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.     

Growth and survival of Atlantic salmon, Salmo salar L., fed different dietary levels of astaxanthin. First-feeding fry

Atlantic salmon fry hatched from pigment-free eggs and from eggs containing the pigment astaxanthin were fed eleven casein/gelatine-based purified diets with varying levels of astaxanthin, ranging from 0 to 317 mg kg^?1, to determine the optimum dietary astaxanthin level for satisfactory growth and survival during the start-feeding period. The fish were fed the experimental diets for a period of 11 weeks. No difference in performance was found between the two types of fry originating from the pigment-free eggs and those containing pigment. However, the dietary astaxanthin concentration was found to have a significant effect on both the growth and the survival of fry. Fish fed diets with astaxanthin concentrations below 5.3 mg kg^?1 were found to have marginal growth. In addition, mortality was high in the groups fed diets with astaxanthin concentrations below 1.0 mg kg^?1. The specific growth rate (SGR) was also affected by the dietary treatment. The lipid content was higher and the moisture content was lower in the fish fed the diets containing astaxanthin concentrations above 5.3 mg kg^?1. The vitamin A and astaxanthin concentrations in whole-body samples of the fry were significantly affected by the dietary level of astaxanthin. A plateau level in whole-body vitamin A concentration was observed at dietary levels of approximately 80 mg astaxanthin kg^?1 and higher, while no maximum astaxanthin concentration in whole-body samples was observed within the dietary levels used. The results suggest the need for a minimum dietary astaxanthin concentration of 5.1 mg kg^?1 to achieve maximum growth and survival during the start-feeding period. The results indicate a low bioavailability of vitamin A palmitate and acetate and the results also suggest a provitamin A function for astaxanthin during the same period.     

Variability in performance in wild Atlantic salmon, Salmo salar L., fry from a single redd

Dispersal and growth were studied in Atlantic salmon, Salmo salar L., fry from a natural isolated redd. The distribution of fry leaving the redd was strongly peaked, 80% being caught within a 2-week period. Early in the dispersal period, all fry leaving the redd had remnants of yolk sac and had not fed; by half way through the dispersal period, no fry had any visible yolk, but 35% still had empty stomachs. Fry leaving the redd during the first half of the dispersal period tended to settle in different first feeding sites than those dispersing later. Predation on fry by larger salmonids was frequent, especially during dispersal. Coefficients of variation for length, weight and condition factor increased significantly over the study period and for individually recognised fry, growth rates varied markedly. Thus, individual salmon fry differ in physical status on emergence from the redd and these differences are amplified during the first few weeks after emergence.     

Determination of the threonine requirement for maintenance in Atlantic salmon (Salmo salar L.) fry with the diet dilution procedure

A study was conducted to evaluate the threonine (Thr) requirement for maintenance in Atlantic salmon using the diet dilution procedure (DDP). Fourteen groups of 60 salmon fry [1.46 g initial body weight (BW)] were fed on seven semi‐purified diets containing graded levels of N (3–64 g kg⁻¹ dry matter) and L‐Thr (1–39 g kg⁻¹ dry matter). Seven doses of Thr represented 1–31% of its ideal level for optimum protein deposition. Indispensable amino acids (AA) other than Thr were included in the same proportion as in the Atlantic salmon fry whole‐body carcass. Protein deposition and Thr accretion were linear functions of Thr intake. At zero Thr intake fry lost 4.4 mg Thr kg BW^−0.75 day⁻¹ and the Thr requirement for maintenance was 5.8 mg kg BW^−0.75 day⁻¹, which represented 11% of the total need for Thr. Increasing doses of Thr did not show any effect on AA concentrations in whole‐body protein, except for cystine. The linear relationship between Thr gain and Thr intake indicates a constant Thr cumulative efficiency (77%) at marginal Thr intake. Finally, our results suggest that (1) both the DDP and the graded supplementation technique can be used in Atlantic salmon fry for the determination of AA maintenance requirement and utilization efficiency, at least for Thr; (2) the dietary protein level has only a minor effect on the Thr maintenance requirement determination.     

