Female Eels Research Articles

Modeling individual growth trajectories of the female European eel in relation to temperature and habitat-use history in the Gironde River, France

AB Aquatic Biology Contact the journal Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AB 19:185-193 (2013) - DOI: https://doi.org/10.3354/ab00526 Modeling individual growth trajectories of the female European eel in relation to temperature and habitat-use history in the Gironde River, France Kazuki Yokouchi1,2,3,*, Françoise Daverat1 1IRSTEA, Unité EPBX, 50 avenue de Verdun, 33612 Cestas cedex, France 2Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa 277-8564, Japan 3Present address: Institute for East China Sea Research, Nagasaki University, 1551-7, Tairamachi, Nagasaki 851-2213, Japan *Email: kazukiy30@gmail.com ABSTRACT: Although the average growth rate of European eel is known to be generally dependent on temperature and habitat at the population scale, large variation in growth among eels in a single river basin can also be widely observed. In this study, individual growth trajectories of female European eels in the Gironde River basin, France, were modeled with habitat-use histories and thermal variations in relation to regional climatic events such as the heat wave in 2003 observed in SW Europe. A mixed-effects model was used to take into account the repeated measures of individual growth of 173 female eels in 1993 to 2003, obtained from otolith structure and chemistry. Temperature effect was tested using a proxy of efficient temperature for growth between 2 thermal thresholds in a year. The models with thresholds for growth between 12 and 15°C (lower) and 18°C (higher) were selected considering Akaike’s information criterion. Individual annual growth estimates were higher in the estuary habitat than in the river habitat. Estimated growth was higher when eels shifted habitats between freshwater and brackish water or seawater. The years 1999 and 2001 had, respectively, the lowest and highest temperature proxy value, showing that a longer growing season (moderate summer/warm winter) had a positive effect on growth. The 2003 heat wave had a negative effect on eel growth, reflected by the lower temperature proxy value. The same approach could be used in the future to estimate the effect of global climate change on the growth and distribution of eels at local and regional scales. KEY WORDS: Anguilla anguilla · Inter-annual temperature variation · Habitat-use history · Mixed-effects model Full text in pdf format PreviousNextCite this article as: Yokouchi K, Daverat F (2013) Modeling individual growth trajectories of the female European eel in relation to temperature and habitat-use history in the Gironde River, France. Aquat Biol 19:185-193. https://doi.org/10.3354/ab00526 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AB Vol. 19, No. 2. Online publication date: October 08, 2013 Print ISSN: 1864-7782; Online ISSN: 1864-7790 Copyright © 2013 Inter-Research.

Detecting genome-wide gene transcription profiles associated with high pollution burden in the critically endangered European eel

The European eel illustrates an example of a critically endangered fish species strongly affected by human stressors throughout its life cycle, in which pollution is considered to be one of the factors responsible for the decline of the stock. The objective of our study was to better understand the transcriptional response of European eels chronically exposed to pollutants in their natural environment. A total of 42 pre-migrating (silver) female eels from lowly, highly and extremely polluted environments in Belgium and, for comparative purposes, a lowly polluted habitat in Italy were measured for polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs) and brominated flame retardants (BFRs). Multipollutant level of bioaccumulation was linked to their genome-wide gene transcription using an eel-specific array of 14,913 annotated cDNAs. Shared responses to pollutant exposure were observed when comparing the highly polluted site in Belgium with the relatively clean sites in Belgium and Italy. First, an altered pattern of transcription of genes was associated with detoxification, with a novel European eel CYP3A gene and gluthatione S-transferase transcriptionally up-regulated. Second, an altered pattern of transcription of genes associated with the oxidative phosphorylation pathway, with the following genes involved in the generation of ATP being transcriptionally down-regulated in individuals from the highly polluted site: NADH dehydrogenase, succinate dehydrogenase, ubiquinol-cytochrome c reductase, cytochrome c oxidase and ATP synthase. Although we did not measure metabolism directly, seeing that the transcription level of many genes encoding enzymes involved in the mitochondrial respiratory chain and oxidative phosphorylation were down-regulated in the highly polluted site suggests that pollutants may have a significant effect on energy metabolism in these fish.

