scale by analyzing swim bladders from elvers, or from adult yellow and silver eels. The DNA of A. crassus was detected in one pool of zooplankton (intermediate host) from the Richelieu River (Montérégie-Québec), as well as in individual swim bladders of 13 elvers from Grande and Petite Trinité rivers (Côte-Nord-Québec). We suggest that our qPCR approach could be used in a quantitative way to estimate the parasitic burden in individual swim bladders of elvers. Our method, which goes beyond most of previous developed protocols that restricted the diagnosis of A. crassus to the moment when it was fully established in its final host, should help to detect early A. crassus infection in nature. [ABSTRACT FROM AUTHOR]

Japanese eels (Anguilla japonica) are a typical migratory fish species with high commercial importance. The Pearl River estuary in southern China is an important natural growing ground for Japanese glass eels, but limited information on Japanese glass eel population characteristics is available, despite their ecological importance. In this paper, we examined the annual patterns of the occurrence of Japanese glass eels in the Pearl River estuary from 2011 to 2022. The most frequently occurring Japanese glass eel's total length is 5.3 cm. The collecting period extended from December to February, and the collection catch-per-unit-effort (CPUE) decreased significantly from 2011 to 2022. The generalized linear model (GLM) indicated that daily changes in Japanese glass eel collection were significantly affected by tidal range, water temperature, and lunar distance. The catch peak appeared when the tidal range rose to 1.7 m, and the water temperature dropped below 8 °C on the full moon days. Overall CPUE analysis showed no significant periodic and inter-annual variability in the period 2011–2022, with the ARIMA model suggesting that the CPUE is expected to remain stable but low in the coming years (2023–2026), although recruitment ultimately depends on the overall spawning stock. [ABSTRACT FROM AUTHOR]

Adjusted tidal barrier management (ATBM, setting tidal doors ajar during tidal rise) is currently applied in Belgium to improve glass eel passage through tidal gates. However, salt intrusion in the receiving waters due to upwelling and accumulation of saline groundwater as a result of intensive drought put severe pressure on the unrestrained use of this cost‐efficient mitigation measure. We evaluated the efficacy of a very restricted ATBM at a tidal barrier on a small canalized waterway, located 3.5 km from the sea: one out of seven gate doors was left 20 cm ajar for ca. 30 min when the water levels on both sides of the tidal barrier were approximately equal. During 20 inflow events differing in tidal flow (i.e., flood or ebb tide) and daytime (i.e., day or night) in March/April 2019, migrating glass eels were caught with a fyke net fixed on the temporally opened gate. In total, 12,853 glass eels were captured, almost all during flood tides at night (96%), with a maximum catch of 3,827 individuals per inflow event. These results show that even small windows of opportunity can help glass eels pass tidal barriers. [ABSTRACT FROM AUTHOR]

This study aimed to explore the bacteria present in the digestive tracts of wild and cultivated Indonesian shortfin eel during the elver phase. The eel has high export potential due to its vitamin and micronutrient content, but slow growth and vulnerability to collapse in farm conditions hinder its cultivation. The microbiota in the eel's digestive tract is crucial for its health, particularly during the elver phase. This study used Next Generation Sequencing to analyze the community structure and diversity of bacteria in the eels' digestive tracts, focusing on the V3–V4 regions of the 16S rRNA gene. Mothur software was used for data analysis and PAST v.3.26 was used to calculate alpha diversity. The results showed that Proteobacteria (64.18%) and Firmicutes (33.55%) were the predominant phyla in the digestive tract of cultivated eels, while Bacteroidetes (54.16%), Firmicutes (14.71%), and Fusobacteria (10.56%) were predominant in wild eels. The most prevalent genera in cultivated and wild elver were Plesiomonas and Cetobacterium, respectively. The microbiota in the digestive tract of cultivated eels was diverse despite uneven distribution. The KEGG database analysis revealed that the primary function of the microbiome was to facilitate the eel's absorption of nutrients by contributing significantly to the metabolism of carbohydrates and amino acids. This study's findings can aid in assessing eel health and improving eel farming conditions. [ABSTRACT FROM AUTHOR]

