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.     

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.).     

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.     

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.     

Feed intake and growth of Atlantic salmon, Salmo salar L., fed diets supplemented with oxytetracycline and squid extract

This study examined the long-term effects of a feeding deterrent, oxytetracycline (OTC), and a feeding stimulant, squid extract, on feed intake, growth and dry matter (DM) digestibility in Atlantic salmon, Salmo salar L. Fish were fed one of four diets for 9 weeks: 1. commercial feed formulation (basic); 2. BM (basic plus 20 g kg⁻¹ OTC); 3. BMS (BM plus 10 g kg⁻¹ squid extract); 4. BS (basic plus 10 g kg⁻¹ squid extract). OTC initially reduced the palatability of the feed, but the fish seemed to become accustomed to the taste of OTC over time. Addition of squid extract to the medicated feed (BMS) seemed to mask the aversive taste of OTC, but the effect on feed consumption was of short duration. Addition of squid extract to the basic feed (BS) had no significant effect on feed intake, growth or feed digestibility. The growth of fish fed medicated diets (BM and BMS) was depressed, probably as a consequence of reduced feed digestibility. The two additives led only to temporary changes in feed acceptability, but both growth and DM digestibility were affected by OTC. Thus, we suggest that short-term studies may be inadequate to test whether deterrent or stimulant properties of feed ingredients are of practical importance in feed formulation.     

Plasma protein changes in Atlantic salmon, Salmo salar L., with infectious salmon anaemia

Moderate to severe anaemia and hypoproteinaemia were reported in a Canadian outbreak of 'haemorrhagic kidney syndrome' in Atlantic salmon, later shown to be caused by a variant of infectious salmon anaemia virus (ISAV). The progressive anaemia associated with ISA has been previously reported, but hypoproteinaemia in salmon infected with European isolates of ISA virus has not been well documented. The present study showed a very significant positive correlation between decreasing haematocrit values and total plasma protein concentrations in Atlantic salmon infected with two Canadian and two Norwegian ISA viral isolates. However, variations in the concentration of individual plasma proteins, typical of acute phase responses in higher vertebrates, were not observed.     

Changes in tissue and plasma free amino acid concentrations after feeding in Atlantic cod

Atlantic cod, Gadus morhua, were maintained on a diet of sandeel and after a 6-day fast were refed a single meal. Concentrations of free amino acids (AAs) were measured in hepatic portal and cardiac blood as well as in the stomach and white muscle at intervals of 6h up to 24h post-feeding. The appearance of both essential and non-essential AAs in the hepatic portal blood was significantly correlated, up to 12h after feeding, to their abundance in the diet. There was a significant decline in total AA concentration in cardiac blood after 6h, followed by a significant increase at 12h. No significant changes in total AA concentration were observed in the other tissues, although mean concentration increased at 12 or 18h. At a more detailed level, the post-prandial changes in concentration of some essential AAs were consistent with their having a role in the stimulation of protein synthesis after feeding.     

Screamer disease in Atlantic salmon, Salmo salar L., in Chile

Investigation of cranial and spinal deformities in Atlantic salmon smolts from Chile immediately after sea transfer, over two successive crops, demonstrated ankylosis of the mandibular articulation, spinal foreshortening, fracture of vertebrae and rarefaction of osseous and cartilaginous tissues including the operculum. As a result the mouth was permanently agape and opercula could be folded back on themselves. All affected fish had been transferred to sea water at 20 °C plus and fed on particular commercial diets. Fish in cooler areas, or on diets high in vitamin C and phosphorus in the high temperature zone, were clinically normal. The condition has not recurred following dietary adjustment.     

Dietary phosphorus requirement of juvenile Atlantic salmon, Salmo salar L.

