A complete check-up of European eel after eight years of restocking in an upland river: Trends in growth, lipid content, sex ratio and health status.

By combining field research and careful laboratory analysis of samples over the course of an eight-year study, we met the challenge of assessing the life history traits and health status of eels restocked in freshwater ecosystems. We found that restocked eels exhibited good growth performance; moreover, the stocks were female-dominated, showed a good Fulton's condition factor (K) and lipid stores and had high survival probability estimated using the best model of Jolly-Seber stock assessment method for open populations. A necropsy revealed the absence of internal lesions. A swim bladder examination revealed the absence of the parasite Anguillicola crassus. Polymerase chain reaction (PCR) analyses revealed an increase of Anguillid herpesvirus-1 (AngHV-1) prevalence throughout the study. Most positive subjects expressed viral loads compatible with a latent infection and correlated positively with K. All restocked eels were contaminated by at least one of the organic pollutant congeners studied, but the pollution loads corresponded to the lowest range of pollutant concentrations reported in the available literature for European eels and did not exceed the maximum residue and contaminant limits in food and feed of several national and international regulations. Pollutant loads were negatively correlated with K, lipid content and eel density for polychlorinated biphenyls PCB 138, 153 and 180 and K for pesticides p.p'-DDE, p.p'-DDD, p.p'-DDT and PBDE47. This study highlights the potential role played by upland aquatic ecosystems in enhancing riverine silver eel production from the perspective of species conservation. To be successful, restocking must be accompanied by improved ecosystem quality and migration routes for eels in inland freshwaters. We also provide some recommendations for future research to improve the management of restocking programmes.

Modulations of lipid metabolism and development of the Atlantic salmon (Salmo salar) fry in response to egg-to-fry rearing conditions.

In stocking program, the use of artificial incubation conditions in hatcheries from the fertilisation of eggs to the release of unfed fry could reduce their ability to adapt to the natural environment. This study evaluates the effects of three factors on the fitness and physiology of salmon fry at their emergence, the origin of water (river vs drilling), the type of support in the incubator (support matrix vs plastic sheets) and the type of incubators (Californian vs vertical trays), and compares them to a semi-natural incubation method in river. Key biological functions including nutritional and immune status were compared among experimental conditions using biometric parameters, lipid composition and gene expression analyses. Our findings demonstrated that fry incubated in vertical trays supplied with river water had no significant difference in growth and lipid composition compared to those in semi-natural incubators. Besides, fry incubated on a substrate matrix in Californian trays exhibited phenotypic characteristics closest to those incubated in river. This support matrix improved fish growth, lipid consumption and distribution compared to fry on plastic sheets. Moreover, the large amounts of several PUFAs in these fry could allow a better membrane fluidity ensuring a better adaptation to temperature variation under cold conditions. In addition, drilling water improved the survival rate compared to river water due to lower numbers of fine particles, known to be responsible for the clogging of eggs. To conclude, using a substrate combined with drilling water in artificial incubators could increase fry fitness and its adaption to wild life.

What are the best upland river characteristics for glass eel restocking practice?

The fitness of restocked European eel (Anguilla anguilla), an endangered fish species, was studied in relation to the environmental variables of habitats in six upland rivers that are typologically different in terms of their hydromorphological and physicochemical characteristics, food resources and fish communities. These rivers received a total of 76,370 imported glass eels in 2017. During a three-year period, we monitored eels with respect to total length, annual growth rate, condition factor and density using capture-mark-recapture experiments to understand the effects of the characteristics of receiving rivers on restocking success levels. Our results showed the survival of the restocked eels in the six rivers and revealed significant differences between them in terms of total length, condition factor and density. Better performance in eel yield variables was observed in a eutrophic alkaline river with greater roughness of riverbed substrates, dominant pool- and riffle-type flow facies and lower brown trout density. The variables conductivity and total hardness had higher explanatory power and were strongly associated with increased eel density. This study suggests that a well-selected habitat/river in a restocking programme can be beneficial for the species and recommends restocking practice as a management tool to achieve eel conservation goals.

A temperature shift on the migratory route similarly impairs hypo-osmoregulatory capacities in two strains of Atlantic salmon (Salmo salar L.) smolts.