How juvenile Atlantic salmon, Salmo salar L., respond to falling water levels: experiments in an artificial stream

The responses of juvenile Atlantic salmon, Salmo salar L., to an experimentally imposed drop in water level were studied in an artificial stream. In a pilot experiment, 20% of fish which had a feeding station in an area of riffle remained there throughout the period of low water levels. The remaining riffle-dwelling fish moved into areas of deeper water, although not necessarily to the pool nearest their feeding station. Out of the fish which left the area, 89% moved in an upstream direction and 11% moved downstream. In a second experiment, which was designed to look in more detail at this response to de-watering, 95% of riffle-dwelling fish left when the water level dropped and moved into a pool, mostly within the 4-h period after water levels started to fall. Seventy-four per cent of fish which left moved upstream and the remainder moved downstream. There was a strong tendency to leave in an upstream direction as shallow areas began to dry out, and this tendency persisted irrespective of variability in fish size, prior social experience and the size of the home range prior to de-watering. It is suggested that the prior opportunity to explore alternative habitat may be an important determinant of response of riffle-dwelling salmon to a sudden drop in water level.     

Effect of sudden increase in discharge in a large river on newly emerged Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) fry

Abstract– Downstrcam displacement of salmonid fry due to flow increase, from 12–15 m³/_s to >100 m³_s, was documented in the river Suldalslåen, Western Norway. In May only fry of brown trout ( Sulmo trutta ) occurred in the drift, while from the beginning of June only newly emerged Atlantic salmon ( Sulmo sular ) were found. The maximum number of Atlantic salmon fry drifting during a single day was estimated to be 17 000 individual. Their density in the drift was higher during the night than during the day, and their appearance in the drift coincided with the predicted period of emergence. Total brown trout numbers in the drift were estimated to vary between 4000 and 16 000 per day. Fry displaced downstream from the lowermost part of the river were lost from the population. The total losses were estimated to be between 75 000 and 100 000 Atlantic salmon fry which represents between 5.6 and 11.1%) of juvenile mortality during the first year of life.     

Effects of diet supplementation of soya‐saponins,isoflavones and phytosterols on Atlantic salmon (Salmo salar,L) fry fed from start‐feeding

A 14‐week trial was conducted to investigate the effects of antinutritional factors (ANFs) commonly present in soybean ingredients, singly and in combination, on Atlantic salmon (Salmo salar L.) fed from start‐feeding. The experimental diets consisted of a negative control fish meal diet (FM), and a positive control diet with 167 g kg^?1 soybean meal inclusion (SBM) and four diets based on the FM diet supplemented with 2 g kg^?1 soya‐saponins (SAP), 1.5 g kg^?1 isoflavones (IFL), 0.3 g kg^?1 phytosterols (PHS) or a mixture of these (MIX). Fish fed the SAP diet showed significantly higher growth performance than those fed FM, while the IFL treatment significantly decreased growth performance of salmon fry. Fish fed the IFL diet had significantly lower maltase activity and higher trypsin activity in proximal intestine than fish fed the FM diet. Histological differences were observed in the liver of fish fed the IFL diet, characterized by reduced size of the hepatocytes. Fish fed the PHS and IFL diets showed the highest frequencies of skeletal deformities among the six treatments. In conclusion, the results indicate that purified isoflavones may negatively affect growth performance, intestinal function, liver metabolism and bone formation of salmon fry.     

The onset of downstream movement of juvenile Atlantic salmon, Salmo salar L., in a chalk stream

The downstream movements of wild Atlantic salmon, Salmo salar L., from their established feeding territories in the River Itchen, Hampshire, UK, were logged continuously over an 11-month period using a passive integrated transponder (PIT) antenna system. The time of these movements was then related to a number of monitored and calculated environmental parameters. Initial downstream movement of smolts in April was correlated with the onset of darkness, at which time salmon moved from their established feeding territories alone. No relationship was found between the number of smolts migrating and river flow or maximum daily water temperature. The timing of downstream movement of parr between October and March was random with regard to sunset and time of maximum daily water temperature, suggesting the environmental cue that initiates movement may be different outside the spring smolt period.     

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.     

