Reevaluation of dietary methionine requirement by plasma methionine and ammonia concentrations in surgically modified rainbow trout,Oncorhynchus mykiss

This study aimed to reevaluate the dietary methionine requirement by means of the plasma methionine and ammonia concentrations in surgically modified rainbow trout, Oncorhynchus mykiss. A total of 35 rainbow trout averaging 505 ± 6.5 g (initial body weight, mean ± SD) were randomly distributed into seven groups with five fish in each group. After 48 h of food deprivation, each group was fed one of seven L‐amino acid‐based diets containing graded levels of methionine (0.25, 0.40, 0.50, 0.60, 0.70, 0.80 or 0.95% of diet, dry matter bases) by intubation at 1% bodyweight. Blood samples were taken at 0, 5 and 24 h after intubation. Post‐prandial plasma free methionine concentrations (PPmet, 5 h after intubation) and post‐absorptive plasma free methionine concentrations (PAmet, 24 h after intubation) of fish fed diets containing 0.60% or more methionine were significantly (P < 0.05) higher than those of fish fed diets containing 0.50% or less methionine. PPmet and PAmet in fish fed diets containing 0.60% or higher methionine were not significantly different except the PPmet of fish fed a diet containing 0.95% methionine. Post‐prandial plasma ammonia concentrations (PPA, 5 h after intubation) of fish fed diets containing 0.70% or more methionine were significantly higher than those of fish fed diets containing 0.60% or less methionine, and PPA of fish fed diets containing 0.25% and up to 0.60% methionine were not significantly different from each other. Broken‐line model analyses on PPmet, PAmet, and PPA indicated that the dietary methionine requirement of rainbow trout was between 0.59% (1.69) and 0.67% (1.91) of diets (% dietary protein bases) when the diets contained 0.5% cystine.     

Survival and changes in growth of juvenile tench (Tinca tinca L.) fed defined diets commonly used to culture non-cyprinid species

The study is an attempt to evaluate the feasibility of intensive tench culture using non‐specific diets as a preliminary step to check the acclimatization of the species under intensive rearing systems. Five‐month‐old juvenile tench were reared in recirculating systems at mean water temperatures of 22°C for 75 days. Fish fed with four different commercial diets (trout starter, trout first feeding, sea‐bass and eel), showed significantly higher final weights than the fish fed either eel or sea‐bass diets. Initial weight for all treatments was 2.3 ± 0.53 g. Final weight for the commercial diet groups was 3.56 ± 0.4 g, compared with the remaining groups that reached 2.09 ± 0.47 g (P < 0.05). Significantly higher survival rates were observed in the eel and sea‐bass groups (84.7 and 51.5%, respectively) than in either of the trout diet groups (38%). Specific growth rates (1.26 vs –0.18) and condition factor (1.26 vs 0.93) were also higher than those fed with salmonid diets (P < 0.05). Results obtained in this study indicate that regardless of the species’ slow growth, when compared with other cyprinids, final growth rates and survival of tench fed exclusively on sea‐bass or eel diets can be considered satisfactory. It must be pointed out that these promising results were obtained at lower temperatures than previous studies of tench in culture systems. The use of belt feeders did not show improvement in growth compared with manually fed fish when trout diets were used.     

Induction of vitellogenin production in male tilapia (Oreochromis mossambicus) by commercial fish diets

Mozambique tilapia, (Oreochromis mossambicus), are a euryhaline teleost and an important biological model species. Captive male tilapia frequently have high levels of the estrogen-induced yolk precursor protein vitellogenin (Vg), a common indicator of exposure to estrogenic compounds. Sex steroids are found in commercial fish diets, but relatively few studies have examined the relationship between commercial diets and Vg production. In a fasting experiment to ascertain a dietary role in male Vg production, plasma Vg was reduced to negligible levels after 2 weeks of fasting, while no change in estrogen receptor (ER) expression was seen. When male tilapia were fed a squid-based diet that replaced the commercial trout diet, plasma Vg was reduced to undetectable levels over 40 days, concomitant with significant reductions in hepatic expression of Vgs A, B, and C, and ERβ, compared with control fish fed commercial trout diet. Female tilapia fed the squid-based for 20 days had no change in these parameters. When male tilapia were fed a defined, soy-based diet, plasma Vg reduced to 20% of levels in fish given either commercial trout diet or a defined, fishmeal-based diet. Overall, results from these studies suggest that estrogens in a commercial trout diet induce vitellogenin production by increasing expression of Vg, but not ER genes in male tilapia.     

