The effect of hepatic passage on postprandial plasma lipid profile of rainbow trout (Oncorhynchus mykiss) after a single meal

For the first time, pre‐ and post‐hepatic plasma lipid profiles were monitored following a single meal in a free‐swimming, non‐anaesthetized fish. Rainbow trout (Oncorhynchus mykiss; 700–1500 g; 10 °C) were equipped with cannulae in the dorsal aorta (DA) and hepatic portal vein (HPV). Simultaneous blood samples, taken from both cannulae at 0, 3, 6, 12, 24 and 48 h postprandial, revealed the time course of the plasma lipid profiles following a single meal (1% of body mass). Primarily monounsaturated fatty acids with the exception of 18:1n ? 9, increased significantly from baseline by 12 h postprandial without greatly affecting total plasma lipid concentrations. Total plasma lipids then showed a small peak at 24 h postprandial, coinciding with a peak in triacylglycerols. We conclude that assimilation of lipids from the digest into the plasma is slower than reported for proteins and carbohydrates in the same species. Furthermore, as there were no significant differences between the HPV and DA, no measurable effect of hepatic passage on plasma lipid levels was resolved. Therefore, we also conclude that, in contrast to that in higher vertebrates, hepatic passage does not seem to have a major role in rainbow trout for modulating the postprandial plasma profile of lipids.     

Dynamics of plasma free amino acids in rainbow trout (Oncorhynchus mykiss under variety of dietary conditions

Plasma amino acid profiles in the systemic circulation of rainbow trout acclimated to diets containing different protein sources, with and without supplementation with free amino acids, were monitored for up to 120 hours after consumption of the diets. Plasma concentrations of lysine, methionine, and isoleucine increased more rapidly after feeding a diet supplemented with these amino acids in free form and, depending upon the increase in plasma concentration, remained at concentrations above those in fish fed the basal diet for at least 24h after feeding. Dietary supplementation with isoleucine increased plasma concentrations of leucine and valine as well as of isoleucine. Maximum plasma concentrations for most amino acids were attained between 12 and 24h postprandial. Dietary inclusion of gelatin caused more rapid elevations in plasma glycine and serine than did free glycine. Feeding at three hour intervals resulted in stable plasma concentrations of individual amino acids in contrast to the fluctuations occurring when fish were fed once daily. Frequent feeding favoured a higher proportion of protein to lipid in body composition.     

Effects of dorsal aorta cannulation on the stress response of channel catfish (Ictalurus punctatus)

The stress response to dorsal aorta cannulation and serial blood sampling was examined in the channel catfish Ictalurus punctatus. Channel catfish cannulated and repetitively sampled once a day for 7 days did not exhibit a change in response to surgery or to the 24h sample regime as measured by plasma cortisol, glucose, and chloride. In fish that were either serially bled (0, 1, 3, 6, 9, 12, 24, and 48h) immediately after surgery or allowed to recover 6 days before being serially bled, overall plasma cortisol levels were higher than those of fish bled every 24h. Catfish serially bled immediately after surgery had significantly higher plasma glucose levels compared with catfish allowed to recover from surgery 6 days before serial sampling. Although channel catfish recover from cannulation surgery in 24h, a longer recovery period may be needed prior to serial sampling if the samples are taken more frequently than every 24h.     

Effects of dietary yeast inclusion and acute stress on postprandial plasma free amino acid profiles of dorsal aorta‐cannulated rainbow trout

In a 4‐week experiment, 15 cannulated rainbow trout were fed three diets based on fish meal (FM), Saccharomyces cerevisiae yeast (SC) and Wickerhamomyces anomalus and S. cerevisiae yeast mix (WA). Fish were fed daily, and blood samples were collected on day 7 of each week at 0, 3, 6, 12 and 24 hr after feeding. In the final week, fish were exposed to a 1‐min netting stressor. All essential and non‐essential plasma amino acid levels except methionine were similar between fish fed diets FM, SC and WA. Plasma methionine and sarcosine were significantly higher in fish fed diets SC and WA, possibly due to the crystalline methionine level, form or feeding regime. Hydroxy‐proline and 3‐methyl‐histidine were higher in fish fed diet FM, which can be explained by the higher levels present in fish meal compared with yeast. In stressed fish, there were no dietary effects on plasma amino acid levels, but significant increases in taurine and cystathionine were found in stressed compared with unstressed fish. These results demonstrate that yeast‐based diets produce similar plasma amino acid profiles to fish meal and suggest that yeast may be a suitable fish meal replacement in diets for rainbow trout.     

