Diets For Juvenile Rainbow Trout Research Articles

Taurine supplementation in plant-based diets for juvenile rainbow trout (Oncorhynchus mykiss): Effects on growth performance, whole body composition, and histomorphological features

Taurine supplementation in plant protein-based diets has been reported to improve growth performance, feed utilization, whole body composition, and gut integrity in carnivorous fish species, but data in rainbow trout are quite scarce. This study aimed to investigate the effects of plant protein-based diets supplemented with different levels of taurine on rainbow trout growth performance, whole body composition, and histomorphological traits of the intestine and main organs. In a 71-day trial, 340 juvenile rainbow trout were randomly distributed to 5 dietary treatments (4 replicate tanks/diet, 17 fish/tank): two control diets (a fishmeal-based diet [positive control; C+] and a soybean meal-soy protein concentrate-based diet [negative control; C-]), and three experimental diets in which the C-diet was supplemented with 2.5, 5 and 10 g/kg of taurine (T2.5, T5 and T10, respectively). At the end of the trial, growth parameters were assessed, and whole fish (12 pools of 3 fish/diet) and gut, liver and kidney samples (12 fish/diet) were collected for whole body composition and histomorphological analyses, respectively. Liver (8 fish/diet) was also sampled for taurine quantification. The fish fed the T10 diet showed higher weight gain, protein efficiency ratio, and specific growth rate – as well as decreased feed conversion ratio – when compared to the C+ group only (P < 0.05), thus suggesting a main effect of the taurine source on the response. The T2.5 rainbow trout showed lower moisture content and increased ether extract and ash contents than the other dietary treatments (P < 0.001). Higher crude protein content was also observed in fish fed the C- and T10 diets in comparison with the other groups (P < 0.001). Hepatic taurine levels were increased in the C+ group when compared to the C-, T2.5 and T5 diets (P < 0.05), with higher contents being also identified in the T10 group than the C- diet (P < 0.01). Differently, gut morphometric indices and histopathological features of the gut, liver, and kidneys were unaffected by dietary taurine supplementation (P > 0.05). In conclusion, dietary taurine supplementation in juvenile rainbow trout fed vegetable protein-based diet is capable of influencing the growth performance and the whole lipid content of the animals, without affecting the histomorphological traits of the intestine, the liver and the kidneys. In particular, the 2.5 and 10 g/kg supplementation levels seem to be capable of exerting the most pronounced effects in terms of composition of the fish body mass and growth parameters, respectively.

Schizochytrium sp. (T18) Oil as a Fish Oil Replacement in Diets for Juvenile Rainbow Trout (Oncorhynchus mykiss): Effects on Growth Performance, Tissue Fatty Acid Content, and Lipid-Related Transcript Expression.

Simple SummaryOne of the main concerns in aquaculture is the overreliance on wild-sourced fish oil as the main source of omega-3 fatty acids in diets for farmed fish. Microbes, such as Schizochytrium, naturally produce high levels of omega-3 fatty acids that could potentially replace fish oil in aquaculture feeds. In this study, we tested the oil from a new strain of Schizochytrium (T18) to replace fish oil in diets for farmed rainbow trout and looked at fish growth performance, muscle and liver fatty acid content, and the expression of transcripts involved in fat metabolism. Trout were raised for 8 weeks and fed diets with either: (1) fish oil control, (2) low inclusion of microbial oil, or (3) higher inclusion of microbial oil. Inclusion of Schizochytrium sp. (T18) oil at high or low levels in the diet resulted in a similar growth performance as seen in trout fed the control diet; however, muscle and liver fatty acid profiles were impacted by diet. Overall, our results showed that Schizochytrium (T18) is an effective source of omega-3 fatty acids in diets for rainbow trout.In this study, we evaluated whether oil extracted from the marine microbe, Schizochytrium sp. (strain T18), with high levels of docosahexaenoic acid (DHA), could replace fish oil (FO) in diets for rainbow trout (Oncorhynchus mykiss). Three experimental diets were tested: (1) a control diet with fish oil (FO diet), (2) a microbial oil (MO) diet with a blend of camelina oil (CO) referred to as MO/CO diet, and (3) a MO diet (at a higher inclusion level). Rainbow trout (18.8 ± 2.9 g fish−1 initial weight ± SD) were fed for 8 weeks and evaluated for growth performance, fatty acid content and transcript expression of lipid-related genes in liver and muscle. There were no differences in growth performance measurements among treatments. In liver and muscle, eicosapentaenoic acid (EPA) was highest in trout fed the FO diet compared to the MO/CO and MO diets. Liver DHA was highest in trout fed the MO/CO diet compared to the FO and MO diets. Muscle DHA was highest in trout fed the MO and MO/CO diets compared to the FO diet. In trout fed the MO/CO diet, compared to the MO diet, fadsd6b was higher in both liver and muscle. In trout fed the FO or MO/CO diets, compared to the MO diet, cox1a was higher in both liver and muscle, cpt1b1a was higher in liver and cpt1a1a, cpt1a1b and cpt1a2a were higher in muscle. Schizochytrium sp. (T18) oil was an effective source of DHA for rainbow trout.