Water temperature manipulation can induce oocyte maturation and ovulation in the common Japanese conger, Conger myriaster

To develop seed production techniques for aquaculture of the common Japanese conger, Conger myriaster, we attempted to induce ovarian maturation and ovulation without exogenous hormone treatment and examined the changes in steroid hormones to understand the endocrine condition of the fish during their maturation process. Juveniles caught in November 2001 (20 to 30cm in total length) were reared for 1.5years in seawater at temperatures ranging from 10 to 20°C before the experiments. Then, 141 randomly selected female eels (58 to 82cm in total length) were maintained at a water temperature of 6°C from April 2003 to March 2004. The initial GSI values (5.0 to 12.0) and oocyte diameters (300 to 400μm) recorded in April 2003 had increased to maximum values of 30.0 and 650μm, respectively, at the end of experiment, in March 2004. In late December 2003, 12 fish with oocytes measuring over 600μm in diameter were selected and divided into an elevated temperature group from 6 to 10°C and a control group of 6°C for 43days. The mean oocyte diameter in the elevated temperature group increased significantly, and one fish (estimated GSI, 47.0) ovulated eggs with an average diameter of 995±46μm on the 41st day at a water temperature of 10°C, in contrast, the control group showed no significant changes in oocyte diameter, and no fish ovulated. Plasma T levels, examined via ELISA, increased with oocyte maturation, with the highest level detected at the migratory nucleus stage, and declined after ovulation. In contrast, plasma E2 was maintained at high levels during the vitellogenic stage, exhibiting the highest value after ovulation. The present study demonstrates that ovarian maturation and ovulation in the common Japanese conger could be achieved through water temperature manipulation.

Molecular characterization of three GnRH receptor paralogs in the European eel, Anguilla anguilla: Tissue-distribution and changes in transcript abundance during artificially induced sexual development

Gonadotropin-releasing hormone receptor (GnRH-R) activation stimulates synthesis and release of gonadotropins in the vertebrate pituitary and also mediates other processes both in the brain and in peripheral tissues. To better understand the differential function of multiple GnRH-R paralogs, three GnRH-R genes (gnrhr1a, 1b, and 2) were isolated and characterized in the European eel. All three gnrhr genes were expressed in the brain and pituitary of pre-pubertal eels, and also in several peripheral tissues, notably gills and kidneys. During hormonally induced sexual maturation, pituitary expression of gnrhr1a (female) and gnrhr2 (male and female) was up-regulated in parallel with gonad development. In the brain, a clear regulation during maturation was seen only for gnrhr2 in the midbrain, with highest levels recorded during early vitellogenesis. These data suggest that GnRH-R2 is the likely hypophysiotropic GnRH-R in male eel, while both GnRH-R1a and GnRH-R2 seems to play this role in female eels.

Using osmotic pumps to deliver hormones to induce sexual maturation of female Japanese eels, Anguilla japonica

This study examines how the continuous administration of various hormones using an osmotic pump, a long-term sustained hormone release system, influences the induction of sexual maturation in females of the Japanese eel, Anguilla japonica. Human chorionic gonadotropin (hCG), salmon pituitary extract (SPE), and gonadotropin-releasing hormone analogue (GnRHa) were tested, and the consequent egg quality was evaluated. The implantation of osmotic pumps loaded with SPE induced vitellogenesis and increased the gonadosomatic index (GSI) at 39–110days. In comparison, pumps loaded with hCG inconsistently induced early vitellogenesis, while those loaded with GnRHa did not exhibit any stimulatory effect. Fewer eels attained full maturity when using the osmotic pump system compared with SPE-injected female eels. However, after the final treatment, the number of eels that ovulated and the time required for ovulation were similar for both the osmotic pump and injection groups. Moreover, more eggs were spawned in the SPE-loaded osmotic pump group than in the SPE-injected group. Egg quality was similar for both experimental groups. Therefore, the implantation of an osmotic pump loaded with SPE represents a reliable method for inducing vitellogenesis and obtaining ovulated eggs from sexually immature Japanese eels.

