Modelling the effects of capture and sea lice [Lepeophtheirus salmonis (Krøyer)] infestation on the cortisol stress response in trout

Measuring baseline cortisol in wild salmonids is problematic because of the stress induced by most capture methods. The cortisol response to different periods of confinement stress was modelled in a laboratory population of rainbow trout [Oncorhynchus mykiss (Walbaum)]. A quadratic model was used to predict baseline (precapture) cortisol levels in these fish. A similar quadratic response pattern of cortisol was observed in wild sea trout (Salmo trutta L.). These were captured in timed sequences on four dates by seine netting in Clew Bay, west coast of Ireland. The estimated precapture cortisol for these fish indicated higher levels soon after migration to sea and a positive correlation with the level of infestation by Lepeophtheirus salmonis (Krøyer). Curvilinearity of the quadratic model was correlated with levels of lice infestation, indicating an enhanced cortisol response to confinement stress during capture when parasitic abundance was high. Estimated precapture cortisol for individual sea trout was positively correlated with numbers of lice although the linear regression had low predictive power. The method provides, for the first time, a means to estimate baseline cortisol levels in wild salmonids, and addresses one of the most difficult problems in working with wild fish. Keywords: Cortisol, trout, Lepeophtheirus salmonis (Krøyer), capture, quadratic model     

Hybrids between Salmonidae species. Hatchability and growth rate in the freshwater period

In 1972 a hybridization experiment with salmonids was initiated at the Fish Breeding Experimental Station, Sunndalsøra. The purpose of this experiment was to study the viability, performance traits and the fertility of hybrids. Hybrids were produced from the following members of the salmon family: Salmon (Salmo salar), sea trout (Salmo trutta), brown trout (Salmo trutta), rainbow trout (Salmo gairdneri) and arctic char (Salvelinus alpinus). All crosses attempted, except those involving the rainbow trout, were relatively successful. Considerable differences in the hatching rates of the different hybrids were observed. All crosses with the arctic char were heavier than the better pure bred specimens at 11 months. In all other crosses the hybrids were lighter than the purebred specimens. The experiment is continuing, it is evident that more research is necessary before the potential value of these hybrids for fish farming can be properly assessed.     

Cold-branding and fin-clipping for marking of salmonids

Identification of individual fish is important under many research circumstances and is essential in breeding and selection work. Results are presented from the marking system used at the Norwegian Research Station for Salmonids. Trained personnel and good light conditions are necessary to obtain a high percentage of correct identifications. The best result obtained was 99% identification 22–28 months after marking. It is estimated that 1.4% of the fish read were incorrectly identified. The frequency of misidentified fish was higher when different cold-branded symbols were included in the system, increasing to a mean of 8% for salmon and 15% for rainbow trout.Cold-branding using five different symbols at two of six different positions on the fish gives 600 permutations when used in conjuction with clipping of the adipose and right or left pelvic fins. A satisfactory degree of correct identification is possible for use in a programme of selection in salmonids.     

A riverscape perspective of Pacific salmonids and aquatic habitats prior to large‐scale dam removal in the Elwha River,Washington, USA

Abstract Dam removal has been increasingly proposed as a river restoration technique. In 2011, two large hydroelectric dams will be removed from Washington State’s Elwha River. Ten anadromous fish populations are expected to recolonise historical habitats after dam removal. A key to understanding watershed recolonisation is the collection of spatially continuous information on fish and aquatic habitats. A riverscape approach with an emphasis on biological data has rarely been applied in mid‐sized, wilderness rivers, particularly in consecutive years prior to dam removal. Concurrent snorkel and habitat surveys were conducted from the headwaters to the mouth (rkm 65–0) of the Elwha River in 2007 and 2008. This riverscape approach characterised the spatial extent, assemblage structure and patterns of relative density of Pacific salmonids. The presence of dams influenced the longitudinal patterns of fish assemblages, and species richness was the highest downstream of the dams, where anadromous salmonids still have access. The percent composition of salmonids was similar in both years for rainbow trout, Oncorhynchus mykiss (Walbaum), coastal cutthroat trout, Oncorhynchus clarkii clarkii (Richardson) (89%; 88%), Chinook salmon, Oncorhynchus tshawytscha (Walbaum) (8%; 9%), and bull trout, Salvelinus confluentus (Suckley) (3% in both years). Spatial patterns of abundance for rainbow and cutthroat trout (r = 0.76) and bull trout (r = 0.70) were also consistent between years. Multivariate and univariate methods detected differences in habitat structure along the river profile caused by natural and anthropogenic factors. The riverscape view highlighted species‐specific biological hotspots and revealed that 60–69% of federally threatened bull trout occurred near or below the dams. Spatially continuous surveys will be vital in evaluating the effectiveness of upcoming dam removal projects at restoring anadromous salmonids.     