The objective of this study was to determine the dietary phosphorus (P) requirement of juvenile Atlantic salmon, Salmon salar L. Triplicate groups of fish (mean initial weight 1.4 g) were fed semipurified, casein-gelatine-based diets containing one of five levels of P (4, 8, 10, 15 and 25 g kg⁻¹) from Ca(H₂PO₄)₂·H₂O, or a commercial feed (17 g kg⁻¹ P) for 9 weeks. Weight gains did not differ significantly among treatment groups fed the experimental diets but were slightly less than gains in fish fed the commercial feed. Feed efficiency (wet weight gain/dry feed consumed) was similar in all groups, averaging 1.45. Availability of dietary P, estimated from apparent retention and apparent digestibility, was 86%. Whole-body P concentrations declined in fish fed diets containing less than 10 g kg⁻¹ P. Fitting a logistic curve to dietary P vs. whole-body P concentrations indicated that a minimum of 11 g kg⁻¹ dietary P (9 g kg⁻¹ digestible P) was required by juvenile Atlantic salmon to maintain whole-body P concentrations at initial levels. Calculation of a dietary requirement using a simple factorial model which incorporated measurements of P availability, feed efficiency and normal whole-body P concentration indicated that the dietary requirement was approximately 10 g kg⁻¹. The dietary requirement established in this study (10–11 g kg⁻¹) is higher than previously reported for Atlantic salmon or other fishes. Possible reasons for the wide range of reported dietary P requirements in fishes are discussed.     

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.     

Density of brown trout, Salmo trutta L., and Atlantic salmon, Salmo salar L., parr after point and scatter stocking of fry

Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., fry were point and scatter stocked in the early part of June at densities of 63–263 fry 100 m⁻² per species in the River Viantienjoki, a small river in northern Finland, and their population densities were assessed in late summer. Both species were always stocked together in similar quantities. Point stocking was used in the first 2 years and scatter stocking in the following 2 years. In point stocking, there was no correlation between the distance from the stocking sites (maximum = 250 m) and parr density in census sites ( r = −0.013 and 0.019 for brown trout and Atlantic salmon, respectively). The stocking density of fry did not influence parr density in August by either method or by species. Stocking density explained only from 11% to 23% of the parr survival depending on the species or stocking method. The mean densities of Atlantic salmon and brown trout parr did not differ significantly from each other at any fishing site ( P > 0.05). Both point and scatter stocking appear to be suitable methods for use in small rivers. The parr densities depend more on the other factors (e.g. habitat quality) than the stocking method, and the choice between methods could be based on the time and labour available.     

Target spawning requirements for Atlantic salmon, Salmo salar L., in Newfoundland rivers

Abstract In recent years, there has been increasing pressure from user groups in Newfoundland and Labrador, Canada, to manage Atlantic salmon, Salmo salar L., on a river-specific basis. The requirement to maximize benefits to users while ensuring that conservation goals are met has resulted in an initiative to develop river-specific target spawning requirements. In Newfoundland, anadromous juvenile Atlantic salmon make widespread use of lacustrine habitat for rearing. Depending on the river, lacustrine habitat can account for a substantial proportion of total production. Therefore, target spawning requirements for a number of rivers in several management areas were derived in terms of the contribution of both riverine and lacustrine habitats. The approach adopted, and derivations of associated parameter values, are reviewed, and limitations and research needs are discussed.     

Infectious pancreatic necrosis in Atlantic salmon, Salmo salar L

A clinical and histopathological review was carried out of 21 outbreaks of acute infectious pancreatic necrosis (IPN) in Scottish Atlantic salmon, Salmo salar L., farms (13 marine and eight fresh water) during 1991-2004. A distinctive syndrome was evident in both post-smolts in sea water and fry in fresh water, where liver lesions, which had not previously been associated with IPN, became a consistent finding in addition to the more typical pancreatic and intestinal changes. Initial cases were described in post-smolts in Shetland, but by the end of the period of investigation this type of pathology had extended down the West coast of Scotland and into Ireland. Limited viral strain analysis suggested that similar strains were involved in both fresh water and sea water and that these differed from earlier isolates from rainbow trout, Oncorhynchus mykiss (Walbaum). In fresh water, recovered fish frequently developed a greatly distended intestine associated with accumulation of undigested food. In sea water, after the initial, often significant (50% or more), losses, there were many fish which failed to grow and became chronically emaciated and prone to sea louse infection. Although use of transfer diets containing immune enhancers and the selection of IPN resistant broodstock has reduced losses the disease remains a serious cause of economic loss.     