Maslinic acid as a feed additive to stimulate growth and hepatic protein-turnover rates in rainbow trout (Onchorhynchus mykiss)

Maslinic acid is a triterpene present in a considerable proportion in solid residues from olive-oil production. In the present work the effects of maslinic acid on growth, protein-turnover rates and nucleic-acid concentration on liver were investigated in the rainbow trout. Five groups of 120 fish of a mean body mass of 20 g were fed for 225 days with diets containing 0, 1, 5, 25 and 250 mg of maslinic acid per kg diet. At the end of the experiment, whole-body and liver weight and growth rate of trout fed with maslinic acid were higher than controls. The highest weight increase was registered for the group fed 250 mg kg(-1), representing a 29% increase over controls. The total hepatic DNA or liver cell hyperplasia levels in trout fed with 25 and 250 mg of maslinic acid kg(-1) were 37% and 68% higher than controls. Also in these same groups of trout, fractional and absolute hepatic protein-synthesis rates were significantly higher than in control, and significant increments in hepatic protein-synthesis efficiency and protein-synthesis capacity were reported. In close agreement with these results, microscopy studies showed that trout fed on 25 and 250 mg kg(-1) hepatocytes appeared to be more compact, with a larger rough-endoplasmic reticulum and larger glycogen stores than controls. These results suggest that maslinic acid can act as a growth factor when added to trout diet.     

The effects of diet composition and ration on biotransformation enzymes and stress parameters in rainbow trout, Oncorhynchus mykiss

Juvenile rainbow trout (Oncorhynchus mykiss) were fed one of three isoenergetic diets varying in protein (35-55%) and lipid content (8-18%), at full satiation levels or half rations for 6 weeks in order to investigate the effects of diet on baseline stress parameters and biotransformation enzyme activity. Growth was greatest in fish fed to satiation on a low protein and high lipid diet. Stress parameters, including plasma lactate, glucose and cortisol concentrations were not significantly affected by dietary treatment or ration. Basal biotransformation enzymes, including glutathione S-transferase (GST) and ethoxyresorufin O-deethylase (EROD) activity, were also unaffected by dietary treatment. Fish exposed to the biotransformation enzyme inducer beta-naphthoflavone did not exhibit an alteration in stress indicators or GST activity; however, EROD activity was increased (87- to 210-fold) in fish receiving all diets and rations. The results of the present study indicate that, unlike mammals, fish may be more recalcitrant to different levels of ingestion of isoenergetic diets varying in protein and lipid concentration with respect to stress responses and the maintenance of basal titers of biotransformation enzymes and their induction.     

Effects of dietary soybean protein levels on metabolic response of the southern catfish, Silurus meridionalis

A closed respirometer was used to measure oxygen consumption of the southern catfish Silurus meridionalis fed with six isonitrogenous (48% crude protein) diets replacing 0%, 13%, 26%, 39%, 52% and 65% fish meal (FM) protein by soybean meal (SBM) protein, in order to investigate the effects of dietary soybean protein level (SPL) (replacing FM) on metabolic rates of the southern catfish. The results showed that there were no significant differences in routine metabolism among dietary treatments. Either the total metabolic rate or specific dynamic action (SDA) was positively correlated with assimilated food energy at each diet, respectively (P<0.05). The SDA coefficient (means the energy spent in metabolism per unit of assimilated dietary energy) significantly increased with increasing dietary SPL (P<0.05). Fish fed the diet with 13% SPL had a significantly lower SDA coefficient (0.1528) than fish fed the diet with 52% or 65% SPL (0.1826 or 0.1932) (P<0.05). However, there were no significant differences in SDA coefficient among fish fed the diets with 13%, 26% and 39% SPL (P>0.05). Results of the present study suggested that an imbalance of essential amino acids at higher dietary SPL resulted in more energy channeled into metabolism, and subsequently increased the SDA coefficient.     

