Introduction and establishment of Gyrodactylus salaris Malmberg, 1957, on Atlantic salmon, Salmo salar L., fry and parr in the River Vefsna, northern Norway

Abstract. Gyrodactylus salaris was probably introduced into the River Vefsna, a large salmon river in northern Norway, by stocking of Atlantic salmon smolts from infected hatcheries in 1975 and 1977. An outbreak of G. salaris among salmon parr was observed in 1978. During the following 2 years, the parasite spread throughout the entire watercourse. There was a rapid rise in prevalence rate and degree of infection. The density of salmon parr decreased from a high level before the Gyrodacrylus outbreak to close to zero after the outbreak. Salmon parr mortality is probably associated with an increase in the parasite burden. Infected fish appear to survive for slightly more than 1 year and this time period is sufficient for transmission of the parasite to the next year class. The total catch of salmon ascending the river has also greatly decreased in recent years.     

The influence of Gyrodactylus salaris Malmberg 1957 (Monogenea) on the epidermis of Atlantic salmon,Salmo salar L., and brook trout,Salvelinus fontinalis (Mitchill): experimental studies

The effect of Gyrodactylus salaris on the epidermal structure of hatchery-reared brook trout parr and Norwegian Atlantic salmon parr was evaluated. Both species were initially susceptible to this parasite, but whereas populations on salmon increased until the host died, brook trout responded to, and eventually eliminated, their infections. Salmon skin samples taken 14 days p.i. showed a reduced mucous cell concentration (less than 1500 cells mm^–2 on the head compared with 2000 cells mm^–2 in controls; the same trend was also seen in other sites) and the epidermis was thinner (48 μm compared with 60 μm on pectoral fins; similar trend seen in other sites) than in uninfected controls kept under identical conditions. Brook trout skin samples were taken 50 days p.i., when the hosts had responded to, and almost eliminated, their infections. No change was then observed in mucous cell density, while the number of epidermal cell layers and the epidermal thickness of brook trout had increased slightly following infection. These results are related to the nature of the host response, and the thinning of the epidermis and loss of mucous cells may in some way be related to the inability of Norwegian salmon parr to respond to the parasite.     

Water temperature regimes affect cataract development in smolting Atlantic salmon, Salmo salar L.

The effect of fluctuation in water temperature during the freshwater rearing stage on cataract development in Atlantic salmon was investigated. Presmolts were exposed to either constant low or high temperatures, or high temperature with cold-water fluctuations. Temperature fluctuations caused the fastest growth rate and most severe cataracts, whilst low temperature resulted in slow growth and minor cataracts. The fish were then individually marked and maintained together in a seawater net pen for 7 months. Cataract progressed in all groups, fish from the low temperature group showed the fastest subsequent growth, and developed cataracts to the same degree as fish from the other groups. There were no statistically significant differences in selenium-dependent glutathione peroxidase levels in pooled samples of aqueous humour, lens and plasma from fish with or without cataract, except from the last freshwater samples, where cataractous fish showed lower plasma activity. Plasma cortisol and glucose levels did not differ, nor did whole eye riboflavin levels. Higher concentrations of some free amino acids in whole eyes were found in fish with cataract. This study shows that fluctuation in water temperature may cause both increased growth rate and cataract development in Atlantic salmon and that cataract development initiated in the freshwater rearing phase continues after transfers to seawater.     

Further development of bithionol therapy as a treatment for amoebic gill disease in Atlantic salmon, Salmo salar L.

This study examined the efficacy of bithionol as a prophylactic or therapeutic oral treatment for Atlantic salmon (AS), Salmo salar , affected by amoebic gill disease (AGD). Furthermore, it explored the interaction of bithionol oral therapy with the current standard treatment (a freshwater bath for at least 3 h). The efficacy of three medicated feeds was determined in the trial by feeding AGD-affected AS at 1% body weight (BW) day⁻¹ either oil coated commercial feed (control) or prophylactic and therapeutic bithionol at 25 mg kg⁻¹ feed. Feeding commenced 2 weeks prior to exposure to Neoparamoeba spp. at 300 cells L⁻¹ and continued for 49 days post-exposure (PE). Bithionol when fed as a 2-week prophylactic or therapeutic treatment at 25 mg kg⁻¹ feed delayed the onset of AGD pathology and reduced the percentage of gill filaments with lesions. Administration of a 3-h freshwater bath at 28 days PE significantly reduced amoeba numbers to a similar level across all treatments; in contrast, gross gill score and percent lesioned filaments were reduced to different extents, the control having a significantly higher score than both bithionol treatments. Following the freshwater bath, clinical signs of AGD increased at a similar level across all treatments, albeit controls were significantly higher than the bithionol treatments immediately following freshwater treatment. This study demonstrated that bithionol at 25 mg kg⁻¹ feed, when fed as a 2-week prophylactic or a therapeutic treatment, delayed and reduced the intensity of AGD pathology and warrants further investigation as a treatment for AGD-affected AS.     