An inventory of the presence and diversity of Anguilla spp. needs to be carried out as a basis for sustainable resource management. Conventional techniques based on morphological characteristics--are often less effective considering the morphological characteristics of Anguilla spp. are very identical and not so many can be observed, especially at the young eel stage. DNA-based molecular identification can be a way to determine diversity and phylogeny for conservation and inventory purposes. This research aimed to determine the diversity and phylogeny of Anguilla spp. obtained at the estuaries of Serayu River in Cilacap, Luk Ulo River in Kebumen, and Jali River in Purworejo in Central Java, Indonesia. A total of 10 samples were taken from each research location were analyzed morphologically, then it is known that there are 4 morphologically different samples at each research location so that a total of 12 samples were taken for molecular analysis. MEGA was used to construct the phylogenetic trees via Neighbour Joining (NJ) algorithms using the Kimura 2-parameter model with uniform rates and obtained by 1000 bootstraps replication. There are four species of Anguilla spp. identified morphologically, namely Anguilla bicolor bicolor, Anguilla bicolor pacifica, Anguilla obscura, and Anguilla australis. The species of Anguilla spp. that is molecularly identified is Anguilla bicolor bicolor with an identity percentage of > 98%. All samples are also known to be related to Anguilla bicolor bicolor identified in other countries. [ABSTRACT FROM AUTHOR]

There is a lack of information on many biological and ecological aspects of the critically endangered European eel during its growth phase in inland waters, such as when the sedentary life stage begins, mobility according to age and response to habitat alteration. We used mobile radio frequency identification (RFID) telemetry technology to track tagged eels over 6 years after their restocking as glass eels in six typologically different rivers. We also cross-referenced telemetry data with those of several electrofishing monitoring sessions to better understand the mobility and behaviour of eels. The relative abundance (maximum 52 individuals km−1) and detection rate (maximum 28%) of eels were not significantly correlated with the time/age after restocking. Eels were present in all restocked rivers, but their abundance was low and mobility was high in a slightly acidified, oligotrophic river that had experienced a great loss of fish habitat heterogeneity. This loss of habitat heterogeneity was due to flooding events and machinery works in riverbeds to restore the altered riverbanks. Four years after glass eel release, restocked eels became sedentary and moved from shallow to deep microhabitats with riverbeds dominated by blocks as the bottom substrate. After this age, they exhibited high fidelity to the residence site. This study provides new insights concerning the biology and ecology of eels restocked as glass eels in freshwaters, which should lead to improved management plans for the species through the implementation of more effective conservation measures and strategies. [ABSTRACT FROM AUTHOR]

This article discusses the issue of data variation in comparative biological analyses, specifically focusing on metabolic rate data. The authors argue against the common practice of removing statistical outliers from regression analyses, providing five reasons to support their decision. They use the example of a biphasic analysis of the ontogenetic metabolic scaling of the American eel to illustrate their points. The authors conclude that their analysis, which includes all data points, better represents the variation in the metabolic data and supports their conclusion of biphasic metabolic scaling in the eel species. [Extracted from the article]

