Feed preferences in juvenile cod estimated by inert lanthanid markers – effects of moisture content in the feed

There are indications that low ingestion ratesof formulated feed may limit the growth ofearly juvenile fish, including cod. This hasparticularly been observed during the weaningperiod, when the fish must change from apresumably palatable live prey to usually a drydiet. The study of the physical properties ofthe diet seems to be an underestimated area ofresearch, compared to studies on thenutritional properties of the diet. In thisstudy we used lanthanide markers as a tool inestimating diet preferences for examining theeffect of dietary moisture content on ingestionrates in juvenile cod (0.8 g). Fishmeal-baseddiets were produced with five levels of water,giving diets with 35, 42, 54, 65 or 95% dryweight. Each diet was marked with a smallamount of a lanthanide or yttrium oxide. Thefish were hand fed on a mixture of these fivediets. Ingestion rates where then estimated bykilling 1/2 of the fish after one day and therest after four days of feeding. The whole fishwith gut content was homogenized and markercontent was measured by ICP-MS. The amountingested of each of the five diets could thenbe calculated from these figures. Despite thehigh variation in diet selection betweenindividuals the results clearly indicate ahigher intake of moist feed. This could becaused by a better palatability or to a higheravailability caused by a lower sinking rate ofthe moist feed.     

Carotenoids in diets for salmonids: I. Pigmentation of rainbow trout with the individual optical isomers of astaxanthin in comparison with canthaxanthin

Visual and chemical determinations revealed that the individual optical isomers of astaxanthin were more efficacious than canthaxanthin in pigmenting the flesh of rainbow trout (Salmo gairdneri, Richardson). The visual assessment of the astaxanthin pigmentation by means of a colour scale correlated well with the chemical determination. The same utilization was found for all three astaxanthin isomers, (3S, 3′S)-, (3R, 3′S)-, (3R, 3′R)-astaxanthin and a 1 : 2 : 1 mixture of the three isomers.No epimerization took place at the chiral centres of C-3 and C-3′ in astaxanthin. Astaxanthin and canthaxanthin were not interconverted. No metabolites were detected in the flesh.     

Temporal changes in texture, gaping, composition and copper status of Atlantic salmon (Salmo salar, L) fed moist feed or extruded dry feed

Temporal development in product quality and copper status was investigated in Atlantic salmon (initial weight 2.3 kg) fed alginate-based moist feed (MF) or commercial extruded dry feed (DF). The fieldwork was conducted from January to May in sea-cages (800 m³) off the Norwegian west coast. The dry matter content (DM) in the muscle of the MF group was stable at about 32% throughout the experiment, while the DM of DF group increased linearly from 32% to 35%. Salmon were sampled for analyses of Cu status, fillet gaping, texture and pH in March and April. The serum Cu content of the MF group was highest in March, but no significant effect of dietary treatment was observed on Cu status of muscle or liver. The Cu level in serum and muscle decreased from March to April in both fish groups, but the hepatic Cu level showed no temporal changes. Gaping incidence and pH were higher in the MF group in April compared with the DF group and, for both fish groups, fillet gaping was higher and hardness was lower in April than in March. Variation in serum Cu level explained 32% of the variation in fillet hardness, and serum and muscle Cu level related negatively with gaping. At the end of the experiment the condition factor, and fillet and visceral fat contents were higher in the DF group, while sensory evaluated hardness and tastiness were similar for both dietary treatments. In conclusion, sampling time had a greater impact on Cu status, texture and gaping than dietary treatment during springtime in adult Atlantic salmon. Dietary treatment significantly affected body shape, and fat and dry matter contents.     

Ration level for salmonids: I. Growth, survival, body composition, and feed conversion in Atlantic salmon fry and fingerlings

Twelve groups of Atlantic salmon (Salmo salar L.), raised in fibre-glass tanks, were fed at 12 ration levels for 7 periods, each period lasting 28 days. The basic ration level was calculated from an expected maximum growth rate, depending on fish size and water temperature. This level was defined as “level 1.00”. The experimental ration levels varied from 0.50 to 3.25 in intervals of 0.25.