Replacing fish oil and astaxanthin by microalgal sources produced different metabolic responses in juvenile rainbow trout fed 2 types of practical diets.

Dietary fish oil supplementation provides n-3 long-chained polyunsaturated fatty acids for supporting fish growth and metabolism and enriching fillet with eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; c22:6n-3). Two experiments were performed as a 3 × 2 factorial arrangement of dietary treatments for 16 wk to determine effects and mechanisms of replacing 0%, 50%, and 100% fish oil with DHA-rich microalgae in combination with synthetic vs. microalgal source of astaxanthin in plant protein meal (PM)- or fishmeal (FM)- based diets for juvenile rainbow trout (Oncorhynchus mykiss). Fish (22 ± 0.26 g) were stocked at 17/tank and 3 tanks/diet. The 100% fish oil replacement impaired (P < 0.0001) growth performance, dietary protein and energy utilization, body indices, and tissue accumulation of DHA and EPA in both diet series. The impairments were associated (P < 0.05) with upregulation of hepatic gene expression related to growth (ghr1and igf1) and biosynthesis of DHA and EPA (fads6 and evol5) that was more dramatic in the FM than PM diet-fed fish, and more pronounced on tissue EPA than DHA concentrations. The source of astaxanthin exerted interaction effects with the fish oil replacement on several measures including muscle total cholesterol concentrations. In conclusion, replacing fish oil by the DHA-rich microalgae produced more negative metabolic responses than the substitution of synthetic astaxanthin by the microalgal source in juvenile rainbow trout fed 2 types of practical diets.

Effects of Dietary Fish Meal Replacement by Red Lentil Meal on Growth and Amino Acid Composition of Rainbow Trout (Oncorhynchus mykiss)

The purpose of this study was to determine the effects of replacing fish meal with red lentil meal (RLM) as an alternative plant protein source in diets for juvenile rainbow trout (10.14±0.04 g mean initial weight) on growth performance and amino acid composition of fish. Four iso-nitrogenous and iso-lipidic experimental diets were prepared to include 15% (RLM15), 20% (RLM20) and 25% (RLM25) of fish meal. At the end of the 60 day feeding trial, the highest mean individual weight gain (30.55±0.08 g) of fish was found in control group but not significantly different from RLM15. Crude protein level of whole body/fillet gradually decreased with increase in RLM percentages in the diets. Generally, essential amino acid (EAA) profiles of whole body/fillets reflected the dietary EAA profile. EAA profile of fish fed RLM15 diet was close to control group (P&gt;0.05). However, lysine levels of fish decreased with increasing dietary RLM levels. RLM20 fed fish had the highest body contents of phenylalanine (P

Effects of enzymatically hydrolyzed fish by-products in diet of juvenile rainbow trout (Oncorhynchus mykiss)

Five experimental diets were formulated to evaluate the effects of dietary enzymatically hydrolyzed tuna by-product on growth, non-specific immune responses, and hematology of juvenile rainbow trout (Oncorhynchus mykiss). A basal diet with 50% of fishmeal was used as control (CON) and four other diets replaced 12.5% (TBB12.5), 25% (TBB25), 37.5% (TBB37.5), and 50% (TBB50) of fish meal in the CON diet. Juvenile rainbow trout (4.87 ± 0.05 g) were randomly distributed into 15 tanks (50 L) and fed 3–4% of wet body weight two times a day. At the end of 7 weeks of feeding trial, weight gain, specific growth rate, feed efficiency, and protein efficiency ratio of fish fed CON diet were significantly higher than those of fish fed TB50 diet (P < 0.05). But there were no significant differences among fish fed CON, TBB12.5, TBB25, and TBB37.5 diets (P > 0.05). There were no significant differences in GPT levels among fish fed CON, TBB12.5, TBB25, and TBB37.5 diets. Also, there were no significant differences in lysozyme, superoxide dismutase, glucose, and total protein levels in all experimental diet (P > 0.05). The broken-line analysis indicated that the minimum dietary level of enzymatically hydrolyzed tuna by-product to replace fishmeal could be 29.7% in rainbow trout. These results indicated that the optimum level of dietary enzymatically hydrolyzed tuna by-product could replace greater than 29.7% but less than 37.5% of fishmeal in juvenile rainbow trout diet.

