Freshwater Eels Research Articles

DNA barcoding of Philippine freshwater eels of families Anguillidae and Synbranchidae

The study used morphological and genetic approach for the DNA barcoding analysis of the inter- and intra-species relationships among catadromous Philippine freshwater eels. Past DNA barcoding studies on Philippine anguillid eels came from Northern Luzon. This study aimed to determine the DNA barcodes of the freshwater eels of the Philippines using the cytochrome c oxidase 1 (CO1) gene of the mitochondrial DNA. Specimens were collected from six sites in the Philippines. Four Anguilla species – Anguilla bicolor pacifica Schmidt, 1928, A. celebesensis Kaup, 1856, A. interioris Whitney, 1938 and A. marmorata Quoy & Gaimard, 1824 and one Monopterus species– Monopterus javanensis Lacepéde, 1800 were collected and identified. Morphological features varied from the taxonomic guides for Anguilla celebesensis and A. interioris with their body color and fin length. Genetic divergence estimates using Kimura 2-parameter substitution model showed an intraspecific variation of 0–0.4% and interspecific variation of 4.1– 27.6%. The ML tree generated was similar to the previous studies and indicated the monophyly of the Indo–Pacific freshwater anguillid eel lineage. This study also reports the first genetic record of M. javanensis sampled from Batac, Ilocos Norte, Philippines.

Contribution of migratory types to the reproduction of migrating silver eels in a tropical eel, Anguilla bicolorbicolor

The recent noticeable depletion of the catadromous eel populations has created global concerns for the eel stocks. Thus, the demand for tropical catadromous eels, which are a major alternate target species to compensate temperate species, is sharply increasing. However, eminently little is known regarding the biology and ecology of tropical eels across the Indo-Pacific region. To elucidate the contribution to reproduction among migratory types in a tropical eel, Anguilla bicolor bicolor, which is currently considered a major commercial target species, the otolith microchemistry was examined in silver (matured) stage eels. The broad range of otolith Sr:Ca ratios indicated that the habitat use was opportunistic among sea, brackish and fresh waters after recruitment to the continental habitats. Two migratory types, estuarine resident and marine resident eels, were found, but no freshwater resident eel, which is considered as the typical catadromous migration, was found. The lack of freshwater resident eels among all silver eels suggests that the estuarine and marine migrants colonizing adjacent to coastal areas might facilitate a substantial contribution to reproduction for the following generation in this area.

Molecular Characterization of Anguilla sp. with Special Notes on Its Population Genetic in the Rivers of Cilacap Central Java, Indonesia

Taxonomic status of Anguilla species in the rivers of Cilacap is vital for further study, such as population genetics and evolutionary history. Taxonomic status, population genetic, and evolutionary histories of Anguilla can be assessed using single genetic marker, such as cytochrome oxidase 1 gene. This study aimed to determine taxonomic status, genetic diversity and connectivity, and evolutionary history of Anguilla populations in the rivers of Cilacap. Anguilla samples were collected from Doplang-Serayu and Segara Anakan watersheds. Sequence homology test to the conspecific sequence in GenBank proved that all samples from Doplang-Serayu watershed were genetically identified as Anguilla bicolor (98.23 to 100% homology). The decision was supported by monophyly between samples and their conspecific references. Anguilla bicolor from both watersheds had high haplotype (gene) diversity. Amova and Fst analysis proved that no genetic difference (p=0.623) was observed between Doplang-Serayu and Segara Anakan watersheds, indicated panmixing. Haplotype network proved that both populations were evolved from two primitive ancestors. This study concluded that Anguilla bicolor was the only freshwater eel observed in the rivers of Cilacap. Anguilla bicolor in the rivers of Cilacap has high genetic diversity but no genetic differentiation was observed among populations. Anguilla bicolor population in the rivers of Cilacap evolved from two primitive ancestors.

