Maintenance ration, protein synthesis capacity, plasma insulin and growth of Atlantic salmon (Salmo salar L.) with genetically different trypsin isozymes

Growth was found to be associated with the changes of trypsin activity in the pyloric caecal tissues and the level of plasma insulin in Atlantic salmon (Salmo salar L.). A decrease in trypsin activity accompanied by an increase in plasma insulin was detected one month before an enhanced growth was observed. There were significant relationships between weight specific consumption rate, plasma insulin levels and fish growth. The correlation of weight specific consumption rate was higher with growth rate (R²=0.7, p<0.0001) than with plasma insulin concentration (R²=0.4, p<0.0001).When the comparison was made between Atlantic salmon carrying and lacking the trypsin variant TRP-2*92, the fish with the variant had lower maintenance ration (p<0.05), higher capacity for protein synthesis in the white muscle (p<0.02), and a greater ability to utilize the feed at a restricted ration than the fish without the variant. In Atlantic salmon lacking the variant, both plasma insulin concentrations and growth rates were significantly lower (p<0.05) in the fish fed 0.5% bw day^–1 than those fed 1% bw day^–1. Whilst the growth rates of TRP-2*92 salmon fed the different rations became similar one month after similar levels of plasma insulin were observed between them. The TRP-2*92 salmon may be defined as a high protein growth efficiency fish with low protein turnover rate.Genetic variation in trypsin isozyme pattern affects feed utilization, plasma insulin levels and growth in Atlantic salmon.     

Effect of phytic acid and phytase on feed intake, growth, digestibility and trypsin activity in Atlantic salmon (Salmo salar, L.)

In the majority of experiments, the effects of phytic acid (with or without phytase) are not separated from the effects of adding plant meals containing phytic acid. A 12‐week experiment was conducted with Atlantic salmon (28.9 g) to determine the separate and combined effects of phytic acid and phytase on feed intake, trypsin activity, digestibility and growth. Diets were prepared without phytic acid and phytase; with 2000 U phytase kg^?1 diet; with 10 g sodium phytate kg^?1 diet; and with 10 g sodium phytate and 2000 U phytase kg^?1 diet. The basal diet contained sufficient phosphorus and other minerals to meet salmonid requirements. The addition of phytic acid had no significant effect on feed intake or weight gain, it significantly (P < 0.05) reduced protein digestibility although there was no reduction in trypsin activity. Phytase inclusion neutralized the effect of phytic acid on protein digestibility. Phytase had no effect on feed intake but significantly enhanced growth whether included with or without phytic acid. Feed efficiency ratio was significantly improved for fish fed the diet containing both phytase and phytic acid but not separately. The significance of this experiment was to separate the direct effects of phytase and the direct effects of phytic acid, added in a pure form, from effects due to other components in ingredients containing phytic acid.     

Protease activities, plasma free amino acids and insulin at different ages of Atlantic salmon (Salmo salar L.) with genetically different trypsin isozymes

We show significant relationships between digestion rate of dietary protein, absorption/transport rate of amino acids, and plasma insulin concentration. Digestion rate of dietary protein was indicated by protease activity ratio of trypsin to chymotrypsin in the pyloric caeca. Plasma total free amino acids and/or the ratio of essential to non-essential free amino acids indicated absorption/transport rate of amino acids. These relationships are associated with and likely to be primarily affected by genetic variation in the expression of different isozymes of trypsin, a key enzyme for feed utilization and growth, affecting digestion, absorption/transport rate of amino acids and plasma insulin levels in Atlantic salmon (Salmo salar L.).     

Protein-nitrogen flux and protein growth efficiency of individual Atlantic salmon (Salmo salar L.)