The Indonesian shortfin eel (Anguilla bicolor bicolor McClelland, 1884) is catadromous fish. The developmental phase of the fish is influenced by the availability and type of feed, and dissolved oxygen levels (DO). Oxygen level fluctuations during migration from sea to fresh waters can cause hypoxia and trigger activation of the Hypoxia-inducible factor 1 alpha gene (HIF-1α). Therefore, undertanding HIF-1α expression is very important in understanding the life cycle and physiology of this organism. This study aimed to perform a histological comparison between the liver structure of eel in the elver eel phase to silver eel and compared the expression of HIF-1α in the liver. The eels were collected from Kulon Progo Regency, Segara Anakan in Cilacap, and Cangkringan in Sleman, respectively. The eel livers were prepared with standard paraffin method. The samples were stained with Hematoxylin-Eosin, Mallory Acid Fuchsin and Periodic Acid-Schiff Alcian Blue for histological observation. The HIF-1α protein expression was detected using immunohistochemistry. The results showed that the structure of liver elver, yellow, and silver eel shared similar tubular hepatocytes. Moreover, there was no significant differences in the parenchymal structure of liver of all stage of eel. Mucin levels were different in each life cycle phase. HIF-1α expression varied by eel phase, with the elver eel showing weak expression, yellow stage showing moderate expression, and silver eels displaying the highest levels of expression. We conclude that the highest expression of HIF-1α was found in the silver eel, which resides in freshwater environments and is found in freshwater while returning to the sea for reproduction. [ABSTRACT FROM AUTHOR]

Visual implant elastomer (VIE) marking was evaluated as a tool for individual tagging of larval Japanese eels, Anguilla japonica , during rearing conditions. Combinations of eight colors of VIE tag were used to individually mark leptocephalus larvae (104–110 days post hatched) by injecting them with either a mixture of a silicone color material and a curing agent, or only color material. Injections were at three locations on the dorso-lateral body surface for individual identification, and a sham-operated control group was also prepared. Tag retention was 100% with or without curing agent, and individuals could be tracked until larvae completely metamorphosed into glass eels. After some initial mortality, the survival and growth rates were not different between each treatment group. These results suggested that VIE tagging could provide good individual larva identification method for research during rearing conditions. • VIE tags were visible and retained for up to 7 months in some eel larvae. • Growth and mortality were not affected by the VIE tags themselves for eel larvae. • VIE tagging can provide good identification resources for research in eel larvae in rearing conditions. [ABSTRACT FROM AUTHOR]

Macroalgae are able to retain environmental microplastics (MPs). The potential ingestion of MP through Atlantic agar Gelidium corneum and different animal species (hake, glass eels, mussels, topshells, anemones, sea cucumbers) that are seafood resources in Spain, was estimated from published MPs data calculating daily dose and annual ingestion rate. The study region was Asturias (SW Bay of Biscay). Lower MP ingestion rate from algae than from any animal analysed revealed a reduced risk of MP intake, probably because the alga is harvested from quite clean subtidal zones. However, MP bioconcentration in Atlantic agar was higher than in sea cucumbers, mussels or glass eels. Compared with other algae, G. corneum ranked the highest for MP retention rate, perhaps for its intricate branching and gelatinous surface, suggesting a possible use in MP bioremediation. More experimental studies in MP uptake by macroalgae are recommended to understand their implication in the accumulation of this pollutant. • Gelidium corneum is a safe seafood regarding microplastics risk. • Coastal organisms are polluted regardless their bioconcentration capacity. • Topshells, anemones, and mussels exhibit high microplastics concentrations. • G. corneum , holothurians, and hake from offshore are relatively clean. • Red algae with high bioconcentration ability may be used for microplastic remediation. [ABSTRACT FROM AUTHOR]

The European eel (Anguilla anguilla) is a catadromous species that inhabits the rivers of the Adriatic watershed in Croatia. It is a critically endangered fish species, according to the IUCN, due to its declining abundance in European rivers caused by overfishing and trafficking and by diseases caused by nematodes, pathogenic bacteria and viruses. An illegal parcel of glass eels was confiscated at the Zagreb Airport and was intended to re-populate Croatian rivers. Barcoding was employed to determine species affiliation, and a thorough health check was carried out. This study reports the evaluation of gross lesions, histological findings, and EVEX virus isolation and identification. Since the confiscated glass eels were of unknown origin and given the serological and genetic similarities of EVA and EVEX, we designed primers and probes for almost whole genome sequencing to elucidate the origin of glass eels based on viral phylogeny. Bayesian phylogeny showed that the isolated strain had the most common ancestor with a Danish isolate and likely evolved from the French isolate of EVEX. These findings are discussed in light of the divergence of recently isolated strains and their possible contribution to the decrease of the abundance of the European eel in European waters. [ABSTRACT FROM AUTHOR]

