American eel, Anguilla rostrata, growth in fresh and salt water: implications for conservation and aquaculture

Abstract  Growth of American eels, Anguilla rostrata (Lesueur), sampled in saltwater bays and adjacent freshwater ponds in the southern Gulf of St. Lawrence, Canada, was estimated by otolith annulus back-calculation, and related to salinity of growth habitat as determined using otolith strontium:calcium ratios. Eels resident in salt water grew on average in length 2.2 times faster than freshwater residents and increased in weight 5.3 times faster than freshwater residents. Previous findings of superior American eel growth rate in brackish water are thus extended to full-strength salt water. Freshwater residents took an estimated 2.4 times longer than saltwater residents to reach the silver eel stage. The extended time to maturation of eels reared in fresh water increases their vulnerability to fisheries and to bio-accumulation of contaminants. Freshwater eel populations should be managed more conservatively than those in saline waters. The increase in eel growth with salinity suggests that rearing in brackish or salt water could improve growth rates in eel aquaculture.     

Vitamin D3 and the renal handling of phosphate in American eels

Cholecalciferol (Vitamin D₃) increased plasma inorganic phosphate concentration in American eels,Anguilla rostrata, in a dose-dependent fashion. This response was more marked in phosphate loaded fish. In control as well as phosphate loaded eels the hyperphosphatemic response to D₃ was associated with a sharp reduction in renal phosphate clearance relative to¹⁴C-polyethelene glycol (PEG) clearance. Glomerular filtration and urine flow rates were not affected by D₃. As renal phosphate clearance, even in phosphate loaded eels, never significantly exceeded that of PEG, it is suggested that D₃ reduced the relative clearance rate of phosphate by increasing renal phosphate reabsorption rather than by reducing the tubular secretion of phosphate.     

Insulin-like growth factor I of Japanese eel, Anguilla japonica: cDNA cloning, tissue distribution, and expression after treatment with growth hormone and seawater acclimation

In an attempt to understand growth regulation in the Japanese eel, Anguilla japonica, we cloned insulin-like growth factor-I (IGF-I) cDNAs and examined their mRNA expression in several tissues. Two eel IGF-I (eIGF-I) cDNAs encoding preprohormones, eIGF-I-Ea1and eIGF-I-Ea2, were cloned from the liver by polymerase chain reaction (PCR). The preproIGF-Is were identical in signal peptide and mature IGF-I, but different in the E domain—eIGF-I-Ea2 mRNA was 36 bp longer than eIGF-I-Ea1 mRNA. Eel IGF-I was 83–94% identical with that of teleosts, 71% identical with that of dogfish, 87% identical with that of bullfrog and chicken, and 83% identical with that of humans. In both males and females the highest eIGF-I-Ea1 mRNA levels were observed in the liver, with detectable levels also found in the gills, heart, stomach, spleen, kidney, intestine, swim-bladder, muscle, and gonads. eIGF-I-Ea1 mRNA levels in the liver were higher in females than in males whereas in the intestine they were lower than in males. eIGF-I-Ea2 mRNA was detected in all the tissues examined and at similar levels in males and females. In this experiment higher eIGF-I-Ea1 mRNA levels were observed in the liver of larger glass eels than in those of smaller fish. eIGF-I-Ea2 mRNA levels were also higher in larger eels, although they were lower than IGF-I-Ea1 mRNA levels. Both eIGF-I mRNA levels in liver were positively correlated with the body size of the␣glass eels. Intraperitoneal injection of recombinant eel GH (reGH), 0.25 μg g⁻¹ body weight, into glass eels resulted in a significant increase in both eIGF-I mRNAs in the liver 1 day after injection compared with control fish, but no elevation was observed 2 days after injection. Incubation of liver slices with reGH at concentrations of 10, 100, and 1,000 ng mL⁻¹ for 24 h resulted in a significant concentration-dependent increase in the levels of both eIGF-I mRNAs. Higher levels of eIGF-I-Ea1 and Ea2 mRNA were observed in the gills ofseawater-reared eels than in those of freshwater-reared fish, but no differenceswere observed in the whole kidney. These results suggest that IGF-I is involved in the regulation of somatic growth and also in adaptation of the Japanese eel to seawater.     