Yersinia ruckeri—A threat not only to rainbow trout

Yersinia ruckeri (Y. ruckeri) can cause mortalities that are contributing to substantial economic losses in the rainbow trout (Oncorhynchus mykiss) aquaculture sector. Because of its most characteristic clinical signs, the disease in rainbow trout caused by this pathogen is called enteric redmouth disease. Although it is considered to affect mainly salmonids, there are reports in the available literature of isolating this bacterium from other fish species, both clinically healthy and diseased. The aim of this study was to analyse the available data concerning yersiniosis in non‐salmonid fish. The analysed data indicate that Y. ruckeri is a threat not only to rainbow trout. Some of the affected species have high commercial importance and mortalities may contribute to high economic losses. The disease symptoms may not be specific and can be different from those characteristic for enteric redmouth in trout, which may lead to misdiagnosis. Collected information indicates that infection with Y. ruckeri should be taken into account in the diagnostic procedures not only in salmonids.     

A comparative tissue distribution study of oxytetracycline in rainbow trout, Oncorhynchus mykiss (Walbaum), and chinook salmon, Oncorhynchus tshawytscha (Walbaum)

Oxytetracycline (OTC), a broad-spectrum antibiotic, is used widely to treat bacterial diseases in farmed fish. In the present study, the time course of OTC concentrations in freshwater rainbow trout, Oncorhynchus mykiss (Walbaum), and seawater chinook salmon, Oncorhynchus tshawytscha (Walbaum), were compared, tissue by tissue, after receiving a bolus dose of the antibiotic (5 mg kg^–1 or 50 mg kg^–1) intra-arterially (i.a.). The OTC concentration–time profiles of rainbow trout tissues were found to be very similar to those of the corresponding tissues in chinook salmon. Therefore, neither water salinity nor fish species seemed to play an important role in the disposition and elimination of OTC in these salmonids. In a separate experiment, rainbow trout were implanted surgically with a urinary cannula and received a single dose of OTC (50 mg kg^–1) i.a. Urine was collected from the cannula daily for 13 days. The amount of OTC excreted into the bile was found to be larger than that eliminated by the urine. These results show the similarity of OTC pharmacokinetics in freshwater rainbow trout and seawater chinook salmon and render support in using a single fish species to study the pharmacokinetics of a drug for other species in the same taxon.     

Mortality of salmonids cultured at low temperature in sea water

Brook trout, rainbow trout, and Atlantic salmon were acclimated to sea water (30‰ S) and placed during autumn in floating cages in a concrete tide pool. The salmon (mean length ca 26 cm in mid-October) grew about 2 cm before they stopped feeding in December. The brook trout (19 cm) and rainbow trout (21 cm) grew little, if any, during December. Feeding by the rainbow trout and salmon continued until temperature fell below 1°C. The brook trout fed little, if any, during December while temperature fell from 4 to 1°C. Brook trout, particularly the mature individuals, had high mortality rates from the time they were put into sea water. Blood plasma osmolalities indicated that brook trout, but not the other species, were under severe osmotic stress in sea water. Most of the fish (all three species) died in a 4-day period during early January when the water cooled to ?0.7 to ?0.8°C. A temperature of ?0.7°C is a reasonable approximation of the lower lethal temperature for these salmonids at 30‰ salinity. Based on our observations, it is not practicable to hold the above species in sea water which is likely to get colder than 1°C because of poor growth and the risk of mortality.     

Triacylglycerol-, wax ester- and sterol ester-hydrolases in midgut of Atlantic salmon (Salmo salar)

Bile salt‐dependent lipase (BSDL) is assumed to be the predominant lipid hydrolase in fish digestive tracts where it hydrolyses dietary triacylglycerols (TAG), sterol esters (SE) and wax esters (WE). BSDL is known to hydrolyse TAG at much faster rates than SE and WE in both fish and mammals. An assay for BSDL has previously been developed for rainbow trout (Oncorhynchus mykiss). However, this setup may not be valid in other fish species. Accordingly, the present study aimed at optimizing previous assays in rainbow trout for use on intestinal luminal contents of Atlantic salmon (Salmo salar L.). Crude intestinal extracts from midgut were desalted before the assay and concentrated bile salts supplemented. In general, the rank order for the degree of hydrolysis in Atlantic salmon was TAG > WE > SE. The optimal assay conditions were determined as being 100 μg protein, 125 μm lipid substrate and 20 mM bile salt (taurocholate) during the 4 h of incubation. Atlantic salmon and rainbow trout of 1500 g showed similar lipolytic activity, while salmon smolts of 300 g showed a significantly lower activity. Furthermore, the inhibition of intestinal lipase activities, especially triacylglycerol hydrolase and sterol ester hydrolase, observed in trout intestinal extracts at bile salt concentrations around 10 mm , was not observed in salmon. This could indicate that the activities in these two salmonids may display different enzyme biochemistry.     