Effects of resveratrol and genistein on growth,nutrient utilization and fatty acid composition of rainbow trout

The replacement of the finite and costly resource fish oil is an important task for aquaculture nutrition. A promising approach could be the use of plant bioactives that may have the potential to influence the metabolism and the synthesis of n-3 long chain polyunsaturated fatty acids, especially EPA (20:5n-3) and DHA (22:6n-3). In this study, the two phytochemicals resveratrol (RV) and genistein (G) were investigated for their effects on fish growth, nutrient utilization and body nutrient composition alongside their effects on whole body fatty acid (FA) composition. In a feeding trial lasting 8 weeks, rainbow trout (initial BW: 81.4±0.5 g) were held in a recirculating aquaculture system and fed six experimental diets with varying fish oil levels as plain variants or supplemented with 0.3% of dry matter (DM) of either RV or G. The six diets were as follows: diet F4 had 4% DM fish oil, diet F0 had 0% DM fish oil, diets F4+RV, F4+G, F0+RV and F0+G were equal to the diets F4 and F0, respectively, and supplemented with the phytochemicals RV and G. The feeding of the F0+RV diet resulted in reduced feed intake, growth rate and slightly reduced whole body lipid levels. At the same time, the amount of polyunsaturated FA and the n-3/n-6 ratio were significantly increased in whole body homogenates of rainbow trout fed diet F0+RV in comparison to the F0 control. The feeding of the F0+G diet led to reduced feed intake, slightly increased protein utilization but did not significantly affect the whole body FA composition. Overall, feeding the fish oil-free diet supplemented with the phytochemicals resulted in more pronounced effects on fish performance and FA composition than the single factors per se (dietary fish oil level or phytochemical). Present data indicate that G might not be of profitable use for trout nutrition. In terms of FA composition, RV could be a potentially useful complement for fish oil. However, the impairment of growth and performance parameters as observed in the present study discourages its use in trout diets.     

The effect of peptide absorption on PepT1 gene expression and digestive system hormones in rainbow trout (Oncorhynchus mykiss)

The present study evaluates the effect of protein source (dipeptides, free amino acids, and intact protein) on development and growth of Salmonid fish alevin. Specifically, we follow the expression of oligopeptide transporter protein PepT1 in the intestine of rainbow trout (Oncorhynchus mykiss). Fish were fed exogenously one of four diets: three formulated (lysyl–glycine dipeptide supplemented diet — PP, free lysine and glycine supplemented diet — AA, control diet with no lysine — CON) or commercial starter (Aller Futura — AF). Fish increased mean body weight 8 fold with PP- and AA-supplemented diets resulting in significantly higher weight gain than fish fed CON. Statistical analysis revealed a significant increase in relative PepT1 expression of fish fed experimental diets. Immunohistochemical staining with PepT1 antibody showed the presence of the transporter protein in the brush border membrane of the proximal intestinal enterocytes of fish from all experimental groups. Leptin immunoreactivity occurred not only in the gastric glands but also in proximal intestine and pyloric caeca of fish fed PP, AA and AF diets. Leptin immunoreactivity was also observed in hepatocyte cytoplasm and pancreatic acinar cells. Gastrin/CCK immunoreactive cells were present in the proximal intestine and pyloric caeca.     