Anthropogenic use of water systems may cause temperature fluctuations between tributaries and large rivers for which physiological population related-effects on osmoregulatory capacity of Atlantic salmon are not well described. We simulated the downstream route in the case of the River Meuse basin to investigate the impact of a 5 °C temperature shift during smoltification on hypo-osmoregulatory capacities of smolts. Three temperature regimes were tested: control temperature-treatment (T1) without temperature shift, early (T2) or late (T3) temperature shift-treatment. Moreover, fish were subjected to seawater challenge during and after the downstream migration peak time. Two allochtonous strains were used: Loire-Allier (LA) and Cong (CG). Without temperature shift (T1), significant differences between the strains were noticed in the peak date and maximum activity of gill Na+/K+ATPase as well as in plasma sodium and potassium concentrations. For early (T2) and late (T3) temperature shift-treatments, gill Na+/K+ATPase activity, plasma osmolality and ion concentrations were negatively influenced in both strains. After salinity challenge, the highest osmolality was measured in smolts subjected to the temperature shift. Predictably circulating levels of GH and IGF-1 changed over the smolting period but they did not explain the observed modifications in hypo-osmoregulatory abilities whatever the population. The results show a negative impact of a temperature shift on hypo-osmoregulatory capacities of smolts regardless of population differences in smoltification timing under conditions without temperature shift. The resilience of such physiological impact was sustained at least for 1 week, comforting the role of high temperature in influencing the rate of changes occurring during smoltification. Therefore, favouring the downstream migration to help smolts reach the sea faster may mitigate the impact of a rapid temperature increase.

Influence of strain origin on osmoregulatory and endocrine parameters of two non-native strains of Atlantic salmon (Salmo salar L.).

Non-native strains of Atlantic salmon are used in reinstatement trials where populations are extinct. Environmental cues like photoperiod and temperature are known to influence the smolting process and there is evidence of strain-, stock- or population-specific differences associated with seaward migration or smoltification. The objective of this study was to compare morphological, osmoregulatory and endorcrine features between two strains, one originating from a cold and short river in Ireland (Cong) and another from a long and warm river in France (Loire-Allier), reared under Belgian conditions in order to highlight major differences in restocking adaptability. Comprehensive endocrine profiles, consistent with their interactive role of mediating changes associated with smolting, have been observed. Na+/K+ATPase activity (1.3-10.5 µmol ADP∗mg prot.-1∗h-1) and hormone plasma levels (e.g. 55-122 ng∗mL-1 of cortisol and 4.5-6.4 ng∗mL-1 of GH) were consistent with reported values. We observed strain-related differences of the influence of temperature and daylength on cortisol, GH and sodium plasma levels. These may be related to the respective environmental conditions prevailing in the river of origin, which have impacted the genetic background for smoltification. Using Na+/K+ATPase activity as an indicator, both strains smoltified successfully and simultaneously testifying a prevailing influence of environmental cues over genetic factors for smoltification.

Relative influence of dietary protein and energy contents on lysine requirements and voluntary feed intake of rainbow trout fry.

The effect of dietary digestible protein (DP) and/or digestible energy (DE) levels on lysine (Lys) requirements, Lys utilisation efficiency and voluntary feed intake (VFI) were studied in rainbow trout fry when Lys was the first limiting indispensable amino acid or in excess in the diet. Two trials were conducted at 11·6°C with eighty-one experimental diets, containing 280 g DP/kg DM (low protein (LP), trial 1), 600 g DP/kg DM (high protein (HP), trial 1) or 440 g DP/kg DM (medium protein (MP), trial 2), 17 MJ DE/kg (low energy (LE)), 19·5 MJ DE/kg (medium energy (ME)) or 22 MJ DE/kg (high energy (HE)), and nine Lys levels from deeply deficient to large excess (2·3-36 g/kg DM). Each diet was given to apparent satiety to one group of fifty fry (initial body weight 0·85 g) for 24 (MP diets, trial 2) or 30 (LP and HP diets, trial 1) feeding days. Based on N gain data fitted with the broken-line model, the relative Lys requirement was significantly different with the dietary DP level, from 13·3-15·7 to 22·9-26·5 g/kg DM for LP and HP diets, respectively, but did not significantly change with the DE level for a same protein level. The Lys utilisation efficiency for protein growth above maintenance was constant across diets, suggesting no effect of either dietary DE or DP levels. In Lys excess, the VFI was markedly decreased by the DP level but not by the extra DE supply. Our results suggest that the relative Lys need is best expressed in terms of percentage of protein content for optimum fish feed formulation, at least in rainbow trout fry.

A n-3 PUFA depletion applied to rainbow trout fry (Oncorhynchus mykiss) does not modulate its subsequent lipid bioconversion capacity.