Mortality of marine cultured rainbow trout, Oncorhynchus my kiss (Walbaum), during the summer in the Netherlands

Abstract. In northern Europe, rearing of rainbow trout, Oncorhynchus mykiss (Walbaum), in sea cages is a substantial part of fish culture. One of the major constraints for the development of this system in other parts of Europe is the unexplained mortality of fish during the summer. To investigate environmental aspects of this mortality, rainbow trout, cultured in cages in sea water, were monitored from May to October for three successive years. No evidence of bacterial involvement could be found, while water temperature and salinity strongly influence the mortality. This supports the suggestion that the mortality has been caused by patho-physiological changes due to higher metabolic requirements for osmoregulation at elevated water temperature in the saline environment which might result in a dissipation of O₂ by CO₂ on the hemoglobin.     

Sleeping disease and pancreas disease: comparative histopathology and acquired cross-protection

Sleeping disease (SD), a disease of freshwater-reared rainbow trout, Oncorhynchus mykiss (Walbaum), was histologically compared to pancreas disease (PD), a disease of sea water-reared Atlantic salmon, Salmo salar L. Some weeks after injection of kidney homogenates from SD- or PD-affected fish in rainbow trout, the fish successively developed pancreatic, heart and muscular lesions in both populations, although the SD-injected population was the most affected. An acquired protection was observed against PD and SD after an initial injection with PD- or SD-infective material. This acquired cross-protection and the similar histopathologies in PD and SD suggest the two diseases could be caused by a similar or identical agent.     

Possibility of anisakid larvae infection in farmed salmon

SUMMARY: Nematodes belonging to the family Anisakidae including Anisakis simplex and Pseudoterranova decipiens are known to cause anisakiasis when their live larvae are ingested by humans. We estimated the possibility of anisakid infection to salmonids, farmed in sea net-pens at Onagawa Bay, Miyagi, Japan, in 1992, 1998 and 1999, by direct examination of the edible muscle and examination of the contents of the alimentary canal. From direct examination of the muscle, no nematode was found in the 249 farmed coho salmon Oncorhynchus kisutch and 40 farmed rainbow trout Oncorhynchus mykiss . In contrast, third-stage larvae of Anisakis simplex were found in seven of 14 wild coho salmon caught in Russia and all the 40 wild chum salmon Oncorhynchus keta caught at Nemuro and Kesennuma in 1998. The stomach and intestines of 521 farmed coho salmon and 40 farmed rainbow trout were examined carefully for the existence of possible carrier organisms such as crustaceans, fish or squid. Such carrier organisms were not found in the stomach and intestines of farmed fish. Thus, we conclude that the possibility of anisakid infection is very low in farmed salmonids.     

Effect of Dietary Inclusion of N-Acetyl Cysteine on Mucus Viscosity and Susceptibility of Rainbow Trout, Oncorhynchus mykiss, and Atlantic Salmon, Salmo salar, to Amoebic Gill Disease

The treatment of amoebic gill disease (AGD) in cultured Atlantic salmon, Salmo salar L., using mucolytic agents has been previously reported. The agent L‐cysteine ethyl ester reduces salmonid mucus viscosity and potentially increases the flushing of the gill. In the present study, the effects of the mucolytic agent N‐acetyl cysteine (NAC) were assessed. Cutaneous mucus from rainbow trout, Oncorhynchus mykiss Walbaum, and Atlantic salmon was shown to have reduced viscosity when mixed in vitro with 100 or 200 μg/mL NAC. Saltwater‐acclimated rainbow trout and Atlantic salmon were fed an oil‐incorporated, NAC‐medicated diet (8 g NAC/kg diet) for up to 24 d and challenged with inoculation of 300 cells/L Neoparamoeba spp., the etiological agent of AGD. Control fish were fed normal oil‐coated pellets and received no NAC. NAC medication failed to reduce the severity of gill lesions associated with AGD even though the mucus viscosity from medicated fish was less than that of controls. Oral NAC medication does not appear to be an effective method for controlling AGD in salmonids despite reducing cutaneous mucus viscosity.     

