Relationships between growth and disease resistance in rainbow trout,Oncorhynchus mykiss (Walbaum)

Rainbow trout from 23 families were evaluated for growth and resistance to the bacterial coldwater disease (BCWD) caused by Flavobacterium psychrophilum and infectious haematopoietic necrosis (IHN) caused by IHN virus. Average family weights were between 161 and 263 g with an average of 225 g at 213 days post‐fertilization with specific growth rates ranging from 2.37 to 2.88. Per cent survival of fish challenged with F. psychrophilum was between 18% and 100%, while for those challenged with IHNV, the range was between 12% and 93%. Significant positive correlations were found for end body weight and resistance to IHN (P < 0.05) and for early body weight and resistance to BCWD (P < 0.1). However, no significant correlations were detected between resistance to both pathogens or disease resistance and overall genetic diversity or diversity within the major histocompatibility locus.     

A test of a feed budget model for rainbow trout, Oncorhynchus mykiss (Walbaum)

A feed budget model, which enables farmers to adapt a general model to local conditions, was tested on rainbow trout, Oncorhynchus mykiss (Walbaum). The model has two major components; component 1 is used to calculate the energy required for one unit of biomass gain (g or kg) and component 2 estimates the daily growth increment (g or kg). Both components can be adjusted in relation to prevailing environmental conditions and, together, they express the daily energetic requirement of the specific species or strain. Fish fed using the model as a guide had a high rate of growth and good feed conversion. The model accurately predicted growth over the experiment, and is presented as a means of calculating the daily feed budget in fish farming. Growth of fish fed using self-feeders is presented for comparison with the results obtained using the model.     

Effect of enzyme supplementation to dehulled lupin-based diets on growth, feed efficiency, nutrient digestibility and carcass composition of rainbow trout, Oncorhynchus mykiss (Walbaum)

High inclusion levels of dehulled lupin (Lupinus angustifolius) in salmonid diets significantly decrease growth rates. This may be caused by the high concentrations of non‐starch polysaccharides including oligosaccharide (OS) in lupin. The antinutritive effects of OS have not yet been fully investigated in fish. The objective of this study was to determine the effect of enzyme supplementation of dehulled lupin‐based diets on the fish performance. There were two control diets: a fish meal‐based diet with no plant protein (FM) and a diet that contained 50% dehulled lupin (L). Four experimental diets based on diet L and containing four different exogenous enzyme supplements were used: diet L(E) (Energex^™); diet L(B) (Bio‐Feed^™ Pro); diet L(α) (Alpha galactosidase^™); and diet L(Mix), which contained all the enzymes. Fish were randomly stocked into tanks in duplicate groups of 38 fish, 16.58±0.169 (SE) g, and were fed twice a day for 6 weeks. The supplemented enzymes did not improve weight gain in fish fed lupin‐based diets. However, mixed enzyme significantly improved Protein Efficiency Ratio (PER). Apparent digestibility of DM, CP and GE significantly improved in fish‐fed L(E) diet. None of the supplemented enzymes affected digestive tract indices or carcass composition. Surprisingly, weight gain was significantly higher in fish‐fed L(α), L(E) and L(Mix) diets as compared with FM diet. Feed intake was significantly higher in fish‐fed L, L(α) and L(E) diets compared with the FM diet. It is concluded that storing of lupin kernel under a suitable condition may have partially hidden the positive effects of exogenous enzymes through activating the endogenous enzymes.     

Effect of protein source and nutrient density on growth efficiency, histology and plasma amino acid concentration of rainbow trout (Oncorhynchus mykiss Walbaum)

The effects of protein source and nutrient density on growth efficiency, nutrient digestibility and plasma amino acid concentrations of rainbow trout were evaluated. A 3 by 2 factorial treatment design with three protein sources, fish meal–barley (F–B), plant concentrates (PC) and plant meals (PM), and two nutrient densities were used. A commercial reference diet was also fed. Triplicate tanks of 30 fish (initial wt. 28 g) were fed each diet, and the final weight averaged 240 g fish⁻¹. Protein source and nutrient density affected feed intake, weight gain and feed conversion ratio. Weight gain of trout fed the PC and PM diets was approximately 10% less than fish fed the F–B diets. Protein retention was affected by protein source, but not nutrient density, and was the highest for the fish fed diets containing fish meal and the lowest for the fish fed PM diets. Apparent digestibility coefficients and apparent amino acid availabilities of the diets corresponded with differences in weight gain. This study provides further evidence that growth rates of trout fed fish meal‐free diets, using conventional and concentrated plant protein ingredients, are good but some limitation to growth exists in the fish meal‐free diets.     