Preliminary success using hydrogen peroxide to treat Atlantic salmon,Salmo salar L., affected with experimentally induced amoebic gill disease (AGD)

Currently, the only effective and commercially used treatment for amoebic gill disease (AGD) in farmed Tasmanian Atlantic salmon is freshwater bathing. Hydrogen peroxide (H₂O₂), commonly used throughout the aquaculture industry for a range of topical skin and gill infections, was trialled in vitro and in vivo to ascertain its potential as an alternative treatment against AGD. Under in vitro conditions, trophozoites of Neoparamoeba perurans were exposed to three concentrations of H₂O₂ in sea water (500, 1000 and 1500 mg L^?1) over four durations (10, 20, 30 and 60 min) each at two temperatures (12 and 18 °C). Trophozoite viability was assessed immediately post‐exposure and after 24 h. A concentration/duration combination of 1000 mg L^?1 for >10 min demonstrated potent amoebicidal activity. Subsequently, Atlantic salmon mildly affected with experimentally induced AGD were treated with H₂O₂ at 12 and 18 °C for 15 min at 1250 mg L^?1 and their re‐infection rate was compared to freshwater‐treated fish over 21 days. Significant differences in the percentage of filaments affected with hyperplastic lesions (in association with amoebae) and plasma osmolality were noted between treatment groups immediately post‐bath. However, the results were largely equivocal in terms of disease resolution over a 3‐week period following treatment. These data suggest that H₂O₂ treatment in sea water successfully ameliorated a clinically light case of AGD under laboratory conditions.     

Population decline in the endemic Atlantic salmon (Salmo salar) in Kapisillit River,Greenland

Atlantic salmon, Salmo salar L., is found throughout the North Atlantic, with thousands of rivers having spawning populations. In Greenland, spawning is limited to one river in West Greenland, the Kapisillit River (64?N), and the salmon are limited to the lower few kilometres of the system. Using mark–recapture, it was estimated the parr population was 5,953 individuals, and that the population size has declined by 52% since 1959. In spite of this decline, parr density remains high, being between 0.26 and 0.62 parr/m². Using a historical age‐length key, an estimated minimum of 635 smolt will have descended to the sea in 2017. These will be caught in a fishery currently subject to no regulatory measures and fishing remains the most likely driver of the population decline. The genetically distinct population is endemic to Greenland, and managers should implement measures to conserve this genetic integrity and local biodiversity.     

The effect of light on the settlement of the salmon louse, Lepeophtheirus salmonis, on Atlantic salmon, Salmo salar L

The salmon louse, Lepeophtheirus salmonis, is an ectoparasitic copepod that infests both wild and farmed salmonid fish. Salmon lice are a major disease problem in the farming of Atlantic salmon, Salmo salar L., and the possibility of salmon lice playing a role in the decline of wild anadromous stocks has also been raised. Lepeophtheirus salmonis can detect a range of stimuli (pressure/moving water, chemicals and light) in the external environment. However, the response thresholds to various stimuli, and the spatial and temporal scales over which they operate in the context of host location, are largely unknown. In this context, we attempted to determine whether salmon lice copepodids settle onto hosts more effectively, or at different locations on the fish's body, under different qualities of light. Lice settlement trials were conducted under three lighting conditions; L1: unpolarized under ultraviolet A (UVA)-through visible; L2: unpolarized without UVA (control); L3: 100% linearly polarized without UVA. A dark control was also conducted. No statistically significant difference in lice settlement was found. While changes in light intensity are involved in host detection at spatial scales on the order of metres, the results presented here suggest that it is not the primary sensory modality underlying host location at smaller spatial scales (cm to mm).     

An experimental investigation on aspects of infectious salmon anaemia virus (ISAV) infection dynamics in seawater Atlantic salmon, Salmo salar L.

This study investigated infection dynamics of infectious salmon anaemia virus (ISAV) by conducting two experiments to examine minimum infective dose and viral shedding of ISAV. In terms of minimum infective dose, the high variability between replicate tanks and the relatively slow spread of infection through the population at 1 × 10¹ TCID₅₀ mL⁻¹ indicated this dose is approaching the minimum infective dose for ISAV in seawater salmon populations. A novel qPCR assay incorporating an influenza virus control standard with each seawater sample was developed that enabled the quantity of ISAV shed from infected populations to be estimated in values equivalent to viral titres. Viral shedding was first detected at 7 days post-challenge (5.8 × 10⁻² TCID₅₀ mL⁻¹ kg⁻¹) and rose to levels above the minimum infective dose (4.2 × 10¹ TCID₅₀ mL⁻¹ kg⁻¹) on day 11 post-challenge, 2 days before mortalities in ISAV inoculated fish started. These results clearly demonstrate that a large viral shedding event occurs before death. Viral titres peaked at 7.0 × 10¹ TCID₅₀ mL⁻¹ kg⁻¹ 15 days post-infection. These data provide important information relevant to the management of ISA.     

Tissue vitamin E concentrations during smoltification and seawater transfer in farmed Atlantic salmon, Salmo salar L.