Growth and protein utilization in Atlantic salmon (Salmo salar L.) given a protease inhibitor in the diet

In a series of experiments the effects of dietary protease inhibitor inclusions on growth and amino acid absorption rate were investigated in Atlantic salmon ( Salmo salar L.). An optimal inclusion of inhibitor was found by conducting dose–response studies with specific growth rate as the main response variable. No negative effects on feed conversion ratio or nitrogen digestibility were observed at this level. In protein sources studies the addition of potato protease inhibitor to a fishmeal based diet increased specific growth rate by 14%. When a proportion of the fishmeal was replaced with hydrolysed protein (fish silage) the addition of the inhibitor resulted in a 31% increase in specific growth rate. Absorption of amino acids from the gastrointestinal tract into the blood was investigated in two experiments using ¹⁴C-labelled algal ( Synechoccus leopoliensis ) protein. The absorption pattern of ¹⁴C-labelled amino acids was altered by adding potato protease inhibitor when the algal protein was supplied in intact form, but not when the algal protein was hydrolysed. The absorption of amino acids from a hydrolysed protein was significantly faster than from intact protein. The enzymatic activity of pepsin in the stomach and of trypsin, chymotrypsin, carboxypeptidases A and B in the different segments of the intestine changed significantly with increasing inclusion of potato protease inhibitors in the diet.     

Cardiac morphology in relation to amoebic gill disease history in Atlantic salmon, Salmo salar L.

Fish from cages with histories of heavy and light amoebic gill disease (AGD) outbreaks were harvested and the morphology, histology and activities of lactate dehydrogenase determined. Although fish with a history of heavy AGD were smaller, their heart somatic indices were similar to those of fish with a history of light AGD. However, morphometrically the ratios of ventricle axis length and width and axis length and height were significantly higher, and there was an overall thickening of the muscularis compactum in the ventricle of fish with heavy AGD history. There was no difference in the lactate dehydrogenase activity of the ventricle muscle in the two fish groups. These results suggest that the change in ventricle shape associated with AGD was a possible compensation for an increased afterload where the lengthening of the ventricle was compensated for by an increase in muscle thickness, but without any overall ventricular hypertrophy or gain in ventricular mass. This suggests that AGD may be associated with cardiovascular compromise in affected fish.     

Heart and skeletal muscle inflammation in Atlantic salmon, Salmo salar L: a new infectious disease

Heart and skeletal muscle inflammation (HSMI) is a disease syndrome of unknown aetiology first observed in farmed Atlantic salmon, Salmo salar, in 1999. In the present study we have demonstrated for the first time that HSMI is an infectious disease. It was induced in Atlantic salmon post-smolts after injection with tissue homogenate from farmed Atlantic salmon previously diagnosed with HSMI. The lesions were also induced in cohabitating salmon given a corresponding injection without tissue homogenate. Six weeks post-challenge the fish that had been injected with tissue homogenate developed a serious epicarditis and myocarditis with mononuclear cell infiltrations in compact and spongy layers of the heart. Similar lesions were found in cohabitants after 10 weeks. The lesions were consistent with samples from field outbreaks of HSMI. No lesions were found in control fish. A viral aetiology is strongly suggested, as no difference in disease induction between an inoculum containing antibiotics and a non-treated inoculum was found. Further investigations are required in order to make conclusions regarding the cause and pathogenesis of HSMI.     

Spinal deformation in commercially cultured Atlantic salmon, Salmo salar L.: a clinical and radiological study

A clinical and radiographic study was carried out on 2016 randomly selected Atlantic salmon parr from six farms, which weighed between 55 and 100 g. In addition, 86 fish from six marine farms were similarly studied. Radiographically detectable vertebral lesions were found in 3.8-8.8% of the parr. The changes ranged from a single vertebra showing features, such as demineralization, increased density and slight loss of structure, to fish with multiple vertebrae affected by collapse, fusion and change in intervertebral space that could give rise to lordosis or kyphosis or abnormal vertebrae at several locations. The predominant location was the vertebral region V21-27, and rarely V44 caudad. The radiographic changes in seawater salmon mirrored those found in parr, but vertebrae V35 caudad were more frequently involved. Very few parr were found to have abnormal vertebrae in the peduncle area which suggests that the development of lesions at this location occurs de novo in the seawater phase, given that 'stumpy' fish are most frequent reported deformity at slaughter. It was concluded that minor vertebral change could be detected radiographically in many parr with no observable effect on external morphology. Whether such minor radiographic alteration would proceed to observable morphological change at slaughter weight is open to question.     