Postprandial kinetics of gene expression of proteins involved in the digestive process in rainbow trout (<Emphasis Type="Italic">O. mykiss</Emphasis>) and impact of diet composition

The impact of increased incorporation of plant ingredients on diets for rainbow trout was evaluated in terms of gene expression of gastric (gastric lipase, pepsinogen) and intestinal (prolidase, maltase, phospholipase A2) digestive enzymes and nutrient transporters (peptide and glucose transporters), as well as of postprandial levels of plasma glucose, triglycerides and total free amino acids. For that purpose, trout alevins were fed from the start of exogenous feeding one of three different experimental diets: a diet rich in fish meal and fish oil (FM–FO), a plant-based diet (noFM–noFO) totally free from fish meal and fish oil, but containing plant ingredients and a Mixed diet (Mixed) intermediate between the FM–FO and noFM–noFO diets. After 16 months of rearing, all fish were left unfed for 72 h and then given a single meal to satiation. Blood, stomach and anterior intestine were sampled before the meal and at 2, 6 and 12 h after this meal. The postprandial kinetics of gene expression of gastric and intestinal digestive enzymes and nutrient transporters were then followed in trout fed the FM–FO diet. The postprandial profiles showed that the expression of almost all genes studied was stimulated by the presence of nutrients in the digestive tract of trout, but the timing (appearance of peaks) varied between genes. Based on these data, we have focused on the molecular response to dietary factors in the stomach and the intestine at 6 and 12 h after feeding, respectively. The reduction in FM and FO levels of dietary incorporation induced a significant decrease in the gene expression of gastric lipase, GLUT2 and PEPT1. The plasma glucose and triglycerides levels were also reduced in trout fed the noFM–noFO diet. Consequently, the present study suggests a decrease in digestive capacities in trout fed a diet rich in plant ingredients.     

Liver damage in brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum), following administration of the cyanobacterial hepatotoxin microcystin-LR via the dorsal aorta

Purified microcystin-LR (MC-LR) was administered via the dorsal aorta to brown trout, Salmo trutta L., or rainbow trout, Oncorhynchus mykiss (Walbaum), and, within 24 h, a dose of 300 μ g MC-LR kg^–1 caused increased activities in the blood by enzymes originating mainly from the liver, i.e. lactate dehydrogenase (LDH) and alanine transaminase (ALT). A dose of 75 μ g MC-LR kg^–1 significantly increased liver enzyme activities in the blood of brown trout at 24 h, but was without effect on rainbow trout, whereas 25 μ g MC-LR kg^–1 had no effect on blood LDH or ALT activities in either species. However, histopathological analysis of liver from both species following administration of the lowest toxin dose showed hepatocyte swelling and necrosis. Liver damage was more severe in brown trout compared to rainbow trout following administration of 300 μ g MC-LR kg^–1, showing disruption of the parenchymal architecture. After 48 h, there was a dose-dependent increase in the hepatosomatic index in both species. It is concluded that brown trout are less tolerant to MC-LR than rainbow trout.     

Effects of oral passive immunization against somatostatin‐14 on growth performance,body composition and IgY delivery in rainbow trout (Oncorhynchus mykiss) and common carp (Cyprinus carpio)

Egg‐yolk antibodies (IgY) against somatostatin‐14 (anti‐SST‐14) were evaluated as orally administered, growth promotants in gastric rainbow trout (Oncorhynchus mykiss) and agastric common carp (Cyprinus carpio) in an 8‐week feeding trial. Feeding groups were compared with fish which did not receive anti‐SST‐14 IgY. Growth responses and IgY plasma contents of the blood were assessed. In contrast to rainbow trout, oral anti‐SST‐14 addition to carp significantly improved protein efficiency ratio (PER, 1.02 ± 0.04) and protein productive value (PPV, 26.7 ± 0.91) after 56 days (P < 0.05) compared to the negative control (PER = 0.91 ± 0.02; PPV = 22.9 ± 0.66). IgY was undetectable in plasma of rainbow trout after oral administration, indicating that gastric degradation of the fed IgY makes this application route for growth promotion in gastric trout challenging.     