Muscle insulin binding and plasma levels in relation to liver glucokinase activity,glucose metabolism and dietary carbohydrates in rainbow trout

Rainbow trout were fed for 10 weeks with either a carbohydrate-free diet (C-free) or with four experimental diets containing various levels (20 or 40%) and sources of starch (extruded wheat or peas) in order to examine metabolic utilisation of dietary vegetable carbohydrates and its endocrine control. The study was focused on the parameters described as limiting in glucose metabolism in fish. Feeding trials were conducted at 8 and 18 degrees C to establish whether carbohydrate-rich diets can be used in trout farming irrespective of water temperature. At both temperatures, pea diets (especially the highest level) resulted in a feed efficiency as high as the C-free diet. Fish had similar growth rates except when fed the low wheat content diet. Glycaemia values 6 h after feeding were significantly higher in trout fed carbohydrate diets than those given the C-free diet, whereas plasma insulin levels were similar independently of the levels of dietary starch. This study provides the first evidence that glucokinase (GK) activity and mRNA level in trout liver increase in proportion to the content of dietary starch. Nevertheless, these changes were not correlated with plasma insulin levels. Insulin-like growth factor-I (IGF-I) binding and number of receptors in skeletal muscle were consistently higher than those for insulin but no diet-induced differences were found for any of these parameters. Temperature clearly affected the postprandial profile of glucose and insulin, which both showed lower levels 6 h after feeding at 8 degrees C than at 18 degrees C, which was consistent with a lower feed intake. Glucose and insulin levels decreased markedly 24 h after feeding at 18 degrees C, while they were still high at 8 degrees C, an observation concordant with delayed transit rate. These findings indicate satisfactory adaptation of rainbow trout to diets with a relatively high vegetable starch content, especially when provided as extruded peas, and indicate that diets with increased levels of carbohydrates can be used in this species even when it is reared at low temperature.     

Dietary protein source affects lipid metabolism in the European seabass (Dicentrarchus labrax)

The study was undertaken to evaluate the effects of dietary protein sources on lipogenesis and fat deposition in a marine teleost, the European seabass (Dicentrarchus labrax). Four isonitrogenous (crude protein (CP, Nx6.25), 44% DM) and isoenergetic (22-23 kJ/g DM) diets were formulated to contain one of the following as the major protein source: fish meal (FM), one of two soy protein concentrates (SPC) and corn gluten meal (CGM). Apparent digestibility coefficients of the diets and raw ingredients, as well as soluble nitrogen (ammonia and urea) and phosphorus excretion were measured. Growth rates of seabass fed plant protein-based diets were significantly lower than those fed fish meal based diet. The protein utilisation was strongly correlated to the dietary essential amino acids index. Measurements of N excretion (ammonia and urea nitrogen) confirmed these data. Daily fat gain at the whole body level ranged between 1.1 to 1.7 g/kg BW, with the highest values being recorded in fish fed the fish meal based diet. Levels of plasma triglycerides and cholesterol were lower in fish fed soy protein diets than in those fed the diet solely based on fish meal. Soy protein rich diets decreased the activities of selected hepatic lipogenic enzymes (glucose 6-phosphate dehydrogenase, malic enzyme, ATP-citrate lysase, acetylcoenzyme A carboxylase and fatty acid synthetase). Highest lipogenic enzyme activities where found in fish fed the fish meal diet, except for fatty acid synthetase which was increased in seabass fed the corn-gluten meal based diets. Overall data suggest that dietary protein sources affects fat deposition and the lipogenic potential in European seabass.     

Influence of dietary composition on gluconeogenesis from L-(U-14C)glutamate in rainbow trout (Salmo gairdneri)

The effect of dietary composition (high-protein, high-carbohydrate and high-fat diets) and starvation on in totum gluconeogenesis from L-(U-14C)glutamate was studied in the rainbow trout. High-fat and high-carbohydrate diets produced a significant hyperglycaemia. Lower blood glucose values were obtained in trout fed on a high-protein diet. Liver glycogen levels were significantly lower in trout fed on carbohydrate-free diets (high-protein and high-fat diets) and in starved fish. Gluconeogenesis from L-(U-14C)glutamate was markedly reduced in fish given the high-carbohydrate diet and significantly enhanced in starved fish. Radioactive liver glycogen was higher in starved fish, although the amount of radioactivity incorporated into glycogen was very low.     

