Eel Samples Research Articles

Polychlorinated dibenzo-p-dioxins, dibenzofurans and biphenyls in fish from the Netherlands: concentrations, profiles and comparison with DR CALUX® bioassay results

Fish from Dutch markets were analysed for concentrations of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) and dioxin-like polychlorinated biphenyls (DL-PCBs) and compared with the new European maximum residue levels (MRLs), set in 2006. In a first study on 11 different fish and shellfish from various locations, concentrations of PCDD/Fs were nearly all below the MRL for PCDD/Fs [4 pg toxic equivalents (TEQ) per gram wet weight (ww)] and nearly all below 8 pg total TEQ/g ww, the new MRL for the sum of PCDD/Fs and DL-PCBs. Some samples exceeded the total TEQ MRL, such as anchovy, tuna and sea bass. Furthermore, 20 (out of 39) wild eel samples exceeded the specific MRL for eel (12 pg total TEQ/g ww), as the study revealed PCDD/F TEQ levels of 0.2-7.9 pg TEQ/g ww and total TEQ values of 0.9 to 52 pg/g ww. TEQ levels in farmed and imported eel were lower and complied with the MRLs. Smoking eel, a popular tradition in the Netherlands, only had marginal effects on PCDD/F and DL-PCB concentrations. Owing to volatilization, concentrations of lower-chlorinated PCBs were reduced to below the limit of quantification after smoking. DL-PCBs contributed 61-97% to the total TEQ in all eel samples. This also holds for other fish and shellfish (except shrimps): DL-PCB contributed (on average) from 53 (herring) to 83% (tuna) to the total TEQ. Principal-component analysis revealed distinctive congener profiles for PCDD/Fs and non-ortho PCBs for mussels, pikeperch, herring and various Mediterranean fish. The application of new TCDD toxic equivalency factors (TEFs) set by the World Health Organization in 2006 (to replace the 1997 TEFs) resulted in lower TEQ values, mainly owing to a decreased mono-ortho PCB contribution. This decrease is most pronounced for eel, owing to the relative high mono-ortho PCB concentrations in eel. Consequently, a larger number of samples would comply with the MRLs when the new TEFs are applied. The DR CALUX(R) assay may be used for screening total TEQ levels in eel, in combination with gas chromatography-high resolution mass spectrometry confirmation of suspected samples. An almost 1:1 correlation was found when the 1997 TEFs were applied, but, surprisingly, a 1.4-fold overestimation occurred with application of the 2006 TEFs.

Detection of Hexabromocyclododecane and Its Metabolite Pentabromocyclododecene in Chicken Egg and Fish from the Official Food Control

During routine gas chromatography with electron capture detection (GC/ECD) analysis of chicken eggs, we observed that the most prominent peak in some samples did not match the retention time of any of the food contaminants screened. Subsequent GC coupled with mass spectrometry (GC/MS) studies clarified that the mass spectrum of the peak was very similar to hexabromocyclododecane (HBCD), which was also identified by GC/MS in the egg. The unknown compound was positively identified as pentabromocyclododecene (PBCDE), a metabolite of HBCD detected for the first time in foodstuffs. Studies of the analytical method used for the analysis of pesticides and contaminants showed that this cleanup method was suitable for the determination of HBCD and PBCDE, but storage of sample extracts resulted in the loss of HBCD when the sample extracts were not sufficiently purified. The concentrations of HBCD and PBCDE in the high polluted sample were 2.0 and 3.6 mg/kg egg fat. HBCD and PBCDE were also detected in two additional eggs at lower levels (<0.15 mg/kg), whereas 75 eggs did not contain these compounds (<0.02 mg/kg). We also detected HBCD and PBCDE in two samples of whitefish (Coregonus sp.), while an eel sample (Anguilla anguilla) positively tested for HBCD did not contain PBCDE. Surprisingly, the potential metabolite of HBCD, PBCDE, has not been detected before in any food or environmental sample. The present results indicate that more attention should be paid to the detection of HBCD and its metabolite PBCDE in chicken eggs.