Modification in Body Fat Content and Fatty Acid Profile of Wild Yellow Shortfin Eel,Anguilla australis,through Short‐term Fattening

Wild‐catch eels with low fat content are either not accepted at all or accepted as a lower‐quality and less valuable product in the global eel market. This study was undertaken with the goal of increasing the fat content of yellow shortfin eel, Anguilla australis (104 ± 11 g initial weight), captured in the wild and kept in captivity for a short period, as well as assessing their rate of growth. An 86‐d feeding trial was conducted indoors in a recirculation aquaculture system, at 25.5 ± 1.6 C. Two commercial extruded dry feeds were tested; one formulated for European eel and one not specifically intended for use on eels. Good values of specific growth rate (1.10 and 1.12%/d) and feed conversion ratio (1.01 and 1.10) were achieved with both tested feeds. At the end of the trial, eels achieved 20–22% total fat, starting from wild fish with 7% fat content. Fat quality of the fattened eel was appreciably superior (higher proportion of omega‐3 fatty acids) than the fat of eels captured in the wild. The results of this study are encouraging for the prospect of fattening yellow shortfin eels and obtaining a market‐oriented eel product.     

Morphological changes in gill mitochondria-rich cells in cultured Japanese eel <Emphasis Type="Italic">Anguilla japonica</Emphasis> acclimated to a wide range of environmental salinity

Morphological changes in gill mitochondria-rich (MR) cells were examined in cultured Japanese eel acclimated to deionized freshwater (DFW), freshwater (FW), 30%-diluted seawater (DSW), and seawater (SW). The gill Na⁺/K⁺-ATPase activity was higher in SW-acclimated eel than in those acclimated to DFW, FW, and DSW. Immunocytochemical observations revealed that MR cells in the gill filaments were most developed in SW, whereas MR cells in the lamellae were preferentially observed in DFW, suggesting that filament and lamellar MR cells are responsible for ion secretion and absorption, respectively. In scanning electron microscopic observations, the apical membrane of lamellar MR cells appeared as a flat or slightly projecting disk with a mesh-like structure on its surface. In contrast, the apical membrane of filament MR cells showed a slightly concave surface. Whole-mount immunocytochemistry revealed that most MR cells showed cystic fibrosis transmembrane conductance regulator immunoreaction in their apical region in fish in DSW and SW, but not in those in DFW and FW, indicating that MR cells developed in DSW and SW function as an ion-secreting site. In addition to MR cells, distinct Na⁺/K⁺-ATPase immunoreaction was observed in the outermost layer of gill epithelia, suggesting that pavement cells are an additional site of ion uptake in the gills.     

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.     

Somatic growth, survival, feed utilization and starvation in European elver Anguilla anguilla (Linnaeus) under two different photoperiods

The aim of the study was to determine the effect of two photoperiod regimes under artificial conditions on growth performance and survival in the European eel Anguilla anguilla during the transition from glass eel to elver. The fish were held in 12-h-light:12-h-dark (12L:12D) and in continuous darkness (DD) in fed and fast trials lasting 80 days. Fed fish kept in the dark showed higher somatic growth, and a better feed conversion ratio, higher protein and lipid content, with significant differences between treatments. Dark environment increased the survival rate in comparison with fish exposed to 12L:12D, during which eels showed lower survival rates and greater variations in size. Fasted fish suffered loss of body mass in both photoperiods at the end of the experimental phase, but fish exposed to 12L:12D lost more body mass. These results indicate that the photoperiod is a key factor during the glass eel–elver stage, which may contribute to optimizing the early stages of eel culture.     

Effects of salinity on osmoregulation,growth and survival in Asian swamp eel (Monopterus albus) (Zuiew 1793)

The Asian swamp eel Monopterus albus is normally considered a freshwater species, but can also occasionally be found in brackish water. It is an obligate air‐breather with highly reduced gills, making its osmoregulation physiology interesting because the gills normally represent the primary site of osmoregulation in teleosts. Being a popular fish for human consumption, the swamp eel is cultured extensively in freshwater ponds in the Mekong Delta of Vietnam. However, very little is known about its salinity tolerance, which is becoming topical due to the increasing salt‐water intrusions into tropical deltas. We therefore studied how increased salinity affects survival, growth, blood pressure, heart rate, blood osmolality and plasma ions. All eels survived prolonged exposure to 5 and 10 g L^?1, although plasma osmolality increased significantly at 10 g L^?1. Further elevation to 15 and 20 g L^?1 was associated with significantly elevated mortality, with a corresponding increase in plasma osmolality and ion con‐centrations. Our results show that swamp eels thrive in 0–10 g L^?1 with an optimum growth between 0 and 9 g L^?1, indicating that utilization of low saline brackish waters for aquaculture is possible.     