Growth differences of Japanese eels Anguilla japonica between fresh and brackish water habitats in relation to annual food consumption in the Kojima Bay‐Asahi River system, Japan

AbstractTo learn about the relationships between feeding and growth of temperate eels in freshwater and brackish water habitats, we analysed 533 yellow‐phase Japanese eels Anguilla japonica collected in both types of habitats in southeastern Japan. Because male eels were very rare in each habitat (FW, N = 1; BW, N = 20), characteristics of female eels were compared between the different habitats. Annual food consumption was evaluated with the consideration of instantaneous food consumption and annual activity period. Stomach fullness index (stomach content weight/body weight) was used as an indicator of instantaneous food consumption. The ratios of number of months with eel catch to those when eel sampling was conducted were used as an indicator of activity period. Female yellow eels tended to be older and slower growing in fresh water (N = 78; age, mean ± SD = 7.9 ± 2.4 years; growth rate, 59.8 ± 14.0 mm year−1) than in brackish water (N = 229; age, 5.5 ± 1.8 years; growth rate, 90.1 ± 24.4 mm year−1). Irrespective of sex, yellow eels in brackish water had a higher stomach fullness index and a greater ratio of months with eel catches, indicating greater annual food consumption by brackish water eels. These results indicate that greater annual food consumption contributes to the greater growth rates of Japanese eels in brackish water habitats.

Induksi Pematangan Gonad dengan Ovaprim-C: untuk Penyediaan Benih Belut Sawah (Monopterus albus) Berkelanjutan

Rice eel ( Monopterus albus Zuiew), have economical value is high in Banyumas, national to international markets, but rarely is grown intensively, the seed is still dependent on the catch. Reproduction of swamp eel unlike other fish species in the presence of phase teleostei intersex / hermafroditus and in nature only spawn once a year. This study will seek to accelerate the scale seed production with natural canvas with several techniques triggers lust and sex organ maturation eels based preparedness aspects of reproductive anatomy, hormones estradiol and testosterone profiles and the intrinsic need for hormonal regulation and the mechanism for the reproductive cycle of the eel rice. In the first year aims to determine (1) the profile of estradiol and testosterone hormonal parent during the natural reproductive cycle, (2) histologic gonadogenesis (development of the testes and ovaries) eel rice during the natural reproductive cycle was evaluated by measuring fecundity. Results: In the control group, body weight gradually decreased during the study. Unlike the control group, the treatment groups were observed every two weeks showed an increase in body weight; Results of the study, in the control group (A 0 ), the value of female eels GSI average of the first two weeks until the fourth two weeks is 0.67 % and 1.78%. While the treatment group A1 was 1.42% - 4.28% (DM-1 s / d DM-4), for the treatment A2 GSI average value is 2.52% - 7.05%; fecundity eggs increased during induction period; calibration titer two types of hormones in the first two weeks to the fourth two weeks showed improvement when compared with the control group; histological profile of rice eel gonad after induction of ovaprim-C show that up to the fourth two weeks, gonad has reached late-stage yolk globule (advanced primary oocyte), even the most mature oocyte has reached the stage: in male eels up to the fourth two weeks has reached the stage of spermatozoa

Dam Removal Increases American Eel Abundance in Distant Headwater Streams

AbstractAmerican eel Anguilla rostrata abundances have undergone significant declines over the last 50 years, and migration barriers have been recognized as a contributing cause. We evaluated eel abundances in headwater streams of Shenandoah National Park, Virginia, to compare sites before and after the removal of a large downstream dam in 2004 (Embrey Dam, Rappahannock River). Eel abundances in headwater streams increased significantly after the removal of Embrey Dam. Observed eel abundances after dam removal exceeded predictions derived from autoregressive models parameterized with data prior to dam removal. Mann–Kendall analyses also revealed consistent increases in eel abundances from 2004 to 2010 but inconsistent temporal trends before dam removal. Increasing eel numbers could not be attributed to changes in local physical habitat (i.e., mean stream depth or substrate size) or regional population dynamics (i.e., abundances in Maryland streams or Virginia estuaries). Dam removal was associated with decreasing minimum eel lengths in headwater streams, suggesting that the dam previously impeded migration of many small‐bodied individuals (<300 mm TL). We hypothesize that restoring connectivity to headwater streams could increase eel population growth rates by increasing female eel numbers and fecundity. This study demonstrated that dams may influence eel abundances in headwater streams up to 150 river kilometers distant, and that dam removal may provide benefits for eel management and conservation at the landscape scale.