The physiologic effects of the anesthetic ketamine hydrochloride on two salmonid species

Adult coho salmon and subadult rainbow trout were used in experiments with the anesthetic ketamine hydrochloride. The drug (30 mg/kg) was injected into the dorsal aorta through an indwelling cannula. Intravascular administration of ketamine caused an immediate cessation of ventilation in both species for 10 s to 300 s and a loss of balance. Ventilation rate recovered to pre-anesthesia values within 1–2 h and arterial oxygen values were at pre-anesthesia levels by 3–24 h. Anesthesia caused a significant acidosis in both species. The blood pH and plasma CO₂ values had returned to pre-anesthesia levels by 4–24 h and 0.5-2 h, respectively.

For adult salmon, five of seven animals were unresponsive to tail grabbing at 4 h while with juvenile trout, three of five fish were fully responsive to touch at 1 to 2 h. This difference in duration of anesthesia was likely size-related. The applications of this injectable anesthetic for commercial fish use, mainly in the transport of animals, are suggested, but its use with food fish has not been assessed.     

Free amino acid distribution in plasma and liver of juvenile cobia (Rachycentron canadum) fed increased levels of lizardfish silage

Juvenile cobia (Rachycentron canadum) (100 g) were fed four moist diets (447–476 g kg^?1 dry wt) where 0, 130, 260 or 390 g kg^?1 of concentrated lizardfish (Saurida undosquamis) silage replaced fresh lizardfish, respectively. Blood and livers were sampled at 0, 6, 12, 24 and 48 h postfeeding at the end of the 3‐week experiment. At 6 h postfeeding in all groups, maximum concentrations of most plasma essential amino acids were observed, while significantly lower levels of most non‐essential amino acid levels were recorded compared to the other sampled times. At 6 and 12 h after feeding, the concentration of most plasma free amino acid (FAA) increased with an increase in dietary fish silage levels. Most FAA in livers of all groups peaked at 12 and 24 h postfeeding. However, at 48 h postfeeding, concentrations of most plasma FAA were significantly higher in fish fed 0% silage‐based diet than in fish fed the other diets (4999 versus 3390–4339 nmol AA mL^?1 plasma). Growth rates and feed utilization were significantly lower in cobia fed 26% or 39% silage‐based diets than in fish fed 0% or 13% silage‐based diets. Different levels of silage protein thus seemed to have effects on growth and feed utilization efficiency of juvenile cobia. Results from this study support the premise that fish silage can be included until 130 g kg^?1 in cobia diets.     

Total fish meal replacement with rapeseed protein concentrate in diets fed to rainbow trout (Oncorhynchus mykiss Walbaum)

The potential of rapeseed protein concentrate as fish meal alternative in diets for rainbow trout (initial average weight 37.8?±?1.4?g) was evaluated. Nine experimental tanks of a freshwater flow-through system were stocked with 12 fish each. Triplicate groups of fish received isonitrogenous (47.9?±?0.5% CP) and isoenergetic (22.4?±?0.2?kJ?g^?1) experimental diets with 0, 66 and 100% of fish meal substituted with rapeseed protein concentrate (71.2% CP), thereby providing 0, 29 and 43% of dietary protein. As the amino acid profile of rapeseed protein concentrate was comparable to fish meal, there was no need to supplement experimental diets with synthetic amino acids. At the end of the 84?days of feeding period, fish growth performance, feed intake and feed efficiencies were not compromised, when 100% of fish meal in the control diet was replaced with rapeseed protein concentrate, revealing a SGR of 1.19 or 1.10, a FCR of 1.09 or 1.18 and a feed intake of 78.5 or 74.7?g in fish fed on the control diet or fed the diet devoid of fish meal, respectively. Intestinal morphology did not reveal any histological abnormalities in all dietary groups. Blood parameters including haematocrit and haemoglobin as well as glucose, triglycerides and total protein in the plasma were not different between treatment groups. Thus, the rapeseed protein concentrate tested here has great potential as an alternative to fish meal in rainbow trout diets.     