Freshwater protected areas: an effective measure to reconcile conservation and exploitation of the threatened European eels (Anguilla anguilla)?

Abstract – For decades, the European eel Anguilla anguilla (L.) population has been declining strongly despite several management attempts, so additional experiments need to be conducted on management measures. The use of freshwater protected areas has been advocated but their efficiency has never been assessed. In this study, we investigated whether the population structure and the silver eel (mature migrating stage) production differ in fished and protected areas within a marsh wetland (Brière, 7000 ha, Northwest France), using an intensive biological study (electrofishing and trapping) and a survey of the traditional fishery (licenses, questionnaires and creel surveys). First, we found that fishermen mainly targeted >320‐mm yellow eels (sedentary stage) using pots and square dipping nets and that harvest by fishermen was highly variable at different locations in the study area. Secondly, we found differences in the size‐class structures and mortality rates between protected and fished areas. Mortality rates of eels >320 mm was positively correlated with harvest by fishermen. Furthermore, the proportion of potentially migrating eels in the total population was found to be higher in the protected areas than in fished areas (6.38% vs. 1.42%, respectively). Thirdly, we found that protected areas potentially produce 8.4% of the total silver eel production whereas they only account for 2.4% of the aquatic habitat area. We estimated that a size adjustment of protected areas to 31.1% with maintaining the current fishery would produce 50% of the potential silver eel of a fully protected marsh. Protection of freshwater areas appears to be a promising management measure and a constructive consensual way to integrate the patrimonial and societal value of the traditional fishery and the international management plans for European eels. Furthermore, freshwater protective measures can be an effective local solution if they are integrated into the framework of freshwater biodiversity management and accompanied by other management measures that focus on all eel life stages.     

Trophic overlap between non‐native brown bullhead (Ameiurus nebulosus) and native shortfin eel (Anguilla australis) in shallow lakes

We quantified trophic overlap between the invasive, non‐native catfish brown bullhead (Ameiurus nebulosus) and the New Zealand native shortfin eel (Anguilla australis) in four peat and riverine lakes using stable isotope (δ¹³C and δ¹⁵N) and gut content analyses. Across all lakes and fish sizes over the austral spring–summer period, shortfin eel guts were dominated numerically by fish prey (57% occurrence cf 42% in brown bullhead), while Diptera larvae were most commonly encountered in guts of brown bullhead (45% cf 14% in eels). Significant differences in % composition of animal contents in guts were detected between fish species and sampling occasions (n = 4) but not between lakes. In contrast, stable isotope signatures of brown bullhead and shortfin eel did differ significantly between lakes but not between sampling occasions, indicating enduring sources of nutrition despite apparently differing ingestion patterns over time. The R mixing model MixSIAR indicated that shortfins likely assimilated higher proportions of fish prey carbon compared to brown bullheads, which appeared to show greater assimilation of invertebrates, consistent with the results of gut content analyses. Isotopic niche regions, calculated in nicheROVER using probabilistic ellipses, indicated that shortfin eels occupied at least c.60% of brown bullhead trophic niche, which occupied less than 30% of eel trophic niche in all but one lake. These estimates suggest that brown bullhead has higher potential to influence shortfin eel nutrition than vice versa, or that a broad trophic niche occupied by eels provides resilience to the effects of overlapping consumption patterns with invasive omnivores.     

Feeding and growth of exercised and unexercised juvenile Atlantic salmon in freshwater,and performance after transfer to seawater

Juvenile Atlantic salmon (Salmo salar), selected from the upper modal group, were first held for 122 days (from December to May) in freshwater, and then for 49 days in seawater (34% salinity). In freshwater, the fish were exposed to either standing water or water currents corresponding to a swimming speed of 1 Bl s^–1. Natural light conditions prevailed throughout the whole experiment.In the freshwater phase, food intake, growth rate and condition factor of the fish increased with increasing daylength. Condition factors, however, decreased slightly during the last month of the freshwater period, suggesting that the fish underwent parr-smolt transformation. The growth rate and food intake of the fish subjected to standing water were approximately 15% lower than those of fish exposed to water currents.Transient suppression of appetite and growth, and an increase in plasma concentration of Cl^–, were seen after exposure of the fish to seawater. Within 28 days, all these parameters had been restored to pre-transfer levels. Previous exercise did not appear to have influenced the hypoosmoregulatory capacity of the fish. Growth rate and food intake observed during the seawater phase were similar, irrespective of the treatment experienced by the fish during freshwater rearing.     