One century of eel growth: changes and implications

Abstract – A cooperative effort gathered a large European length‐at‐age data set (N = 45,759, Lat. 36S–61N Long. 10W–27E) for Anguilla anguilla, covering one century. To assess the effect of global warming during the last century and habitat effects on growth, a model was fitted on the data representing the conditions met at the distribution area scale. Two GLMs were designed to predict eel log(GR): one model was fitted to the whole data and the other was fitted to the female data subset. A model selection procedure was applied to select the best predictors among sex, age class, five temperature parameters and six habitat parameters (depth, salinity and four variables related to the position in the catchment). The yearly sum of temperatures above 13 °C (TempSUP13), the relative distance within the catchment, sex, age class, salinity class and depth class were finally selected. The best model predicted eel log(GR) with a 64.46% accuracy for the whole data and 66.91% for the female eel data. Growth rate (GR) was greater in habitats close to the sea and in deep habitats. TempSUP13 variable had one of the greatest predictive powers in the model, showing that global warming had affected eel growth during the last century.

11-ketotestosterone Synchronously Induces Oocyte Development and Silvering-Related Changes in the Japanese Eel,Anguilla japonica

To evaluate the effects of sex steroids on silvering in the Japanese eel, Anguilla japonica, the development of oocytes, eye size, digestive tract, and swim bladder were studied in relation to observations of the profiles of plasma levels of sex steroids (estradiol 17β, E2; testosterone, T; 11-ketotestosterone; 11-KT) during silvering for each sex and by administrating 11-KT to yellow eels. All steroids examined in the study increased in female eels after silvering had begun, whereas in males, only 11-KT increased significantly, and no statistical differences were found in plasma levels of E2 and T between eels in both developmental stages. 11-KT appeared to induce the early stage of oocyte growth, enlargement of the eyes, degeneration of the digestive tract and the development of the swim bladder. This suggested that 11-KT synchronously accelerates early development of the ovaries and the morphological changes, possibly in adaption to oceanic migration, and that 11-KT is one of the most important factors in early stages of development in the Japanese eel, as it appears to be in other anguillid eels.

Molecular Characterization and Gene Expression of Japanese Eel (Anguilla japonica) Gonadotropin Receptors

A luteinizing hormone receptor (lhr) cDNA with high identity to other fish lhrs was fully cloned from the ovary of the Japanese eel (Anguilla japonica). The genes for two gonadotropin receptors (Gthr), follicle-stimulating hormone receptor (fshr) and lhr, were differentially expressed during oogenesis, which was artificially induced by salmon pituitary extract, a gonadotropin-rich source. Transcript abundance of fshr was significantly elevated at the early vitellogenic stage and peaked at the late vitellogenic stage, while lhr gene expression rapidly induced at the late vitellogenic stage and thereafter remained at a high level. The abundance of fshr and lhr transcripts was highest in the ovary in female eels. In addition to the ovary, forebrain was a major site for the fshr transcript, although the level did not change with reproductive status. Furthermore, it was examined how eel Gthrs were activated by two mammalian chrionic gonadtropin (CG), equine CG (eCG) and human CG (hCG), that have been used for study of fish reproduction as substitutes for homologous Gths. Both CGs specifically activated eel Lhr, but not Fshr, although the degree of effectiveness was different; thus the concentration of hCG (0.1 ng/ml) required for significant activation of Lhr was much lower than that of eCG (100 ng/ml). These data on gene expression and ligand-activation of Gthrs suggest that Fsh and Lh act differentially in the regulation of reproductive function in Japanese eel.

