Distribution of the flagellate Hexamita salmonis Moore, 1922 and the microsporidian Loma salmonae Putz, Hoffman and Dunbar, 1965 in brown trout, Salmo trutta L., and rainbow trout, Salmo gairdneri Richardson, in the River Itchen (U.K.) and three of its fish farms

The occurrence of Hexamita salmonis Moore, 1922 and Loma salmonae Putz, Hoffman and Dunbar, 1965 was investigated at 10 sites on the R. Itchen (five for brown trout only, three for rainbow trout only, and two for both brown trout and rainbow trout) and at three of its nine fish farms (two for rainbow trout, one for brown trout). Hexamita salmonis was recorded in brown trout from three river sites and the farm, and in rainbow trout from both farms and four river sites. Prevalence of Hexamita salmonis in farmed rainbow trout was higher than in farmed brown trout and was consistent with the former species being more susceptible to infection. H. salmonis was at significantly higher prevalence in rainbow trout from farm no. 5 than farm no. 2 for three size classes of fish. In wild brown trout and feral rainbow trout, the highest prevalences of H. salmonis were recorded at sites in the vicinity of farm no. 2. This distribution was consistent with an area of naturally high infection levels, and with infected fish unintentionally released from farm no. 2 serving as a source of infection, the infection subsequently becoming established in the river fish. Loma salmonae was recorded in wild brown trout and in rainbow trout from both farms. This appears to be the first recording of this parasite from British salmonids and also the first recording of the parasite from brown trout. The distribution of the parasite (particularly the prevalence being higher at farm no. 2 than farm no. 5) was consistent with it being introduced into the R. Itchen via rainbow trout from farm no. 2 (and probably no. 3) much of whose stock derived from imported Californian 'Shasta' rainbow trout.     

Comparative susceptibility of rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta to Myxobolus cerebralis, the cause of salmonid whirling disease.

The susceptibility of rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta to Myxobolus cerebralis, the cause of salmonid whirling disease, was assessed following dosed exposures to the infectious stages (triactinomyxons). Parallel groups of age-matched brown trout and rainbow trout were exposed to 10, 100, 1000 or 10,000 triactinomyxons per fish for 2 h and then placed in aquaria receiving single pass 15 degrees C well water. Severity of infection was evaluated by presence of clinical signs (whirling and/or black tail), prevalence of infection, severity of microscopic lesions, and spore counts 5 mo after exposure. Clinical signs of whirling disease, including a darkened caudal region (black tail) and radical tail chasing swimming (whirling), occurred first among rainbow trout at the highest dose at 6 to 7 wk post exposure. Black tail and whirling occurred among rainbow trout receiving 1000 and 100 triactinomyxons per fish at 8 to 9 wk post exposure. Only 1 of 20 fish had a black tail among rainbow trout receiving 10 triactinomyxons per fish, although 30% of the fish were infected at 5 mo post exposure. Black tails were observed in brown trout at 1000 and 10,000 triactinomyxons per fish beginning at 11 and 7 wk post exposure, respectively. There was no evidence of the tail chasing swimming (whirling) in any group of brown trout. The prevalence of infection, spore numbers, and severity of microscopic lesions due to M. cerebralis among brown trout were less at each exposure dose when compared to rainbow trout. Infections were found among rainbow trout at all doses of exposure but only among brown trout exposed to doses of 100 triactinomyxons per fish or greater. Risk of infection analyses showed that rainbow trout were more apt to be infected at each exposure dose than brown trout. Spore counts reached 1.7 x 10(6) per head among rainbow trout at the highest dose of exposure compared to 1.7 x 10(4) at the same exposure dose among brown trout. Spore numbers increased with dose of exposure in rainbow trout but not in brown trout. As microscopic lesion scores increased from mild to moderate, spore numbers increased in rainbow trout but not brown trout. The mechanisms by which brown trout resist infections with M. cerebralis were not determined. Cellular immune functions, including those of eosinophilic granular leukocytes that were more prominent in brown trout than rainbow trout, may be involved.     

Active biomonitoring with brown trout and rainbow trout in diluted sewage plant effluents

Brown trout Salmo trutta populations of numerous Swiss rivers are declining. Sewage plant effluents are discussed as a possible cause. To investigate the influence of sewage plant effluents, brown trout as well as rainbow trout Oncorhynchus mykiss were exposed to 10% diluted waste water over a period of 12 months. The effects were compared to those on trout kept in commercial tap water. The mortality rate was low and no pathogenic bacteria or viruses were recorded in exposed and tap-water animals. Parasitological examination revealed a mild infestation with Gryodactylus sp. in all groups. Macroscopically and histologically, only minor changes in gills, skin, and kidney of exposed animals were found when compared to fish kept in tap water. Degenerative and inflammatory reactions in the liver of exposed animals were the most prominent findings. Several brown trout caught in the River Langete showed marked proliferative, degenerative and inflammatory lesions of gills, liver, and kidney. The results do not suggest that waste-water effects would explain the decrease of fish populations. However, it is conceivable that the effluents in combination with other factors in the river enhance the development of changes.     

