An experimental study of the susceptibility of Atlantic salmon fry, Salmo salar L., to viral haemorrhagic septicaemia

Abstract. Infection trials using two serotypes of VHS viruses (type 1 and 23/75) demonstrated that Atlantic salmon fry were susceptible to the disease when injected intraperitoneally (i.p.) with 10³ pfu of virus/fish but resistant to infection by a bath method when exposed for 3 h in water containing 5 × 10⁴ pfu of virus/ml. In the i.p.-infected fish, mortality reached 78 and 67% within 13 days with VHSV₁ nad 23/75 serotypes, respectively. High virus yields were recovered from infected fish and virus shedding was demonstrated by the onset of VHS in rainbow trout kept in the outflow water from the aquaria containing infected salmon. Neither mortality nor virus shedding occurred in salmon infected by the water route but virus multiplication was demonstrated in 2 of 60 fish with VHSV₁ and 3 of 60 fish with virus 23/75. On day 79 post-infection the sera from surviving salmon of both i.p. and bath infection trials exhibited good neutralizing titres (around 1000) against the homologous viruses.     

An experiment on aquaculture potential of Atlantic salmon, Salmo salar L., kelts in Newfoundland, Canada

Abstract. Post-spawning Atlantic salmon. Salmo salar L ., were obtained from a salmon enhancement project in the autumn of 1984, These salmon were overwintered in submerged cages in fresh water, retrieved in the spring of 1985, and transferred to a marine cage after vaccination for Vibrio anguillarum . Salmon mortality was highest immediately after their retrieval from overwintering cages, Mortality through the duration of the marine cage interval was 0.22% per day. Growth reached a maximum of 1.76% per day. Average final weight reached by reconditioned kelt in the 101 days of marine cage confinement was greater than the average weight of natural repeal spawners in the stock that provided kelt to the aquaculture experiment. Reconditioned salmon were slaughtered in November and marketed, both fresh and smoked. The incidence of maturation among slaughtered salmon was less than 2%. Consumer response to reconditioned kelt indicated that flesh colour was inferior but thai the products were acceptable and that 84% of the consumers polled would purchase the product again.     

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.     

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.     

Atlantic salmon, Salmo salar L., feed in Scottish coastal waters

Abstract. The stomach contents of 256 Atlantic salmon, Salmo salar L., of fork lengths 53–66cm, caught in bag nets on the Scottish west coast near Ullapool, were examined between June 1983 and July 1986. A total of 61 fish contained fish in the stomach or had faecal pellets containing fish bones in the gut. All recognizable whole fish were sandeels, Ammodytes marinus Raitt, ranging in size from 4.5 to 15cm. No evidence for crustacean or other non-fish prey items was found. Results indicate that feeding salmon were caught up to a certain cut-off point in June or early July, after which all salmon sampled were not feeding. It is suggested that either there is a local feeding stock of salmon or that fish feed during migration from the Faroe Isles or other possible distant water origins.     

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.     

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.     

Biotin in practical fish-meal based diet for Atlantic salmon Salmo salar L. fry

Groups of Atlantic salmon fry (0.19 g initial weight) were fed a fish-meal based starter diet with different supplements of biotin (0, 0.5, 1.0 and 1.5 mg biotin kg^–1) in triplicate tanks for 18 weeks. The basal diet contained 0.3 mg total biotin kg^–1. The experimental design included a negative control diet made by replacing 10% of the fish-meal with spray-dried raw hen's egg white.
Throughout the experimental period the fish grew to about 5 g and there were no significant differences in growth and mortality among the groups of fish fed the fish-meal diets. At the end of the experiment there were no significant differences in biotin level in the liver, while whole-body biotin concentrations correlated significantly with the dietary biotin concentrations. No significant increase in pyruvate carboxylase (PC) activity was found in livers from fish given different dietary levels of biotin. The control diet with egg white resulted in severe growth reduction and increased mortality compared with the other dietary groups. The concentrations of biotin in liver and whole body were decreased in fish fed egg white. Increased levels of glycogen and reduced PC activity in the liver were observed in this group after 18 weeks.
Histology of the gills showed no differences in appearance when fish were fed fish-meal based diets while the addition of egg white resulted in hypertrophy and hyperplasia of the gill tissue and extensive fusions of the secondary gill lamellae.
The results show that there is no need for supplemental biotin in practical fish-meal based diets for Atlantic salmon fry to achieve optimal growth, survival and maximal liver PC activity.     