Economics of freshwater Mud Eel fattening (Monopterus cuchia) in polyculture system: a case study from Nilphamari, Bangladesh

The present study was carried out to analyze the cost and returns as well as to determine the effect of variable inputs on outputs of Monopterus cuchia fattening in polyculture systems. A total of 60 Cuchia farmers were selected from Saidpur upazila in the Nilphamari District. Data were collected through face-to-face interviews from September 2018-April 2019. Average stocking density (no/acre) for M. cuchia, Heteropneustes fossilis, Clarias batrachus and Oreochromis niloticus were 2288±228, 2066±144, 1883±197 and 4842±715, respectively. The average production of Cuchia, shing, magur and tilapia were 979.17±128.65, 368.33±66.36, 330±51.42 and 1027.5±118.76 kg per acre, respectively. The total variable cost was BDT 191,225.9±96565.7 per acre whereas total fixed cost was BDT 85,731.45±6276.64 per acre. In the total variable cost, percentages of labor, feed, fertilization, liming, stocking and other cost were 12.01%, 36.01%, 0.38%, 1.47%, 14.55% and 6.59%, respectively. This study also revealed that per acre gross revenue and net return were BDT 452,025.58±35829 and 173,907.72±24560 respectively. The benefit cost ratio (BCR) value in Cuchia polyculture farming was 1.65. Based on the Cobb-Douglas production function model, fingerling cost and feed cost were significant of Cuchia fattening in polyculture system. The production function analysis also reveals positive effects of different variable costs indicating enough scope to increase production and income from Cuchia fattening in polyculture system with a reduction in production costs and increased market price

Morphological and Allometric Changes in Anguilla japonica Larvae.

Simple SummaryThe freshwater eel Anguilla japonica is a commercially important in Northeast Asia. However, eels farming is entirely dependent on natural catches, and eel resources are declining. This study was conducted to provide baseline information on the development of mass seed production technology necessary for the conservation of species and the maintenance of aquaculture. This study was conducted for 200 days after hatching (DAH) and analyzed morphometry and allometry. In this study, cultured eel larvae stages were divided in size similar to wild eel larvae, but cultured eel differed from wild eel in growth rate and the number of preanal myomeres. In addition, as eel larvae rarely have mixed feeding periods, it is important to determine the optimal first feeding time. The eel larvae may need a change in diet type to prevent lower jaw deformity in the leptocephalus stage. As eel larvae changed to a willow leaf-like form, the relative growth pattern of the eel larval stages was unique. This growth pattern may reflect the early life history of long distance and diel vertical migration. Meanwhile, the inflection point in the body parts’ growth patterns showed only before 30 DAH and was similar to the period of mass mortality. Therefore, future studies should focus on developing an optimal feeding and rearing protocol from the first feeding to 30 DAH.The freshwater eel Anguilla japonica is rapidly decreasing in number and has not yet been successfully mass produced. This may be at least partially attributable to the unique and long early life history of the eel. Therefore, we investigated its ontogeny of morphometry and growth pattern in larval stages to provide baseline information for understanding the early life history and improving seed rearing technology. This study was conducted for 200 days after hatching (DAH) and analyzed morphometry and allometry for eel larvae. The following cultured eel larval stages were identified: the yolk sac larvae stage (0–6 DAH, 3.23–6.85 mm total length (TL)), the pre-leptocephalus stage (7–30 DAH, 6.85–15.31 mm TL), and the leptocephalus stage (50–200 DAH, 15.31–60.06 mm TL). Cultured and wild eel larvae could be divided into characteristic larval stages at similar sizes. However, compared to wild eels, cultured eels had a slower growth rate and fewer preanal myomeres. Meanwhile, cultured eel larvae rarely had a mixed feeding period as the absorption of endogenous reserves was completed by 7 DAH. The lower jaw of eel larvae was significantly longer than the upper jaw from 50 DAH. In the pre-leptocephalus and leptocephalus stages, eel larvae showed continuous positive allometric growth at trunk height and tail muscle height with change to the willow leaf-like form. These growth characteristics may be the result of adaptation to the migration over long distances and to a diel vertical migration. The inflection point in the body parts growth patterns showed only before 30 DAH, and mass mortality appeared at this period. Therefore, to improve the growth and survival rates of cultured eel seed, it is necessary to focus on improving the feeding and rearing protocol until 30 DAH.