Protein-nitrogen flux (the proportions of consumed and absorbed protein-nitrogen partitioned into protein synthesis and growth) was examined in Atlantic salmon, Salmo salar L. Salmon were held in groups and fed high or low rations or starved. Individual food consumption rates were measured using radiography. Fish varied widely in protein growth efficiency (protein growth divided by protein consumption), but this did not correlate with consumption rate, digestive capacity (as measured by absorption efficiency, trypsin levels and pyloric caecal size) or feeding hierarchy rank. Protein synthesis rates, measured in whole-animals, were linearly correlated with protein consumption and assimilation. There was a significant correlation between protein growth efficiency and the efficiency of retention of synthesised proteins. The capacity for protein synthesis and RNA activity were positively correlated with rates of food consumption and growth but were not correlated with protein growth efficiency. It was concluded that individual differences in protein growth efficiency related to differences in synthesis retention efficiency, but not to differences in the capacity for protein synthesis, RNA activity, digestive capacity or feeding hierarchy rank.     

Effect of fish feed processing conditions on digestive protease activities, free amino acid pools, feed conversion efficiency and growth in Atlantic salmon (Salmo salar L.)

The responses of the digestive proteases trypsin and chymotrypsin and protein metabolism to differences in feed protein quality were investigated in Atlantic salmon (Salmo salar L.). Two sets of experimental feeds were produced. Each set of high and low quality feeds was provided to either 150 g or 2 kg salmon. Protein in the high quality feeds had significantly higher percentages of free (reactive) sulphydryl (SH) groups than the corresponding feeds based on low quality meals. After 90 days feeding, groups given high and low quality feeds did not differ in their specific growth rates (SGR) in either experiment. However, feed conversion efficiency (FCE) was significantly different between the high and low quality feed groups in 2 kg salmon, where the difference between the high and low feed protein qualities was larger, 10% versus 4% SH/[SH + (S–S)] in 150 g salmon. Higher FCE was preceded by significantly higher trypsin and chymotrypsin specific activities on day 60. SGR, in general, changed after the first month and was stable during the last 2 months in both experiments. Concurrently, both trypsin (T) and chymotrypsin (C) decreased with an increased activity ratio of trypsin to chymotrypsin (T/C ratio), and resulting in significantly lower T/C ratio on day 90 in salmon feeding on high quality feeds in both sizes of fish. Differences in FCE were associated with significant differences in levels of total free amino acids (TFAA) in the plasma and the white muscle, as well as in the ratio of essential to non‐essential free amino acids (EAA/NEAA ratio), free hydroxyproline, and RNA in the white muscle. Interestingly, after 3 days starvation (day 93), 5–7 h postprandial EAA/NEAA ratio in the plasma was significantly lower in the high quality diet groups in both experiments. Trypsin specific activity inversely correlated with muscle TFAA levels in 2 kg salmon, concurrent with higher muscle levels of RNA, lower free hydroxyproline and higher FCE in fish fed higher quality diets.     

Feed intake, growth and feed conversion efficiency of Atlantic halibut, Hippoglossus hippoglossus (L.)

The individual food intake of each fish in each of four groups of Atlantic halibut, Hippoglossus hippoglossus (L.) (mean weight: 422 g) was monitored by direct observation over a period of 21 days. Gross feed conversion efficiency (= growth·feed intake^?1), net feed conversion efficiency and maintenance ration were estimated by regression analysis. Specific growth rates were found to be linearly related to weight-specific consumption at a temperature of 8–9°C: growth = 1.922· feed intake ? 0.242. Maintenance ration was 0.126% of body weight day^?1. The gross feed conversion efficiency increased asymptotically with increasing feed intakes and growth rates, and was found to approach 1.9 at high growth rates (0.5 on a dry weight basis). One feeding per day seemed to be sufficient for maximum food intake and growth rate.     