Acclimation of rainbow trout to sea water

In some fish farms it is advantageous to transfer rainbow trout from fresh water to sea water at as small a size as possible. It has been shown that a relatively small reduction in salinity at the marine site can give appreciable improvements in survival rates during acclimation by smaller fish, thus rainbow trout of 15 g have routinely been put directly into sea cages at a salinity of 22% with mortality levels of from 1 to 8%.     

Comparison of growth rate in Atlantic salmon,pink salmon,Arctic char,sea trout and rainbow trout under Norwegian farming conditions

Growth rates of Atlantic salmon, pink salmon, Arctic char, sea trout and rainbow trout were compared under Norwegian farming conditions. During the juvenile, freshwater period, growth was fastest in pink salmon, followed by rainbow trout and Arctic char. Freshwater growth of sea trout and, especially, Atlantic salmon, was slow. After transfer of smolts or fingerling to sea water, Arctic char failed to survive the autumn. Sea water growth of sea trout was slow, but the three species, rainbow trout, Atlantic salmon and pink salmon, all grew rapidly through all seasons. When in sea water, rainbow trout and pink salmon were regularly attacked by vibriosis, while Atlantic salmon were rarely attacked, and sea trout never. It is concluded that, for commercial farming in Norway, rainbow trout are of value for production of fish of any size up to 3–4 kg, and pink salmon for production of small fish of 0.5–1.5 kg. Atlantic salmon is the only species suitable for production of a very large salmonid, i.e., more than 4–5 kg.     

THE EFFECT OF FISH MEAL CONTAINING DIFFERENT LEVELS OF FISH SOLUBLES ON THE GROWTH OF RAINBOW TROUT (Salmo gairdneri)

Two experiments were conducted to examine the effect of fish soluble additives in fish meal used as a protein source in dry diets to rainbow trout. Experiment I was made up by 3 dietary groups, run with 4 replicates of 100 trout on each diet. The protein sources examined were as follows: (1) presscakemeal, (2) whole meal, and (3) whole meal + lysine to the same levels as (1). The experiment was run for 12 weeks with ad libitum feeding three times a day. The results showed significantly lower (P < 0.05) weight gain for trout given presscakemeal as compared with whole meal. Experiment II evaluated 3 diets, run in triplicates with 100 trout in each aquarium. Presscakemeal (1), whole meal (2), and whole meal + extra addition of fish solubles (3) were examined as protein sources. The experiment was conducted for 16 weeks. The average weight gains were not significantly different among the dietary groups. The conclusion drawn from the experiments is that fish meal with fish soluble additives is preferred as a protein source in feed for rainbow trout.     

Are brown trout Salmo trutta fario and rainbow trout Oncorhynchus mykiss two of a kind? A comparative study of salmonids to temperature‐influenced Tetracapsuloides bryosalmonae infection

Proliferative kidney disease (PKD) of salmonids caused by Tetracapsuloides bryosalmonae causes high mortalities of wild brown trout (Salmo trutta fario) and farmed rainbow trout (Oncorhynchus mykiss) at elevated water temperatures. Here the aim was to compare the temperature‐dependent modulation of T. bryosalmonae in the two salmonid host species, which display different temperature optima. We used a novel experimental set‐up in which we exposed brown trout and rainbow trout to an identical quantified low concentration of T. bryosalmonae for a short time period (1 hr). We followed the development of the parasite in the fish hosts for 70 days. PKD prevalence and parasite kinetics were assessed using qPCR. Exposures were performed at temperatures (12°C and 15°C) that reflect an environmental scenario that may occur in the natural habitat of salmonids. T. bryosalmonae infection was confirmed earliest in brown trout kept at 15°C (day 7 post‐exposure) while, in all other groups, T. bryosalmonae was not confirmed until day 15 post‐exposure. Moreover, significantly greater infection prevalence and a faster increase of parasite intensity were observed in brown trout kept at 15°C than in all other groups. These results indicate that PKD is differentially modulated by water temperature in related host species.     

The role of Lake IJsselmeer, a closed-off estuary of the River Rhine, in rehabilitation of salmonid populations

After many technical measures and restocking activities in the River Rhine, there are now signs of salmonids being able to complete their anadromous life cycle. In Lake IJsselmeer, a former estuary of the River Rhine, the large effort associated with professional fisheries has been used to collect bycatch data on rare, migratory fish. To clarify the function of the lake for salmonids, catch data collected throughout the year between 1995 and 1999 were related to various possible migratory strategies known for these species. Species-specific differences were found in timing, length–frequency distribution and maturity stage reflecting different behaviour of Atlantic salmon, Salmo salar L. ( n =249) and sea trout, S. trutta L. ( n =3962). There was evidence that salmon exhibited traditional anadromous behaviour and use the lake only as a corridor. By contrast, sea trout appears to use the lake both as a corridor and as feeding habitat.     