Occurrence of canthaxanthin in Atlantic salmon, Salmo salar L., fry in Irish rivers as an indicator of escaped farmed salmon

The rapid growth of Atlantic salmon, Salmo salar L., culture in north-western Europe has given rise to concerns regarding the biological consequences of fish farm escapes on wild salmonid populations. Canthaxanthin, a carotenoid pigment additive to farmed salmon feed which is passed from females to their progeny, may be used as an indicator of the numbers of escaped farmed salmon which spawn in the wild. In the present study, thin-layer chromatography and high-performance liquid chromatography (HPLC) were used to screen emergent Atlantic salmon fry sampled from seven river catchments in Ireland for canthaxanthin. The incidence of fry containing canthaxanthin at greater than trace levels (<5% of total carotenoid pigment) was 0–4%, with an average of 1.7%, among the seven rivers sampled, indicating that the progeny of farmed salmon were present at similarly low frequencies. Canthaxanthin was detected at trace levels in an unexpectedly high proportion (35%) of salmon fry. Canthaxanthin was present at levels exceeding trace amounts in 24% of 21 non-anadromous brown trout, Salmo trutta L., sampled from six Irish rivers and present at trace levels in a further 57% of the fish, indicating that dietary canthaxanthin is freely available to salmonids in Irish rivers. The widespread presence of trace levels in salmon fry may be attributable, at least in part, to the increased sensitivity of the HPLC methods and to rapid dietary uptake during early post-emergence feeding.     

Changes in the innate immune response of Atlantic salmon, Salmo salar L., exposed to experimental infection with Neoparamoeba sp

An experiment was conducted to determine the effect of Neoparamoeba sp. infection on the innate immune responses of Atlantic salmon. Atlantic salmon were experimentally infected with Neoparamoeba sp. and serially sampled 0, 1, 4, 6, 8 and 11 days post-exposure (dpe). Histological analysis of infected fish gill arches identified the presence of characteristic amoebic gill disease lesions as early as 1 dpe with a steady increase in the number of affected gill filaments over time. Immune parameters investigated were anterior kidney phagocyte function (respiratory burst, chemotaxis and phagocytosis) and total plasma protein and lysozyme. In comparison with non-exposed control fish basal respiratory burst responses were suppressed at 8 and 11 dpe, while phorbol myristate acetate-stimulated activity was significantly suppressed at 11 dpe. Variable differences in phagocytic activity and phagocytic rate following infection were identified. There was an increase in the chemotactic response of anterior kidney macrophages isolated from exposed fish relative to control fish at 8 dpe. Total protein and lysozyme levels were not affected by Neoparamoeba sp. exposure.     

Humoral response and susceptibility of five full-sib families of Atlantic salmon, Salmo salar L., to the haemoflagellate, Cryptobia salmositica

Susceptibility and antibody production against pathogenic and vaccine strains of the haemoflagellate, Cryptobia salmositica were investigated in five full‐sib families (A–E) of Atlantic salmon, Salmo salar. Humoral response and susceptibility of families were compared within three treatments: infection, vaccination and vaccination followed by challenge. Parasitaemias caused by the vaccine strain of C. salmositica were considerably lower than those caused by the pathogenic strain. All vaccinated families were protected when challenged with the pathogenic strain. Family B had significantly lower parasitaemias (with both strains) than the other families. When naïve fish were infected with the pathogenic strain, this family had a significantly lower and earlier peak parasitaemia (4.3 ±1.3 × 10⁶ parasites mL^?1 blood at 3 weeks post‐infection; w.p.i.) than the other families. Family C had the highest peak (11.1 ± 1.2 × 10⁶ parasites mL^?1 blood), which occurred at 4 w.p.i. Antibodies against C. salmositica were detected earlier in Family B (3 w.p.i.) than in Family C (5 w.p.i.). This demonstrates an association of increased susceptibility with a delayed antibody response. Western immunoblot identified antibodies against 112, 181 and 200 kDa antigens earlier in more resistant fish (Family B). Antigenic stimulation leading to a stronger antibody response was shown with the vaccine strain and in the later stages of infection.     