Nutritional strategies are currently developed to produce farmed fish rich in n-3 long-chain PUFA (LC-PUFA) whilst replacing fish oil by plant-derived oils in aquafeeds. The optimisation of such strategies requires a thorough understanding of fish lipid metabolism and its nutritional modulation. The present study evaluated the fatty acid bioconversion capacity of rainbow trout (Oncorhynchus mykiss) fry previously depleted in n-3 PUFA through a 60-d pre-experimental feeding period with a sunflower oil-based diet (SO) followed by a 36-d experimental period during which fish were fed either a linseed oil-based diet (LO) (this treatment being called SO/LO) or a fish oil-based diet (FO) (this treatment being called SO/FO). These treatments were compared with fish continuously fed on SO, LO or FO for 96 d. At the end of the 36-d experimental period, SO/LO and SO/FO fish recovered >80 % of the n-3 LC-PUFA reported for LO and FO fish, respectively. Fish fed on LO showed high apparent in vivo elongation and desaturation activities along the n-3 biosynthesis pathway. However, at the end of the experimental period, no impact of the fish n-3 PUFA depletion was observed on apparent in vivo elongation and desaturation activities of SO/LO fish as compared with LO fish. In contrast, the fish n-3 PUFA depletion negatively modulated the n-6 PUFA bioconversion capacity of fish in terms of reduced apparent in vivo elongation and desaturation activities. The effects were similar after 10 or 36 d of the experimental period, indicating the absence of short-term effects.

Temperature Increase Negatively Affects the Fatty Acid Bioconversion Capacity of Rainbow Trout (Oncorhynchus mykiss) Fed a Linseed Oil-Based Diet.

Aquaculture is meant to provide fish rich in omega-3 long chain polyunsaturated fatty acids (n-3 LC-PUFA). This objective must be reached despite (1) the necessity to replace the finite and limited fish oil in feed production and (2) the increased temperature of the supply water induced by the global warming. The objective of the present paper was to determine to what extent increased water temperature influences the fatty acid bioconversion capacity of rainbow trout (Oncorhynchus mykiss) fed a plant-derived diet. Fish were fed two diets formulated with fish oil (FO) or linseed oil (LO) as only added lipid source at the optimal water temperature of 15°C or at the increased water temperature of 19°C for 60 days. We observed that a temperature increase close to the upper limit of the species temperature tolerance range negatively affected the feed efficiency of rainbow trout fed LO despite a higher feed intake. The negative impact of increased water temperature on fatty acid bioconversion capacity appeared also to be quite clear considering the reduced expression of fatty acid desaturase 2 in liver and intestine and the reduced Δ6 desaturase enzymatic activity in intestinal microsomes. The present results also highlighted a negative impact of increased temperature on the apparent in vivo enzymatic activity of Δ5 and Δ6 desaturases of fish fed LO. Interestingly, this last parameter appeared less affected than those mentioned above. This study highlights that the increased temperature that rainbow trout may face due to global warming could reduce their fatty acid bioconversion capacity. The unavoidable replacement of finite fish oil by more sustainable, readily available and economically viable alternative lipid sources in aquaculture feeds should take this undeniable environmental issue on aquaculture productivity into account.

Nutritive value of three tropical forage legumes and their influence on growth performance, carcass traits and organ weights of pigs.

The effects of tropical forage legumes on feed intake, growth performance and carcass traits were investigated in 16 groups of two Large White × Duroc pigs. The diets consisted of a commercial corn-soybean meal diet as the basal diet and three forage-supplemented diets. Four groups of control pigs received daily 4 % of body weight of the basal diet, and 12 groups of experimental pigs were fed the basal diet at 3.2 % of body weight completed with fresh leaves of one of the three forage legumes (Psophocarpus scandens, Stylosanthes guianensis and Vigna unguiculata) ad libitum. The study lasted 90 days. The in vitro digestion and fermentation of the forage legumes were also determined. The in vitro digestible energy content of the legumes was between 0.72 and 0.77 that of the basal diet (14.4 MJ/kg dry matter (DM)). V . unguiculata was the most digestible forage legume expected for crude protein digestibility. Feeding forage legumes lowered the dry matter intake by 4.5 to 9.6 % (P < 0.05), final body weight (P = 0.013), slaughter weight, average daily gain and hot carcass weight (P < 0.05) without affecting the feed conversion ratio (FCR), dressing percentage and back fat thickness. In conclusion, using forage to feed pig could be interesting in pig smallholder production with limited access to concentrate, as FCR was not significantly affected.

Protein level affects the relative lysine requirement of growing rainbow trout (Oncorhynchus mykiss) fry.