The giant mottled eel, Anguilla marmorata, is at high risk of extinction due to overfishing of glass eels and elvers to provide stock for eel farming. In Vietnam, information on the genetic diversity and population structure of this species, which is necessary for resource management, is limited. In order to address this paucity of information, sequencing of the entire mitochondrial control region (mtDNA) was carried out for 176 individuals collected from central Vietnam. The sequences were investigated using various genetic, phylogenetic and population analyses. A total of 165 distinct haplotypes were identified. The percentage of variation within and among populations was 99.26% and 0.74%, respectively. The fixation index was low (0.007) and not significant (p = 0.096). Therefore, panmixia and a lack of significant population genetic structuring seem likely for A. marmorata in central Vietnam. Most sampled eels were genetically similar to eels found in North Pacific populations (Japan, Taiwan and the Philippines), except for one sample from Quang Tri and two samples from Binh Dinh, which had high values of genetic identity (97% to 99%) with South Pacific populations (Tahiti, Fiji, New Caledonia and Papua New Guinea). Taken together, we suggest that A. marmorata from the North and South Pacific populations co-exist in central Vietnam. [ABSTRACT FROM AUTHOR]

The transition from marine to fresh water is a challenging task for juvenile eels. This critical step in the early eels' life is preceded by a metamorphosis from the oceanic larval to the continental glass eel stage, requiring major energy‐demanding morphological, physiological and behavioural modifications during which time these animals do not feed. The success of the glass eels' inland migration after metamorphosis will largely depend on remaining energy levels, which can be supplemented only by resuming food uptake. Although it is crucial for their survival and the maintenance of the population, the feeding behaviour of glass eels is still an understudied aspect of the eels' complex life cycle. Many uncertainties about the phenology, diet, potential prey preferences and their relation with migration modus (migratory vs. sedentary) still remain. In this study, the authors analysed the stomach and gut contents of 458 European glass eels (Anguilla anguilla L. 1758) captured in a drainage canal connecting a small mesotidal estuary with an adjacent polder area during the spring migration seasons of 2016 and 2017. They demonstrated that although glass eels started feeding briefly upon arrival in the estuary, food uptake for early arrivals was restricted to a minority that sparsely feed on detritus and some worm‐like benthic invertebrates. Along the season, food uptake intensified eventually engaging all glass eels and their dietary palette diversified including a wide array of planktonic and benthic organisms. Crustacean plankton (mainly cyclopoid copepods) was an important part of the glass eel diet, whereas benthic oligochaetes were less abundant as food source in spite of their high presence in the sediments. No clear differences in feeding behaviour could be observed between migratory and sedentary glass eels. This study showed that glass eels can use highly artificial and dynamic drainage canals as feeding ground during their critical marine/freshwater transition. This outcome is also a plea to improve the accessibility of alternative (unnatural) migration routes between the ocean and suitable freshwater growth habitats for the European eel. [ABSTRACT FROM AUTHOR]