Evaluation of feed protein quality by measuring plasma free amino acids in Atlantic salmon (Salmo salar L.) after dorsal aorta cannulation

Two successive experiments were conducted in order to assess plasma free amino acid (FAA) profiles as a method for evaluating protein quality of fish feeds for Atlantic salmon, Salmo salar (L.). In experiment 1, the importance of meal size and inter‐fish variation was assessed by using dorsal aorta cannulated fish and diets that contained different sources of fishmeal (menhaden versus herring) which in the case of herring, had been dried at either 70 or 100 °C. In experiment 2, an attempt was made to mimic a production situation by comparing the FAA profiles in salmon fed diets containing two commercially available fishmeals that had been produced in accordance with industrial standards (Norse‐LT94® and NorSeaMink®; Norsildmel AL, Fyllingsdalen, Norway). FAA profiles in plasma 6 h after feeding were compared with feed true protein digestibility as determined in mink. Cannulated fish, held in individual tanks, were hand‐fed twice daily to pellet rejection (satiety) and daily records of the actual rations consumed were maintained. A total of 24 different amino acids and other amino‐containing compounds were detected using high‐pressure liquid chromatography. Morning and evening meal size showed significant correlations. Meal size had a significant effect on blood levels of the majority of essential free amino acids (EAA) as well as the total sum of FAA (TFAA). In experiment 1, a marked inter‐individual effect was found, possibly because of incipient sexual maturation. FAA profiles were therefore corrected for meal size by linear regression while repeated sampling via the permanently implanted cannula allowed paired comparisons of the different test diets, minimizing inter‐individual variation. Significant differences in plasma FAA profile, EAA and TFAA were detected between fish fed all diets in both experiments. The preceding parameters for fish ingesting each feed were directly related to their respective mink protein digestibility in experiment 1, but not in experiment 2. Our results show that dietary protein quality can be differentiated by the aforementioned protocol, and by using fish with a low metabolic rate feed qualities could be ranked correctly as in experiment 1.     

Evaluation of dietary natural mineral materials as an antibiotic replacer on growth performance,non‐specific immune responses and disease resistance in rainbow trout,Oncorhynchus mykiss

We evaluated the effects of some dietary natural mineral materials as an antibiotic replacer based on growth performance, non‐specific immune responses and disease resistance in juvenile and subadult rainbow trout, Oncorhynchus mykiss. First experiment, juvenile rainbow trout averaging 2.7 ± 0.02 g (mean ± SD) were fed one of the six experimental diets; a basal commercial diet as a control (CON), CON with oxytetracycline (OTC), with yellow loess (YL), with Macsumsuk^® (MS), with Song‐Gang^® stone (SG) and with barley stone (BS) at 0.4% of each diet. At the end of 8‐week feeding trial, weight gain (WG), specific growth rate (SGR), feed efficiency (FE) and protein efficiency ratio (PER) of fish fed YL diet were significantly higher than those of fish fed CON diet. Non‐specific immune responses such as superoxide dismutase (SOD), myeloperoxidase (MPO), lysozyme (LYS) activity and oxidative radical production of fish fed YL diet were higher than those of fish fed CON diet. At the end of 15 days of challenge test with Aeromonas salmonicida, average cumulative survival rate of fish fed YL diet was significantly higher than that of fish fed BS and CON diets. However, there were no significant differences among fish fed YL, SG and OTC diets. Second experiment, subadult rainbow trout averaging 261.5 ± 3.5 g (mean ± SD) were fed one of the four experimental diets for 22 weeks: CON, and CON with OTC, YL or SG at 0.4% of each diet. At the end of feeding, growth performance of fish fed SG and YL diets was significantly higher than that of fish fed CON diet. Non‐specific immune responses in terms of SOD, MPO, LYS and NBT of fish fed SG and YL diets were significantly higher than those of fish fed CON diet. However, there were no significant differences among the fish fed YL, SG and OTC diets. The results indicate that dietary yellow loess or Song‐gang^® stone at 0.4% of diet could replace oxytetracycline in juvenile and subadult rainbow trout.     

Effects of Dietary Protein Source and Feeding Regime on Growth Performance,Nutrient Digestibility,Fatty Acids,and Quality Characteristics of Rainbow Trout,Oncorhynchus mykiss,Fillets

This study investigated the effects of dietary protein source and feeding regime (apparent satiation and rationed) on growth performance, nutrient digestibility, fatty acid (FA) profile, and fillet quality traits in rainbow trout. A stock of 1200 juvenile trout (mean weight 114.6 ± 0.2 g) were randomly distributed into 24 fiberglass tanks (four diets × three replications × two feeding regimes). The experimental diets were formulated to be isoproteic and isoenergetic based on bacterial protein meal (BPM), pea protein concentrate (PPC), mixture thereof (MIX), and fish meal (FM), respectively. The feeding trial lasted 77 d with water temperature of 13 C. Statistical differences appeared among the diets only in terms of crude protein digestibility. Growth performance and somatic indexes were significantly affected by the diet, while only the condition factor was influenced by the feeding regime. None of the parameters appeared to be affected by the interaction effects. Differences appeared between the FA profiles of the dorsal muscle. Oleic, linoleic, α‐linolenic, and docosahexaenoic acid contents were influenced by diet, while only minor FAs were influenced by feeding regime. In conclusion, growth performance and nutrient digestibility resulted lower in fish fed BPM diet than other groups, while PPC group was similar to FM group.     