Migration ecology in the freshwater eels of the genus Anguilla Schrank, 1798

Migration ecology in the freshwater eels of the genus Anguilla Schrank, 1798

Demographics and exploitation of two Near Threatened freshwater eels, Anguilla bengalensis and Anguilla bicolor, in small‐scale subsistence fisheries and implications for conservation

Abstract Tropical freshwater eels (Anguilla bengalensis and Anguilla bicolor) contribute a major share of the world's wild‐caught eel production, having become the next major target owing to the declines in availability of both Anguilla japonica and Anguilla anguilla, species that have traditionally contributed to eel aquaculture and trade. Although both A. bengalensis and A. bicolor are assessed as Near Threatened on the International Union for Conservation of Nature Red List, these assessments are primarily based on anecdotal information and local knowledge on population declines. Demographics and exploitation levels of A. bengalensis and A. bicolor were determined from small coastal river systems, and their adjoining aquascapes in the Western Ghats hotspot of southern peninsular India, and the value of these data for future conservation planning discussed. The computed estimates of annual catch data for freshwater eels from the study region are between 0.17 (A. bicolor) to 0.30 t (A. bengalensis). Virtual population analysis of exploitation showed a drastic decline (in number) in the length groups >45 cm for both species, suggesting that they were less likely to attain their asymptotic length in the region. Current exploitation rates of A. bengalensis are unsustainable, and those for A. bicolor are almost close to reaching this level. Exploitation of all life stages from yellow eels to adults (29–171 cm) is likely to cause recruitment failure and significant mortality of spawning individuals of both species. An integrative conservation approach, including raising awareness (leading to voluntary restrictions), fishing closures in reservoirs, village‐level quotas, and regular monitoring of populations, will ensure a sustainable future for the freshwater eel species in the Western Ghats hotspot, and elsewhere in the tropics where these species are exploited.

Oxygen isotope fractionation of otoliths formed across the maximum thermal range for somatic growth of the American eel Anguilla rostrata

Stable isotope compositions of otoliths can be used to provide thermal histories of fish. This tool, however, is currently unrefined for catadromous species, such as the American eel Anguilla rostrata, where the otolith is formed in both marine and freshwater environments. We reared 2000 elvers in fresh water for 32 weeks in 10 temperature treatments ranging from 10 to 34 °C, and then measured δ18O and δ13C of otoliths to 1) determine how somatic and otolith growth responded to temperature; 2) evaluate the relationship between water temperature and δ18O and δ13C of otoliths; and 3) develop a species-specific isotopic fractionation equation that isolates the freshwater portion of the otolith. Our results show that eel elvers have a high optimum temperature for somatic growth (27–28 °C). Optimum temperature for otolith growth was slightly higher, suggesting that otolith growth can be decoupled from body growth. The expression for carbonate-water isotopic fractionation of American eel otoliths over the temperature range examined was 1000lnαaragonite-water = 14.30(103 T−1) – 18.651. This equation accurately predicted water temperature (predictive error 0.49 °C) for 180 American eel otoliths obtained from a different controlled rearing study (conducted at 22 and 28 °C). This experiment validated the use of the oxygen and carbon isotope compositions of otoliths to determine the thermal history of American eels in fresh water, thus providing a method that can be applied to wild eels.