Studies on limiting essential amino acids in grieves as source of dietary protein for rainbow trout (oncorhynchus mykiss)

Diets were computed to contain equal concentrations of digestible crude protein either of wheat gluten (diet 1) or of grieves (diets 2–8). Per kg dry diet, 41 g crystalline amino acids were supplemented. All diets contained at least 1.2 g Lys per MJ digestible energy (DE). In diet 2, ratios of Met + Cys, Trp, Leu, Ile and Phe to Lys were about equal to those in diet 1. In each of diets 3–7, one of the respective amino acids, in diet 8 all five were replaced by Glu in the supplemented mixture of amino acids.

Each diet was fed to triplciate groups of 20 trout during a trial lasting 66 days. Trout fed the diet containing wheat gluten consumed more dry matter and showed higher growth rates as well as higher protein contents in their gained body mass than trout fed diets based on grieves. Supplementing Met plus Trp significantly improved dry matter intake, growth rate and protein content of gain, though not to the level of trout fed the wheat gluten diet, whereas Leu, Ile and Phe showed no such effect. When grieves were not supplemented with both Met and Trp, gain in body mass contained significantly more lipids. DE required per kg gain by trout fed wheat gluten, grieves + Met + Trp or grieves without supplementation of Met and Trp was 20.1, 21.2 and 29.9 MJ, respectively.     

Growth and feed utilization by F4 human growth hormone transgenic carp fed diets with different protein levels

The F₄ generation of human growth hormone (hGH) transgenic red common carp Cyprinus carpio had significantly higher growth rates than the non-transgenic controls. Protein and energy intakes were significantly higher in the transgenic carp than in the controls fed the 20% protein diet, but were not different between the two strains fed diets with 30 and 40% protein. Faecal protein loss, as a proportion of protein intake, was significantly lower in the transgenics than in the controls fed diets with 20 and 30% protein, but was not different between the two strains fed diet with 40% protein. Faecal energy loss, as a proportion of energy intake, was significantly lower in the transgenics than in the controls fed diet with 20% protein, but was not different between the two strains fed diets with 30 and 40% protein. Recovered protein, as a proportion of protein intake, was significantly higher in the transgenics than in the controls fed all diets, whereas recovered energy was significantly higher in the transgenic fish fed the 40% protein diet. For fish fed each diet, the transgenics had significantly higher body contents of dry matter and protein, but lower contents of lipid than the controls. It was concluded that transgenics were more efficient in utilizing dietary protein than the controls. At a lower dietary protein level, transgenics achieved higher growth rates mainly by increasing feed intake; at higher levels of dietary protein, transgenics achieved higher growth rates mainly through a higher energy conversion efficiency.     

The red alga Porphyra dioica as a fish-feed ingredient for rainbow trout (Oncorhynchus mykiss): effects on growth, feed efficiency, and carcass composition

Porphyra dioica meal was added at levels of 5, 10 and 15% to a diet for rainbow trout formulated to be isonitrogenous and isolipidic. The control diet was a commercial trout diet without seaweed meal. The experimental groups were fed in triplicate for 12.5 weeks, during which fish weight increased on average from 107–261 g. Seaweed meal inclusion did not affect significantly weight gain (WG), specific growth rate (SGR), feed efficiency (FE), protein efficiency ratio (PER) and apparent digestibility coefficient of the dry matter (ADC_dm) for any of the diets. Voluntary feed intake (VFI) increased for all seaweed diets compared to the control diet but not significantly (P?>?0.05). Final weight (FW) was significantly smaller for the 15% P. dioica inclusion and hepatosomatic index (HSI) for the 10% and 15% inclusion. Carcass protein content increased for all three experimental diets, and was significantly higher for the diet with 10% seaweed inclusion. Rainbow trout fed with Porphyra meal presented a dark orange pigmentation of the flesh at the end of the trial, compared to the whitish color from the control fish. These results suggest that P. dioica can effectively be included in diets for rainbow trout up to 10% without significant negative effects on weight gain and growth performance. The pigmentation effect of the fish flesh by adding P. dioica meal to the feed is of a considerable interest to the organic salmon-farming industry.     