Organ and phospholipid class fatty acid specificity in response to dietary depletion of essential n‐3 fatty acids in Atlantic salmon (Salmo salar L.)

The cell membrane phospholipid composition is of major importance for normal cell functions. However, it is not known how complete depletion of both shorter and longer chain omega‐3 fatty acids in salmon diets influences fatty acid composition of phospholipid subclasses in different organs of Atlantic salmon. We describe here the fatty acid composition in phospholipid subclasses of liver, muscle, heart and intestine in Atlantic salmon after 18 months of dietary n‐3 essential fatty acids deprivation. The percentage of 22:6n‐3 was markedly reduced in almost all phospholipid subclasses, and except for muscle phosphatidylcholine, phosphatidylethanolamine (PE) and phosphatidylinositol (PI), phospholipids in deficient fish were totally devoid of 20:5n‐3. As compensation, we observed significant increases in 20:4n‐6, and especially in 20:3n‐9 (Mead acid) and 22:5n‐6, varying among phospholipids and organs. High amounts of 20:3n‐9 were found in liver and intestinal PE, little in PE from heart and muscle. For 22:5n‐6, we saw a small incorporation in PI in liver and intestine compared to heart and muscle. Generally in PI, the preference for 20:4n‐6 to 20:5n‐3 differed significantly between organs. Overall, changes upon n‐3 deprivation seemed to be strongest in liver and intestine, the lipid‐secreting organs, and less in muscle and heart.     

Assessment of infectious salmon anaemia virus prevalence for different groups of farmed Atlantic salmon, Salmo salar L., in New Brunswick

Abstract Infectious salmon anaemia (ISA) virus (ISAV) has been causing disease in New Brunswick since 1996. As a control measure, all fish in an outbreak cage are killed. The objective of this study was to compare ISAV prevalence in cages experiencing an outbreak with healthy cages from the same farm, neighbouring farms and distant farms. Atlantic salmon from five different groups were tested using an RT-PCR test. Groups included moribund fish from a cage experiencing an outbreak (A), healthy fish from an outbreak cage (B), healthy fish from a negative cage from a farm experiencing an outbreak in a different cage (C), healthy fish from a negative farm near an outbreak farm (D), and healthy fish sampled at a negative farm located in an area with only negative farms (E). Apparent prevalences (standard error) for the different groups (A-E) were 0.94 (+/-0.026), 0.41 (+/-0.062), 0.29 (+/-0.040), 0.08 (+/-0.037) and 0.08 (+/-0.037), respectively. All groups were significantly different (P < 0.002) from each other except for groups B and C and groups D and E. Because the prevalence of the virus was significantly higher in the outbreak cage (B) compared with other sites, early harvest of outbreak cages will remove one source of virus. However, ISA negative cages (C) that remain on the positive farm may potentially act as a viral reservoir.     

An estimation of dietary iron requirement of Atlantic salmon, Salmo salar L., parr

A study was conducted to determine the dietary iron requirement of fingerling Atlantic salmon Salmo salar L. During the first 4 weeks of the experiment, fish with an initial weight of 5 g were fed a casein–gelatine-based purified diet which contained 11 mg iron kg^?1. Thereafter duplicate tanks (200 fish in each) were fed the casein–gelatine purified diets containing supplemental iron levels of 0, 10, 20, 30, 40, 60, 100, 200 or 400 mg iron kg^?1 (added as FeSO_?4* 7H₂O) for 12 weeks. Weight gain, body length and mortality were monitored. Liver iron and ascorbic acid concentration were analysed in addition to whole-body iron, manganese and zinc concentration. Several haematological parameters were also measured. There were no significant differences in weight gain and survival of salmon fed diets containing different iron levels. Haematological values, hepatic and whole-body iron concentrations were, however, significantly affected by the dietary iron content. Liver vitamin C concentration decreased with increasing dietary iron levels. Dietary supplementation with iron significantly reduced whole-body manganese, but no effect of dietary iron on whole-body zinc was found. Based on haematology and hepatic iron concentration, the iron requirement of Atlantic salmon was determined to be between 60 and 100 mg iron kg¹.