Conservation of freshwater eels in food‐web studies: Non‐lethal stable isotope analyses substitute fin for muscle tissue with lipid correction

AbstractAquatic food‐web studies involving fish and using stable isotope analysis typically sample fish white muscle, which generally requires the fish to be euthanised. Alternatively, fin tissue can be sampled non‐lethally, avoiding the need to euthanise an animal to obtain what is usually a relatively small tissue sample. We set out to develop equations for converting δ13C and δ15N values from derived non‐lethally sampled fin tissue to equivalent white muscle tissue, for use in food‐web studies. We explored δ13C and δ15N values of paired fin‐clips and white muscle from two species of anguillid fishes, the endemic longfin eel (Anguilla dieffenbachii), and the Australasian shortfin eel (A. australis), sourced from multiple studies spread throughout New Zealand. We also examined the effect of lipid extraction on δ13C and δ15N, which reduced variability of δ13C when compared to untreated samples. We were also able to estimate δ13C values of equivalent lipid‐extracted muscle samples directly from untreated fin. For δ15N, paired untreated fin and muscle values were not statistically different so fin δ15N can be used as a direct estimate of muscle δ15N. Solvent lipid extraction with a methanol–chloroform mixture, however, altered δ15N values and increased variability, so should be avoided. We present equations to estimate lipid‐corrected δ13C values of muscle tissue for use in stable isotope studies of freshwater anguillids, avoiding euthanasia. Our results demonstrate that δ13C and δ15N values can be accurately derived from non‐lethally obtained fin clips, reducing unnecessary mortality in fish species of conservation concern.

Phylogenetic analysis and characterization of a new parasitic cnidarian (Myxosporea: Myxobolidae) parasitizing skin of the giant mottled eel from the Solomon Islands.

Myxosporeans are microscopic cnidarians associated with severe diseases in aquaculture and wild fish populations. This group of parasitic cnidarians thus warrants close attention concerning its potential impact on susceptible fish stocks. At present, little is known about this group of parasites infecting anguillid eels. From myxospore specimens collected from a freshwater eel (Anguilla marmorata) in the Solomon Islands, we describe a new species belonging to the genus Myxobolus based on an integrative taxonomic analysis of morphological, biological traits and molecular data. Furthermore, we determined the phylogenetic position and relationships of this species among other platysporine myxosporeans. Molecular phylogenetic assessment of small subunit ribosomal DNA showed that the species clusters together with Myxobolus portucalensis and Echinactinomyxon type 5 Özer, Wootten and Shinn, 2002, in a well-supported subclade. This is the first report of a myxosporean parasite infecting fish from the Solomon Islands.

Development of droplet digital Polymerase Chain Reaction assays for the detection of long-finned (Anguilla dieffenbachii) and short-finned (Anguilla australis) eels in environmental samples.

Freshwater eels are ecologically, and culturally important worldwide. The New Zealand long-finned eel (Anguilla dieffenbachii) and short-finned eel (Anguilla australis) are apex predators, playing an important role in ecosystem functioning of rivers and lakes. Recently, there has been a national decline in their populations due to habitat destruction and commercial harvest. The emergence of targeted environmental DNA detection methodologies provides an opportunity to enhance information about their past and present distributions. In this study we successfully developed species-specific droplet digital Polymerase Chain Reaction (ddPCR) assays to detect A. dieffenbachii and A. australis DNA in water and sediment samples. Assays utilized primers and probes designed for regions of the mitochondrial cytochrome b and 16S ribosomal RNA genes in A. dieffenbachii and A. australis, respectively. River water samples (n = 27) were analyzed using metabarcoding of fish taxa and were compared with the ddPCR assays. The presence of A. dieffenbachii and A. australis DNA was detected in a greater number of water samples using ddPCR in comparison to metabarcoding. There was a strong and positive correlation between gene copies (ddPCR analyses) and relative eel sequence reads (metabarcoding analyses) when compared to eel biomass. These ddPCR assays provide a new method for assessing spatial distributions of A. dieffenbachii and A. australis in a range of environments and sample types.

