Optimum dietary ratio of digestible protein and energy for juvenile American eel, Anguilla rostrata, fed practical diets

Triplicate groups of juvenile American eel, Anguilla rostrata, initial weight 8.2 ± 0.24 g, were fed to satiation herring meal based diets formulated with digestible protein/digestible energy (DP/DE) ratios of 19, 20, 21, 22 and 23 g DP MJ DE^?1 (as‐fed basis) for 84 days. Data were collected to determine the effect of dietary DP/DE ratio on feed intake (FI), mean weight (MW), specific growth rate (SGR), feed conversion ratio (FCR), apparent digestibility (AD) of major nutrients, rate of phosphate excretion (RPE) and nutrient retention efficiency (RE). Highest MW, SGR and lowest FCR (P < 0.05) were achieved by feeding 22 g DP MJ DE^?1 with values (mean ± SE) of 22.9 ± 0.07 g fish^?1, 1.23 ± 0.033% day^?1 and 0.91 ± 0.075 g feed g gain^?1, respectively. With exception of lipid, digestibility of all nutrients were the same (P > 0.05) with mean AD coefficients for organic matter, protein, energy and phosphorous of 86.3, 94.1, 89.2 and 34.7%, respectively. Lipid AD was significantly higher (P < 0.05) when DP/DE ratio was 21, 22 or 23 g DP MJ DE^?1 at 92.3% as opposed to when DP/DE ratio was 19 or 20 g DP MJ DE^?1 at 90.3%. The DP/DE ratio had no significant effect (P > 0.05) on RPE and it averaged 0.05 ± 0.002 g phosphate kg fish^?1 day^?1. Nitrogen retention efficiency (NRE) significantly (P < 0.05) increased as DP/DE ratio increased to 21 g DP MJ DE^?1 and was similar thereafter (P > 0.05) at an average of 31.6 ± 0.67%. Energy retention efficiency (ERE) significantly (P < 0.05) increased to 42.9 ± 1.24% as DP/DE ratio increased to 22 g DP MJ DE^?1 and thereafter significantly (P < 0.05) decreased. Lipid retention efficiency (LRE) increased significantly (P < 0.05) to 75.7 ± 0.85% as dietary DP/DE ratio increased to 23 g DP MJ DE^?1. Non‐linear quadratic regression of ERE against dietary DP/DE ratio yielded an estimated optimum DP/DE ratio for juvenile American eel of 22.1 g DP MJ DE^?1.     

Effects of ambient ion concentrations on gill ATPases in fresh water eel,Anguilla anguilla

Branchial activities of Na⁺,K⁺-ATPase (ouabain sensitive), Mg²⁺ ATPase (ouabain insensitive) and kinetic analysis of high and low affinity Ca²⁺ ATPase were measured inAnguilla anguilla that had been acclimated to demineralized water (DW, Ca < 10 M), freshwater (FW, Ca = 2 mM), and Low calcium freshwater (L-Ca, Ca = 0.9 mM). Na⁺,K⁺-ATPase activity decreased while ouabain insensitive activity increased when ambient Ca²⁺ decreased. Two kinetic forms of Ca²⁺ ATPase could be resolved in each environmental condition. The stimulation coefficients of both sites or enzymes were not affected by ambient Ca²⁺ concentrations. The maximal velocity of both the high and the low affinity Ca²⁺ ATPase was increased when external Ca²⁺ was decreased during acclimation. The low affinity Ca²⁺ ATPase and the Mg²⁺ stimulated enzyme could be a non specific enzyme accepting either Ca²⁺ or Mg²⁺. Results are compared with previous results in the literature and in relation to the branchial morphology and ionic exchanges in fish.     

Age and growth of yellow eel, Anguilla anguilla (L.), determined by two different methods

Abstract– Ageing and calculation of growth rate of eels has always been controversial, and remarkably little validation has been reported. This article describes the growth of yellow eels from three lakes in the Burrishoole system, Ireland, as determined by back-calculation from otoliths prepared by burning and cracking. Eels captured by fyke net ranged in length from 25.0 cm to 98.5 cm and the ages ranged from 6 to 49 years. Back-calculated growth was slow, with mean annual increments of 2.08 cm, 1.40 cm and 1.97 cm for the three lakes. Yellow eels captured in the Burrishoole system in 1987 and 1988 were marked by Pan Jet inoculated alcian blue dye and released back into the three lakes. Recaptures made over the following three years yielded considerable growth data. Mean annual growth rates recorded were 2.30 cm, 1.07 cm and 0.97 cm. These data compare favorably with the calculated growth rates, thereby validating the otolith analysis for the eels in the Burrishoole system.     