Changes in the blood chemistry of rainbow trout, Salmo gairdneri Rich, in relation to dietary protein level, and an anabolic steroid hormone, ethylestrenol

The effect of feeding isocaloric test diets containing 35, 45 and 55% protein, with and without inclusion of the anabolic steroid hormone, ethylestrenol, was studied in rainbow trout, Salmo gairdneri , in relation to blood chemistry. The control (no hormone) and experimental diets (hormone-supplemented), were fed for a total of 60 days, after which time all groups of fish were fed an identical commercial diet with no hormone for a further 30 days (withdrawal period). After 60 days, the body weights offish fed the 35 and 45% experimental diets were significantly greater than their respective controls, and after hormone withdrawal, increased growth was still apparent in the 35% experimental group.
No significant changes in serum amino acid nitrogen (AAN), protein, or glucose in relation to the dietary protein level, or inclusion of steroid, were observed after 30 days. Serum creatinine, however, increased with an increase in dietary protein, and was significantly higher in the 35% experimental group than the respective control. After 60 days, the most significant observation was the marked increase in serum glucose with an increase in dietary protein, but respective control and experimental values were not significantly different at this time. Following a 30-day withdrawal period, serum AAN in the 55% experimental group was significantly higher than the control, whereas serum protein, creatinine, and glucose stabilized to similar concentrations in all groups. Over the 90-day period of feeding, in both control and experimental groups, serum AAN and protein tended to increase, serum creatinine and glucose to decrease, whilst haematocrit remained constant. It is concluded that addition of ethylestrenol to trout diets has apparently little effect on serum metabolite concentrations and haematocrit, the most significant variations being related more to diet composition and duration of study.     

Growth performance and body composition in response to dietary protein and lipid levels in Nile tilapia (Oreochromis niloticus Linnaeus, 1758) subjected to normal and temporally restricted feeding regimes

A factorial experiment was designed to examine the effect on compensatory growth (CG) of Nile tilapia Oreochromis niloticus fed diets containing different protein and lipid levels under normal and temporally restricted feeding regimes. Four diets were formulated to contain either 30% or 36% crude protein, and 5% or 11% crude lipid. Triplicate replicates of each treatment were assigned to 24 150‐L tanks (20 fish/tank density). Fish (mean initial weight ± SD = 8.79 ± 0.34 g) were then fed either the normal feeding regime (thrice daily to apparent satiation) or the restricted regime (1 day feed deprivation followed by 3 days of feeding to apparent satiation) over a 44‐days study period. Fish receiving a diet under the restricted regime achieved weight gains (WG) comparable to fish consuming the diet containing 30% protein and 5% lipids under the normal feeding regime. Fish maintained on the restricted feeding regime exhibited reduced feed intake (FI), WG, feed efficiency ratio (FE), protein efficiency rate (PER) and hepatosomatic index versus fish on the normal feeding regime, except WG in fish fed the diet with 30% protein and 5% lipids. However, the resultant FI (85%~94%) was higher than the excepted 75% intake when fish were subjected to the restricted regime. Feeding 11% lipid diets led to improved FI, WG, FE, and PER compared to feeding the 5% lipid diets. Increased FI, WG, and FE, but reduced PER were observed in fish fed with 36% protein versus fish fed 30% protein. Fish receiving the 36% protein diets had lower whole‐body moisture and ash contents, but elevated whole‐body protein and lipid contents compared to those receiving the 30% protein diets. Whole‐body moisture contents were lower, but whole‐body protein, lipid and ash contents were higher in fish fed 11% lipid diets than in fish fed 5% lipid diets. There was an increase in whole‐body moisture content, but a decrease in protein and lipid content in response to the restricted feeding regime. Ash content was not affected by the feeding regime. The present study shows that Nile tilapia fed diets subjected to a restricted feeding regime exhibited growth comparable to those fed the diet at 30% protein and 5% lipid levels under a normal feeding regime. This positive effect was more pronounced in diets at a high protein level or in a combination of high protein and lipid levels.     