Reemplazo de la harina de pescado con gluten de maíz en dietas de juveniles de trucha arcoíris (Oncorhynchus mykiss): efectos en crecimiento y otros parámetros fisiológicos

Background: Plant-origin meals have been proposed as an alternative to fishmeal as a source of protein in diets for rainbow trout (Oncorhynchus mykiss) and one suggested alternative is corn gluten. Goals: Determine the effects on the growth and other physiological parameters of rainbow trout juveniles fed diets made with corn gluten as substitute for fishmeal. Methods: Three diets that substituted corn gluten in ratios of 50, 75, and 100% (G50%, G75% and G100%, respectively) for fishmeal were fed during 90 days to juveniles initially weighing14.7 ± 0.2 g (mean ± standard error). We measured growth performance, oxygen consumption, P and N excretion, protein and lipid contents in muscle and liver, as well the number of lymphocytes and burst activity of the kidney macrophage. Results: Growth performance (weight gain and specific growth rate) decreased as corn gluten increased in the diet. The organisms fed with G75% and G100% diets had a significantly lower de­position of protein in the muscle with respect to the other groups, while lipid deposition increased in the same tissue. Increasing levels of the corn gluten in the diet caused increased oxygen consumption and N ammonium excretion but did not affect the number of lymphocyte and the macrophage activity. Conclusions: Corn gluten may be used at a substitution level up to 50% in diets for juvenile rainbow trout.

Land cover influences on juvenile Rainbow Trout diet composition and condition in Lake Michigan tributaries

AbstractWatershed development may alter tributaries in ways that influence the growth and development of juvenile fish. For Rainbow Trout Oncorhynchus mykiss, the extent of land cover influence on the diet composition of stream‐residing juveniles is still an open but important question, particularly given their broad global distribution. Our study evaluated the effect of land cover on diet composition and condition metrics of juvenile Rainbow Trout in Lake Michigan tributaries. Juveniles were collected in the fall of 2014 and 2015 from 18 sampling locations across the Lake Michigan basin and included a wide variety of land cover types. Multivariate statistical procedures were used to quantitatively score sites along axes that maximized variation in land cover and diet composition. Subsequently, Spearman's rank correlations were used to test for significance of correlations between (a) land cover and diet composition, (b) land cover and fish condition, and (c) diet composition and fish condition. Our results indicated that land cover had an influence on the diet compositions of juvenile Rainbow Trout, but neither land cover nor diet composition had a significant influence on Rainbow Trout condition. These findings contrast with previously published work focusing on the growth responses of other fish species and provide insight into the plastic nature of Rainbow Trout. Further, we discuss potential ways in which fish may mitigate the effect of land cover changes as they are transmitted through stream food webs; however, the magnitude of the mitigation likely varies among fish species.

Evaluation of solid-state fermented protein concentrates as a fish meal replacer in the diets of juvenile rainbow trout, Oncorhynchus mykiss

Evaluation of solid-state fermented protein concentrates as a fish meal replacer in the diets of juvenile rainbow trout, <i>Oncorhynchus mykiss</i>

Refining Inclusion Levels of Grain Distiller's Dried Yeast in Commercial‐type and Plant‐based Diets for Juvenile Rainbow Trout, Oncorhynchus mykiss