Quality attributes of farmed eel ( Anguilla anguilla) stored under air, vacuum and modified atmosphere packaging at 0 °C

The shelf life of fresh eel in various packaging conditions of atmospheric air, vacuum and modified atmosphere packaging (MAP) (40% CO 2, 30% N 2 and 30% O 2) at 0 °C was investigated. All raw eel samples received acceptable sensory scores during the first 11±1 days of storage in atmospheric air, 11±1 days of storage in vacuum and finally 18±1 days of storage in MAP conditions. Using the microbial quality indicators the shelf life of eel packed in air, vacuum and MAP was estimated to be more than 18, 28 and 34 days, respectively. The main spoilage microorganisms under MAP conditions were lactic acid producing bacteria followed by Shewanella spp., pseudomonads, Enterobacteriaceae and yeasts. Chemical data revealed that pH, ammonia, glucose and lactate examinations might not be useful for monitoring eel quality differences.

The use of the DR CALUX® bioassay and indicator polychlorinated biphenyls for screening of elevated levels of dioxins and dioxin‐like polychlorinated biphenyls in eel

The DR CALUX bioassay is a very suitable screening method for dioxins and dioxin-like-PCBs in feed and food. This was, e. g. demonstrated in a survey in the Netherlands to control the dioxin levels in eel. The DR CALUX assay, but also indicator polychlorinated biphenyls (PCB) were evaluated as a screening method. Based on the limit for polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/F) [at that time 8 pg toxic equivalents (TEQ)/g eel], and the relation between PCDD/F and dioxin-like-PCB, a decision limit of 30 pg TEQ/g eel was used for screening of 153 field samples. Suspected samples (21) and part of the higher contaminated negative samples (35) were analyzed by GC/MS for dioxins, non-ortho, mono-ortho and indicator PCB, revealing 13 samples exceeding the action limit of 30 pg TEQ/g eel. Only one sample slightly exceeded the dioxin level of 8 pg TEQ/g eel. The relatively low sensitivity for mono-ortho PCB was overcome by the use of reference samples, as shown by the correlation of 0.93 between GC/MS and CALUX determined total TEQ levels. The present data show that the DR CALUX assay can be used for screening of total TEQ levels in eel. The use for dioxins only requires a safe, and therefore relatively low, decision limit. The indicator PCB also showed a good correlation with total TEQ levels, mainly due to the large contribution of the mono-ortho PCB at higher concentrations. The relation with dioxins was very poor and as such indicator PCB seem less suitable than the DR CALUX assay for screening for dioxins only. The present study clearly shows that part of the wild eel samples contains high total TEQ levels and will exceed the future European Union limit of 12 pg TEQ/g eel for dioxins and dioxin-like PCB. Especially at high TEQ levels, dioxin-like PCB contribute most to the total TEQ. In practice, wild eel presents only a minor part of the eel consumed.

Development and Optimization of an Indirect Enzyme‐Linked Immunosorbent Assay for Thiamphenicol

The specificity of an enzyme‐linked immunosorbent assay (ELISA) was modified by changing the chemical structure of the haptenated coating antigen. A competitive indirect immunoassay was developed, evaluated, and validated to measure thiamphenicol (TAP) in eels. The working range of the assay was placed between 0.45 and 8.35 µg/L. Moreover, the influence of several physicochemical parameters, such as incubation time, ionic strength, detergent concentration, and pH, were studied. The specificity of this immunoassay was evaluated using 23 structurally related compounds. Finally, a good correlation was observed between the chromatographic and immunoassay techniques while analyzing five certified eel samples.

How does the European eel (Anguilla anguilla) retain its population structure during its larval migration across the North Atlantic Ocean?

A Lagrangian model is presented of the current-carried migration of the leptocephali (larvae) of the European eel (Anguilla anguilla) across the North Atlantic Ocean from the spawning area in the Sargasso Sea to the adult range in Europe and North Africa. The success of larvae in crossing the Atlantic Ocean and reaching particular latitude bins on the eastern side depended strongly on starting location in the Sargasso Sea and migration depth. In the model domain, silver eel spawners can develop strategies for spawning location and migration depth to preferentially target particular regions in the adult range. This observation may help to explain the presence of gradients in molecular markers in eel samples collected across Europe. Spawning in the period of late winter – spring maximizes the average food availability along the 2-year larval trajectory. The fastest transatlantic larval migration in the model is about 2 years, and the route to Europe takes most of the larvae past the east coast of North America in the first year. These model results are consistent with the hypothesis that the European and American eel (Anguilla rostrata) could separate themselves on different sides of the Atlantic Ocean on the basis of the different durations of their larval stages.