Increase of Metallothionein-immunopositive Chloride Cells in the Gills of Brown Trout and Rainbow Trout After Exposure to Sewage Treatment Plant Effluents

Metallothionein, a biomarker of exposure and toxicity of heavy metals, has been detected in the gills of brown trout (Salmo trutta fario L.) and rainbow trout (Oncorhynchus mykiss Richardson) by means of immunohistochemistry. A very prominent labelling of chloride cells was found after exposure to diluted sewage plant effluents. No significant increase was observed in either the number of labelled cells or their labelling intensity after exposure to water of a polluted river compared to fish kept in tap water. These results do not correlate with findings of a histopathological study, suggesting that the metal levels at the sewage treatment plant were too low to produce gross histopathology. A comparison between the species indicated that the rainbow trout showed a generally higher metallothionein expression than the brown trout.     

Morphological organ alterations and infectious diseases in brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss exposed to polluted river water

Poor water quality is discussed as a major factor causing a decline of brown trout populations in Swiss rivers. For our study we have chosen a river in the Swiss midlands, where the brown trout population has decreased dramatically during the last 10 yr and where feral fish have shown distinctive pathological alterations. The objective of our study was to investigate whether river water may be responsible for impaired fish health leading to an increased mortality in the river. In an active monitoring program, groups of brown and rainbow trout were exposed to polluted river water for 24 mo. Fish held in tap water served as a reference. Mortality, macroscopic and histopathologic changes, and infectious agents were investigated. Compared with the reference group, high mortality rates and severe pathological alterations of the inner organs were observed in fish held in river water. Especially gills, liver and kidney of these fish showed significantly higher changes than fish from tap water. These changes were dominated by degenerative and inflammatory reactions. Additionally, several infectious agents were diagnosed in fish exposed to river water. The most important findings were furunculosis and proliferative kidney disease. Brown trout seemed to be more sensitive than rainbow trout to environmental stress and infectious agents.     

Mass-marking of brown trout (Salmo trutta L.) larvae by alizarin: method and evaluation of stocking

This study describes otolith marking of brown trout (Salmo trutta L.) larvae by immersion in different solutions of alizarin red S (ARS). The best results were obtained after marking with ARS at a concentration of 150 mg L^?1. To evaluate the efficiency of stocking with brown trout fry, 10 000 20‐day‐old larvae were marked in years 2002 and 2003 with ARS and released 2 weeks later into sections of a river with natural brown trout reproduction. Electro‐fishing surveys carried out 2 months after stocking in 2002 revealed that only 4.8% of all caught young‐of‐the‐year trout originated from stocking; in 2003 the percentage was 8.9%. Based on the substantial natural reproduction and the low ratio of stocked to wild trout, it was recommended to discontinue stocking.     

Overwinter feeding in rainbow trout

The contents of the stomachs of 38 rainbow trout stocked in Llyn Alaw, Anglesey, in August 1969 and caught between October 1969 and February 1970 were analysed. The fish were actively feeding on the bottom fauna throughout the winter and 21 of the stomachs were full or distended. The mean volume of the contents of the stomachs was 2–8 times greater than that of the contents of stomachs of similarly sized brown trout caught at the same time.     

Natural reproduction of anadromous rainbow trout in Norway

Six naturally reproduced 0+ rainbow trout were caught in a Norwegian river in 1994. This is the second report and first recorded sample of descendants from anadromous parents found in a Norwegian river. If this is not a unique event, the finding bodes ill for future conservation of native anadromous Salmonids.     