Relationship between dietary tyrosine and plasma cholesterol in the rat

The present study was undertaken to measure the effects of dietary tyrosine added to fish protein and peanut meal on plasma cholesterol and plasma thyroid hormone levels in the rat. These dietary proteins were chosen because they contain similar amounts of tyrosine but release it at different rates during enzymatic hydrolysis. Casein was chosen as the reference protein. Supplementation was used to obtain tyrosine levels similar to that of casein. Male Sprague-Dawley rats were fed cholesterol-enriched diets containing 15% protein. After 3 weeks of experimental feeding, total postprandial plasma cholesterol was similar in the casein and peanut meal groups and significantly lower in the fish group. When added to the fish diet, tyrosine caused an increase in plasma cholesterol to a level similar to that of the casein group, whereas supplementation had no effect on plasma cholesterol of rats fed the peanut meal diet. The effects of dietary proteins or of tyrosine supplementation on cholesterol levels of the (density less than 1.006 g/mL) lipoprotein fraction were comparable, but not all significant, to those observed on total plasma cholesterol. In addition, casein and fish diets induced significantly higher levels of plasma triiodothyronine (T3) and lower levels of plasma thyroxine (T4) than did the peanut meal diet. However, the addition of tyrosine to the fish or the peanut meal diet did not modify the plasma thyroid hormone levels.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of a commercial trout diet on liver ultrastructure of fed and fasted yearling coho salmon, Oncorhynchus kisutch Walbaum

Yearling coho salmon were fed either a commercial trout (TC) diet or a diet consisting of homogenized adult Pacific Ocean coho salmon (PS), supplemented with vitamins and minerals. The hepatosomatic index of the TC-fed fish killed 12 h after the last feed or after 4 or 8 weeks of fasting were significantly larger ( P 0.05) than comparable PS-fed groups.
'Light' and'dark' hepatocyte cell types were evident in all groups although the dark cells were less numerous in the fasted fish. Hepatocytes of fasted fish contained markedly more endoplasmic reticulum than fed fish. Hepatocytic glycogen content of the TC-fed fish was higher than in PS-fed fish. Whereas in the TC-fed group hepatic glycogen was still evident in fish fasted for 8 weeks, the hepatocytes of the fasted PS-fed group contained little or no glycogen. The suitability of commercial fish diets for salmonids is discussed.     

Effect of dietary tryptophan on plasma and brain tryptophan, brain serotonin, and brain 5-hydroxyindoleacetic acid in rainbow trout

In order to determine the effect of dietary tryptophan level on plasma and brain tryptophan, brain serotonin, and brain 5-hydroxyindoleacetic acid levels, juvenile rainbow trout (Salmo gairdneri) were raised for 16 weeks on semipurified diets containing 0.06%, 0.16%, 0.21%, 0.26%, 0.39%, or 0.59% tryptophan. After 14 weeks, feed intake was depressed in fish fed the diets containing 0.06% or 0.16% tryptophan. No further differences in feed intake were noted between the remaining treatments. In addition, body weight was lower in fish fed diets containing 0.06%, 0.16%, or 0.21% tryptophan compared with fish fed higher levels. After 16 weeks of feeding the test diets, plasma tryptophan levels were found to be directly related to dietary tryptophan levels. Similarly, increased dietary levels of tryptophan resulted in increased brain levels of tryptophan, serotonin, and 5-hydroxyindoleacetic acid. These results demonstrate that in rainbow trout, as in mammals, altered dietary levels of tryptophan result in alterations in plasma and brain tryptophan, brain serotonin, and brain 5-hydroxyindoleacetic acid.