中草药添加剂对日本鳗鲡生长和非特异性免疫效应的研究

在基础饲料中分别添加0.5 g/kg、1.0 g/kg和1.5 g/kg的自制中草药免疫添加剂,配制成3种试验饲料,以基础饲料为对照组饲料,每处理设3个平行样,对体重为30~40 g的日本鳗鲡(Anguilla japonica)进行为期60 d的饲养试验,每20 d取样一次,以相对增重率、成活率和血清中的溶菌酶(LZM)、一氧化氮合酶(NOS)、碱性磷酸酶(AKP)和超氧化物歧化酶(SOD)活性为指标,探讨了中草药免疫添加剂对日本鳗鲡生长和非特异性免疫效应的影响。结果显示:饲料中添加中草药免疫添加剂可以显著提高日本鳗鲡的增重率,降低饲料系数(P<0.05)。试验组日本鳗鲡血清中的LZM、NOS、AKP和SOD等活性均高于对照组。因此,中草药免疫添加剂具有显著促生长和增强机体免疫能力的作用,建议在实际生产中的添加量为1.0 g/kg较为合适。     

The role of glycogen phosphorylase in the regulation of glycogenolysis by insulin and glucagon in isolated eel (Anguilla rostrata) hepatocytes

The effects of porcine, scombroid, and salmon insulins, and bovine and anglerfish glucagons on glycogen depletion and glycogen phosphorylase (GPase) activities were examined in freshly isolated American eel (Anguilla rostrata) hepatocytes. Eel liver GPase in crude homogenates was activated (increase in % GPase a) by phosphorylating conditions and was rapidly inactivated (less than 1 h) when a phosphatase inhibitor (fluoride) was absent. Caffeine inhibits, and AMP activates, the b form of GPase consistent with their effects on rat liver GPase. Both mammalian and fish glucagons increased glucose production in eel hepatocytes, but had more ambiguous effects on glycogen levels and GPase activities. The magnitude of bovine glucagon effects were dependent on the initial glycogen content of the cells; only at glycogen concentrations less than approximately 70 μmoles.g⁻¹ did glucagon significantly increase % GPase a. Anglerfish glucagon significantly increased cyclic AMP (cAMP) concentrations by 90% at 10⁻⁷ M, but had no effects at 10⁻⁹ M and 10⁻⁸ M. Scombroid and salmon insulins maintained hepatocyte glycogen concentrations and decreased glucose production, with these effects more pronounced at low (10⁻⁹ to 10⁻⁸ M) rather than high (10⁻⁷ M) hormone concentrations. Porcine and salmon insulins decreased total GPase and % GPase a activities, and salmon insulin decreased CAMP levels, but only at 10⁻⁸ M (by 44%). Glycogen is, therefore, depleted by glucagon and maintained by insulin in freshly isolated American eel hepatocytes, and these changes are accomplished, at least in part, by changes in the activities of GPase. Changes in cAMP do not explain all of the observed hormone effects.     

吡喹酮、甲苯咪唑、溴氰菊酯对花鳗鲡的急性毒性

采用静水式急性毒性试验法研究了吡喹酮、甲苯咪唑和溴氰菊酯对花鳗鲡(Anguilla marmorata)(体重为0.87±0.19 g)的急性毒性。试验结果表明,在水温28±2℃时,吡喹酮对花鳗鲡的24 h LC50、48 h LC50、72 h LC50、96 h LC50分别为73.85 mg/L、66.71 mg/L、56.57 mg/L、52.72 mg/L,安全浓度为16.34 mg/L;甲苯咪唑对花鳗鲡的24 h LC50、48 h LC50、72 h LC50、96 h LC50分别为142.81mg/L、13.80 mg/L、5.19mg/L、1.74mg/L,安全浓度为0.023 mg/L;溴氰菊酯对花鳗鲡的24 h LC50、48 h LC50、72 h LC50、96 h LC50分别为9.56μg/L、9.34μg/L、7.69μg/L、7.67μg/L,安全浓度为2.68μg/L。吡喹酮、甲苯咪唑、溴氰菊酯对花鳗鲡分别属于低毒、中毒、剧毒药物。甲苯咪唑对花鳗鲡的安全浓度0.023 mg/L远低于欧洲鳗鲡(Anguilla anguilla)的常用浓度1～2 mg/L,因此在花鳗鲡养殖中使用甲苯咪唑制剂需十分谨慎。     