Plasma and liver vitamin E concentrations in a population of farmed Atlantic salmon, Salmo salar L., fed commercial diets were measured by high-performance liquid chromatography. In healthy fish fed diets containing 160-210 mg kg^?1all-racα-tocopheryl acetate, vitamin E accumulated in the liver and plasma. Over a 20-month period, beginning 4 months prior to seawater transfer, mean vitamin E concentrations ranged from 51 to 754 μg g^?1 wet tissue in liver and from 7 to 68 μg mL^?1 in plasma. In liver, a sharp increase in vitamin E concentrations was recorded between 6 and 10 weeks after transfer. In plasma a similar increase occurred between 4 and 5 months post transfer. Total lipid and polyunsaturated fatty acid concentrations were also measured in these tissues. Liver total lipid concentrations fell during the period of smoltification, but there was no relationship between vitamin E and either total lipid or total polyunsaturated fatty acid concentrations in liver or plasma.     

Impact of stocked Atlantic salmon (Salmo salar L.) on habitat use by the wild population

Laffaille P. Impact of stocked Atlantic salmon (Salmo salar L.) on habitat use by the wild population.
Ecology of Freshwater Fish 2011: 20: 67–73. © 2010 John Wiley & Sons A/S Abstract – We investigated the summer habitat occupied by populations of young‐of‐the‐year wild and stocked (farmed populations released into the native range) Atlantic salmon under allopatric and sympatric conditions. Under allopatric conditions, farmed and wild salmon occupied habitats with the same characteristics. The salmon preferentially occupied the riffle areas. However, under sympatric conditions, the fish occupied meso‐ and micro‐habitats with different characteristics. Wild salmon avoided habitats used by farmed salmon and preferred glide areas with considerable vegetation cover. This study suggests that differences in the pattern of habitats used by young Atlantic salmon were both size‐ and origin‐dependent and may result from intra‐species competition between farmed and wild populations. Given that stocking with farmed Atlantic salmon is carried out intensively to enhance recreational angling or to conserve salmon populations, this study warns that this can have a negative impact on the extant wild Atlantic salmon population.     

Effect of hydrogen peroxide as treatment for amoebic gill disease in Atlantic salmon (Salmo salar L.) in different temperatures

Amoebic gill disease (AGD) is a pathogenic disease in salmonids caused by Neoparamoeba perurans. Treatment of AGD infection has been through freshwater bathing of the fish. However, as the availability of fresh water is often limited, hydrogen peroxide has been introduced as an alternative treatment. This study investigated the effect of hydrogen peroxide as treatment for AGD‐infected salmon (Salmo salar L.,) at different seawater temperatures and hydrogen peroxide dosages. In total, 600 fish were challenged with N. perurans and the severity of the AGD infection was measured using a gill score scale. After challenge and disease development, the fish were distributed into 12 tanks. The treatment was performed at different seawater temperatures (8°C, 12°C, 17°C) using different hydrogen peroxide doses. Each temperature included an untreated control group. Linear models were used to analyse gill score. A significant effect of treatment was found (?0.68 ± 0.05) regardless of dose and temperature, suggesting that hydrogen peroxide was effective in treating AGD. When the model included dose, a negative linear relationship between dose and gill score was found. The study proved that treatment of AGD with hydrogen peroxide was successful, as gills partially recovered following treatment and further disease development was delayed.     

Comparison of population dynamics of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in a small tributary of the River Scorff (Brittany, France)

The population dynamics of Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) were compared in a small tributary of the River Scorff (Brittany, France) from spawning time to the beginning of the third growing season. The spawning and fry emergence of the two species took place at approximately the same time. In the first autumn, the densityof 0+ juveniles and settling rate from the egg stage were much higher in trout than in salmon. The emigration rate from 0+ population was much higher in trout than in salmon. The size of resident and migrating fish was always smaller in salmon than in trout, whatever the age. The low level of salmon production in the brook, compared with trout, was the result of low survival from egg to 0+ stage in autumn, combined with the small proportion of juveniles migrating after the first growing season. This was not compensated by a high number of migrants the next year. The role of physical habitat, inter-and intraspecific competition, predation and migration dependence on size and early sexual maturity is discussed. Indications are that small tributaries of the type studied are of great value for recruitment in trout but not very productive for juvenile salmon.     

Effect of marine Eubothrium sp. (Cestoda: Pseudophyllidea) on the growth of Atlantic salmon, Salmo salar L.

Atlantic Salmon, Salmo salar L., experimentally infected with marine Eubothrium sp. were kept together with uninfected salmon in the laboratory for 11 months in two tanks, 80 infected and 80 uninfected in each tank. The infected fish had a reduced growth rate compared with the uninfected fish. Significant differences in growth between infected and uninfected fish were not observed until several months post-infection. There was no correlation between the number of Eubothrium sp. and fish weight, indicating that even low intensity alters the growth rate of the salmon. The cestode had the same effect on both sexes of salmon. Haematocrit level was found to be significantly lower in infected compared with uninfected salmon in one of the samples during the experiment.