During the start-feeding period there was an average survival rate of 95%. After the remaining 6 periods the group which was fed least had a survival rate of 48%. The other groups had an average survival rate of 81%. In all 7 periods, increasing the ration levels from 0.50 to 1.00 resulted in increased growth. A further increase in ration level did not result in extra growth. The weight distribution of the fingerlings was bimodal, with the majority of fish in the scantiest fed groups being in the lower mode. The majority of the fish in the adequately fed groups were distributed in the upper mode.

The water temperature varied from 10 to 17°C, and the thermal sum was 2430 day-degrees. The average final weight of the fish which received ration levels from 1.00 to 3.25 was 28.3 g, while the fish on the scantiest ration weighed only 9.6 g at the end of the study. The feed conversion rate, which was 0.9–1.0 kg dry feed/kg growth for the two underfed groups, ranged from 1.0 to 3.0 in the other groups. The chemical composition of the fish varied according to fish size and ration level.     

Ration level for salmonids II. Growth, feed intake, protein digestibility, body composition, and feed conversion in rainbow trout weighing 0.5–1.0 kg

Six groups of rainbow trout (Salmo gairdneri Richardson), raised in fibre-glass tanks, were fed at six different ration levels for two periods of 21 days each. Each tank contained 20 individually tagged fish with a mean initial weight of 415 g. The water temperature averaged 8.4°C and 10.2°C in the two periods. The basic ration level was calculated from an expected maximum growth rate, depending on fish size and water temperature. This level was defined as “level 1.000”. The other experimental levels were 0.125, 0.250, 0.500, 2.000, and 4.000 relative to this, respectively.

No fish died during the experiment. The growth of the fish increased significantly with increasing rations up to the level 2.000. The fish lost weight at the ration level 0.125. The growth rate was near zero at the ration level 0.250. The growth of the fish receiving the highest ration level was 1.72% of their weight per day. The variation in growth of the fish within various weight classes was influenced by the ration level so that the growth distribution of the scantiest fed groups was skewed to the left. There was no skewness in the distribution of the most plentifully fed groups. The ration level had no significant effect on the apparent protein digestibility. Measurements of the feed intake using a radioactive isotope in a 6-h meal showed that the fish ingested all the feed at the ration levels from 0.125 to 1.000. The feed intake recorded was equivalent to the requirements for fish growth at the ration level 2.000, while it was lower at the ration level 4.000. The liver weight percentage increased with increasing ration while the dressed-out carcass percentage decreased. The chemical compostion of the carcass was influenced to a greater extent by the feeding than that of the intestines. The feed conversion efficiency (FCE) and the productive protein value (PPV) of the feed were highest at the ration level 1.000. The values obtained were an FCE of 1.0 kg growth per kg dry feed ingredients and a PPV of 41%.     

Effect of varying dietary protein level in different families of rainbow trout

Groups of rainbow trout (Salmo gairdneri) fingerlings from five different families and five inbred groups were fed for 153 days on each of four diets similar in energy content, but differing in their percentage of protein. The protein contents were 24, 33, 42 and 51%, respectively. Significant differences between fish families were found for growth, condition factor, chemical composition of the carcass, and digestibility of protein. At the end of the experiment, interaction between diet fed and family was found close to significance for weight and relative growth rate, and significant for length and condition factor. This interaction indicates that prospects for selectively breeding strains of rainbow trout specifically better able to utilize protein are promising.Fish growth rate increased with increasing dietary protein content. The corresponding condition factor, content of dry matter and energy in the carcass decreased. The high protein diets gave more protein in the carcass and a more discoloured liver than the low protein diets. Apparent protein digestibility increased with increasing protein level.     