Influence of environmental parameters on habitat use by sympatric freshwater eels Anguilla marmorata and Anguilla japonica on Yakushima Island, Japan

It is known that environmental parameters, such as water depth, size of substrate material, and current velocity, influence the organization of stream fish communities. However, few studies have investigated the effects of these parameters on the sympatric freshwater eels. Here, the habitat use of two anguillid eels, the Indo-Pacific eel (Anguilla marmorata Quoy and Gaimard, 1824) and the Japanese eel (Anguilla japonica Temminck and Schlegel, 1846), coexisting in the same river systems in Japan, was investigated. It was found that the tropical species A. marmorata exclusively used habitats with larger substrate materials, fewer fallen leaves and less leaf detritus, higher current velocity, and lower turbidity than the temperate species A. japonica, and the distributions of the two anguillid eels were clearly segregated. Thus, when both species co-occur in the same river systems, clear flowing mainstems and tributaries are preferred habitats for A. marmorata, whereas stagnant muddy estuaries, backwater areas, irrigation channels, and reservoirs are preferred habitats for A. japonica. These habitat segregations were consistent as body size increased and life-stage developments from elver to yellow eel and did not show ontogenetic shifts. The findings indicated that both species had strong habitat preferences, and these environmental parameters must be considered in the conservation of anguillid eels in sympatric zones.

Migrating silver eels return from the sea to the river of origin after a false start.

The European eel's singular spawning migration from European waters towards the Sargasso Sea remains elusive, including the early phase of migration at sea. During spawning migration, the movement of freshwater resident eels from river to sea has been thought to be irreversible. We report the first recorded incidents of eels returning to the river of origin after spending up to a year in the marine environment. After migrating to the Baltic Sea, 21% of the silver eels, tagged with acoustic transmitters, returned to the Narva River. Half returned 11-12 months after moving to the sea, with 15 km being the longest upstream movement. The returned eels spent up to 33 days in the river and migrated to the sea again. The fastest specimen migrated to the outlet of the Baltic Sea in 68 days after the second start-roughly 1300 km. The surprising occurrence of returning migrants has implications for sustainable management and protection of this critically endangered species.

Investigation of mutations (L41F, F17M, N57E, Y99F_Y134W) effects on the TolAIII-UnaG fluorescence protein's unconjugated bilirubin (UC-BR) binding ability and thermal stability properties

The UnaG protein is a ligand (unconjugated bilirubin) dependent fluorescence protein isolated from Unagi freshwater eel larvae and expressed as fusion in heterologous expression systems. Bilirubin is a tetrapyrrole molecule mainly produced from heme catabolism by the destruction of erythrocytes in the body. Bilirubin can cause kernicterus, a serious condition associated with permanent neurological damage in neonates with the passage of brain tissue. Different methods have been developed for plasma bilirubin analysis and quantification. The use of UnaG fluorescence protein triggered by bilirubin has become a new approach in bilirubin studies. In this study, we aimed to investigate the biophysical characterization of ligand interactions with the proteins obtained as a result of mutations (UnaGY99F_Y134W, UnaGN57E, UnaGL41F, and UnaGF17M) on the amino acid sequence of TolAIII-UnaG protein. After the purity levels of the expressed proteins have been analyzed by SDS-PAGE, secondary structures and thermal melting temperatures of the proteins have been examined by circular dichroism spectroscopy. Then determination of excitation and emission points by fluorescence spectroscopy, titration studies have been performed with bilirubin, and dissociation constant was calculated. According to the biophysical characterization studies, UnaGL41F has the highest affinity and stability among the mutants.

Genetic Characterization of Swamp Eel of Bangladesh Through Dna Barcoding and Rapd Techniques

The freshwater air-breathing swamp eel Monopterus spp. are native to the freshwater of Bangladesh and throughout the Indian subcontinent. To identify the different swamp eel species and to check the genetic diversity among them, a total of twelve swamp eel specimens were collected from four districts (Tangail, Bogura, Bagerhat and Sylhet) representing the four division of Bangladesh. The extracted DNA from twelve fish samples was amplified by the PCR technique for DNA barcoding and RAPD analysis. Among 12 specimens, 8 specimens showed a 95-100% similarity with M. cuchia species published in the NCBI GenBank database and BOLD system. The studied mct3 (collected from Tangail region), mcs1, mcs2 and mcs3 (collected from Sylhet region) specimens showed about 83% homology with Ophisternon sp. MFIV306-10 as per BLAST search; whereas BOLD private database showed 99% similarity with Ophisternon bengalense (Bengal eel). From the phylogenetic tree analysis, 8 samples were clustered with M. cuchia and 4 samples showed similarity with Ophisternon sp. MFIV306-10 and Ophisternon bengalense _ANGBF45828-19. In RAPD-PCR based analysis, it was found that the maximum genetic distance (1.6094) was observed between mcba2 and mcs3, while between mct1 and mct2, the minimum genetic distance was 0.000. A total of 192 bands, of which 35 were polymorphic with 17.88% polymorphisms among swamp eel species and the size of the amplified DNA fragments ranged from 250 to 1800 bp. The information on DNA barcoding and RAPD analysis help measure the genetic diversity among swamp eel species, ensure the reliability of the published taxonomic information, and initiate proper management programs to conserve these vulnerable species to meet future export demand.
 Asiat. Soc. Bangladesh, Sci. 46(2): 117-131, December 2020