Effects of Microbial Phytase Supplementation in Corn Distiller's Dried Grain with Solubles on Nutrient Digestibility and Growth Performance of Rainbow Trout,Oncorhynchus mykiss

Abstract

Two experiments were conducted to evaluate the effects of microbial phytase in corn distiller's dried grain with solubles (DDGS) on apparent digestibility coefficients (ADCs) and growth performance of rainbow trout, Oncorhynchus mykiss. In experiment 1, DDGS was supplemented with phytase at 0,300,600,900, and 1200 FTU/kg diet. A total of 180 fish (129.1±9.3 g) were stocked into six 140-L tanks with 30 fish/tank. Fish were assigned randomly to a reference diet and each of the 5 diets containing DDGS. Yttrium oxide was used as an inert marker. Results showed that ADCs in DDGS supplemented with different dosages of phytase were: dry matter, 49.1-58.6%; crude fat, 78.9-88.9%; crude protein, 80.0-91.9%; gross energy, 50.5-66.6%; minerals, -7.3-99.7%; and amino acids: 73.9 to 96.8%. In experiment 2, abasal diet containing 15% DDGS supplemented with lysine and methionine was used to determine if trace mineral supplemental levels in rainbow trout diets could be reduced if microbial phytase was supplemented. Six diets were made with trace mineral premix supplementation at 0.1, 0.08, 0.06, 0.04, 0.02, and 0%. Phytase was not supplemented in the basal diet, but supplemented at 500 FTU/kg diet in all other diets. Ten-week results showed that there were no significant differences in fish weight gain, feed conversion ratio, and survival (P < 0.05). There were no significant differences in fish body composition and apparent nutrient retention among fish fed all diets, except that fish fed a diet without trace mineral supplementation had the lowest zinc level and the highest manganese retention. Results indicated that phytase was effective in releasing most of minerals, and that trace mineral supplementation level could be reduced when phytase was used in rainbow trout diets.     

Real-time PCR detection of Parvicapsula pseudobranchicola (Myxozoa: Myxosporea) in wild salmonids in Norway

The myxozoan genus Parvicapsula contains 14 species infecting fish, some of which are known to cause severe disease in farmed and wild salmonids. Parvicapsula pseudobranchicola infections were first reported from seawater-reared Atlantic salmon, Salmo salar, in Norway in 2002 and have since then been an increasing problem. The present study describes a Taqman real-time PCR assay for specific detection of P. pseudobranchicola. The Taqman assay targets the 18S rRNA gene of P. pseudobranchicola and is able to detect as few as ten copies of the target sequence. Using the described assay, P. pseudobranchicola was detected in both farmed and wild salmonids, indicating that wild Atlantic salmon, sea trout, Salmo trutta, and Arctic char, Salvelinus alpinus, may be natural hosts of the parasite. Parvicapsula pseudobranchicola was found in samples from wild salmonids in the far south and the far north of Norway, displaying a wide geographic range of the parasite. Farmed salmonids showed P. pseudobranchicola infection levels many folds higher than that observed for wild sea trout, indicating that farmed Atlantic salmon are subjected to an elevated infection pressure compared with wild salmonids.     

Comparison of mortality and viral load in rainbow trout (Oncorhynchus mykiss) infected with infectious pancreatic necrosis virus (IPNV) genogroups 1 and 5

Infectious pancreatic necrosis virus (IPNV) is the aetiological agent of a highly contagious disease that affects farmed salmonids. IPNV isolates have been phylogenetically classified into eight genogroups, of which two are present in Chile, genogroups 1 and 5. Here, we compare the mortality rate caused by isolates from both genogroups in rainbow trout (Oncorhynchus mykiss) fry to determine if there is an association between host susceptibility and phylogenetic characterization of IPNV. Fish were challenged by immersion with one of four isolates (two for each genogroup), and mortality curves were assessed after 30 days. Viral load was measured in all mortalities and in live fish sampled at 1, 7 and 20 days post-infection. Although mortality was low throughout the challenge, differences were found between fish infected with different isolates. Both isolates from genogroup 1 caused greater cumulative mortalities than either of the isolates from genogroup 5. When combined, the overall mortality rate of fish challenged with genogroup 1 isolates was significantly higher than those infected with genogroup 5. However, viral load was lower on trout infected with genogroup 1 isolates. These results suggest that rainbow trout are more susceptible to IPNV isolates from genogroup 1 than genogroup 5.