Glucose tolerance in Atlantic salmon, Salmo salar L., dependence on adaption to dietary starch and water temperature

Individually tagged Atlantic salmon, Salmo salar L., were fed three different adaptation diets containing low, medium or high levels of extruded starch, before intraperitoneal injection of glucose. The injections were done at winter and at summer temperatures. This was to investigate whether the tolerance for glucose varied according to adaption diet and/or according to season. The results showed a temperature-dependent response. At winter temperatures (2°C) the Atlantic salmon had problems adapting to both medium and high levels of dietary starch. This was indicated by lower growth response in the adaptation period, as well as by delayed glucose regulation. At summer temperatures (12.5°C) the Atlantic salmon could tolerate and utilize both medium and high levels of dietary starch. In summer also all groups responded evenly to the tolerance tests with respect to blood glucose regulation. Incorporation of liver glycogen was higher at summer than at winter temperatures in the high- carbohydrate group. No effect was found on haematocrit values as a response to diet or season.     

Randomized clinical field trial of a bacterial kidney disease vaccine in Atlantic salmon,Salmo salar L.

A randomized, blinded clinical trial was performed to assess the relative effectiveness of five commercial and one experimental vaccine in a population of farmed fish experiencing a bacterial kidney disease (BKD) outbreak that occurred in one study cage that was part of a larger clinical field trial. A total of 6000 uniquely identified Atlantic salmon S1 presmolts were randomly assigned to vaccine groups in the hatchery and transferred to a commercial marine aquaculture site. Repeated sampling events to evaluate growth, inherent physical conditions and health status were carried out over the entire production cycle. During the second summer at sea, the study cage developed an outbreak of BKD that lasted approximately 240 days. The effectiveness of the selected vaccines was evaluated using survival analysis methods. The sole vaccine group offering protection for BKD was found to significantly decrease the hazard of dying (hazard ratio, HR = 0.68, P = 0.018) during the outbreak, compared to the industry standard, vaccine group. Additionally, during the outbreak, fish with a shortened operculum had a significantly decreased hazard (HR = 0.38, P = 0.033) compared to those fish with a normal operculum, while fish with jaw deformities had a significantly increased hazard (HR = 2.55, P = 0.001) compared to fish with normal jaw status.     

Metabolic enzyme activities in Atlantic salmon (Salmo salar L.) embryos respond more to chronic changes in oxygen availability than to environmental temperature

Cronic exposure to an increased temperature (e.g., 10 °C as compared to 5 °C) and to a different oxygen tension can dramatically affect muscle cellularity in Atlantic salmon embryos at a developmental stage close to hatching. Maximal activities of enzymes of aerobic and anaerobic metabolism have been shown to vary with temperature and growth rates in fish but only limited data are available for embryonic fish. In order to obtain data on Atlantic salmon embryos and to be able to compare temperature and oxygen effects, the maximal activities of lactate dehydrogenase (LDH) and cytochrome c oxidase (CCO) were determined in body tissue containing mainly muscle as measures of anaerobic and aerobic metabolism, respectively. Temperatures of 5 and 10 °C, different O₂ tensions (50% normoxic, normoxic, 150% normoxic), and growth within (chorionated) or without (dechorionated) the egg capsule were chosen as environmental conditions. Temperature affected CCO activities and thus the CCO/LDH ratio in dechorionated but not in chorionated embryos (5 < 10 °C) and had no effect on LDH activities. However, changes in oxygen availability had an effect on all parameters measured. Tissue protein concentration increased after dechorionation but no temperature effect was found. Both LDH and CCO activities undergo complex responses to oxygen availability depending on incubation temperature. There thus appeared to be an interaction of temperature and oxygen availability with regard to maximum activities of key enzymes of the aerobic and anaerobic metabolism in the salmon embryos.