An aeromonad epidemic in the brown trout (Salmo trutta L.)

The pathology and bacteriology of an aeromonad epidemic in the spawning population of brown trout ( Salmo trutta L.) of the freshwater Loch Leven, Kinross, Scotland, is described, together with estimates of its quantitative effect on the stock of trout.     

Initial feeding in migratory brown trout (Salmo trutta L.) alevins

Initial feeding of brown trout was investigated under laboratory conditions. Fifty per cent feeding occurred when yolk constituted approximately 31% of total alevin dry weight, and feeding rate was positively correlated to developmental stage. The possible ecological implications of initial feeding are discussed.     

Effects of substrate embeddedness on juvenile brown trout (Salmo trutta L.) growth and behaviour

Young salmonids may use substratum as hiding stations and/or shelter and they depend on invertebrates, which develop on substratum, for their feeding. For several decades, human activities have contributed to increase siltation in streams, and negative consequences on trout production have sometimes been highlighted. In the research devoted to the understanding of that negative effect, most studies have focused on embryo‐larval survival, and consequences of substrate embeddedness on later stages have rarely been investigated. In the present work we attempt at studying the impact of embeddedness on brown trout juveniles. In an experimental channel, trout growth was compared in embedded and non‐embedded sections. Growth was reduced with embeddedness due to change in trophic conditions and/or in habitat. To investigate the direct role of substratum for fish, trouts behaviour was observed from an under water observation room in two cages offering embedded and non‐embedded substrate conditions but similar trophic conditions. Competition appeared heavier in the embedded cage where dominated fishes stayed almost motionless. The effect of substratum quality on intra‐specific competition is discussed in relation with visual isolation and territory size.     

Electrophoretic variation in six populations of brown trout (Salmo trutta L.)

Starch gel electrophoretic studies of 16 enzymes encoded by 34 Loci were performed on six brown trout populations. One new polymorphism is described at the Pmi-2 locus. Breeding data were analysed for both single and joint segregation of six loci: Aat-1, Cpk-1, G3p-2, Mdh-2, Mdh-3, and Pmi-2. All the loci are shown to segregate in simple mendelian ratios and one nonrandom joint segregation was observed. The polymorphism level, heterozygosities, and genetic distances were estimated and compared with those reported in other studies on brown trout and closely related salmonid species. The polymorphism level (25%) and average heterozygosity (9%) were high. Significant genetic distances were observed, but the average degree of differentiation between populations appeared to be small (9% of the total heterozygosity).     

New biochemical genetic markers in the brown trout (Salmo trutta L.)

Three 'new' polymorphic loci in the brown trout (Salmo trutta L.) was detected using starch gel electrophoresis. Evidence for simple Mendelian inheritance at all three loci are presented.     

Seasonal changes in the epidermis of the brown trout Salmo trutta (L.)

A sexual dimorphism was found in the epidermis of hatchery-reared brown trout. The epidermis of both male and female fish undergoes rhythmical changes in thickness during successive spawning cycles. For most of the year the male has a significantly thicker epidermis than the female. During the spawning period (December and January) there is a highly significant drop in the concentration of mucous cells in the epidermis of the male brown trout. These findings are discussed in the light of our knowledge of the skin structure in other salmonid fish and in relation to possible hormonal factors influencing the epidermis of teleosts.     

Activity cycles in the brown trout (Salmo trutta Lin.)

Summary The annual and diurnal activity cycle for four naturally feeding brown trout separately confined in netting cages on the bed of Windermere is described. All the fish showed a similar annual cycle of maximum activity during May and June, one fish showing a second activity peak during the autumn. The fish also showed a similar diurnal activity rhythm of low activity during the night and increased activity during the day with a pronounced increase at dawn.The possible influence of light and temperature on the fishes activity is briefly discussed.     

Effects of six trace metals on calcium fluxes in brown trout (Salmo trutta L.) in soft water

Summary Calcium fluxes were measured simultaneously in brown trout fry maintained in an artificial soft water medium of [Ca] 20 mol·l^-1 and pH 5.6, and exposed to each of six trace metals (Al, Cu, Fe, Ni, Pb, and Zn). The trace metal concentrations represented typical and maximum levels found in acid waters experiencing declining fishery status. In the absence of trace metals, evidence is presented which suggests that ca. 91% of Ca taken up from the external medium was by extraintestinal active transport. Calcium efflux was stimulated by both concentrations of Al, Cu, Fe, and Pb. Efflux was also stimulated by [Ni] 170 nmol·l^-1 and [Zn] 3000 nmol·l^-1. In some cases, response to increased efflux was stimulation of influx. Lack of stimulation of influx resulted in negative net Ca fluxes. Net Ca losses were recorded at both concentrations of Al, Pb, and Ni, lower concentrations only of Fe, and higher concentrations only of Cu and Zn.Abbreviations J _in influx - J _net net flux - J _out efflux Henceforward in this paper, chemical elements are referred to by their chemical symbols rather than by full names     