Improvement in hatching rates in the Japanese eel Anguilla japonica by the control of rearing temperatures in the late stage of maturation in the female parents

In order to improve the hatching rate of eggs from artificially matured female eels, the environmental temperature during late stages of oocyte development was lowered from 20°C (control) to 18°C. Thus, female eels in which artificial maturation was induced at 20°C using chum salmon pituitary extract (SPE) were transferred to water at either 20 or 18°C at the beginning of the late stage of maturation. Biopsy by cannulation was carried out on the basis of an increase in body weight index (BWI; BWI=body weight×100/initial body weight). If the eel was judged to be nearing final maturation, a priming dose of SPE was administered, followed by a dose of maturation-inducing hormone (MIH). The average hatching rates of the eggs from the eels maintained at 20 and 18°C were 16.6% (n=10) and 45.9% (n=11), respectively. The hatching rates at 18°C were significantly higher than those at 20°C. At 20°C, the optimal timing of cannulation may often be missed due to both the rapid rate of oocyte development in the late stage and the unreliability of the BWI-based timing criterion. At 18°C, rapid development of maturing oocytes in the late stage was prevented, and the optimal ovulation induction period was prolonged. Accordingly, the likelihood for optimal timing of cannulation was increased, as reflected in high hatching rates of fertilised eggs. On the basis of these results, a temperature of 18°C in late stages of oocyte development is recommended to help in improving and stabilising the hatching rate.

The changes of serum biochemical components during carp pituitary extract and HCG induced maturation of the female Japanese eel (Anguilla japonica)

Based on the gonadosomatic indexes (GSI) of different developmental ovary stages of the fe-male Japanese eel during carp pituitary extract and HCG induced maturation, the paper measured and analyzed six biochemical components of the serum, namely total protein (TP), triglycerides (TG), total cholesterol (TC), blood glucose (Glu), calcium (Ca) and phosphorus(P) by medical inspection methods. The results showed that the gonad of treatment group got matured gradually during 84 days with GSI in-creasing significantly to 46.32%±1.75% at the 12th injection; while GSI of the control group stayed at 2.45% ± 0.30%. There was no difference between the TP level of treatment group and control group (P 0.05) which remained relatively stable. However, the blood lipid displayed significant difference (P 0.05) until 56th day: TG and TC of the treatment group maintained a high level, averaged at (12.67 ± 2.83) mmol/L and (15.54 ± 3.01) mmol/L respectively; while control group kept a low level, averaged at (6.33 ± 4.14) mmol/L and (8.53 ± 3.30) mmol/L respectively. The Glu level of both groups carried on rising from a low level at the beginning. Correlation analysis indicated that there was significant negative correlations between TG and GSI in treatment group (r =?0.732, n=6, P 0.01). Significant correlations were found between Ca and GSI (r =0.961, n=6, P 0.01), P and GSI (r =0.775, n=6, P 0.01)in treatment group: as the growth rates of GSI, and the level of Ca and P increased dramatically from 28th day, while the control group stayed at a low level. It turned out that there is a close relationship between ovary development and lipid metabolism, and some inorganic ions (Ca, P) have played an important role in this process. Accord-ing to the nutrition requirement and metabolic rule of the female Japanese eel, it’s a new potential way to judge ovary developmental stages by measuring and analyzing the level of blood lipid or serum Ca and P.

Induced oogenesis of the European eel (Anguilla anguillaL.) in freshwater condition

European eel is a catadromous fish species, which means that after living in freshwater premature individuals adapt to sea water, and migrate to the Sargasso Sea for spawning. Although male eel can be sexually matured even in freshwater, to date, it was believed that female eel can be matured only in seawater. Here we show that the process of sexual maturation may be induced in freshwater by treating female eels with carp pituitary (GSI = 9.87 ± 1.55%). It is thus proposed that seawater condition is not an obligatory environment for stimulating gametogenesis and for artificial maturation of the European eel in neither gender.