Digestibility, growth and excretion response of rainbow trout (Oncorhynchus mykiss Walbaum) to feeds of different ingredient particle sizes

The effects of feed ingredient particle size on apparent digestibility, growth and excretion responses of rainbow trout were investigated in pilot production systems. Particle size of the feed ingredients (ground using a hammer mill with 0.6 and 3.0 mm screen sizes) had no significant effect (P > 0.05, d.f. = 1/6) on the apparent digestibility of dry matter, phosphorus and crude protein in cold‐extruded diets. Feed ingredient particle size had an impact on the feed conversion ratio over an 85‐day feeding trial when key ingredients (78.65% of diet) were ground using a pulverizer and a hammer mill with 3.0 mm screen size and heat extruded. No significant differences (P > 0.05, d.f. = 1/12) were observed in the excretion ratios of TAN (total ammonia nitrogen), TKN (total Kjeldahl nitrogen), TP (total phosphorus), BOD₅ (5‐day biochemical oxygen demand) and TSS (total suspended solids) among three feed types, which included a wide ingredient particle size distribution.     

Effect and interaction of rainbow trout strain (Oncorhynchus mykiss) and diet type on growth and nutrient retention

Eight strains of rainbow trout were introgressed to develop a single strain (H‐ARS) that was selected for faster growth when fed a fishmeal‐free, plant‐based diet (Selection Diet). For four generations, families from these crosses were fed the Selection Diet and selected for increased weight gain. Growth and nutrient retention were compared among H‐ARS and two parental strains, the House Creek (HSC) and Fish Lake (FL) fed either a fish meal or Selection diet for 12 weeks. There was a significant effect of strain (P < 0.01), but not diet on weight gain, and a significant interaction of strain by diet (P < 0.05). The H‐ARS trout gained more weight averaged across diet (991% of initial wt.) than the HC (924%) or FL trout (483%). The FL trout fed the fish meal diet gained more weight than FL trout fed the selection diet (510% vs 456%). Conversely, H‐ARS trout fed the plant‐based diet gained more weight than those fed the fish meal diet (1009% vs 974%). HSC trout had similar weight gain fed either diet (922% vs 926%). A significant effect of strain on protein retention (P < 0.01) was observed, along with a significant strain by diet interaction (P < 0.02). The results demonstrate that rainbow trout can be selectively improved to grow on a plant‐based diet.     

Use of probiotics to control furunculosis in rainbow trout, Oncorhynchus mykiss (Walbaum)

Aerobic heterotrophic bacteria were isolated from the intestinal contents of Atlantic salmon, Salmo salar , rainbow trout, Oncorhynchus mykiss , and turbot, Scophthalmus maximus , on tryptone soya agar and De Man Rogosa and Sharpe agar, of which 11 of 177 (6% of the total) of the isolates were antagonistic to Aeromonas salmonicida . Four of these cultures, which were identified tentatively as A. hydrophila , Vibrio fluvialis , Carnobacterium sp. and an unidentified Gram-positive coccus, were beneficial to fish when fed singly or as an equi-mixture. Feed supplemented with the putative probiotics indicated survival of the organisms in the gastrointestinal tract for 7 days. Feeding with the probiotics for 7 and 14 days led to better survival following challenge with A. salmonicida . There was no indication of serum or mucus antibodies to A. salmonicida , but there was an increased number of erythrocytes, macrophages, lymphocytes and leucocytes, and enhanced lysozyme activity in the fish.     

Digestibility, growth and nutrient retention in rainbow trout (Oncorhynchus mykiss) fed diets extruded at two different temperatures

Digestibility and utilization of a fishmeal‐based diet extruded at 103 or 137 °C were examined. Each of the diets was fed to 0.5‐kg rainbow trout in nine tanks supplied with freshwater. Specific growth rate was significantly (P < 0.05) elevated for trout fed the feed extruded at 137 °C. The apparent digestibilities of protein and energy, feed conversion ratio, retentions of nitrogen and energy were not significantly affected by extrusion temperature. Digestibility of ash was significantly reduced in the diets produced at 137 °C.     