AbstractRecent studies with rainbow trout, Oncorhynchus mykiss, demonstrated that grain distiller's dried yeast (GDDY) at up to 12% inclusion was an effective dietary protein source when replacing fish meal. To examine the effectiveness of GDDY when replacing an increased variety of dietary protein sources, two feeding trials were conducted. In the first trial, six commercial‐type diets were formulated to contain 42% digestible protein and 20% crude lipid with GDDY included at 0, 6, 9, 12, 15, and 18% to replace all dietary protein sources except fish meal. In the second trial, four plant‐based diets with GDDY at 0, 9, 12, and 15% were examined. Experimental diets were fed twice daily, to apparent satiation, to three replicate tanks of fish/diet (initial weight = 22.3 ± 0.7 g) for 10 wk in a 15 C recirculating system. Results demonstrated excellent fish growth and the inclusion rate of GDDY did not negatively affect growth or feed conversion. No significant negative effects of GDDY on body indices or whole‐body proximate composition were observed. Based on these results, GDDY can be included in both commercial‐type diets and plant‐based diets for rainbow trout at up to 18 or 15%, respectively, without decreasing growth performance.

Evaluation of grain distillers dried yeast as a fish meal substitute in practical-type diets of juvenile rainbow trout, Oncorhynchus mykiss

Abstract Grain distillers dried yeast (GDDY) is a single-cell protein obtained as a co-product during the production of fuel ethanol that may have potential as a protein replacement for rainbow trout. The goal of this study was to examine the suitability of GDDY as a replacement for fish meal on a digestible protein basis in rainbow trout diets. An in-vivo digestibility study was performed to determine the nutrient availability of GDDY. Subsequently, a control diet containing 42% digestible protein and 20% lipid was formulated to replace fish meal protein with GDDY protein at eight different levels (0, 25, 37.5, 50, 62.5, 75, 87.5, and 100%). Diets were fed to juvenile rainbow trout stocked into four replicate tanks per dietary treatment (30 fish/tank) and fed twice daily for nine weeks. High GDDY inclusion rates significantly altered rainbow trout growth and feed conversion but not feed intake. There were no significant differences in production performance in fish fed the 25% GDDY and 37.5% GDDY diets when compared to fish fed the control diet, but further dietary fish meal replacement generally decreased fish performance. Further research is warranted to determine why fish performance decreased with higher inclusion levels of GDDY in spite of similar feed intake among levels.

Partial to total replacement of fishmeal by poultry by-product meal in diets for juvenile rainbow trout (Oncorhynchus mykiss) and their effect on fatty acids from muscle tissue and the time required to retrieve the effect

Triplicate groups of rainbow trout (Oncorhynchus mykiss) (1.4 ± 0.1 g) were fed experimental diets containing four levels of protein substitution to totally replace fishmeal with poultry by-product meal (0, 33, 67 and 100%). Fishmeal (FM) from tuna fish by-products and poultry by-product meal ‘pet food grade' (PBM) were used. Diets were formulated to be isonitrogenous (43.5% crude protein) and isolipidic (12.5% crude fat). After 80 days of feeding, no significant differences were found in terms of thermal unit growth coefficient (TGC). However, the EPA levels in muscle tissue among treatments 0PBM, 24PBM and 44PBM were found to be significantly higher than those for the 59PBM diet. Nevertheless, a re-feeding period with the control diet showed that 28 days was long enough to regain the FA profile needed. We concluded that PBM used up to 44% in diets for juvenile rainbow trout can be used without a significantly decrease in EPA and DHA under the present conditions. The use of a whole fishmeal diet could be an important strategy to recuperate the fatty acid profile obtained when trout is feed on PBM basis. It will be important to perform longer experiments with larger fish to confirm these results.

Effects of dietary propolis and vitamin E on growth performance and antioxidant status in juvenile rainbow trout (Oncorhynchus mykiss) under different flow rate

The present study investigated the effects of propolis and vitamin E supplementation in diets of juvenile rainbow trout subjected to two different flow rates (0.9 and 2.1 L min−1) on growth performance, and vitamin A, C and E concentrations in tissues as well as malondialdehyde (MDA) levels. Juvenile rainbow trout were fed with diets containing 10 and 30 g propolis kg−1, 60 mg kg−1 vitamin E (Rovimix E-50 adsorbate; min.%50 dl-α-tokopherly acetate) and without supplemented basal diet for 12 weeks. Weight gain (WG) in the C group was significantly lower (P 0.05). Survival rate (SUR) was higher in propolis and vitamin E supplemented diet groups (P < 0.05) than control diet group at 0.9 L min−1. Fish fed on diet E60 had higher (P < 0.05) tissue vitamin E concentration than fishes fed on other diets groups. Vitamin C concentration in rainbow trout tissues was significantly affected by the 30 g propolis supplemented diet group (P < 0.05), followed by the 10 g propolis supplemented diet group (P < 0.05). MDA level of E60 group was found significantly decreased instead of different than other groups (P < 0.05). The results of Student's t-test revealed that WG, SGR, SUR values, vitamin (A, C, E) concentrations and MDA levels of tissues were negatively affected by 0.9 L min−1flow rate treatment in juvenile rainbow trout.