Alizarin detection in fish fins is extensively employed because it is easy to use. However, in eels, the eelGFP fluorescent protein may impede the detection of the fluorescent markers in the eel tissues. The study tests the effectiveness of three of the most up-to-date alizarin-detecting technologies on the living body and fins of European glass eels (Anguilla anguilla L.). The findings demonstrated that the control group had a high autofluorescence at alizarin and eelGFP maxima bands. With fluorescence reflectance imaging (FRI), the eel living body autofluorescence impeded the detection of the marked eels. In contrast with experimental excitation-emission-matrix (EEM) fluorescence analyses, 99% of the marked eels were correctly assigned to their group from fluorescence analyses of their fin cellular contents. With epifluorometry (EPI), 100% of the marked eels were detected with the caudal fin tips when excited at 450–490 nm wavelengths due to a weaker autofluorescence signal. EEM and FRI assays unveiled an average fluorescence quenching 60% and 44% of the marked group respectively, in the alizarin and eelGFP maxima bands. The fluorescence quenching observed is discussed. Results will benefit experimental design by examining autofluorescence effects on mark detection and the development of non-invasive detection methods in this critically endangered species. [ABSTRACT FROM AUTHOR]

The spread of viral diseases in eels is suggested to severely affect the European eel (Anguilla anguilla) panmictic population. The European Commission has initiated the Eel Recovery Plan (Council Regulation No. 1100/2007) to try to return the European eel stock to more sustainable levels within that measures eel restocking. However, scientific evidence evaluating the efficacy of stocking remains scarce. In addition, knowledge about the impact and contribution of eel stocking on the distribution of infectious diseases is insufficient. In this study, we aimed to investigate virus infections in batches of eels intended for restocking. We analysed samples of glass eels from certified fisheries and farmed European eels from different aquaculture farms. All analysed eels were purchased within a North Rhine Westphalian conservation program. Via a combination of cell culture and qPCR‐based techniques, we detected infections of glass eels with the rhabdovirus Eel Virus European X and anguillid herpesvirus 1 infections in farmed eels (10–15 cm). [ABSTRACT FROM AUTHOR]

The European eel (Anguilla anguilla) has undergone an unprecedented population decline since the 1980s, with current recruitment levels fluctuating from 3 to 15% of historical levels for the last 20 years. Monitoring of glass eels and elvers as 0 + recruitment is an essential step in helping to understand the trend in recruitment and to better quantify the current recruitment time series. Two locations within the Shannon estuary on the west coast of Ireland were monitored for glass eel recruitment from January to April in 2017 and 2018. This study used a generalised linear mixed model to examine a range of environmental variables impacting on glass eel abundance in transitional waters. Results found that water temperature and moon phase were the most important variables. Tidal height and cloud cover also influenced the abundance of glass eels but to a lesser extent. This study found that focussing survey efforts on nights around the full moon when water temperatures exceed 5℃ will allow a catch which is representative of the population in an estuary. Glass eel monitoring needs a long-term sampling plan in order to account for annual fluctuations apparent in glass eel recruitment. [ABSTRACT FROM AUTHOR]

Understanding recruitment of glass eels in estuaries is crucial for the conservation of the European eel (Anguilla anguilla). However, basic knowledge on estuarine-specific glass eel migration, including in estuarine harbours, is mostly lacking. Therefore, we studied glass eel migration in the Dutch–German Ems estuary and the harbour at Delfzijl (The Netherlands) and tagged glass eels with Visual Implant Elastomer tags (VIE tags). We released 2000 tagged glass eels into the Ems estuary itself and 1000 tagged glass eels into the tidal harbour at Delfzijl. At three estuarine locations, i.e., Delfzijl–Duurswold, Termunterzijl, and Nieuwe Statenzijl, glass eel collectors were strategically placed, each location being progressively situated further upstream in the Ems estuary. Most glass eels (nuntagged = 97,089, ntagged = 74) were caught at Nieuwe Statenzijl, although this location is much further upstream. Lower numbers of glass eels (nuntagged = 1856, n tagged = 31) were caught at Delfzijl–Duurswold and Termunterzijl (nuntagged = 1192, ntagged = 7). Glass eels arrived approximately a week earlier at Nieuwe Statenzijl than at the other two locations, and the migration speed of tagged glass eels was highest at Nieuwe Statenzijl (>2 km/day) and lower (