Carbohydrate in rainbow trout diets. II. Influence of carbohydrate levels on chemical composition and feed utilization of fish from different families

Groups of rainbow trout fingerlings from ten different families were sampled after 12 and 24 weeks feeding on each of three diets which were similar in protein and energy content but had different percentages of metabolizable energy present as carbohydrate: 17, 25 and 38%. Fish fed different diets had the same dressed carcass weights, expressed as percentage of body weight, but fish fed higher carbohydrate diets had heavier livers and a higher percentage of discoloured livers. Chemical analyses of fish bodies showed lower fat and energy but higher protein and ash content for those fed higher carbohydrate diets. Livers contained more fat and carbohydrate but the same dry matter, and faeces showed less dry matter, more protein and the same energy content in fish with higher dietary carbohydrate. Utilization of dietary energy and protein was poorer in fish fed high levels of carbohydrate, but health of all fish appeared good. Little variation between different fish families, and no interaction between fish family and diet was found for any of the above parameters, indicating that prospects for change through selective breeding are poor.     

Long-term effects of dietary fat level and feeding rate on growth, feed utilization and carcass quality of Atlantic salmon

Two extruded diets designed to have fat levels of 220 g kg⁻¹ (F22) and 300 g kg⁻¹ (F30) were fed to Atlantic salmon Salmo salar L. in two different experiments during the sea rearing period (from 0.2–0.3 kg to 3–4 kg). Each diet was fed restricted and isoenergetically at two feeding rates to fish in triplicate groups. In one of the experiments, a supplementary group of fish was fed to satiation with the F30 diet. All fish were slaughtered and evaluated for quality according to a commercial standard.
No difference in growth was observed between fish fed the two diets at similar feeding rates and the growth was proportional to the amount of dietary energy offered. Feed conversion ratios decreased according to higher energy content in the F30 diet, and the nitrogen and phosphorus retention increased significantly. Fish fed the F30 diet revealed a higher incidence of sexual maturity. Fat content in cutlets and dressed carcasses were significantly affected by feeding rate but not by dietary fat level. Fish fed the F30 diet had more visceral fat and, consequently, lower dress-out percentage. Mortality, liver size and liver colour were not significantly affected by dietary fat level.
These experiments showed that even large differences in dietary fat level employed for the entire sea rearing period of Atlantic salmon, did not, or only marginally affected the cutlet and dressed carcass fat content. The high fat diet improved the feed utilization, thus decreasing the discharge to the environments. Furthermore, it resulted in greater growth at ad libitum feeding.     

Digestibility of fat and fatty acids in rainbow trout and mink

The digestibility of different fats in rainbow trout and mink showed somewhat higher values in mink than in fish. Digestibility coefficients in fish did not exhibit distinct differences between two water temperatures (3 and 11°C).Rainbow trout and mink responded very similarly to digestibility differences between dietary fats and fatty acids. Both species revealed decreasing digestibility of total lipids and fatty acids with increasing melting point. Thus soybean oil, cod liver oil and capelin oil were efficiently digested, while hydrogenation of capelin oil resulted in decreased digestibility. Mild hydrogenation to 21°C melting point had a slight effect. The digestibility of saturated and unsaturated fatty acids were similarly influenced by hydrogenation. The digestibility of individual fatty acids decreased with increasing chain length up to C₁₈. A further increase in chain length up to C₂₂ caused increased digestibility. Unsaturated acids showed higher digestibility than their saturated counterparts.     

Fat-containing bleaching earth as a feed constituent for rainbow trout

Fat-containing bleaching earth (FBE) is a waste product from the bleaching of edible oils in the fat industry. The experiments described were carried out to investigate if FBE may be utilized as an ingredient in feed for rainbow trout (Salmo gairdneri, Richardson). Using bomb calorimetry, a sample of FBE from the bleaching of marine oils was found to contain 3400 kcal/kg. This indicates a fat content of about 36% even though analysis showed an extractable crude fat content of only 23%. The digestibility of the crude fat was estimated at about 80%.Fish growth was very good on all four experimental diets, which contained 0, 10, 20 and 30% FBE. Level of dietary FBE had no measureable effect on fish mortality, condition factor, dressed-out weight, fat content, or fatty acid composition. Fish fed diets containing FBE showed a significantly higher frequency of discoloured livers than fish fed control diets, indicating that the quality of the fat in FBE might be reduced. An organoleptic evaluation conducted on the fish showed differences between fish fed different diets, but no negative effects of FBE.