An initial investigation replacing fish meal with a commercial fermented soybean meal product in the diets of juvenile rainbow trout

The inclusion of PepSoyGen (PSG), a commercially-available fermented soybean meal product, was evaluated with juvenile rainbow trout, Oncorhynchus mykiss in an initial 70-day feeding trial, with a supplemental trial involving a subset of the experimental diets continuing for an additional 40 d. Six diets containing 0%, 10%, 20%, 30%, 40%, or 50% PSG, with the PSG directly replacing fish meal, were used in the first trial. There were no significant differences in weight gain or feed conversion ratio between the fish meal-based control diet and diets containing up to 30% PSG. However, weight gain was significantly reduced and feed conversion ratio significantly increased with the 40% and 50% PSG diets. No health assessment differences were observed in fish receiving any of the diets, and no evidence of gross gut inflammation was evident. There were no significant differences in weight gain or feed conversion ratio among the four dietary treatments ranging from 0% to 30% PSG which were fed for an additional 40 d after the initial 70-d trial (110 days total). Based on these results, juvenile rainbow trout diets can contain up to 30% PSG without any loss of rearing performance, thereby replacing at least 60% of the fish meal.

Effects of Total Replacement of Fishmeal with Spirulina Powder and Soybean Meal on Juvenile Rainbow Trout (Oncorhynchus mykiss Walbaum)

Juvenile rainbow trout (Oncorhynchus mykiss) were fed diets in which fishmeal was totally replaced by mixtures of Spirulina powder and soybean meal. Three experimental diets were formulated with 75% Spirulina and 25% soybean meal, 50% Spirulina and 50% soybean meal, and 25% Spirulina and 75% soybean meal. A commercial diet and a diet with 100% fishmeal served as controls. Diets were fed to triplicate groups of 10 juveniles with an initial weight of 6.8±0.4 g for 50 days. At the end of the experiment, growth performance, oxygen consumption, nitrogen and phosphorus excretion, protein digestibility, and serum protein content and lysozyme activity were evaluated. Data were analyzed by one-way ANOVA. Growth performance was not significantly affected by the experimental diets, but values tended to be lower as the level of dietary soybean meal increased. Among the experimental diets, the 75% Spirulina/25% soybean diet produced the best growth performance. Further, in fish fed this diet, the apparent coefficient of digestibility of protein was higher than 98% (as with all diets), oxygen consumption was lower and lysozyme activity was higher than in fish fed other replacement diets, and dissolved phosphorus and phosphorus in feces were similar or lower than in the controls. The present research shows the possibility of using mixtures of Spirulina powder and soybean meal as total substitutes of fishmeal in diets for juvenile rainbow trout.

Effect of replacing fish meal with soybean meal on growth, feed utilization and nitrogen and phosphorus excretion on rainbow trout (Oncorhynchus mykiss)

The effect of replacing fish meal with soybean meal (SBM) pretreated with phytase on feeds of juvenile rainbow trout was evaluated in a 90-day feeding trial. The rainbow trout (initial body weight, 4.01 ± 0.02 g) were fed five isonitrogenous (crude protein, 44.97%) and isolipidic (crude lipid, 13.42%) feeds. Diets were formulated to contain phytase-pretreated SBM replacing 0, 20, 40, 60 and 80% of fish meal protein, respectively. The results showed that there was no significant difference in weight gain (WG) among fish fed S0, S20, S40 and S60 diets; however, a significant reduction of this variable occurred when 80% of fish meal protein was replaced by phytase-treated SBM (P < 0.05). Similarly, specific growth rate and protein efficiency ratio had a similar trend with WG. Apparent digestibility coefficient (ADC) of protein and lipid in the S80 diet was significantly lower than that of the other diets, and the ADC of phosphorus significantly increased with the increase of dietary phytase-treated SBM level. No significant differences among treatments were detected for moisture, protein, lipid and ash content in whole body and muscle samples. Nitrogen and phosphorus excretion indicated that fish meal replacement by phytase-treated SBM led to an increase in nitrogen excretion, but led to a reduction in total phosphorus excretion. The results of the present study show that 60% of fish meal could be replaced by phytase-treated SBM in diets of juvenile rainbow trout without compromising weight gain or feed efficiency. A quadratic equation according to regression analysis of weight gain against dietary phytase-treated SBM level indicated that the optimal level of dietary phytase-treated SBM replacement for maximum growth was 26.90%.