The effect of two digestible protein levels (310 and 469 g/kg DM) on the relative lysine (Lys; g Lys/kg DM or g Lys/100 g protein) and the absolute Lys (g Lys intake/kg 0.75 per d) requirements was studied in rainbow trout fry using a dose-response trial. At each protein level, sixteen isoenergetic (22-23 MJ digestible energy/kg DM) diets were tested, involving a full range (2-70 g/kg DM) of sixteen Lys levels. Each diet was given to one group of sixty rainbow trout fry (mean initial body weight 0.78 g) reared at 15 degrees C for 31 feeding d. The Lys requirements were estimated based on the relationships between weight, protein, and Lys gains (g/kg 0.75 per d) and Lys concentration (g/kg DM or g/100 g protein) or Lys intake (g/kg 0.75 per d), using the broken-line model (BLM) and the non-linear four-parameter saturation kinetics model (SKM-4). Both the model and the response criterion chosen markedly impacted the relative Lys requirement. The relative Lys requirement for Lys gain of rainbow trout estimated with the BLM (and SKM-4 at 90 % of the maximum response) increased from 16.8 (19.6) g/kg DM at a low protein level to 23.4 (24.5) g/kg DM at a high protein level. However, the dietary protein content affected neither the absolute Lys requirement nor the relative Lys requirement expressed as g Lys/100 g protein nor the Lys requirement for maintenance (21 mg Lys/kg 0.75 per d).

Maintenance threonine requirement and efficiency of its use for accretion of whole-body threonine and protein in Atlantic salmon (Salmo salar L.) fry.

Eighteen groups of seventy Atlantic salmon (Salmo salar L.) fry (initial mean body weight 0.8 (sd 0.01) g) were fed on semi-purified diets containing graded levels of l-threonine (Thr) in 15 litres aquaria at a temperature of 14.5+/-1 degrees C. Doses of Thr represented 1, 31, 41, 51, 62, 72, 83 and 93 % of its ideal level for optimum protein deposition. Indispensable amino acids other than Thr were included in the same proportion (on a g/16 g N basis) as in the Atlantic salmon fry whole-body carcass. Following 36 d of feeding and a 36 h fast, fry were killed for whole-body protein and amino acid analysis. Weight gain (r2 0.98), protein accretion (r2 0.97), and Thr accretion (r2 0.97) were linear (P<0.01) functions of Thr intake. Slope of the Thr accretion regression line showed that the efficiency of Thr utilisation above maintenance was 76 %. At zero Thr intake, fry lost 5.4 mg Thr/kg body weight0.75 per d. The Thr maintenance requirement was 7.2 mg/kg body weight0.75 per d and the Thr requirement for growth was 66 mg for 1 g protein deposition. Increasing doses of Thr resulted in increased (P<0.05) concentrations of histidine and lysine, and decreased concentrations of isoleucine in whole-body protein. The maintenance need for Thr represented 13.4 % of the total need for Thr. The data suggest that efficiency of Thr utilisation above maintenance is constant at all levels of Thr intake between 1 and 93 % of the level required for optimum protein deposition.

The optimum dietary indispensable amino acid pattern for growing Atlantic salmon (Salmo salar L.) fry.

To determine the optimum indispensable (I) amino acid (AA) balance in Atlantic salmon (Salmo salar L.) fry, a single protocol established for the pig was adapted. The balance was calculated from the reduction in N gain after replacing about 45% of a single IAA by a mixture of dispensable AA in isonitrogenous diets. We confirmed that the mixture of AA simulating the AA pattern of cod-meal protein and gelatine (46:3, w/w) was used with the same efficiency as cod-meal protein and gelatine. From the deletion experiment an optimum balance between the IAA was derived. Expressed relative to lysine = 100, the optimal balance was: arginine 76 (SE 0.2), histidine 28 (SE 2.2), methionine + cystine 64 (SE 1.7), phenylalanine + tyrosine 105 (SE 1.6), threonine 51 (SE 2.4), tryptophan 14 (SE 0.7), valine 59 (SE 1.7). No estimates were made for isoleucine and leucine. Expressed as g/16 g N, the optimal balance was: arginine 4.0 (SE 0.0), histidine 1.5 (SE 0.1), lysine 5.3 (SE 0.2), methionine + cystine 3.4 (SE 0.1), phenylaline + tyrosine 5.6 (SE 0.1), threonine 2.7 (SE 0.1), tryptophan 0.7 (SE 0.0), valine 3.1 (SE 0.1). This AA composition is close to that of the Atlantic salmon whole-body, but using it as an estimation of the IAA requirements may lead to an overestimation of the branched-chain AA requirements and an underestimation of aromatic and S-containing AA requirements. The results are discussed in accordance with the key assumptions associated with the model used (broken-line model, IAA efficiencies and maintenance requirements).