The effect of different numbers of feeding days on feed consumption and growth of rainbow trout [Oncorhynchus mykiss (Walbaum)]

A trial was conducted to evaluate the effects of the number of feeding days on feed consumption and growth of rainbow trout, Oncorhynchus mykiss (Walbaum) (initial mean weight 53.6 ± 11.3 g). Fish were held under natural winter–spring (40°57′N) photoperiod (L:8 D:16 – L:14 D: 10) and ambient water temperatures (4–12 °C) for 128 days, and fed on day 7 (F₇), day 5 (F_5.1: no feeding on weekends and F_5.2: no feeding on Sundays and Wednesdays) or every other day (F_EOD). Feeding treatments were replicated with 50 fish held in 200‐L tanks. Number of feeding days affected final live weight, with fish in group F₇ achieving a mean weight of 285.6 g, followed by F_5.1 (256.6 g), F_5.2 (247.6 g) and F_EOD (223.2 g). Final condition factors (CF) differed between the F_EOD group and the others. Feed conversion ratios (FCR) exhibited significant differences among some of the groups. The results suggest that depriving rainbow trout of feed for more than 1 day a week has a negative effect on growth, but little influence on feed conversion.     

Feeding behaviour of fast- and slow-growing strains of rainbow trout, Oncorhynchus mykiss (Walbaum), during first feeding

The development of feeding behaviour in two rainbow trout strains and their hybrid was compared at the transition from endogenous nutrition to exogenous feeding. After 60 min of acclimatization, the swimming pattern, spatial distribution, agonistic behaviour and snapping of fish were quantified by making 5-min observations on groups of five fish. Behavioural patterns were recorded in four replicate groups per treatment, before and after the introduction of dry trout feed or Artemia nauplii (actometric tests) or extracts of these (olfactometric tests). Fish of the slow growing strain were less active than those of the other two strains. Greater swimming activity observed in the fish of the fast growing strain was associated with more rapid feeding and a higher number of snapping responses. Such behavioural precocity is of interest because it could provide the basis for the growth differences between the fast and slow growing strains.     

Effect of feeding on the tissue free amino acid concentrations in rainbow trout (Oncorhynchus mykiss Walbaum)

This study investigates whether tissue free amino acid (FAA) pools in rainbow trout,Oncorhynchus mykiss (Walbaum), are altered following feeding and the relationships between the amount of food consumed and the FAA pool size. Trout were starved for 7 days to provide baseline data and then refed on day 8. Individual food intake was measured by radiography and the consumption of amino acids (AA) calculated from dietary protein consumption. Total FAA concentrations in the stomach, liver and white muscle were little changed at various times after the meal and this pattern was repeated for the majority of individual FAA. Overall, the most notable change was a reduction in essential FAA concentrations (principally in valine, leucine and isoleucine) in the white muscle following feeding. However, in the caeca total FAA, total essential FAA and a number of individual FAA were significantly elevated at 4, 9 and 15h following feeding. There were few significant correlations between dietary amino acid consumption and total tissue FAA and essential FAA concentration in the stomach, caecum and white muscle; correlations were stronger in the liver. In order to explain the relative constancy of total FAA concentrations in the tissues following food intake (which represents over 100% of the total FAA pool) a model is presented that quantifies the AA flux through the free pools and considers the role of protein turnover in regulating FAA pool size.     

Coastal movement and growth of domesticated rainbow trout (Oncorhynchus mykiss (Walbaum)) in Norway

Between 1979 and 1991, hatchery-reared 1+ and 2+ rainbow trout were released in May and June in the Boknafjord at the mouth of the River Imsa, and from April to June in the Oslofjord, southern Norway. The fish were recaptured in the fjords of release, in rivers emptying there and from rivers and sea further along the Norwegian coast. Of the total recaptures, 35.6% ascended rivers in the year of release. Some fish released at the River Imsa returned to the area of release after spending 1 to 3 years at sea. Among those that were recaptured in rivers, 11.9% were recaptured in other rivers than the Imsa. The growth rate of the fish varied inversely with size at release. The growth rate was higher in the Oslofjord than in the Boknafjord, and in both areas the growth rate decreased with time after release. Rainbow trout that had escaped from local fish farms entered the River Imsa between March and December. Adults ascending in spring remained a shorter time in the river (37 days) than those ascending in autumn (200 days). Fish entering in autumn decreased more in weight during their river stay than those ascending in spring, and females more than males. Spawning took place in April-May. The monthly mean river stay of juvenile entrants ranged from 2 weeks to 6 months. In total, 27.8% of the rainbow trout escaped from fish farms and captured in the Imsa strayed to other rivers for repeat spawning. There is no evidence for continuing survival of escaped Norwegian rainbow trout in the wild.     