Distribution of genetic variation in farmed and natural stocks of european eel

European eel (Anguilla anguilla; Teleostei) is a valuable commercial species. However, over the past 25 years, the population of European eel has been declining to such a degree that major concerns have been raised for its long‐term conservation. Since little information is available on the life‐cycle and genetic structure of European eel, it has been difficult to evaluate the existence of any population substructuring. Molecular genetic methods contribute to a better knowledge of the demography and population structure in marine fish. In addition, management strategies and conservation goals must consider information on genetic substructuring as well as on life history patterns.The aim of the study is to provide more detailed knowledge on the genetic variability, demography and population substructuring of European eel by analysing and comparing natural and farmed individuals. Natural eel samples have been obtained in two geographical sites (Netherlands, France) including temporal samples in a short‐scale (within years) and a long‐scale (between years). Simultaneously, farmed glass eels have been grown in two separate batches during one year. Batches have been monitored and genetic samples have been obtained during the year.A combination of selection‐sensitive (allozymes) and selection‐neutral markers (microsatellites) has been used in the study since selection seems to play an important role in the determination of the quality of future eel spawners. Results suggest a positive correlation between growth and genetic variability since individuals attaining a large length and mass present significant higher heterozygosities.

ＬＣ／ＭＳ／ＭＳによる食品中のキノロン剤の同時定量

A simple method was developed for the simultaneous determination of seven quinolones (enoxacin, ofloxacin, ciprofloxacin, danofloxacin, lomefloxacin, enrofloxacin and sarafloxacin) in foods using liquid chromatography coupled with electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). The seven quinolones were extracted with acetonitrile containing 0.2% formic acid, and the extracted solution was cleaned up on a C18 cartridge. The extract was diluted with 5 mmol/L IPCC-MS3 for injection into the LC-ESI-MS/MS. The LC separation was carried out on an ODS column with gradient elution of 5 mmol/L IPCC-MS3-acetonitrile as the mobile phase. Mass spectral acquisition was done in the positive ion mode by applying selected reaction monitoring (SRM). The recoveries of the seven quinolones were mostly greater than 60% from foods fortified at 10 ng/g. The detection limits in foods were 2 ng/g for enoxacin and ciprofloxacin, and 1 ng/g for the other drugs. Twenty cattle muscle, 7 swine muscle, 9 chicken muscle, 16 milk, 19 prawn and 20 broiled eel samples from retail markets were analyzed by this method. Enrofloxacin and its metabolite ciprofloxacin were detected in 9 broiled eel at the level of trace (tr)-34 ng/g and tr-10 ng/g, respectively.

Study on PCBs, PCDD/Fs, organochlorine pesticides, heavy metals and arsenic content in freshwater fish species from the River Turia (Spain)

In this study samples of common trout, European eel and barbel have been analysed for the levels of heavy metals such as lead (Pb), cadmium (Cd), zinc (Zn), copper (Cu), and the metalloid arsenic (As). Polychlorinated biphenyls (PCBs), including three non- ortho substituted PCBs, 2,3,7,8-substituted polychlorinated dibenzo- p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and DDT and its two main metabolites were also analysed in some selected samples, in order to evaluate the extent of contamination of the River Turia basin. All samples presented detectable levels of the elements analysed, presenting zinc the highest concentrations. European eel was the species found to highly accumulate these contaminants. Regarding organochlorinated compounds, seven samples exhibited detectable levels of the most toxic PCDD/Fs congener, the 2,3,7,8-TCDD. Non- ortho PCBs were not detected or not quantified in almost all samples and the sum of the individual PCB congeners determined were low compared to other aquatic ecosystems with similar fish species.

INITIAL INFECTION, DISTRIBUTION, AND SEASONAL OCCURRENCE OFHOFERELLUS GILSONI(MYXOZOA) WITHIN A POPULATION OFANGUILLA ROSTRATA(ANGUILLIDAE) IN SOUTHERN NOVA SCOTIA