Short- and long-term nutritional modulation of acetyl-CoA carboxylase activity in selected tissues of rainbow trout (Oncorhynchus mykiss).

Acetyl-CoA carboxylase (ACoAC) catalyses the carboxylation of acetyl-CoA into malonyl-CoA. This product plays a pivotal role in the regulation of energy metabolism since it is both a substrate for fatty acid synthesis and an inhibitor of the oxidative pathway. The present study was initiated to analyse the modulation of ACoAC activity in liver and selected extrahepatic tissues of rainbow trout (Oncorhynchus mykiss) by dietary changes as a contribution to the understanding of the nutritional control of lipid metabolism in fish. Short-term effects of food intake were studied by measuring ACoAC activity in the liver and dorsal white muscle at different time intervals after a meal. Only slight variations were observed in the muscle during the period 2-72 h after the meal. The long-term effects of an increase in dietary lipids or carbohydrates levels were examined by measuring ACoAC activity in the liver, adipose tissue, intestine, kidney, red muscle, dorsal and ventral white muscles of trout after 3 months of feeding with different diets. ACoAC activity is stimulated by a high-digestible starch diet in the abdominal adipose tissue and the white muscle. A high-lipid diet decreases ACoAC activity in the liver and the intestine, but not in other tissues. Contrary to mammals, a rapid adaptation of ACoAC activity to food supply is not effective in rainbow trout. However, a long-term nutritional control of ACoAC activity does occur in this species, but the target tissue differs with the predominant non-protein energy sources in the diet. The present results suggest the potential existence of two ACoAC isoforms with different tissue distribution as has been observed in mammals and birds.

Polyunsaturated fatty acid profiles of whole body phospholipids and triacylglycerols in anadromous and landlocked Atlantic salmon (Salmo salar L.) fry.

We compared the fatty acid compositions and gains of whole body triacylglycerols (TAG) and phospholipids (PL) in anadromous and landlocked Atlantic salmon (Salmo salar) fry, of the same age, fed the same commercial marine oil-rich diet over a 42-day feeding trial. The landlocked strain exhibited significantly (P<0.05) higher growth rate and feed efficiency, due principally to a higher fat retention, particularly of monounsaturated and saturated fatty acids (SFA). n-3 and n-6 long-chain polyunsaturated fatty acid (PUFA) gains and retentions were significantly higher (P<0.05) in the landlocked fry. Great similarities were found in the fatty acid profiles of whole body TAG of both strains. However, marked genotypic differences were observed in the PUFA profiles of whole body PL fractions. The total PUFA, n-3 PUFA and docosahexaenoic acid (DHA) level in PL was significantly higher (P<0.05) while the SFA level, and the PUFA C18/C20 and eicosapentaenoic acid/arachidonic acid ratios were significantly lower (P<0.05) in the anadromous fry than in landlocked fry. Our results indicate that the level of DHA in salmon PL is under strong genetic control and that the capacity for incorporation, and possibly for the conversion of dietary n-3 and n-6 PUFA, is higher in the landlocked strain.

The effects of dietary lipid and strain difference on polyunsaturated fatty acid composition and conversion in anadromous and landlocked salmon (Salmo salar L.) parr.

Five experimental diets containing different proportions of olive, sunflower and linseed oils were used in a 55-day feeding trial on both anadromous and landlocked parr of Atlantic salmon (Salmo salar) of the same age, in order to study the effects of diet and strain on growth and fatty acid composition and absolute gains in fish whole body triacylglycerols (TAG) and phospholipids (PL). Growth rate was higher in landlocked than in anadromous parr, but not between the different diets. By contrast, the effect of diet on whole body fatty acid composition was much more pronounced than that of strain difference. The fatty acids deposition results establish significant (P<0.05) positive correlations and linear relationships between the percentage of several fatty acids (18:1n-9, 18:2n-6, 18:3n-3) in dietary lipids and their absolute gains in whole body TAG and PL of both stocks. They also indicate the selective deposition of 18:1n-9 compared with linoleic acid (LLA) and linolenic acid (LNA). Finally, the results suggest the occurrence of the conversion of LLA and LNA to long-chain polyunsaturated fatty acids, its stimulation by increased substrate availability, a significantly higher n-3 and n-6 polyunsaturated fatty acids conversion capacity in landlocked than in anadromous parr and a strong genetic influence on docosahexaenoic acid content in salmon parr PL.