Partial or total replacement of fishmeal by solvent-extracted cottonseed meal in diets for juvenile rainbow trout (Oncorhynchus mykiss)

The effect of solvent-extracted cottonseed meal (SCSM) as a partial or total replacement of fishmeal was studied in juvenile rainbow trout (Oncorhynchus mykiss). Six experimental diets SCSM0, SCSM25, SCSM50, SCSM75, SCSM75A and SCSMT, containing a gradient of SCSM 0, 152, 305, 465, 460 and 610 g kg−1 to replace 0, 112.5, 225, 337.5, 337.5 and 450 g kg−1 fishmeal protein were fed to triplicate groups (initial body weight of 39.2 ± 0.1 g) for 8 weeks. The diet SCSM75A was supplemented with lysine and methionine, to be similar to SCSM0 for juvenile rainbow trout. Faeces were colleted after 4 weeks of normal feeding for apparent digestibility coefficients (ADC) of dry matter, crude protein and gross energy determination. Total replacement of fishmeal adversely affected growth performance. Fish fed with diet SCSMT had significantly (P < 0.05) lower weight gain, specific growth ratio, feed conversion efficiency (FCE) and protein efficiency ratio than fish fed with other diets. The FCE of SCSM75 and SCSM75A were significantly lower (P < 0.05) than those of fish fed with SCSM0 diets. The ADC of the dry matter of SCSM75 and SCSMT were significantly lower than the SCSM0 diet, and the ADC of crude protein and the energy of SCSMT were the lowest (P < 0.05). The ADC of threonine, proline, alanine, valine, isoleucine, leucine, lysine and methionine of fish fed with diet SCSMT were lower. Lysine and methionine supplement positively affected the ADC of SCS75A diet. There were no significant differences in the fish body composition. It is shown that SCSM can be utilized in the juvenile rainbow trout diet up to 305 g kg−1, to replace about 50% of fishmeal protein in this experiment.

Effect of dietary conjugated linoleic acid on muscle, liver and visceral lipid deposition in rainbow trout juveniles ( Oncorhynchus mykiss)

Some of the conjugated linoleic acids (CLA), cis-9, trans-11 and trans-10, cis-12 isomers, may present a wide range of health benefits. Taking into account these beneficial effects and the relative low amounts of CLA in fish products, commercial diets for juvenile rainbow trout were supplemented with graded levels (0, 0.5, 0.75, 1 and 2%) of CLA. Duplicated groups of 5.3 g fish were fed to satiation, twice a day, over a period of 12 weeks with a CLA mixture, containing mainly the bioactive cis-9, trans-11 and trans-10, cis-12 isomers. The increase of dietary CLA levels did not affect growth performance, feed conversion ratio (FCR), hepatosomatic index (HSI) or viscerosomatic index (VSI) of the rainbow trout juveniles. Total lipids in muscle and liver were similar among treatments. In viscera, CLA supplementation reduced total lipids, but only the 0.5% CLA treatment differed significantly from the control. The fatty acid profile was affected by the dietary inclusion of CLA, resulting in a significant increase of the saturated and polyunsaturated fractions and a general decrease of the monounsaturated one, in both muscle and viscera. Nevertheless, in liver only fish fed 1% CLA had a significantly higher saturated fraction. The general decreases of monounsaturated fatty acid level in tissues with the saturated increase suggest a reduction of Δ-9 desaturase activity. CLA was deposited in all tissues and reflected the dietary CLA level. The highest concentration was attained when the 2% diet was used (6.9 g/100 g lipids in muscle). In conclusion, rainbow trout are capable of incorporating CLA in muscle, liver and viscera better than any other vertebrate, showing that this species is a potential route to increase human CLA ingestion.