Feeding habitat and silvering stage affect lipid content and fatty acid composition of European eel Anguilla anguilla tissues.

Lipids, particularly fatty acids (FAs), are major sources of energy and nutrients in aquatic ecosystems and play key roles during vertebrate development. The European eel Anguilla anguilla goes through major biochemical and physiological changes throughout its lifecycle as it inhabits sea- (SW), and/or brackish- (BW) and/or freshwater (FW) habitats. With the ultimate goal being to understand the reasons for eels adopting a certain life history strategy (FW or SW residency vs. 'habitat shifting'), we explored differences in lipid content and FA composition of muscle, liver and eyes from eels collected across Norwegian SW, BW and FW habitats, and at different lifecycle stages (yellow to silver). FW and SW eels had a higher lipid content overall compared to BW eels, reflecting differences in food availability and life history strategies. SW eels had higher proportions of certain monounsaturated FAs (MUFAs; 18:1n-9, 20:1n-9), and of the essential polyunsaturated FAs 20:5n-3 (eicosapentaenoic acid, EPA) and 22:6n-3 (docosahexaenoic acid) than FW eels, reflecting a marine-based diet. In contrast, the muscle of FW eels had higher proportions of 18:3n-3, 18:2n-6 and 20:4n-6 (arachidonic acid), as is typical of FW organisms. MUFA proportions increased in later stage eels, consistent with the hypothesis that the eels accumulate energy stores prior to migration. In addition, the decrease of EPA with advancing stage may be associated with the critical role that this FA plays in eel sexual development. Lipid and FA information provided further understanding of the habitat use and overall ecology of this critically endangered species.

Trophic ecology of the European eel (Anguilla anguilla) across different salinity habitats inferred from fatty acid and stable isotope analysis

We combined fatty acid (FA) and stable isotope (SI) analyses to investigate the trophic ecology of different stages of European eels (Anguilla anguilla) across seawater (SW), brackish water (BW), and freshwater (FW) habitats. Salinity was the main driver of differences in the biochemical composition, and the greatest variation occurred between SW and FW eels. SW eels had a higher content of the FA indicator of carnivory, as well as the highest stable isotope ratios (C, N). In contrast, FW eels exhibited the highest lipid content and omega-6 polyunsaturated FA, but the lowest stable isotope ratios, suggesting major dietary differences between the eels in these two habitats. While the biochemical composition of BW eels was closer to those of SW eels, BW eels had the largest SI range, indicating higher dietary plasticity. FW individuals had better overall condition compared to SW eels. Independent of habitat, larger individuals were in the best condition, and had higher lipid content and monounsaturated FA. These findings suggest a biological advantage for eels to maintain a catadromous life history strategy.