Recently transformed elvers of Anguilla rostrata, entering the mouth of the East River in the Timber/Connaught Lake watershed (Nova Scotia) from the sea in mid-May, did not develop infections of the urinary bladder parasite Hoferellus gilsoni (Myxozoa) after nearly 3 mo in captivity. However, 40% of elvers collected from the mouth of the river in early June and 80% in late June exhibited patent infections by mid-August. It is speculated that given this initial level of infection, the summer run of elvers (estimated to be 208,000-963,000 individuals annually) represents a significant upstream dispersal of spores. Infections in yellow eel, which has a limited home range, probably contribute to spore dispersal within the home range and downstream through spore drift. Sporogenesis occurred in samples of yellow eel collected over an 8-mo period from April to November. Eel length was similar in 3 widely separated sampling locations in the watershed. Prevalences at these localities were 44, 36, and 57% in April and, by June, reached 76, 70, and 90%, respectively. Prevalences remained near the June levels until sampling ceased in November 1998. Studies on captive fish suggest that H. gilsoni can overwinter in the eel. Data from the study indicate that immediate infection of young elvers entering the system, effective spore dispersal, and a seasonally extended sporogenesis all contribute to the ubiquitous presence of the parasite in the eel population.

Genetic structure of New Zealand eels Anguilla dieffenbachii and A. australis with allozyme markers

Abstract – Glass eels and adults of Anguilla australis and A. dieffenbachii were collected from rivers in the North and South Islands of New Zealand and tested for nine and ten allozyme loci, respectively. There was no heterogeneity among glass eel samples, but there was heterogeneity among adult samples within species. The results indicate that there may be single panmictic populations subject to sweepstake events or different selective pressures in the adult environments.

Population structure of the Japanese eel Anguilla japonica as examined by mitochondrial DNA sequencing

SUMMARY: To examine the population structure of the Japanese eel Anguilla japonica, mitochondrial DNA sequence analysis was made for various samples of glass eels that might reflect genetic characteristics of spawning aggregates. The mitochondrial DNA from a total of 51 glass eels collected at Tanegashima Island, Kanagawa and Ibaraki in different periods within an inshore migrating season was sequenced for a 615 base-pair fragment from the tRNAThr gene to the central part of the control region. The DNA region was so variable that no individual was found to possess the same sequence, although average sequence differences within samples (1.07–1.63) were not large. Average sequence differences between samples (1.22–1.57) were comparable to those within samples, suggesting no genetic heterogeneity among samples. Tree analysis of the sequences showed neither a geographical nor temporal structure of population. Furthermore, although all DNA sequences from the present study were different from one another, two sequences were found to be the same with those reported from individuals collected in different localities in different years. Altogether, the present mitochondrial DNA sequence analysis of glass eels did not provide any evidence for genetic subdivision of the Japanese eel population.

Polycyclic musk fragrances in different environmental compartments in Berlin (Germany)

The aim of this study was to obtain data about the contamination of different environmental compartments (102 surface water samples, 59 sediment samples and 165 eel samples) by polycyclic musks (HHCB, AHTN, ADBI, AHMI, and ATII) within the framework of an exposure monitoring program. Results for HHCB (Galaxolide ®) gave the following mean values in areas strongly polluted with sewage: surface water 1.59 μg l −1; sediment 0.92 mg kg −1 d.w. and eel 1513 μg kg −1 f.w. (in the edible portion) (6471 μg kg −1 lipid). The following average concentrations were found in waters hardly contaminated with sewage: surface water 0.07 μg l −1, sediment <0.02 mg kg −1 and eel 52 μg kg −1 f.w. (445 μg kg −1 lipid). Mean concentrations of 6.85 μg l −1 (maximum: 13.3 μg/l) could be measured at sewage treatment plants’ outlets. It could be shown that these polycyclics are highly suited to use as indicators of the degree of contamination of waters with organic substances originating from sewage. A mean bioconcentration factor (BCF) on wet weight of 862 (HHCB) and 1069 (AHTN) for the transfer from water to eel under natural conditions could be calculated. The corresponding BCF-values based on the lipid content of eel were 3504 (HHCB) and 5017 (AHTN).

Distribution of polychlorinated biphenyls in marine species from French Frigate Shoals, North Pacific Ocean