Implications of the precautionary approach for the management of the European eel, Anguilla anguilla

The European eel, Anguilla anguilla (L.), has a complex life history and many aspects of the biology and population dynamics of this species remain unknown or, at best, poorly understood. Relatively little is also known about the status of the stocks and fisheries, but available data suggest that recruitment of glass eels has been falling for the last 20 years and is at historically low levels. Yellow and silver eel catches have also been falling in many parts of the species range over a similar time‐scale. Re‐examination of the principles applied to fisheries management over recent years has resulted in the adoption of a ‘precautionary approach’ to the conservation, management and exploitation of fish stocks, and in an explicit need to take account of uncertainties in management to reduce risks to stocks and their environment. Such an approach is highly relevant to the management of the European eel and requires that urgent consideration is given to harvest strategies and decision structures for the national and international management of stocks and fisheries. Provisional biological reference levels should be established to provide an equable assessment of the status of stocks in all parts of Europe and to evaluate the need for management measures in all fisheries. These will need to be reviewed as further information comes available. Monitoring and research on eel stocks should therefore be enhanced and co‐ordinated to improve our understanding of the status of stocks throughout Europe and the biology of the species.     

日本鳗鲡人工育苗及仔鱼饲料研究

日本通过40年对鳗鲡成熟控制技术的研究,特别是近几年来采用17,20β-dihydroxy-4-pregnen-3-one(DHP)的鳗鲡雌鱼成熟诱导技术,使高质量鳗鲡受精卵的取得具有相当高的把握性。日本水产厅养殖研究所采用以鲨鱼卵冷冻干燥粉末为主要成份并含有磷虾或其它鲜鱼虾抽提液的糊状饲料,喂食人工孵化的日本鳗鲡仔鱼取得举世嘱目的突破性进展,但柳叶状仔鱼全长达到30mm以后几乎不再摄食,出现生长停滞,并已变得十分衰弱。全面攻克日本鳗鲡人工育苗难题尚待时日,特别是研制能有效促进后期柳叶状仔鱼生长并变态的全价育苗饲料已成为目前关注的焦点。     

Effects of salinity and temperature on the growth and survival of New Zealand shortfin,Anguilla australis,and longfin,A. dieffenbachii,glass eels

The ideal water conditions for maximizing the performance of the nursery culture of glass eels harvested from the wild for aquaculture need to be determined for the New Zealand shortfin (Anguilla australis) and longfin (Anguilla dieffenbachii) eels. This study determined the survival and growth of glass eels reared under different temperature and salinity conditions in the laboratory. The growth and survival of shortfin and longfin glass eels reared in salt water (35‰) maintained at 25 °C was examined over 84 days from capture. The mean specific growth rate (SGR) was higher in shortfin [2.30±0.29% body weight (b.w.) day^?1] than longfin glass eels (1.52±0.06% b.w. day^?1), and survival was also higher in shortfin (76.0±4.16%) than for longfin glass eels (28.7±6.36%). A second experiment identified the effect of salinity (0, 17.5‰ and 35‰) and temperature (17.5 and 26.5 °C) on the acclimation, growth performance and survival of shortfin and longfin glass eels over a period of 84 days from capture. There was no incidence of mortality for either shortfin or longfin glass eels reared across all salinity treatments (0‰, 17.5‰ and 35‰) at 26.5 °C, while survival of shortfin and longfin glass eels reared at 17.5 °C was the highest in 17.5‰, followed by 35‰ and 0‰ treatments. Both temperature and salinity affected the SGR of shortfin glass eels, with the highest SGR observed for shortfin glass eels reared in 0‰ water maintained at 26.5 °C. In longfin glass eels, salinity alone had an effect on the SGR, with the highest SGR observed in glass eels reared in 0‰ water regardless of the water temperature (17.5 and 26.5 °C). In addition, the adaptability of glass eels to salinity was evaluated from the development and the physiological responses of gill chloride cell (CC) morphology. The number and size of CCs increased significantly with increasing salinity in both shortfin and longfin eels.     

Temporal changes in European eel, Anguilla anguilla, stocks in a small catchment after installation of fish passes

Abstract  Changes in the abundance of European eel, Anguilla anguilla L., in the River Frémur, France, were examined over an 8-year period. Natural connectivity of the river was disturbed by three high dams that inhibited eel upstream migration and reduced recruitment by elvers and yellow eels. After eel passes were installed, fish became more abundant upstream (mean density 0.5 eel m⁻²). Moreover, except in the more upstream areas, no decline in eel numbers and biomass was found, in contrast to the general decline of eel throughout its distribution range. It was concluded that eel passes are important to conserve and/or to recover eel stocks.     