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.     

An experimental means of transmitting pancreas disease in Atlantic salmon Salmo salar L. fry in freshwater

A challenge model for pancreas disease in Atlantic salmon, Salmo salar L. fry, was developed comparing two salmonid alphavirus (SAV) subtypes: SAV1 and SAV5. Viral doses of 3 × 10⁵ TCID₅₀ mL⁻¹ for SAV1 and 3 × 10⁴ for SAV5 were tested in triplicate tanks, each containing 450 salmon fry. Cumulative mortalities of 1.2% were recorded. Titres of virus recovered from the mortalities ranged from 10² to 10⁷ TCID₅₀ mL⁻¹. Fry were sampled at 3, 5 and 7.5 weeks post-challenge. Sampling after 3 weeks revealed a high prevalence of infection in the absence of clinical signs, and infectious virus was recovered from 80% and 43% of sampled fry infected with SAV1 and SAV5, respectively. After 5 weeks pancreas, heart and red skeletal muscle lesions were generally observed, whilst degeneration in white skeletal muscle was observed only in fish infected with SAV1. In situ hybridisation confirmed the presence of viral genome in infected pancreas, heart and muscle. After 7.5 weeks, infectious virus (both isolates) was recovered from 13.3% of the fish sampled, with a viral titre of 10² TCID₅₀ mL⁻¹. Clearly, salmon fry are susceptible to SAV infection and pancreas disease.     

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.     

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 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.     

Distribution of Atlantic salmon, Salmo salar L., in freshwater bodies of Europe

Data are presented on the current geographic distribution of Atlantic salmon, Salmo salar L., in freshwater bodies of Europe. They dwell in 14 hydrological systems, including basins of U lakes and three rivers. Nine of the lakes (Ladoga, Onega, Kuito and others) arc located in Russia, one in Finland (Saimaa) and one in Sweden (Vänern). All the rivers inhabited by freshwater salmon flow within Norwegian borders. Most of the lakes and rivers have good connection with the Baltic, White or Norwegian Seas. Over the past 20 years the freshwater salmon stock in Lake Imandra (the Kola Peninsula) has completely disappeared as a result of heavy human impact. The situation with such non-migratory salmon, which used to be important to fishing and trade, is also critical in all other water bodies. It is necessary, therefore, to elaborate effective measures for protecting and restoring freshwater salmon in the lakes and rivers of Europe.     

Incidence of hybridization between Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L., in Ireland

The substantial growth of the farmed salmon industry in Europe since the 1970s has highlighted concerns regarding the genetic impact of escaped farmed salmon on wild salmonid stocks. High incidences of salmon × trout hybrids have been recorded in rivers situated near intensive salmon farming in Norway and Scotland, which may be indicative of a breakdown in reproductive isolation between salmon, Salmo salar L., and brown trout, Salmo trutta L. In the present study, salmonid fry and 0+ parr were collected from rivers in western Ireland. Allozyme and minisatellite DNA analysis were carried out on fry to determine the frequency of F1 hybrids from 10 rivers located within 38 km of salmon farms and three rivers at least 80 km from salmon farms. A total of 49 hybrids were recorded from 4135 salmonid fry (frequency = 1.2%). Mitochondrial DNA analysis showed that all hybrids arose from Atlantic salmon female × brown trout male crosses. Hybrid parr were recorded from one of the low-risk rivers (1.0%), but were present in seven out of the 10 catchments located within 38 km of salmon farms, with frequencies ranging from 0.7% to 3.1%. The results of the present survey, which represents the first extensive record of the levels of salmon-trout hybridization in Ireland, are discussed in relation to the continued growth of salmon farming in this country.