Assisted propagation of the European eel will lead to a closed production cycle supplying the aquaculture industry with juvenile glass eels. Females require long-term weekly treatment with pituitary extract (PE), which is stressful and causes abnormalities in oogenesis. We tested the effects of 17α- methyltestosterone (17 MT), as potent androgen activating the androgen receptor, and 17β-estradiol (E2), as an inducer of vitellogenesis, to shorten the duration of PE treatment.Four groups of feminized eels were subjected to a simulated migration and subsequent injection with implants containing 17 MT (17 MT-group), E2 (E2-group) or 17 MT plus E2 (17 MT + E2-group) to test for synergistic effects, or without any steroids as controls (C-group). The effects of a 2-months treatment were investigated by determining the eye index (EI), hepatosomatic and gonadosomatic index (HSI and GSI, respectively), plasma steroid concentrations by liquid chromatography mass spectrometry (LCMS), gonadal histology, expression of androgen receptors a and b (ara, arb); estrogen receptor 1 (esr1); FSH receptor (fshr); vitellogenin receptor (vtgr) and aromatase (cyp19), and the required number of weekly PE injections to fully mature. For many parameters, both the 17 MT and E2 groups showed an increase vs. controls, with the 17 MT + E2 group showing a synergistic effect, as seen for EI, GSI (3.4 for 17 MT and for E2, 6.6 for 17 MT + E2), oocyte diameter and ara, arb and esr1 expression. Concentrations of almost all focal steroids decreased with simulated migration and steroid treatment. Only eels of the 17 MT-group showed increased expression of cyp19 and of fshr, while fshr expression increased 44-fold in the 17 MT + E2 group, highlighting that co-implantation is most effective in raising fshr mRNA levels. Specific for eels of the E2 groups were vitellogenesis-associated changes such as an increase of HSI, plasma E2, and presence of yolk in the oocytes. Steroid treatments reduced the duration of PE treatment, again synergistically for co-implantation. In conclusion, E2 is necessary to start vitellogenesis, but 17 MT has specific effects on cyp19 and fshr expression. The combination is necessary for synergistic effects and as such, steroid implants could be applied in assisted reproduction protocols for European eel to improve oocyte quality leading to the production of more vital larvae. [ABSTRACT FROM AUTHOR]

The role of methylation in adaptive, developmental and speciation processes has attracted considerable interest, but interpretation of results is complicated by diffuse boundaries between genetic and non‐genetic variation. We studied whole genome genetic and methylation variation in the European eel, distributed from subarctic to subtropical environments, but with panmixia precluding genetically based local adaptation beyond single‐generation responses. Overall methylation was 70.9%, with hypomethylation predominantly found in promoters and first exons. Redundancy analyses involving juvenile glass eels showed 0.06% and 0.03% of the variance at SNPs to be explained by localities and environmental variables, respectively, with GO terms of genes associated with outliers primarily involving neural system functioning. For CpGs 2.98% and 1.36% of variance was explained by localities and environmental variables. Differentially methylated regions particularly included genes involved in developmental processes, with Hox clusters featuring prominently. Life stage (adult versus glass eels) was the most important source of inter‐individual variation in methylation, probably reflecting both ageing and developmental processes. Demethylation of transposable elements relative to pure European eel was observed in European X American eel hybrids, possibly representing postzygotic barriers in this system characterized by prolonged speciation and ongoing gene flow. Whereas the genetic data are consistent with a role of single‐generation selective responses, the methylation results underpin the importance of epigenetics in the life cycle of eels and suggest interactions between local environments, development and phenotypic variation mediated by methylation variation. Eels are remarkable by having retained eight Hox clusters, and the results suggest important roles of methylation at Hox genes for adaptive processes. [ABSTRACT FROM AUTHOR]