Differential regulation of the expression of cytochrome P450 aromatase, estrogen and androgen receptor subtypes in the brain–pituitary–ovarian axis of the Japanese eel ( Anguilla japonica) reveals steroid dependent and independent mechanisms

This study aimed at investigating the role of sexual steroids in the regulation of the expression of the single aromatase gene and steroid receptor subtypes in the brain–pituitary–ovarian axis of the Japanese eel. Unlike other teleosts, which possess duplicated genes for aromatase, cyp19a1a and cyp19a1b, expressed in the gonads and in the brain, respectively, eel species possess a single cyp19a1. Phylogenetic analysis indicated that eel brain/gonadal cyp19a1 branches at the basis of both teleost gonadal cyp19a1a and brain cyp19a1b clades. Female eels treated with catfish pituitary homogenate (CPH) to induce sexual maturation showed an increase in the expression of cyp19a1 and aromatase enzymatic activity in the brain and in the ovaries. Treatments with sex steroids (estradiol-17 β, E 2 or testosterone, T) revealed that the increase in cyp19a1 expression in the brain may result from E 2-specific induction. In contrast, the increase in cyp19a1 expression in the ovaries of CPH-treated eels is a result of steroid-independent control, probably from a direct effect of gonadotropins contained in the pituitary extract. Analysis of the expression of estrogen and androgen receptor subtypes, esr-α, esr-β, ar-α and ar-β, in eels treated with CPH or sex steroids revealed differential regulations. In CPH-treated eels, the expression of esr-α and ar-α was significantly increased in the brain, while the expression of ar- α and ar- β was increased in the ovaries. No change was observed in esr- β in any organ. Steroid treatments induced an upregulation by E 2 of esr- α, but not esr- β expression, in the brain, pituitary and ovaries, while no autoregulation by T of its own receptors could be observed. These results reveal both steroid-dependent and -independent mechanisms in the regulation of cyp19a1 and steroid receptor subtype expression in the eel.

Fusion of lipid droplets in Japanese eel oocytes: Stage classification and its use as a biomarker for induction of final oocyte maturation and ovulation

The quality of eggs obtained from maturation-induced Japanese eel, Anguilla japonica, is unstable. One of the causes for low quality eggs is that females are artificially induced to ovulate at an inappropriate maturational status. Here we investigated the feasibility to use the morphological changes of lipid droplets in oocytes as a biomarker to show the optimum timing for injections of salmon pituitary extract (SPE) for priming and maturation-inducing steroid (MIS) for final oocyte maturation and ovulation. Various sizes of oocytes (700 to 1000 μm) were collected by cannulation from maturation-induced female eels during five days until ovulation. Morphology of the lipid droplets in these oocytes were classified into 10 stages mainly on the basis of their diameter, which increased with the progress of maturation as the droplets fused together. To assess the maturational status of each female, the lipid droplet stage was determined for 10 oocytes randomly chosen among the most advanced mode of developing oocytes and the median value was defined as the stage for the individual. Relationship between the lipid droplet stage at induction of ovulation and hatchability of the ovulated eggs was examined in 23 females, in which the hatching rate markedly fluctuated. Stages of females from which good quality eggs were obtained converged around stage 4 (five largest droplets, 55 to 70 μm) at priming with SPE, stage 6 (90 to 110 μm) at injection with MIS, and stage 7 (110 to 130 μm) at ovulation, while those of females from which low quality eggs were obtained showed distributions over broader ranges. Furthermore, 466 females were induced to mature and the lipid droplet stage at induction of ovulation was examined in 13 females from which high quality eggs (hatching rate >80%) were obtained. Most of the 13 females were at stage 3 (40 to 55 μm) or 4 when primed with SPE, at stage 6 when injected with MIS, and at stage 7 when they ovulated. These results suggest that the optimum stages for SPE priming, MIS injection, and ovulation are stages 3 to 4, 6, and 7, respectively, though the optimum stage for priming will be variable if the temperature and/or the time between priming and MIS injection is varied. We conclude that the lipid droplet stage can be used as a sensitive biomarker to show the optimum timing for inducing ovulation by hormonal treatments.