Soybean meal-induced enteritis in Atlantic salmon (Salmo salar L.) at different temperatures

This study evaluates the effect of temperature on the development of intestinal disorders when Atlantic salmon are fed soybean meal (SBM). In this study 20% of the dietary fishmeal was replaced by solvent-extracted Hipro SBM. Atlantic salmon reared at two different water temperatures (8 °C and 12 °C), were fed a control diet and an experimental diet for 20 days. Samples were taken at days 7 and 20. The extent of the morphological changes was assessed using a semi-quantitative scoring system developed for this purpose. The study demonstrates that enteritis is affected by temperature. The intestinal disorders were more severe in fish reared at 12 °C compared with those reared at 8 °C. It can be concluded from this study that temperature changes the speed but not the type of SBM-induced enteritis expressed as a delay on the response when Atlantic salmon are kept at lower temperatures.     

Seasonally changing metabolism in Atlantic salmon (Salmo salar L.) I – Growth and feed conversion ratio

To determine seasonal variation in growth and feed conversion ratio (FCR), Atlantic salmon postsmolts (Salmo salar L.) were exposed to either simulated natural photoperiod (SNP) for 12 months or continuous light (LL) from January to June followed by SNP until December. Feed was given to excess and uneaten feed pellets were collected after every meal for estimation of feed intake and calculation of FCR. Body weight increased from 1086 ± 9 g (mean ± SEM) in January to 4970 ± 7 g (SNP) and 5190 ± 23 g (LL) in December. Specific growth rate (SGR), condition factor and feed intake displayed strong seasonal variation in both groups. Measurements of the thermal growth coefficient correlated highly with SGR (r = 0.98, P < 0.05), indicating that the seasonal variation in SGR was independent of temperature and fish size. Continuous light treatment resulted in increased growth from spring, while the fish exposed to simulated natural light had increased growth rate in late summer. Furthermore, LL improved FCR. Periods of high SGR were concurrent with periods of low FCR in both groups.     

Growth, feed conversion efficiency and growth heterogeneity in Atlantic halibut (Hippoglossus hippoglossus) reared at three different photoperiods

Juvenile Atlantic halibut, Hippoglossus hippoglossus (initial weight (SD) 191.3 (±44.7)) g, were reared for 99 days at a constant temperature of 11°C and subjected to three different light regimes from 13 September to 21 December: continuous light (LD24:0), simulated natural photoperiod of Bergen (60°25′N, LDN) and constant 20 h light:4 h dark (LD20:4). The fish reared on the different photoperiod regimes differed in their growth patterns as juveniles exposed to long days, i.e. LD20:4 and LD24:0, exhibited faster growth than those experiencing a natural photoperiod. The LD20:4 group showed the highest average specific growth rate (0.72% body weight day^?1), whereas fish on LDN displayed the lowest average specific growth rate (0.60% body weight day^?1). The final mean weights of the LD20:4 and the LD24:0 groups were 15% and 12% higher than those of the LDN group. Dividing the duration of the experiment into three time periods shows that the LD20:4 and LD24:0 had a higher feed conversion efficiency (FCE) as compared with the LDN group during the first and the last period, while a reversed situation was observed in the second period. Our data indicate a larger variation in growth rates among individuals in the best‐performing groups (here LD20:4 and LD24:0). This may indicate that formation of size hierarchies is more pronounced in groups with more homogenous growth (here LDN). Overall, our findings indicate that extended light regimes result in faster growth and better feed conversion in juvenile Atlantic halibut. In line with findings on other flatfish species, this supports the concept that constant long day:short night or continuous light regimes should be used by the farmer in order to maximize growth and improve feed conversion in Atlantic halibut.     

Quantification of dietary DNA in tissues of Atlantic salmon (Salmo salar L.) fed genetically modified feed ingredients

The impact of dietary DNA on metabolism and health of animals and humans has received little attention, except in the context of genetically modified organisms (GMOs) and horizontal gene transfer. In a series of studies, we have investigated the uptake and persistence of dietary DNA in Atlantic salmon (Salmo salar L.). The objective of this study was to investigate the uptake and persistence of dietary DNA of soybean and maize origin. A feeding experiment on salmon was started at late yolk sac stage and lasted for 7 months. The fish were randomly distributed in groups in indoor tanks and fed different types of feed. After the last feeding, the fish were starved for 24 h before samples were dissected. Using the polymerase chain reaction (PCR) amplification of short targets from the chloroplast ribulose‐1,5‐carboxylase large subunit (rbcL) gene present in some of the feed components, the uptake and transport of dietary DNA from plant ingredients to tissues could be studied. The dietary DNA, of plant origin, was found to be present in all tissues investigated and their concentrations were determined.     