Polychlorinated biphenyls (PCBs) were analyzed in: sediment; coral ( Porites evermanni); fish ( Stegastes fasciolatus, Neoniphon sammara, Acanthurus triostegus and Mulloidichthys vanicolensis); crab ( Grapsus tenuicrustatus); lobster ( Panulirus marginatus); and eel ( Conger cinereus, Gymnothorax flavimarginatus, G. undulatus and G. meleagris) samples collected from Tern Island and the corresponding reference samples from Disappearing Island. The two islands are part of French Frigate Shoals, a national wildlife refuge in the North Pacific Ocean. The dominant congeners 118, 138 and 153 represent 22–25, 32–34, 12–39, 37–46 and 30–55% of the sum of PCBs in the coral, sediment, fish, crab and eel, respectively. In general, high trophic species such as eels were found to highly bioaccumulate PCBs. The total average PCB concentrations were as high as 96 and 29 μg/g dry wt. in eels and damselfish, respectively, from Tern Island. The localized behavior and high bioaccumulation potential for PCBs suggest that damselfish are an excellent species for monitoring PCBs in small areas in the ocean. The high average concentrations of the sum of PCBs in different food chain levels suggest that pollution source(s) are around Tern Island and possibly around Disappearing Island. Aroclor 1254 and its analogs are suspected sources responsible for PCBs in the samples.

Assessing acceptability of eel (Anguilla anguilla) fed three different diets

Acceptability and preference of eels fed on three different diets—a commercial type (control) and two experimental types, one enriched with polyunsaturated fatty acids (fish-oil) and the other with saturated fatty acids (coconut-oil)—have been studied in three Spanish consumer panels from towns in three different regions (La Coruna, Seville and Valencia). The results of the Wilcoxon test indicate that in general the diet does not have significant effects (p < 0.05) on the overall acceptability of the eel samples, either in the overall evaluation or in the evaluation of the sensory attributes of appearance, aroma, flavour and texture, although in one of the panels (Seville) the overall and flavour acceptability scores were significantly higher for the fish-oil-fed eel than for the control-fed eel. Using the Friedman test, we did not observe significant differences (p < 0.05) in the preferences, except in one panel (Seville) where there was a significant preference for the fish-oil sample rather than the control sample. The multinomial logit indicates that the choice of preferred sample appears to be associated with the acceptability of its flavour, texture and aroma and overall impression. © 1999 Society of Chemical Industry

Levels of synthetic musks; Bromocyclene and PCBs in eel (anguilla anguilla) and PCBs in sediment samples from some waters of Berlin / Germany

The purpose of this study was to identify firstly the extent of contamination both of the aquatic biota and of sediments and secondly any regional differences in the concentrations of these compounds. In 1996, polychlorinated biphenyls (PCBs, including their coplanar congeners) were measured in 58 eel and 50 sediment samples. Furthermore, the veterinary pharmaceutical bromocyclene and the nitro musks were determined in 84 (1995) and 122 (1996) eel samples. Polycyclic musk fragrances were additionally determined in analyses carried out in 1996. The mean values obtained during the two measurement periods (1995 and 1996) were 24 and 12 μg/kg fw. for musk xylene, 41 and 39 pg/kg fw. for musk ketone, 14 and 7 μg/kg fw. for bromocyclene and, in 1996, 592 μg/kg fw. for HHCB (maximum: 4131 μg/kg fw.). Decreasing contamination levels are seen for musk xylene and bromocylene but not for musk ketone. This tendency, and the high amounts of polycyclic musk fragrances in the edible parts of eel show that these musks are widely being used in place of nitro musks and that they reach the aquatic system via waste waters, especially those of sewage treatment plants. The mean levels of PCBs in the biota samples of a highly polluted area were 1227 μg/kg fw. or 119 pgTEQ/g (sediment: 203 μg/kg dw.) and of a slightly polluted area 340 μg/kg fw. or 49 TEQ/g (sediment: 47 μg/kg dw.).

Biomonitoring aquatic pollution with feral eel ( Anguilla anguilla). III. Statistical analyses of relationships between contaminant exposure and biomarkers

In a large-scale field study, sediments and eel ( Anguilla anguilla) samples were collected from six Amsterdam freshwater sites with varying degrees of pollution. All sediment and eel samples were analyzed for organic trace pollutants, such as polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs) and polycyclic aromatic hydrocarbons (PAHs). In addition, the pollution-induced responses of a suite of 21 biochemical parameters in eel (notably phase I and phase II biotransformation enzymes, antioxidant enzymes, PAH metabolites, DNA adducts and serum transaminases) were measured. The resulting comprehensive database was subjected to statistical analyses in order to determine the biomarkers which were most suitable to assess inland water pollution and to classify the environmental quality of the sites. Bivariate correlation analysis, principal component analysis (PCA) and residual maximum likelihood analysis (REML) all revealed that the eel tissue levels of most PCB and OCP analyte groups were suitable to assess exposure to these contaminants, whereas PAH tissue levels were not. The phase I biotransformation enzymes in eel were found to be the most responsive to organic pollutants in the environment. Phase II enzymes and cofactors, as well as DNA adducts, were found to be less sensitive biomarkers, whereas the antioxidant enzymes and the serum transaminases did not show statistically significant correlations with pollutant levels. Similar results were obtained by means of the postulated bivariate correlation-significance index (CSI) and the multivariate PCA analysis. Discriminant analysis (DA) was used to classify the pollution status of the various sites. It appeared that the best discrimination between reference sites, moderately polluted sites and heavily polluted sites was obtained using DA on data of the nine most responsive biochemical markers. The importance of monitoring biota for the classification of the pollution status or environmental quality of freshwater sites was demonstrated in the present study, since no clear discrimination between moderately and heavily polluted sites could be made using sediment pollutant levels only. The results indicate that biological effect monitoring is the only appropriate method providing a reliable environmental risk assessment.