Relaciones tróficas de Trucha Marrón,Salmo fario Linné, y Trucha Arco Iris,Salmo gairdneri Richardson, (Osteichthyes,Salmoniformes) en un embalse norpatagónico

A detailed comparative study of the diet of wild brown and rainbow trout in the Ramos Mexía reservoir, Neuquén Province, during 1982 and 1983 was made. The seasonal composition of both diets was determined, calculating number, volume, and occurrence. The feeding habits of the brown trout are based mainly on perch, silverside, galaxiids, molluscs, and crustaceans; for rainbow trout, galaxiids, terrestrial insects, ostracods, crustaceans, and molluscs were recorded. Fishes were mainly caught throughout the year by brown trouts, while rainbow trouts capture mostly ostracods during spring and galaxiids throughout the rest of the year. A decrease in the percentage of galaxiids, insects, and molluscs was observed with the increase of brown trout size, while increasing that of silverside. Rainbow trouts also showed an increase in perch and galaxiids predation with size, with a decrease in crustaceans and insects. A diet overlap was recorded only in spring. Trouts occupy both the 3rd and 5th level in the trophic chain, being very active ichthyofagous predators.     

The status of brown and rainbow trout, Salmo trutta and S. gairdneri as hosts of the acanthocephalan, Pomphorhynchus laevis

The status of brown and rainbow trout as hosts of Pomphorhynchus laevis was studied in the field and by means of laboratory investigations. Field data indicated that rainbow trout might belong to the group of preferred hosts of P. laevis , whereas brown trout belonged to the group in which the parasite achieved less than optimal growth and maturation. This was confirmed by laboratory infections. In rainbow trout P. laevis attained up to three times the growth rate in brown trout and maturation occurred whereas in brown trout establishment was lower, growth slower and no parasites matured. Changes in the behaviour of infected Gammarus pulex induced by the presence of P. laevis cystacanths were such as to render the shrimps more vulnerable to predation by trout and other surface and mid-water feeding fish, and selective predation upon infeged G. pulex by fish was demonstrated. nvestigations into the stimuli necessary for eversion of cystacanths of P. laevis revealed that the most important factor was a non-specific component of bile, and it was concluded that cystacanths were likely to evert in any species of fish. Recognition of the different status of brown and rainbow trout as hosts of P. laevis still fails to explain some peculiarities in the distribution of the parasite in the British Isles, where in Britain it occurs in trout in only one river but in Ireland in all rivers throughout the country. It is suggested that the Irish parasites may constitute a different strain of P. laevis , since they use a different species of intermediate host and are better able to survive in brown trout.     

Limestone Treatment of Whetstone Brook, Massachusetts. II. Changes in the Brown Trout (Salmo trutta)and Brook Trout (Salvelinus fontinalis)Fishery

Density, age structure, and growth rates of wild brook trout (Salvelinus fontinalis)and brown trout (Salmo trutta)in Whetstone Brook in northcentral Massachusetts were monitored for 4 years before and 3 years during limestone treatment to mitigate acidic conditions. The population density of brook trout increased significantly during treatment. Liming did not have any significant effects on the growth rates of brook trout or brown trout. Actual survival rates of brook trout and brown trout were not calculated due to the low density of both species, but more older individuals of both species were captured during the treatment period. Fulton condition factors (an index of fish condition) increased significantly for both brook trout and brown trout during treatment. Seven-day in situ bioassays of brown trout and rainbow trout demonstrated that liming improved the chemical environment for fish in Whetstone Brook. During a pretreatment bioassay in 1987, 100% rainbow trout mortality was observed at both the control and treatment stations in Whetstone Brook. Brown trout mortality was 67% in the control station and 70% in the treatment station. The pH during the 1987 bioassay averaged 4.90 in the control station and 4.99 in the treated station. During a bioassay conducted in 1990 after treatment began, rainbow trout mortality was 100% in the control station and 0% in the treatment station. Brown trout mortality was 17% in the control station and 0% in the treatment station. The pH during the 1990 bioassay averaged 5.23 in the control station and 6.60 in the treatment station. Analysis of total aluminum in the gills of fish from the 1990 bioassay revealed higher levels in fish from the control station than in those from the treatment station.     

Habitat shifts in rainbow trout: competitive influences of brown trout

Summary We compared habitat use by rainbow trout sympatric (three streams) and allopatric (two streams) with brown trout to determine whether competition occurred between these two species in the southern Appalachian Mountains. We measured water depth, water velocity, substrate, distance to overhead vegetation, sunlight, and surface turbulence both where we collected trout and for the streams in general. This enabled us to separate the effects of habitat availability from possible competitive effects. The results provided strong evidence for asymmetrical interspecific competition. Habitat use varied significantly between allopatric and sympatric rainbow trout in 68% of the comparisons made. Portions of some differences refelected differences in habitats available in the several streams. However, for all habitat variables measured except sunlight, rainbow trout used their preferred habitats less in sympatry with brown trout than in allopatry if brown trout also preferred the same habitats. Multivariate analysis indicated that water velocity and its correlates (substrate particle size and surface turbulence) were the most critical habitat variables in the interaction between the species, cover in the form of shade and close overhead vegetation was second most important, and water depth was least important.     