Lipostatic regulation of feed intake in Atlantic salmon Salmo salar L. defending adiposity at the expense of growth?

Production of Atlantic salmon Salmo salar L. utilizes feeds with high fat concentrations to give low feed:gain. However, increased dietary fat content inevitably leads to increased body fat deposition, and salmon with very high fat concentrations in the muscle (fillet) are regarded as being of inferior quality. Adiposity is thought to participate in the regulation of feed intake by means of negative feedback control. Thus, elevated adiposity is predicted to result in lower feed consumption, and thereby possibly impair growth. We tested the hypothesis that high body fat content in salmon would lead to reduced feed intake and growth. Salmon (740 g) were preconditioned with high‐ or low‐fat feed (38.8 and 27.8% dietary fat content) for 10 weeks to establish differences in body fat storage (build‐up phase). Thereafter, fat and lean fish (19.4% and 16.7% body fat content) were fed the high‐ and low‐fat feeds for an additional 7 weeks (Phase Two). During Phase Two, the fat fish consumed 30% less feed than lean fish, which resulted in corresponding differences in growth. The differences in adiposity seen at the end of the build‐up phase were still evident at trial end. Groups of fish, which were preconditioned with the same feed during the build‐up phase, had similar feed consumption and growth to each other during Phase Two, indicating that body fat was playing an important role in the regulation of feed intake.     

Stress responses of Atlantic salmon (Salmo salar L.) elicited by water level reduction in rearing tanks

The Atlantic salmon (Salmo salar) stress response was examined by measuring plasma cortisol, glucose and chloride in fish after water level reduction within rearing tanks. Maximum plasma cortisol levels (366±43 (SD) nM and 534±280 nM for Groups 1 and 2, respectively) were observed 20 min after application of the stressor. Cortisol levels were down to control levels 24 h later. The pattern for cortisol changes observed within these two groups were comparable. Plasma glucose and chloride concentrations did not change significantly in either of the experimental groups. The fish in group 2 were then repeatedly subjected to the same stressor every third day. After the fifth exposure to the stressor, blood was obtained from fish at times corresponding to sampling after the first exposure. Their maximum plasma cortisol level only reached 223 (±96) nM, and was down to prestress levels within 2 h. Plasma glucose and chloride concentrations did not change significantly in this second experiment. This challenge test revealed an acute primary response in Atlantic salmon without any apparent harmful secondary responses, that may thus serve as a standardized reference stressor using other fish groups under comparable conditions.     

Growth, feed conversion and chemical composition of Atlantic salmon (Salmo salar L.) and Atlantic halibut (Hippoglossus hippoglossus L.) fed diets supplemented with krill or amphipods

The effects of partial replacement of fish meal (FM) with meal made from northern krill (Thysanoessa inermis), Antarctic krill (Euphausia superba) or Arctic amphipod (Themsto libellula) as protein source in the diets for Atlantic salmon (Salmo salar L.) and Atlantic halibut (Hippoglossus hippoglossus L.) on growth, feed conversion, macro‐nutrient utilization, muscle chemical composition and fish welfare were studied. Six experimental diets were prepared using a low‐temperature FM diet as control. The other diets included northern krill where 20, 40 or 60% of the dietary FM protein was replaced with protein from northern krill, and two diets where the FM protein was replaced with protein from Antarctic krill or Arctic amphipod at 40% protein replacement level. All diets were iso‐nitrogenous and iso‐caloric. Atlantic salmon grew from 410 g to approximately 1500 g during the 160 day experiment, and Atlantic halibut grew from 345 g to 500–600 g during the 150 day experiment. Inclusion of krill in the diets enhanced specific growth rate in salmon, especially during the first 100 days (P < 0.01), and in a dose–response manner in halibut for over the 150 day feeding period (P < 0.05). Feed conversion ratio did not differ between dietary treatments, and no difference was found in dry matter digestibility, protein digestibility and fish muscle composition. Good growth rates, blood parameters within normal ranges and low mortalities in all experimental treatments indicted that fish health was not affected either Atlantic salmon or Atlantic halibut fed the various zooplankton diets.     