Biomonitoring aquatic pollution with feral eel ( Anguilla anguilla) I. Bioaccumulation: biota-sediment ratios of PCBs, OCPs, PCDDs and PCDFs

Samples of sediments and eel taken from six Amsterdam freshwater sites were analyzed for polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), polychlorinated dibenzofurans (PCDFs) and polychlorinated dibenzo- p-dioxins (PCDDs). Biota-sediment accumulation factors (BSAFs) were determined from levels in sediments [ng/g organic matter (OM)] and eel muscle tissues [ng/g lipid weight (LW)]. Bioaccumulation patterns were both site- and analyte-specific. Bioaccumulation of these persistent organic trace pollutants in eel is discussed, considering partitioning, uptake from contaminated water (bioconcentration) and food (biomagnification), clearance, and bioavailability. Trends in bioaccumulation profiles indicated the following. Levels of PCBs, DDTs, HCB, PCDFs and PCDDs in eel tissues appeared to be exposure related since they increased with increasing levels of pollution in sediments. High BSAF values indicated that biomagnification contributed significantly to the total bioaccumulation process for PCBs, DDTs and HCB. Higher BSAF values for the higher chlorinated PCBs might be due to a more efficient biomagnification of these congeners and to a selective metabolism of the lower chlorinated congeners. Bioaccumulation of HCHs, drins and heptachlor compounds was low in eel. Bioaccumulation of PCDF and PCDD congeners was extremely low in eel. BSAF values of these congeners decreased with increasing chlorine substitution. A large variation in BSAF values for most of the hydrophobic chemicals was observed between different sites. This suggests that bioaccumulation not only depends upon type of organism and analyte, but that site-specific factors should also be taken into account. This might be due to differences in bioavailability (including desorption kinetics) of the compounds or to differences in the diet of the fish when biomagnification is involved. Estimated PCB toxic equivalent values (TEQs) were higher than calculated PCDF/ D TEQs at the reference site. The opposite was found at the most polluted site.

Gas Chromatographic and Mass Spectrometric Identification of Tetrachloroalkanoic and Dichloroalkenoic Acids in Eel Lipids

Chlorinated fatty acid methyl esters, transesterified from the lipids of eel (Anguilla anguilla; obtained from the receiving waters of a chlorine bleaching pulp mill), were studied by gas chromatography (GC) using electrolytic conductivity detection and mass spectrometry (MS) with electron impact and ammonia positive-ion chemical ionization. GC retention indices and column difference values in combination with GC/MS demonstrated the presence of isomers of methyl dichlorotetradecenoate, methyl dichlorohexadecenoate, methyl dichlorooctadecenoate and methyl tetrachlorotetradecanoate. Isomers of methyl threo,threo-tetrachlorooctadecanoate were identified by co-injection of the eel sample and synthesized reference compounds. The results were confirmed by ammonia positive-ion chemical ionization high-resolution selective-ion monitoring of the ammonium adduct molecular ions.

Sexual dimorphism in yellow European eels, Anguilla anguilla (L.)

Sexual dimorphism in yellow eels, Anguilla anguilla (L.). was studied to clarify the differences between sexes. The focus was on the relationship between length/weight and sex. Three samples of eels, grown in captivity in different places in Israel, were studied. Females were significantly lighter than undifferentiated eels in the length range 33-47 cm. The regression coefficient did not differ significantly between females and undifferentiated eels. Sex induction, by a single injection of human chorionic gonadotropin, resulted in a higher loss of body weight during differentiation into the female than into the male.