The endangered European eel biological life cycle consists of five stages of metamorphosis: leptocephalus, glass eel, elver, yellow eel, and silver eel. Montenegro has limited data on biological and population parameters of glass eel e.g. accurate time and the number of migration waves, condition, rate of survival, recruitment success, etc. Since the early 1980s, the occurrence of glass eels in European waters has decreased significantly. Accordingly, after almost 25 years the first research was conducted on glass eel in the river Bojana (Montenegro). This study was done during the winter and spring of 2021 and 2022. Stage of pigmentation and condition were examined on 50 registered individuals of glass eel. Four pigmentation stages: VIA0, VIA1, VIA2, and VII were registered. Its biometric characteristics indicate that one migration wave was registered with the domination of stage VIA1. This research indicates the necessity of further research on the glass eel, as well as the threats they are facing in the river Bojana and another small river in the Adriatic catchment of Montenegro, with the aim of establishing the management plan for better protection conservation of this important but vulnerable species. [ABSTRACT FROM AUTHOR]

Freshwater eels are of tropical marine origin and have spread worldwide. Of the 19 recognised species or subspecies, all 6 temperate species are listed within IUCN Threatened Species categories, together with 7 tropical species. Many reasons have been advanced to explain the significant declines of eels including habitat fragmentation and loss, spread of parasites and diseases, pollution, impacts of hydro dams, overfishing and oceanic changes. The present paper reviews some of the unique life history characteristics of Anguillids—fecundity and semelparous spawning, adaptability, resilience, and being energetically conservative—but despite such remarkable adaptations, according to the IUCN the genus is facing possible extinction. It is suggested that Anguillid life history strategies rely on two additional components—environmental determination of sex, and the need for surplus production (the Surplus Production Hypothesis, SPH). Surplus production is required to counteract the many hazards that eels face throughout an often long and complex life history. Recruitment of temperate species has fallen by over 90%, and resulting lower densities of juvenile eels in tidal and lower river reaches provides less incentive for upstream migration and dispersal. Female eels are generally found further inland than males, with development of sex associated with lower densities. With fewer juveniles migrating to inland areas, the numbers of females will become reduced, and further compound the reduced spawner biomass and consequent reduced recruitment of glass eels. The SPH hypothesis emphasises the importance of conservative harvest of glass eels, habitat connectivity, the need to ensure maximum female escapement, and the importance of stocking inland waterways with juvenile eels. [ABSTRACT FROM AUTHOR]

To evaluate the quality of glass eels that were derived from late-stage Anguilla japonica leptocephali raised on an old-type of shark egg-based diet (SE) or a new-type of fish meal-based diet (FSY), specific gravity (SG), feeding tests, and body compositional and RNA-seq gene-expression analyses were conducted. The largest content difference between the two dietary groups was in lipids, with the body-compositional index (BCI) being 29.2% lower in FSY-leptocephali and 43.1% lower in FSY-glass eels at day 3 post-metamorphosis, but the difference between groups narrowed to 21.6% at post-metamorphosis first feeding. FSY-leptocephali had heavier SG, a larger number of differentially expressed genes (DEGs) related to muscle development and/or movement, and higher creatine kinase expression, suggesting that they settled easily, and were forced to swim more against fast bottom currents in the tanks while consuming more energy. It was found that the days until first feeding in FSY-glass eels was significantly shorter than in SE glass eels, with lower variability among eels. The number of DEGs was larger in FSY-glass eels, with the largest difference of 3.6 times occurring at day 10 post-metamorphosis, especially in the metabolism category, but at first feeding, they decreased drastically in both groups narrowing the difference. In the FSY-glass eels, the number of DEGs related to lipid, amino acid, and protein metabolisms were larger at days 3–20 after metamorphosis. Total enzyme expression levels for acylglycerol degradation, ketone body biosynthesis and citric acid cycle pathways also tended to be higher, suggesting that they obtained energy aerobically using lipids and other materials. Based on the patterns of up- and down-regulation of expression levels, growth hormone, ghrelin/obestatin prepropeptide and pro-melanin-concentrating hormone-like were detected as possible orexigenic hormones, and proopiomelanocortin and nucleobindin-2 as possible anorexigenic hormones. These results indicate that diet quality affects not only SG and other characteristics of leptocephali, but even body composition, metabolism and time to first feeding in the post-metamorphic glass eels for use as seedlings for aquaculture. • Anguilla japonica leptocephali and glass eels, reared using two diets, were examined • Type of diet affected the body composition of leptocephali and glass eels • Diet affected leptocephalus buoyancy and their muscle-related gene expressions • The days until first feeding of glass eels after metamorphosis was affected by diet • First feeding started when stored nutrition such as lipid decreased after consumption [ABSTRACT FROM AUTHOR]