Differences in head shape of the European eel, Anguilla anguilla (L.)

Differences in eel, Anguilla anguilla (L.), head shape were the result of greater increases in head width, rather than changes in head length. The ratio of head width:total length (HW:TL) increased significantly ( t -test, P < 0.05) from the glass-yellow eel stage. Cultured yellow eels were exclusively narrow headed (mean HW:TL = 0.027, range = 0.023–0.032), while wild yellow eels displayed an array of head shapes (mean HW:TL = 0.033, range = 0.023–0.046). Therefore, broad heads (HW:TL 0.033) occurred only among wild yellow eels sampled and may have resulted from diet. Cultured yellow eels consumed only small pellet material. Of wild yellow eels stomachs containing food, 78% of broad-headed eels consumed large and/or hard-bodied organisms (e.g. beetles, fish, molluscs and Notonecta sp.), while 83% of narrow-headed eels consumed exclusively small/soft-bodied prey (e.g. amphipods and chironomids).     

Migration of eels tagged in the Baltic Sea and Lake Mälaren—in the context of the stocking question

Eels (Anguilla spp.) are in decline worldwide, and the signs of reduced recruitment have been observed in continental Europe since the early 1970s. To aid recovery of the European eel, stocking is used by many European countries as a management option. In this study, tagging experiments were conducted to follow eel migration from Lake Mälaren and four sites along the Swedish east coast in the Baltic Sea. The recaptured tagged eels were retrieved from fishermen, allowing for the opportunity to investigate their origin (brackish water, stocked in freshwater or a mix in between) by otolith microchemistry and to assess for morphological differences after tagging. Several changes took place; for example, eye index increased while weight and condition decreased with migrated distance and time until recapture. In Lake Mälaren, the majority of tagged eels did not migrate out of the outlets, irrespective of their origin. Most of them were caught in the opposite direction and continued to be caught in the lake 1–3 years after tagging, with significant weight losses. Overall, overwintering is suggested to be an inferior option, but it is uncertain whether this is a natural behaviour or a result of translocation and restocking. For coastal eels, origin had no effect on migratory behaviour; a majority of the tagged eels migrated towards the outlet of the Baltic Sea. Interestingly, a minority of the recaptured eels originated from stocked fish. Instead, recaptures were dominated by natural immigrants that had spent most of their lives in brackish waters.     

Effects of temperature on vitellogenesis in Japanese eel Anguilla japonica

ABSTRACT:   Cultured immature female Japanese eels acclimated to sea water at either 10 or 20°C were treated weekly with salmon gonadotropin fraction (sGTH) in order to investigate the effects of water temperature on artificial induction of ovarian maturation. In eels maintained at 20°C, ovulation was induced in 11 of 18 fish during the experimental period of 13 weeks, whereas at 10°C all fish showed a low gonadosomatic index (GSI) at the end of the experiment. Plasma vitellogenin levels were higher in eels kept at 20°C than in eels at 10°C throughout the experiment. However, no significant differences were observed in the plasma testosterone and estradiol-17β levels between groups. Eels pretreated with sGTHs at 10°C for 13 weeks were separated to two different temperature (10 and 20°C) groups, and received the same weekly sGTH injections. In eels transferred to 20°C, plasma vitellogenin levels, GSI and oocyte diameter were increased, but these values were maintained at low levels in eels that remained at 10°C. These results clearly indicate that water temperature is an important factor regulating vitellogenesis in the Japanese eel.     

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

To 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.     

Migration of silver eel,Anguilla anguilla,through three water pumping stations in The Netherlands

European eel, Anguilla anguilla (L.), migrating to sea encounter many man-made structures that can hamper and delay migration or induce mortality. Three pumping stations in Friesland, the Netherlands, were covered with acoustic receivers. Ninety-three silver eels tagged with acoustic transmitters were released in the polders upstream of the stations and 89% were detected passing a pumping station. The majority of silver eels passed the stations within a day after arriving at the station. Four silver eels stayed for longer than 2 weeks before passage, and 18 were detected at the receiver downstream the pumping station for more than one day, with detections up to several weeks. These detections probably indicated a dead eel, but could also indicate a live eel remaining at the site. Most of the silver eels passed the pumping stations within a day after release, so fish-friendly pumps will benefit the migrating population most. In the Netherlands, there are several thousand pumping stations. Installing fish passages near these stations is not feasible due to high costs. Prioritising all these sites in relation to the degree of blockage, mortality rates and its relative importance for migratory fish, can maximise the effectiveness of measures and mitigation taken.     