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.     

Estuarine and fluvial recruitment of the European glass eel, Anguilla anguilla, in an exploited Atlantic estuary

A glass eel fishery exists downstream of the Arzal estuarine dam in the Vilaine (Brittany, France). Catch statistics were collected between 1996 and 2000, and processed using a subsampling technique which allowed data from a reliable subsample to be extrapolated to the whole fishery. During the same period, glass eel migration into fresh water was monitored using a glass eel trap located on the dam – the upstream limit of the fishery. The analysis of glass eel biology and exploitation shows that the glass eel fishery is very intensive and that there was more or less no escapement during the fishing seasons studied. The proportion of the stock successfully migrating towards fresh water, as compared with the total catch is estimated to range between 0.3 and 3.9%. In such a fishery, the fishing effort affects the abundance. Consequently, the total catch has to be used instead of catch‐per‐unit‐effort to estimate abundance.     

Growth hormone secretion during longterm incubation of the pituitary of the Japanese eel,Anguilla japonica

Growth hormone (GH) secretion from organ-cultured pituitaries of the eel (Anguilla japonica) was studied during incubation in a defined medium for 2 weeks, using a homologous radioimmunoassay which does not distinguish between the two molecular forms of eel GH. The total amount of GH secreted increased gradually during the incubation period; so that the amount of GH released on day 14 was about 30 times greater than that on day 1. On day 14, the proportion of GH released relative to the total amount of GH present (the sum of GH released into the medium and residual content in the pituitary) was 96% and the amount produced on day 14 was 4 times greater than the content in the unincubated pituitary. Somatostatin (SRIF, 1.8 × 10^-7 M) inhibited the increase in GH release. On day 7, the proportion of GH released by pituitaries treated with SRIF (28%) was less than that released by the control pituitary (91%). There was no significant difference in GH release between the pituitaries incubated in isotonic medium (300 mOsm) and those in hypotonic medium (240 mOsm) for 2 weeks except for the first 3 days, when the pituitaries in hypotonic medium secreted significantly greater amounts of GH than those incubated under isotonic condition. Hypertonic medium (350 mOsm) had no effect on GH release except for significant inhibition on days 6 and 14. When secretion of the two forms of GH (GH I and II) was examined after separation by polyacrylamide gel electrophoresis followed by densitometry, slightly more GH I tended to be secreted than GH II during the culture period, although the effects of SRIF and osmolality of the media on GH I release were similar to those on GH II. It is concluded that GH secretion and production in the eel is mainly under the inhibitory control of hypothalamus, and that osmolality has a minimum influence on the GH release.     

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).     

The use of clove oil as an anaesthetic for the longfinned eel, Anguilla reinhardtii (Steindachner)

To handle large river eels during procedures such as measuring and tagging for field and aquaculture studies, they must be anaesthetized. During our initial biological studies of Anguilla reinhardtii (Steindachner) it was found that the anaesthetic benzocaine was relatively expensive and elicited a variable response, even when used at relatively high concentrations. Human health risks are also a concern when using benzocaine, as some of the eels may later be sold for human consumption. Therefore, experiments were done to evaluate the use of clove oil (a safe, naturally occurring product) for anaesthesia of this species at a range of temperatures (17 and 25 °C) and salinities (0–32 g L^?1). It was found that clove oil provided a suitable anaesthetic response through this wide range of temperatures and salinities. Response times were found to be relatively variable for both benzocaine and clove oil. This variability may be related to stress, environmental factors, or the condition of the fish. Clove oil is recommended for anaesthesia of anguillid eels because it is effective, relatively inexpensive, and poses little risk to human health.     

Effects of feeding stimulants on the feed consumption,growth and survival at glass eel and elver stages in the European eel (Anguilla anguilla)