Effect of Thermal Regime on the Expression of Sex-related Genes in Female European Eel

Effect of Thermal Regime on the Expression of Sex-related Genes in Female European Eel

Effects of Estradiol Valerate on Steroid Hormones and Sex Reversal of Female Rice Field Eel, Monopterus albus (Zuiew)

The rice field eel, Monopterus albus (Zuiew), is an important aquaculture fish in many Asian countries. Productivity, however, is limited by low fecundity as a result of sex reversal of female juveniles of this fish. This study was conducted to investigate the effects of estradiol valerate on the sex reversal process of this fish by determining sex changes of initial 2-yr-old female eels (26.5 ± 2.6 cm, 21.9 ± 2.1 g) treated with three concentrations of estradiol valerate: 0 mg/kg (Control, Group A), 10 mg/kg (Group B), and 50 mg/kg (Group C). In Group A, sex reversal took place with significantly higher percentage of sex change: about 45% of female eels to intersexes and males. Only about 22% of female eels changed to intersexes and males in Group B, but no sex reversal occurred in Group C during the experimental period. The result also indicated that the gonadal development and the concentrations of serum estradiol (E2) and testosterone (T) varied significantly with different concentrations of dietary estradiol valerate. Serum T significantly reduced but serum E2 significantly increased in Group C compared to the control Group A. The results demonstrated that estradiol valerate affected the secretion of sex steroid hormones in rice field eels and thus inhibited the sex reversal process at a low concentration (10 mg/kg) or stopped the sex reversal process at a high concentration (50 mg/kg) of estradiol valerate. These results suggest that a higher yield of mature female eels could be achieved by treatments with estradiol valerate, thus enhancing the fecundity of this rice field eel.

First capture of post-spawning female of the Japanese eel Anguilla japonica at the southern West Mariana Ridge

Two adult male freshwater eels, Anguilla japonica, were captured in June 2008 in the West Mariana Ridge (13°N, 142°E) in the North Pacific, but collections of females have yet to be reported. In September 2008, we successfully caught two adult female A. japonica, 55.5 and 66.2 cm in total length, in the adjacent but northern area (14°N, 143°E). Six newly hatched eel larvae (pre-leptocephali) were also collected by subsequent plankton sampling conducted near the female catch area. Female adults appeared to be in the post-spawning state, probably a considerable time after spawning, since a small number of remarkably regressed oocytes (50–250 μm in diameter) were observed in the ovaries. Capture of post-spawning female eels and newly hatched larvae near the Suruga Seamount (14°N) together with the previous collection of mature males in the southern area (13°N) corroborates that the area along the West Mariana Ridge is the spawning area of this species, but suggests that the eel spawning may occur over a wider area than previously expected.

Effects of dietary lipids on tissue fatty acids profile, growth and reproductive performance of female rice field eel (Monopterus albus)

The effects of different lipids on tissue fatty acid profile and reproductive performance in female rice field eel were investigated in this study. Virgin female eels were fed with six diets containing different lipids (diets FO, LO, SO, PO and PL with fish oil, linseed oil, soybean oil, peanut oil and pork lard, respectively; diet APO with arachidonic acid and peanut oil). The results showed that there were positive correlations between the contents of 18:2n-6, 18:3n-3, arachidonic acid (ARA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in the tissues of eels and those of the corresponding fatty acids in their diets. The specific growth rate of eels fed with diet PO was the lowest and significantly lower than that of FO and SO. Gonad of eels fed with diets PO and PL showed hypogonadism. The long chain polyunsaturated fatty acids (LC-PUFA) can be synthesized by eels, but the quantity was not enough to meet their reproduction requirement completely. The fatty acid desaturation, rather than elongation probably was one of the limiting factors. Addition of proper amount of ARA in diet was favorable to the increase of the hatching rate of fertilized eggs, while EPA and DHA in diet were beneficial to the increase of the survival rate of larva. Both n-3PUFA and a suitable n-6/n-3PUFA ratio were necessary for growth and reproduction of eels.