The use of fyke nets as a quantitative capture technique for freshwater eels (Anguilla spp.) in rivers

Abstract  Fyke netting is a convenient and effective technique for capturing freshwater eels, and catch-per-unit-effort is usually assumed to be an index of eel abundance. The present study investigated the potential of depletion fishing using baited fyke nets to obtain population estimates of longfin eels Anguilla dieffenbachii Gray, in a river in the South Island of New Zealand. The probability of capture (capture efficiency) of a single night's fishing increased with increasing size of eel, and ranged from 0.2 to 0.9 for eels <400 mm, to 0.7–0.9 for eels ≥500 mm. The capture efficiency of baited vs unbaited nets was assessed in a small stream that was electric fished after netting trials were completed. Baited nets proved to be an effective method of assessing abundance of longfin eels (>400 mm) but not shortfins ( A. australis Richardson); capture efficiency of the population of longfins (the sum of fyke caught and electric fished eels) was 0.4 for a single night's fyke netting, increasing to 0.8 over four nights. Comparable efficiencies for shortfins were <0.1 and 0.3 respectively. Unbaited nets were markedly less efficient for both species.     

Behaviour,escapement and mortality of female European silver eels within a regulated lowland river draining into the Baltic Sea

Migration patterns of 146 female silver eels on their way through fresh and brackish water stretches of the Warnow River into the Baltic Sea were studied using acoustic telemetry. The majority of eels (77%) regained their expected downstream migration behaviour after a median rehabilitation time of 2 days. Positive rheotactic behaviour was observed for 23% of tagged eels after release, probably influenced by experimental procedures. The observed silver eel escapement rate was 63%. Two migratory groups were classified by significantly different migratory speed, time of residence in brackish waters and biometric silvering characteristics. Although the classification of silvering stages is based on these measures, the proportions of these stages did not reflect the observed differences in migratory behaviour. In the context of a permanent silver eel descent in the freshwater part of the Warnow River, a permanent escapement was likewise occurring into the Baltic Sea.     

Cortisol and liver metabolism of immature American eels,Anguilla rostrata (LeSueur)

Plasma and liver composition, liver enzyme activities, and metabolite flux in isolated hepatocytes have been studied in immature American eels,Anguilla rostrata, injected daily IP with saline or cortisol (0.35 mg/kg eel). Plasma cortisol values were significantly increased above saline controls in those eels receiving cortisol at 3h and 6h following the final (tenth) injection. On day 6 and 10 of injection plasma cortisol levels were significantly below saline controls 24h following cortisol injection. Plasma glucose values were significantly depressed in the cortisol-injected eels at both 6 and 24h following the final (tenth) injection. At the 24h sampling time, plasma protein had significantly increased, but there was no change in either plasma amino acid or fatty acid levels. An increased hepatosomatic index was attributed to a major increase in total lipids, as both protein and glycogen contents were decreased. Of the liver enzymes assayed, significant activity changes occurred only for lactate dehydrogenase (decreased), mitochondrial citrate synthase (increased) and phosphoenolpyruvate carboxykinase (increased) 24h following the final (tenth) cortisol injection. Although enzyme activity changes implied increased liver gluconeogenesis, the absolute rate of lactate, alanine, and aspartate incorporation into glucose declined in viable hepatocytes isolated from cortisol-injected eels compared to the saline controls. Relative changes in metabolite flux did support a preferential increase in gluconeogenesis from amino acids. These results are consistent with the increase in amino acid gluconeogenesis as a result of cortisol administration implied in previous studies, but failed to show a definitive cortisol effect on this pathway in the eel liver. It is suggested that other hormones (e.g. thyroxine, catecholamines, glucagon) may interact in a complex way with cortisolin vivo to bring about the biochemical changes observed in this study. The rapid clearance of exogenously injected cortisol noted in this study makes causal relationships between the injected hormone and any observed metabolic effect in the intact animal difficult.