Elvers and glass eels of Anguilla anguilla were fed diets containing two types of feeding stimulants (FS) that were based on processed marine (MBFS) and yeast proteins (YBFS). Elvers (1.5 ± 0.3 g) were fed seven diets (MBFS and YBFS diets at 20 g kg^?1, 40 g kg^?1 and 60 g kg^?1 plus control) for 60 days. Glass eels (250 ± 100 mg) were weaned to 60 g kg^?1 MBFS, 60 g kg^?1 YBFS and control diets for 30 days. Diets containing 60 g kg^?1 FS had a beneficial effect in terms of growth, homogenous size distribution and feed intake in elvers. Elvers fed 60 g kg^?1 MBFS and YBFS diets grew 11.9% and 5.6% faster than the control group. No differences in growth and size distribution were detected in glass eels fed 60 g kg^?1 MBFS and YBFS diets. However, FS affected the digestive system maturation; fish fed the 60 g kg^?1 MBFS and YBFS diets showed higher and intermediate values in the specific enzyme activities in comparison with the control group. This study revealed that the incorporation of FS into a pelleted diet was beneficial on the overall performance of European glass eels and elvers. However, the observed results were different depending on the eel’s stage of development, as well as the type and inclusion level of the FS.     

Migratory history of the giant mottled eel (Anguilla marmorata) in the Bonin Islands of Japan

Chino N, Arai T. Migratory history of the giant mottled eel ( Anguilla marmorata ) in the Bonin Islands of Japan.
Ecology of Freshwater Fish 2010: 19: 19–25. © 2009 John Wiley & Sons A/S
Abstract –  In order to elucidate the migratory history and habitat use of the giant mottled eel Anguilla marmorata, in the Bonin Islands of Japan, strontium (Sr) and calcium (Ca) concentrations in otoliths were examined by X-ray electron microprobe analysis. The X-ray intensity map and line analyses showed three fluctuation patterns: (i) freshwater residence, (ii) continuous residence in brackish water, and (iii) residence in freshwater after recruitment, while returning to brackish water. Based on these results and previous studies, the migratory behaviours of A. marmorata appear to differ in each habitat in response to inter- and intra-specific competition. The results suggest that A. marmorata has a flexible pattern of migration, with an ability to adapt to various habitats and salinities.     

Growth of Australian shortfin eel (Anguilla australis) elvers given different dietary protein and lipid levels

The Australian shortfin eel, Anguilla australis is a potential candidate for intensive aquaculture. The present study was undertaken to evaluate the growth of elvers (5.4 g ± 0.1 initial weight) fed with diets of varying protein and lipid content, and to assess the potential of using soya‐bean meal as a dietary ingredient. A 10 week experiment was conducted at 24 (±1.0) °C by rearing fish, in 60 L conical fibre glass tanks using a closed recirculation system. Diets having protein concentrations of 40 or 50% (by dry weight) were tested at three lipid levels (15, 20, 25%); diets being designated P₄₀L₁₅, P₄₀L₂₀, P₄₀L₂₅, P₅₀L_15, etc. All these diets contained 5% soya‐bean meal. In addition P₅₀L₂₀ diets were formulated to contain 10 and 20% soya‐bean meal in the diet (Diets S1 & S2). Shortfin eel grew best on the P₅₀L₁₅ diet, with an average specific growth rate of 2.26%. Food conservation ratio (FCR) and Protein efficiency ratio (PER) ranged from 1.21 (P₅₀L₁₅) to 2.12 (P₄₀L₂₅), and 0.92 (P₅₀L₂₅) to 1.65 (P₅₀L₁₅), respectively. Based on all criteria the best growth performance of shortfin eel was on the P₅₀L₁₅ diet, followed by P₄₀L₂₀ and P₄₀L₁₅. At both protein levels fish reared on diets with 25% lipid performed poorly. The performance of shortfin eel was not affected by the amount of soya‐bean meal in the diet, up to a maximum of 20% dietary inclusion. No significant differences in muscle protein were evident in shortfin eel reared on different dietary treatments, nor was the lipid content of muscle related to dietary lipid level.     

Winners and losers of conservation policies for European eel,Anguilla anguilla: an economic welfare analysis for differently specialised eel anglers

Abstract Recreational specialisation theory was coupled with a discrete choice experiment to understand eel, Anguilla anguilla L., angler’s heterogeneity in their reaction to regulatory changes and the associated welfare changes. Differently specialised eel anglers exhibited distinct preferences for catch variables and eel angling regulations. All anglers preferred slightly to moderately stricter regulations than are currently in place; however, such policies particularly benefited casual eel anglers. In contrast, advanced eel anglers would be most penalised by highly restrictive regulations as indicated by substantial reductions in economic welfare. Aversions to stricter regulations found for advanced anglers contradicted predictions from specialisation theory. From an eel management perspective, the implementation of some simple tools such as increased minimum‐size limits will reduce angling mortality on eel and simultaneously increase the welfare of anglers. By contrast, highly restrictive eel angling regulations will result in considerable economic welfare losses of several million € per year for northern Germany alone.