Retrospective analysis of Atlantic salmon (Salmo salar) marine growth and condition in the northwest Atlantic based on tag‐recovery data

The life history of North American Atlantic salmon (Salmo salar) is characterized by extensive round‐trip migrations between freshwater rearing habitats and marine feeding grounds off the coasts of Canada and Greenland. Growth is rapid during the marine migration, and growth rate and condition factor may be indicators of salmon health during this period. Growth data were evaluated from a tag‐recovery program conducted from 1969 to 1991 using hatchery‐reared Atlantic salmon smolts released in the Penobscot River, Maine, U.S.A. Information from recaptures of 3167 salmon that were at large in the marine environment for 1 month to 3 yr was analyzed. Length–weight measurements coupled with time‐at‐large data were used to estimate von Bertalanffy and allometric growth parameters specific to the marine phase. Variations in growth and condition factor in relation to smolt age, release date, and temperature conditions in the northwest Atlantic were also examined. The von Bertalanffy k parameter declined with ordinal release date, indicating faster growth rates during the first year of smolts released earlier in the spring. The 2‐yr‐old smolts had a larger k than 1‐yr‐old smolts, although 1‐yr‐old smolts grew to a larger asymptotic size. Sea surface temperature had variable effects on growth parameters and condition factor, with temperature at the beginning of the migration and in overwintering habitat during the first year at sea having the greatest influence on length–weight relationships. Determining the mechanisms that influence growth of individuals during the marine phase will help elucidate the factors responsible for historic growth trends, establishing a baseline for current research.     

Different expressions of trypsin and chymotrypsin in relation to growth in Atlantic salmon (Salmo salar L.)

The expressions of trypsin and chymotrypsin in the pyloric caeca of Atlantic salmon (Salmo salar L.) were studied in three experiments. Two internal (trypsin phenotypes, life stages) and three common external factors (starvation, feeding, temperatures) influencing growth rates were varied. Growth was stimulated by increased temperature and higher feeding rate, and it was depressed during starvation. The interaction between trypsin phenotype and start-feeding temperature affected specific activity of trypsin, but not of chymotrypsin. Trypsin specific activity and the activity ratio of trypsin to chymotrypsin (T/C ratio) increased when growth was promoted. Chymotrypsin specific activity, on the other hand, increased when there was a reduction in growth rate whereas fish with higher growth had higher chymotrypsin specific activity resulting in lower T/C ratio value. During a rapid growth phase, trypsin specific activity did not correlate with chymotrypsin specific activity. On the other hand, a relationship between specific activities of trypsin and chymotrypsin could be observed when growth declined, such as during food deprivation. Trypsin is the sensitive key protease under conditions favouring growth and genetically and environmentally affected, while chymotrypsin plays a major role when growth is limited or depressed. Trypsin specific activity and the T/C ratio value are shown to be important factors in the digestion process affecting growth rate, and could be applicable as indicators for growth studies of fish in captive cultures and in the wild, especially when food consumption rate cannot be measured.     

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.     