An in vitro method to determine phosphorus digestibility of rainbow trout Oncorhynchus mykiss (Walbaum) feed ingredients

An in vitro method was developed to assess the digestibility of phosphorus in 12 plant and animal feed ingredients for rainbow trout Oncorhynchus mykiss (Walbaum). The method simulates the gastrointestinal tract of the rainbow trout with regard to pH and gastrointestinal enzymes. Phosphorus solubility was measured after acid digestion (pH 3) with and without gastric enzymes, after alkaline digestion (pH 9) with and without intestinal enzymes, and after a two-step process involving acid and alkaline digestion. Commercially available digestive enzymes from mammals were compared with digestive enzymes from rainbow trout. Correlating in vitro digestibility with in vivo digestibility showed that acid digestion with both commercial enzymes ( r ²=0.98, P  < 0.05) and trout enzymes ( r ²=0.94, P  < 0.05) predicted the in vivo digestibility of animal feed ingredients. Alkaline digestion with both enzyme systems (commercial r ²=0.79; trout r ²=0.74, P  < 0.05) or without ( r ²=0.82, P  < 0.05) enzymes predicted the in vivo digestibility of ingredients from animal byproducts but not those from plant products. The in vitro digestibility with two enzyme steps (acid and alkaline) predicted in vivo digestibility of plant and animal ingredients ( r ²=0.79 for commercial enzymes and r ²=0.74 for trout enzymes) better than did one-step acid or alkaline digestion.     

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.     

Covariation between diploid and triploid progenies from common breeders in rainbow trout, Oncorhynchus mykiss (Walbaum)

The covariation between diploid and triploid progenies from common breeders was investigated in various progeny-testing experiments where either dams or sires were sampled from rainbow trout, Oncorhynchus mykiss (Walbaum), stocks. Triploidiza tion was found to frequently reduce the performance in the traits studied: body length and weight, growth, coefficient of condition and pyloric caeca number. Triploidization also generated significant interactions with the parental breeding value. These interactions were caused in part by the familial variance not being the same in triploids as in diploids, but also by actual ranking differences between diploid and triploid familial performances. However, the effect of these interactions was minor as compared with the amount of variation common to both ploidy levels (genetic correlations averaged 0.7–0.9). Therefore, selection of diploid breeders appeared efficient enough for improving triploid progeny, unless family selection methods including triploid progeny testing were preferred for other reasons. Lastly, it was observed that variances from maternal origin tended to be larger in triploids, whereas variances from paternal origin tended to be smaller, as compared with diploids. This point was discussed referring to the genetic make-up of triploids and in the scope of dams and sires for selective breeding.     

Effects of ration level in rainbow trout, Oncorhynchus mykiss (Walbaum), on sensory characteristics, lipid content and fatty acid composition

Effects of feed ration levels (RLs) and age on sensory characteristics, fillet lipid content (LC) and fatty acid composition (FA) of rainbow trout, Oncorhynchus mykiss (Walbaum), fillet were investigated in a longitudinal feeding experiment. Daily RLs were administered as a percentage of the ration size necessary for expected maximum growth (RL100). Fish on RL50, RL75, RL100 and RL200 were sampled at 1.4, 2.0 and 2.4 years of age. Analyses of LC and FA were carried out on a fillet cross-section. Sensory analyses were performed by an expert panel using ranking and conventional profiling. RL had no effect on the intensity of fresh smell or any sensory characteristic of taste or consistency of fish on RLs over 75%, irrespective of age; fish on RL50, however, scored significantly lower for fresh taste and firmness. Age had no effect on the intensity of total taste, whereas fresh taste increased and rancidity decreased with increasing age irrespective of RL. LC related strongly to RL and age, and increased up to 2.0 years of age. An increase in LC resulted in a higher level of monounsaturated FA and a lower level of polyunsaturated FA. A restriction to RL75 had no negative effects on sensory characteristics, LC or FA of the fillet.