Partial replacement of fish oil by soybean oil on lipid distribution and liver histology in European sea bass (Dicentrarchus labrax) and rainbow trout (Oncorhynchus mykiss) juveniles

A 12‐week feeding trial was conducted to evaluate the effects of fish oil replacement by soybean oil, on lipid distribution and liver histology of two commercially important finfish species: rainbow trout (Oncorhynchus mykiss) and European sea bass (Dicentrarchus labrax). Sea bass (16.2 ± 0.5 g; mean ± SD) and rainbow trout (52.1 ± 0.5 g) juveniles were fed one of three isonitrogenous (500 g kg^?1 CP) and isoenergetic (19 kJ g^?1) diets, containing 0% (control, diet A), 25% (diet B) and 50% (diet C) soybean oil. At the end of the experiment, lipid deposition was evaluated in muscle, liver and viscera. Cholesterol and triglycerides levels were also determined in plasma. Tissue total, neutral and polar lipid composition (g kg^?1 total lipids) showed no significant differences within species, regardless the dietary treatment. The same trend was observed for plasma parameters (P > 0.05). Viscera were the preferential tissue of lipid deposition, with 252–276 and 469–513 g kg^?1 total lipid content in trout and sea bass, respectively. Dietary fish oil replacement had no effect on either hepatic lipid droplets accumulation or degree and pattern of vacuolization in the observed liver sections. These data suggest that both sea bass and trout can be fed diets containing up to 50% soybean oil without adverse effects on tissue lipid composition or liver histology.     

Changes of Non-Specific Immune Parameters in Rainbow Trout,Oncorhynchus mykiss,After Exposure to Antimicrobial Agents Used in Aquaculture

Changes of plasma lysozyme and serum bactericidal activity, ceruloplasmin, total plasma protein, serum glucose, hematocrit and leucocrit levels in rainbow trout (Oncorhynchus mykiss) were studied after exposure to formalin and chloramine-T, which are used in aquaculture as anti-ectoparasitic and antibacterial agents. Fish were exposed to formalin at a concentration of 250 ppm for 60 min and exposed to chloramine-T for 5 ppm for 3 h. The parameters were analyzed for rainbow trout exposed to formalin and chloramine-T soon after treatments and rainbow trout exposed to freshwater after 24 h (return) following the treatments. Plasma lysozyme activity decreased after exposure to formalin; however, it was not affected by chloramine-T exposure. Serum bactericidal activity reduced after both formalin and chloramine-T exposure and did not change after returning to freshwater. Ceruloplasmin and total plasma protein were not affected by both treatments. Serum glucose levels of exposed fish elevated and remained at higher levels during recovery period when compared to control values. Hematocrit values of fish exposed to formalin increased; however, no significant change was observed in fish exposed to chloramine-T. Leucocrit levels of fish treated with both formalin and chloramine-T were higher than that of controls.     

Supplementation of maca (Lepidium meyenii) tuber meal in diets improves growth rate and survival of rainbow trout Oncorhynchus mykiss (Walbaum) alevins and juveniles

Maca tuber meal is used in fish diet formulations in Andean trout culture and knowledge of its effects on fish growth is paramount to healthy human food production. In the first experiment with rainbow trout alevins (0.096±0.002 g), starter diets were offered from first feeding until 15 weeks. We formulated high protein content (～60%) semi‐purified starter diets supplemented with 0%, 5%, 10%, or 15% maca tuber meal (control, M‐5, M‐10, and M‐15 respectively). The second feeding trial was conducted with juveniles (1.56±0.02 g) fed one of three diets (control, M‐15, and commercial) for 8 weeks. In the first experiment, fish fed M‐10 and M‐15 diets exhibited significantly higher growth rates than the other dietary groups. Survival was significantly improved in the groups fed diets supplemented with maca tuber meal (60.0–69.2%) in comparison with the group fed a control diet (21.7%). The second experiment showed a higher growth rate in the M‐15 group compared with the control and a commercial diet fed group. Leucocyte numbers were increased by dietary supplementation of maca tuber meal. The findings of the present study suggest that a maca tuber meal inclusion at least 5% improves growth rate, feed utilization, immunity by increased leucocyte number, and survival of rainbow trout alevins and juveniles.     