Brassica by-products in diets of rainbow trout ( Oncorhynchus mykiss) and their effects on performance, body composition, thyroid status and liver histology

An experiment was designed to assess the inclusion rate of cabbage meal in diets for juvenile rainbow trout ( Oncorhynchus mykiss). Cabbage meal was evaluated at 320 (C4), 250 (C3) and 160 g kg −1 (C2) inclusion rate and compared to a control diet (C1) without cabbage meal. Triplicate groups of trout (initial body weight: 16.6 g) were fed by hand with isonitrogenous and isoenergetic experimental diets, twice a day for 10 weeks. The results showed very high significant differences ( P<0.001) between diets on final body weight, specific growth rate and voluntary feed intake and a linear effect of substitution. Diets effected a highly significant difference ( P<0.01) on feed gain ratio and protein efficiency ratio with the substitution being explained by a quadratic contrast. The diets also promoted very highly significant differences ( P<0.001) in all coefficient of total tract apparent digestibility (CTTAD) except for crude protein in which differences were of smaller magnitude ( P<0.01). The partial replacement of gelatinised starch and fish meal by cabbage meal seems to have a negative influence on all CTTAD except ash. Diets containing cabbage meal were associated with significantly lower plasma T 3 levels ( P<0.05) than diet C1 due to the presence of anti-nutritional factors. Thus, inclusion of cabbage meal in diets for rainbow trout is acceptable up to a certain level.

Effects of Rapeseed Meal-Glucosinolates on Thyroid Metabolism and Feed Utilization in Rainbow Trout

Two rapeseed meals (RM1 and RM2), containing glucosinolates at a concentration of 26 and 40 μmol/g, respectively, were incorporated at increasing levels (10, 20, and 30% for RM1 and 30 and 50% for RM2) in diets of juvenile rainbow trout. Disturbances in the thyroid axis appeared after 14 days of feeding (with a dietary incorporation level of 10%). The dietary supplementation with T3 or iodine induced an increase in plasma T3 levels, compared to that in fish fed the RM diets, and reduced the deleterious effect of RM on growth. When trout were reared in seawater, there was also a slight increase in thyroid hormone levels. TSH treatment had no effect on the thyroid hormone plasma levels. The incorporation of 30% of RM1, which induced a lower dietary content of toxic compounds than RM2, led to a rapid decrease of plasma T4 and T3 levels, but growth was affected only after 6 months of feeding. During these studies, the deiodinase activities responded in a complex manner to restore plasma and tissue levels of T3.

Replacement of Fish Meal by a Mixture of Animal By-Products in Juvenile Rainbow Trout Diets

A mixture of animal by-products (MAB) was tested to replace fish meal (FM) in diets for juvenile rainbow trout Oncorhynchus mykiss. Fish averaging (±SD) 0.96 ± 0.07 g were divided into 15 groups, and 3 groups were fed one of five isonitrogenous diets replacing 0, 20, 40, 60, or 100% of FM protein with similar percentages of MAB (control, MAB20, MAB40, MAB60, or MAB100, respectively). The MAB consisted of 25% meat and bone meal, 24.5% leather meal, 20% squid liver powder, 15% feather meal, 7.5% blood meal (spray-dried), 7.5% poultry by-product meal, and 0.25% each methionine and lysine. After 16 weeks of feeding, fish fed diets MAB40, MAB60, and MAB100 exhibited significantly lower growth performance than the fish fed the control and MAB20 diets. Apparent digestibility estimates of protein with different collection times (every 40 min and after 12 and 48 h) did not show, in a feces collection system, protein leaching into the water for up to 48 h. The MAB substitution of up to 60% of FM protein in diets did not show differences in apparent protein digestibility (85.8 ± 1.05% for MAB20, 83.1 ± 0.45% for MAB40, and 82.7 ± 2.67% for MAB60) compared with the control (85.0 ± 1.88%), whereas in the MAB100 group digestibility (77.7 ± 4.42%) was significantly lower than in the other groups. The apparent phosphorus absorption of test diet groups was significantly higher (36.9 ± 11.12% for MAB20, 24.0 ± 6.20% for MAB40, 57.1 ± 5.22% for MAB60, and 57.4 ± 5.34% for MAB100) than that of the control (8.3 ± 0.15%). Concentrations of protein and ash in the whole body increased as MAB substitution in diets increased. The findings suggest that MAB could replace up to 28% of FM protein in diets for juvenile rainbow trout for 16 weeks without adverse growth effects.