Vertical distribution and substrate preference of brown trout in a littoral zone

Synopsis We studied vertical distribution, substrate preference and food choice of brown trout, Salmo trutta, from benthic gillnet catches at four littoral sampling locations in a Norwegian hydroelectric reservoir. The sampling locations had different bottom substrates; at one location the bottom substrate consisted of sand, while at the other three, substrates consisted of stones ranging 2–5 cm, 10–30 cm and 30–150 cm in diameter, respectively. Small-sized (< 160 cm) and intermediate-size (164–269 mm) brown trout were mainly caught close to the bottom (0–0.5 m above). Small-sized brown trout were caught in the highest frequency at the location with substrate consisting of 10–30 cm large stones. Intermediate-sized brown trout were also caught in highest frequency at this location, but were also caught in a high frequency at the location with sandy substrate. In contrast, the catches of large-sized ( 270 mm) brown trout did not vary with distance from the bottom or with substrate coarseness. The most important food items for the brown trout were aquatic insects, surface insects, Eurycercus lamellatus and crustacean zooplankton, mainly Daphnia longispina, Bythotrephes longimanus, and Holopedium gibberum. In accordance with the differences in vertical distribution, benthic food was more important to small than to large brown trout. We argue that small brown trout stayed close to the bottom to reduce aggressive behaviour from larger specimens, and that small brown trout were therefore more dependent on benthic food items. We also argue that the observed differences in substrate preference between the size groups of brown trout is explained by variation in access to shelter, visual isolation between individuals and benthic feeding conditions between locations.     

Regional variation in the microhardness and mineralization of vertebrae from brown and rainbow trout

Regional variation in properties of vertebral bone from brown Salmo trutta and rainbow trout Oncorhychus mykiss were explored by using microhardness tests. Statistically‐significant positive correlations were identified between the microhardness of bone and its mineral content. In both brown and rainbow trout, the vertebrae from the caudal region were harder than those of the trunk region. There was a significant difference between the species; microhardness of bone from vertebrae of rainbow trout was greater than those from brown trout.     

Characteristics of the spawning migrations of brown trout, Salmo trutta L., and rainbow trout, S. gairdneri Richardson, in Great Lake, Tasmania

Upstream spawning migrations of mature brown trout, S. trutta , and rainbow trout, S. gairdneri , were studied in Liawenee Canal, Great Lake from 1949 to 1985. Brown trout migrations normally occurred from early April to mid-May and rainbow trout from late August to early November. In 1983, 16 425 brown trout and 1338 rainbow trout passed through a fixed upstream diversion trap. Brown trout spawning migrations occurred predominantly over the temperature range 6–10° C, while rainbow trout migrated predominantly over the range 5–11° C. Migrations peaked at water temperatures of 7.6°C (males) and 7.8°C (females) for brown trout, and 8.3°C (males) and 9.6°C (females) for rainbow trout. Rainbow trout migrations occurred at high flow conditions and were positively correlated with canal flow increases, while brown trout migrated under low canal flow. Mean length, weight and condition of rainbow trout of both sexes decreased significantly during migrations. Female brown trout decreased in weight and condition but not in length; male brown trout did not change in condition despite decreases in both length and weight during migrations. Overall sex ratio was 2:1 (female:male) for both species, with the relative proportion of male fish decreasing as migrations progressed. Age composition changed during migrations; dominant age classes were 3 < 4 < 5 + years for both species. Comparison of length, weight, condition and age revealed minor changes during the 37-year period 1949–1985.     

Chromosome banding in salmonid fish: nucleolar organizer regions in Salmo and Salvelinus

Chromosome banding patterns obtained by silver staining (Ag-NORs) were analyzed in three species of Salmo (rainbow, brown trout, and Atlantic salmon) and three species of Salvelinus (brook trout, lake trout, and arctic char). In rainbow trout and Atlantic salmon the Ag-NORs were found at the secondary constrictions of a single chromosome pair, while in brown trout the Ag-NORs were found on the short arms of one or two of the two longest subtelocentric or acrocentric chromosome pairs. The location of the Ag-NORs was multichromosomal in the three Salvelinus species, occurring on one or both members of four to six different chromosome pairs in different individuals. The Ag-NOR sites were on the short arms of some acrocentric pairs and at the telomeres of other acrocentric pairs and one or two metacentric pairs. Chromomycin A3 positive bands were found at the same sites as the Ag-NORs in all species. In the species with multichromosomal location of Ag-NORs, polymorphisms in the size and location of the NORs were extremely common, so that almost every individual fish had a different pattern of Ag-NOR sites.     