Climbing for dummies: recommendation for multi‐specific fishways for the conservation of tropical eels and gobies

AbstractDams and other man‐made barriers impair upstream fish migration and thus threaten fish populations that need access to upper river reaches to complete their life cycle. For many years, fishways have been used to mitigate this impact. Fishways around the globe are typically built based on recommendations made for northern hemisphere species, particularly salmonids. These recommendations do not consider the locomotor characteristics and skills of other species, especially those living in the tropics. Among tropical species, freshwater eels and gobies of the Sicydiinae subfamily are important cultural and economic species that are particularly sensitive to the impact of man‐made barriers. Our experimental study aimed to test different substrates and slopes for ramp‐like fishways adapted to tropical eels and sicydiines. Among the five substrates tested for 368 eels Anguilla marmorata, elastomer pins appeared to be the most efficient. Elastomer pins also appeared to be more efficient than the fine concrete which is currently used in fishways for sicydiines (Sicyopterus lagocephalus, N = 1797, and Cotylopus acutipinnis, N = 1303). The slope had a lesser effect on the climbing success of sicydiines compared to substrate type, except for gradients greater than 50° that induced a slight decrease in success. Our results indicated that ramp‐like fishways fitted with 1.0 cm diameter elastomer pins, positioned in staggered rows with a diagonal spacing of 1.3 cm, wetted with low‐flow and angled less than 50°, are well adapted to accommodate the different locomotor characteristics and skills of tropical eels and sicydiines.

An extensive clean-up method for extraction of 17β-estradiol from eel aquaculture waste solids for quantitation via high-performance liquid chromatography tandem-mass spectrometry

The use of 17β-estradiol (E2) in aquaculture feed increases growth and promotes feminization in freshwater eels. E2 and its metabolites (estrone (E1) and estriol (E3)) must be accurately measured in solid and liquid waste streams to ensure proper management. For reliable quantitation of estrogens in complex aquaculture waste solids, a rigorous clean-up method is imperative. The focus of this study was to assess, optimize, and validate a quick, easy, cheap, effective, rugged, and safe (QuEChERS) technique for extraction of estrogens from aquaculture waste solids and quantitation via liquid chromatography tandem-mass spectrometry (LC-MS/MS). Waste solids were collected from a recirculating aquaculture system containing American eels. Waste solids samples (2 g) were spiked to 50 μg kg−1 of E1, E2, and E3 and processed using 800 mg magnesium sulfate (MgSO4) and 400 mg of sodium acetate (NaOAc) as extraction salts. Two dispersive solid phase extraction (dSPE) steps were implemented to achieve optimal E2 recovery efficiency (RE (%)) in preliminary experiments (RE (%) = 102%; RSD = 5%). RE (%) for E1 and E3 were 87% (RSD = 54%) and 60% (RSD = 83%), respectively. Method performance characteristics were further assessed for E2 only and included specificity, linearity, accuracy (RE (%) and process efficiency (PE (%))), precision (intra-day precision, and inter-day precision), matrix effects, method detection limit (MDL), and limit of quantitation (LOQ). MDL and LOQ values for E2 were 0.7 and 2.3 μg kg−1, respectively. PE (%) and RE (%) were excellent (99–101% and 99–100%, respectively) and matrix effects were minimal (99 and 101%). The results of this study indicate that this method can be used for quantitation of E2 in aquaculture waste solids.

Contrasting patterns of genetic population structure in tropical freshwater eels of genus Anguilla in the Indo-Pacific

Freshwater eels, genus Anguilla, have a distinctive catadromous life history, which could be associated with certain oceanic current systems and offshore spawning sites. Thus, migration and dispersion patterns are believed to be important factors influencing the population structure of each species. Temperate eel species are well studied, while little research has been conducted on the tropical counterparts that comprise two-thirds of all eel species. The population structure of three tropical species, A. marmorata, A. bicolor bicolor and A. bengalensis bengalensis, which are distributed widely in the Indo-Pacific region, were explored by means of DNA sequence analysis of mitochondrial cytochrome c oxidase subunit I (COI). This study suggests that A. bicolor bicolor might have two genetically distinct populations (fixation index, FST = 0.891; p < 0.001) that co-occur geographically in the Indo-Pacific region, while A. marmorata and A. bengalensis bengalensis might have a panmictic-population structure in this region. This study is the first to explore the population genetic structure of A. bengalensis bengalensis. The present results also suggest plausible dispersion and migration of these tropical species into their continental habitats.