Japanese eels (Anguilla japonica) are a typical migratory fish species with high commercial importance. The Pearl River estuary in southern China is an important natural growing ground for Japanese glass eels, but limited information on Japanese glass eel population characteristics is available, despite their ecological importance. In this paper, we examined the annual patterns of the occurrence of Japanese glass eels in the Pearl River estuary from 2011 to 2022. The most frequently occurring Japanese glass eel’s total length is 5.3 cm. The collecting period extended from December to February, and the collection catch-per-unit-effort (CPUE) decreased significantly from 2011 to 2022. The generalized linear model (GLM) indicated that daily changes in Japanese glass eel collection were significantly affected by tidal range, water temperature, and lunar distance. The catch peak appeared when the tidal range rose to 1.7 m, and the water temperature dropped below 8 °C on the full moon days. Overall CPUE analysis showed no significant periodic and inter-annual variability in the period 2011–2022, with the ARIMA model suggesting that the CPUE is expected to remain stable but low in the coming years (2023–2026), although recruitment ultimately depends on the overall spawning stock.

Japanese eel, Anguilla japonica , holds significant importance in Taiwanese aquaculture. With the intensification of eel farming, the impact of Edwardsiella tarda has become increasingly severe. Consequently, the abusive use of antibiotics has risen. Bacillus subtilis natto NTU-18, a strain of Bacillus with a high survival rate in feed processing, plays a crucial role in promoting intestinal health through competitive rejection, enhancing immune responses against bacterial pathogens, and improving intestinal health by modulating gastrointestinal microbiota to produce beneficial metabolites of mice and grass carp, Ctenopharyngodon idella. This study investigated the effects of different proportions (control, 0.25 %, 0.5 %, 1 %, and 2 %) of B. subtilis natto NTU-18 added to paste feed on the growth performance, intestinal morphology, and microbiota, expression of immune-related genes, and resistance to E. tarda in Japanese glass eel. The results indicated that the growth performance of all groups with B. subtilis natto NTU-18 added was significantly higher than that of the control group and did not impact the villi morphology. The expression of immune-related genes in the kidney, specifically HSP70 and SOD, was significantly higher from 0.5 % and above than the control; however, no significant differences were observed in CAT, POD, and HSP90. In the liver, significant differences were found in HSP70 and IgM above 0.25 % compared to the control group, with no significant differences in SOD, CAT, POD, and HSP90 among all groups. Additionally, intestinal microbiota analysis revealed that the 2 % additional group had significantly lower diversity than other groups, with Cetobacterium as the dominant species. The challenge test observed that the survival rates of the 0.5 % and 1 % groups were significantly higher. This research suggests that adding 0.5 % and 1 % of B. subtilis natto NTU-18 to the diet is beneficial for Japanese glass eel's immunity, growth performance, and disease resistance. • Bacillus subtilis natto NTU-18 showed as great eel paste feed additive. • 0.5–1 % B. subtilis natto NTU-18 supplement was the most optimal for Anguilla japonica glass eel. • Overdosed of B. subtilis natto NTU-18 would reduce the gastrointestinal microbiota diversity of Japanese glass eel. [ABSTRACT FROM AUTHOR]