Changes in haematocrit values in blood samples treated with and without oxygen: a comparative study with four salmonid species

Changes in haematocrit values under lowered oxygen tension and aerobically treated blood samples of rainbow trout Salmo gairdneri (Richardson), landlocked Baltic salmon S. salar (L.), brown trout S. trutta lacustris (L.) and lake trout Salvelinus namaycush (Walbaum) have been studied in vitro . The mean haematocrit value increased during 2 h incubation under lowered oxygen tension by 32.7 ± 3.1% in rainbow trout, 28.2 ± 2.8% in landlocked Baltic salmon, 29.2 ± 5.6% in brown trout and 25.2 ± 2.8% in lake trout. During corresponding incubation with oxygen the mean haematocrit value decreased below the starting level by 18.1 ± 1.9% in rainbow trout, 18.3 ± 4.8% in landlocked Baltic salmon, 22.4 ± 0.7% in brown trout and 11.8 ± 1.7% in lake trout. Consequently, the changes in the haematocrit values were greater in the species belonging to the genus Salmo than in that of the genus Salvelinus . The increase in the haematocrit values seems to have resulted from swelling of the erythrocytes and their decrease is similarly attributable to shrinking of the cells. The reasons for this swelling, which may be complicated, and its apparent significance for haematocrit determinations are discussed.     

Temporal variations in the diet of brown trout (Salmo trutta L.) and rainbow trout (S. gairdneri R.) in Rutland Water

The diet of brown trout (Salmo trutta L.) and rainbow trout (S. gairdneri Richardson) in Rutland Water were compared during the first two fishing seasons (April–October 1977 and 1978).Fortnightly samples of approximately forty stomachs were obtained from boat and bank, rod-and-line caught trout giving a total of 1046 stomachs over the two seasons.During 1977 seasonal changes in the diet were divided into two phases; the first being a period of abundant drowned terrestrial food until June. This was followed by a period of more stable water level from July onwards when chironomid larvae and pupae were consistently the most important food items and the diversity of food also increased.In 1978 the proportion of chironomid pupae and larvae declined and they were replaced in the diet by Gammarus and Asellus.     

Strontium content of scales as a marker for distinguishing between sea trout and brown trout

Strontium was determined in trout scales from a river where it is often difficult to distinguish between sea trout and resident brown trout by coloration or other visual marks. Sr values were compared with values in scales from brown trout caught above the anadromous stretch of the same river and in scales from a river where sea trout coloration is typical. In the first river, the Sr concentration was generally low, and as a mean only 50 ppm higher in scales from individuals classified as sea trout from the anadromous stretch than in brown trout scales from the upper stretch. There was no consistency between fish coloration and Sr concentration in scales from presumed sea trout on the anadromous stretch. Individuals with a typical sea trout coloration could have a lower concentration of Sr than individuals that were classified as uncertain sea trout by coloration. Fish weight did not seem to influence Sr levels. The mean Sr concentration in scales from the typical sea trout colored population in the second river was 2.8 times higher than that of the anadromous part of the first river. The high variability of Sr concentration in sea trout scales may be explained by differences in individual and population life history. The Sr levels reflect differences in saltwater exposure, either expressed by length of stay or concentration of salt in marine habitats. The study has shown that fish coloration is an inadequate mean to distinguish between resident and migratory trout. Nor is Sr determination of scales alone sufficient, because of low inter-group and high intra-group variability in some rivers. However, Sr values can give valuable information on individual and population migration on a large scale.     

Modelling expected trout ranges under current and future water temperature regimes in the Eastern Cape,South Africa

Different values have resulted in conflicts between anglers and conservation lobbies in the management of trout in South Africa. Key to the conflict is the demarcation of boundaries to areas in which brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss currently occur, or are likely to establish following stocking for angling. To provide a longer-term perspective on these areas, we developed models to link salmonid biological thermal thresholds to elevation. These, when applied spatially using a digital elevation model with a probability of occurrence model, provided the basis for estimating potentially available thermal habitat for these two cold water species. Here, we acknowledge that other variables (stocking history; river connectivity) also play a role in understanding trout distributions. Using a simple scenario of an increase in mean daily water temperatures of 2 °C, we demonstrated that both brown and rainbow trout are likely to exhibit considerable range reductions in the future. Because it is possible that these range restrictions will result in an increasing desire to introduce trout into areas above their current distribution limits for the maintenance of angling opportunities, conservation managers should prioritise these areas, with management interventions seeking to understand what will help to limit introductions.