Effects of inositol supplementation on growth, chemical composition and blood chemistry in Atlantic salmon, Salmo salar L., fry

The present experiment reports on the effects of inositol supplementation from 0 to 1600 mg kg⁻¹ to a fishmeal-based diet on growth, inositol retention, proximate composition (dry matter, protein and lipid) and blood chemistry in Atlantic salmon, Salmo salar L., fry during a 28-week feeding experiment. Growth was affected to a minor extent, and only during the first 4 weeks of the experiment. Mortality was low and not related to dietary inositol. The inositol concentration in whole fish and liver was affected by dietary inositol supplementation. Proximate composition of whole fish was similar among dietary groups, except for positive correlations between dry matter, lipid and protein and dietary inositol supplementation after 8 weeks. Analyses of hepatic lipid classes after 8 weeks showed non-significant differences among the groups, mostly explained by somewhat reduced total hepatic lipid accumulation with increasing dietary inositol. Triacylglycerol and phosphatidylcholine accounted for 80% and 8–10% of the hepatic lipids (approximately 80 mg lipid g⁻¹ wet weight) in all groups. Blood haemoglobin was positively correlated and plasma triacylglycerol was negatively correlated to dietary inositol supplementation at week 28, while plasma protein and cholesterol were unaffected. The present results indicate that the requirement of Atlantic salmon fry for inositol is covered through the natural content of inositol in practical feed ingredients at around 300 mg inositol kg⁻¹. It may, however, be advisable to supplement starter diets with moderate amounts of inositol, around 200 mg kg⁻¹, to compensate for fluctuations in inositol concentrations in natural ingredients, leaching loss of inositol from the diet and for any potential increased inositol requirement in salmon fry.     

Is there lipostatic regulation of feed intake in Atlantic salmon Salmo salar L.?

High‐energy feeds, with increased fat concentrations, are often used in salmon farming to improve feed:gain ratio. However, fish are thought to regulate ingestion to meet their energy and nutrient intake requirements. Further, feeds with excessive fat content will lead to increased adiposity, which is thought to exert a negative feedback on feed intake via lipostatic regulation mechanisms. A test of the lipostatic model of feed intake regulation was carried out on juvenile Atlantic salmon Salmo salar L. (c. 165 g) in which body fat content (5.6% and 9.4% body fat) had been manipulated by feeding feeds with different fat concentrations. Thereafter, the fish were offered the high‐ and low‐fat feeds (15.6% and 26.5% dietary fat content) simultaneously, and our hypothesis was that feed preference and intake would reflect the state of fat storage. Thus, we predicted that, when given a choice of feeds, the lean fish would eat more, and that the body fat status of the groups would converge over time. The results indicated a general preference for the leaner feed irrespective of adiposity level, but leaner fish consumed more feed, grew faster and deposited more body fat than their fatter counterparts. Over time, body compositions converged among treatments, and differences in feed intake ablated. These findings seem to provide supportive evidence for a lipostatic regulation of feed intake in fish.     

Influence of long term ammonia exposure on Atlantic salmon (Salmo salar L.) parr growth and welfare

The objective of this study was to determine the long‐term effects of ambient unionized ammonia nitrogen (NH₃‐N) combined with different feeding regimes on Atlantic salmon Salmo salar L parr growth, welfare and smoltification. Previous studies on the parr stage of Atlantic salmon have mostly focused on acute exposure, or at low temperatures. Atlantic salmon parr were exposed for 105 days (at 12°C, pH 6.8) to four sublethal ammonia concentrations ranging from 0.1 to 35 μg L^?1 NH₃‐N (0.1–25 mg L^?1 TAN) at two feeding levels: full feed strength (+20% overfeeding) and 1/3 of full feed strength. After 21 days, it was observed that 32 μg L^?1 NH₃‐N reduced growth rate of parr fed full ration, but this effect was not evident at the end of the exposure. Feed utilization was not affected by ammonia exposure at any sampling point. Increasing ammonia levels were associated with a higher prevalence and severity of gill damage at 22 days but not at the end of the exposure. The examination of welfare indicators revealed only a few pathologies, not related to ammonia exposure. In addition, higher ammonia concentrations did not appear to influence the development of hypo‐osmoregulatory ability during parr‐smolt transformation.