Modulation of the response of rainbow trout (Oncorhynchus mykiss Walbaum) to confinement, by an ectoparasitic (Argulus foliaceus L.) infestation and cortisol feeding

The effects of a 21 day infestation with the ectoparasite Argulus foliaceus on the response of rainbow trout Oncorhynchus mykiss to an acute stressor (confinement) were investigated. Fish were divided into a control and two parasitized groups, with one parasitized group having received cortisol feeding 72 and 24 h before infection. Prior to confinement only control fish showed a significant increase in body mass and condition factor. Following 4 h confinement, plasma glucose levels in the normal- fed/infested group were significantly higher than in the controls, while the cortisol-fed/infested group had significantly lower plasma levels of adrenocorticotropic hormone (ACTH) and glucose. Both infested groups, but not controls, had higher plasma ACTH and cortisol levels 48 h after confinement when compared with their pre-confinement levels. Stressor-induced immunomodulation was more evident in the parasitized fish after confinement, as was concluded from decreased lysozyme activity, alternative complement activity and oxygen radical production either immediately after, or 48 h post 4 h confinement. Circulating lymphocyte numbers were significantly reduced in the control and normal-fed/infested fish. Results indicate that low ectoparasitic infestations can, for a prolonged period, lead to a stress experience with effects that impair the response to subsequent stressors. Such effects may be further confounded by a previous elevation of plasma cortisol levels.     

Poor utilization of dietary free amino acids by white sturgeon

Three separate experiments were conducted to investigate the utilization of dietary free amino acids by white sturgeon,Acipenser transmontanus. In the first experiment, sturgeon were fed for 8 weeks with isoenergetic and isonitrogenous diets containing either intact protein, an amino acid mixture, or a neutralized amino acid mixture of similar composition. Sturgeon fed the two amino acid diets grew significantly (p<0.05) less, mainly because of reduced feed intake. In the second experiment, postprandial changes in plasma free amino acids of sturgeon force-fed the above diets were monitored at 2, 4, 6, 8, 12, and 24 h Total plasma amino acids in sturgeon force-fed the intact protein diet showed a gradual increase and reached peak levels at 8 h. In contrast, plasma amino acids of fish force-fed the amino acid diets showed a rapid increase and reached maximal levels at 2 h. In the third experiment, free amino acid diets showed a urine of sturgeon force-fed the three diets was monitored at 4, 8, 12, 16, 20, and 24 h. In all dietary groups, peak excretion of indispensable amino acids was detected at 8 h after feeding but with significantly higher levels from fish force-fed the amino acid diets. Sturgeon force-fed the neutralized amino acid diet excreted 35% less amino acids compared to fish force-fed the non-neutralized diet. Sturgeon force-fed the intact protein diet excreted significantly higher amounts of ammonia. Under amino acid feeding conditions, sturgeon excreted more amino acids in its urine, but the urinary excretion of amino acids was not a major route in dealing with the rapid absorption of dietary free amino acids.     

Circadian rhythms and individual variability of self-feeding activity in groups of rainbow trout Oncorhynchus mykiss (Walbaum)

An experiment was conducted to examine whether circadian self‐feeding rhythms exist in groups of rainbow trout Oncorhynchus mykiss (Walbaum) and to investigate individual variability of self‐ feeding and its influence on growth. Ten trout of 144.9 ± 12.0 g (mean ± SD) were transferred to each of eight 290‐L tanks and given free access to feed via a self‐feeder. Fish in four of the eight groups were individually tagged, and the individual activating the trigger was identified using video observations. The self‐feeding activity of each group was recorded under a light:dark cycle of 16.5:7.5 h and under continuous light (LL) conditions. Although diel self‐feeding rhythms in the groups of trout were, in general, diurnal, night‐feeding activity was observed in the initial LD phase, but there was a progressive reduction and eventual cessation in most groups. Endogenous circadian self‐feeding rhythms were detected in the groups of trout under LL conditions. The trout formed dominance hierarchies of self‐feeding activity, with one or two individuals accounting for the majority of trigger actuations. However, the dominants did not occupy the area surrounding the trigger. Growth rates did not differ significantly between the dominants and the subordinates within the groups.