Maintenance of a small anadromous subpopulation of brown trout (Salmo trutta L.) by straying

1. Microsatellite and isozyme loci variation were used to study structure and dynamics of a brown trout (Salmo trutta) population heavily affected by damming. The downstream area accessible for spawning was drastically reduced to a stream 1 km long influenced by regulated discharge. 2. Stocking of hatchery‐reared juveniles failed and the population is entirely supported by anadromous adults from neighbouring populations. 3. Temporal genetic stability is reported here. Some punctual between‐river genetic differences are likely because of different contribution from each neighbouring river through years. 4. High anadromy‐mediated gene flow produces a lack of genetic substructure in the region. The role of anadromous brown trout on maintenance of endangered small populations is emphasised.     

Selective predation on Cladocera by brown trout Salmo trutta L.

The size and species composition of zooplankton ingested by brown trout in Lough Sheelin was studied. Zooplankton and fish samples were taken simultaneously. The zooplankton samples consisted of Daphnia hyalina var. galeata , Copepoda and Bythotrephes longimanus in the proportion of 246: 10: 1. The entire stomach contents of the fish were examined with particular attention paid to the zooplankton component. The results indicated a high degree of selection for B. longimanus .     

Genome‐wide methylation study of diploid and triploid brown trout (Salmo trutta L.)

The induction of triploidization in fish is a very common practice in aquaculture. Although triploidization has been applied successfully in many salmonid species, little is known about the epigenetic mechanisms implicated in the maintenance of the normal functions of the new polyploid genome. By means of methylation‐sensitive amplified polymorphism (MSAP) techniques, genome‐wide methylation changes associated with triploidization were assessed in DNA samples obtained from diploid and triploid siblings of brown trout (Salmo trutta). Simple comparative body measurements showed that the triploid trout used in the study were statistically bigger, however, not heavier than their diploid counterparts. The statistical analysis of the MSAP data showed no significant differences between diploid and triploid brown trout in respect to brain, gill, heart, liver, kidney or muscle samples. Nonetheless, local analysis pointed to the possibility of differences in connection with concrete loci. This is the first study that has investigated DNA methylation alterations associated with triploidization in brown trout. Our results set the basis for new studies to be undertaken and provide a new approach concerning triploidization effects of the salmonid genome while also contributing to the better understanding of the genome‐wide methylation processes.     

Redundancy analysis,genome-wide association studies and the pigmentation of brown trout (Salmo trutta L.)

The association of molecular variants with phenotypic variation is a main issue in biology, often tackled with genome-wide association studies (GWAS). GWAS are challenging, with increasing, but still limited, use in evolutionary biology. We used redundancy analysis (RDA) as a complimentary ordination approach to single- and multitrait GWAS to explore the molecular basis of pigmentation variation in brown trout (Salmo trutta) belonging to wild populations impacted by hatchery fish. Based on 75,684 single nucleotide polymorphic (SNP) markers, RDA, single- and multitrait GWAS allowed the extraction of 337 independent colour patterning loci (CPLs) associated with trout pigmentation traits, such as the number of red and black spots on flanks. Collectively, these CPLs (i) mapped onto 35 out of 40 brown trout linkage groups indicating a polygenic genomic architecture of pigmentation, (ii) were found to be associated with 218 candidate genes, including 197 genes formerly mentioned in the literature associated to skin pigmentation, skin patterning, differentiation or structure notably in a close relative, the rainbow trout (Onchorhynchus mykiss), and (iii) related to functions relevant to pigmentation variation (e.g., calcium- and ion-binding, cell adhesion). Annotated CPLs include genes with well-known pigmentation effects (e.g., PMEL, SLC45A2, SOX10), but also markers associated with genes formerly found expressed in rainbow or brown trout skins. RDA was also shown to be useful to investigate management issues, especially the dynamics of trout pigmentation submitted to several generations of hatchery introgression.     

The immune response of brown trout (Salmo trutta L.) to injection with soluble antigens.

The immune response of brown trout (Salmo trutta) to horse serum and keyhole limpet haemocyanin was studied. Intraperitoneal and intramuscular injections were used, with and without adjuvant, in 209 fish. Complement-fixing antibodies (CFA) and precipitins were produced to both antigens. CFA were detected after 8 days to haemocyanin and after 13 days to horse serum. Maximum CFA titres to a single intraperitoneal injection of horse serum or haemocyanin were reached at 44 and 43-46 days respectively. Precipitins to a single injection of haemocyanin given intraperitoneally were detected after 19 days using gel diffusion. Similarly using the intramuscular route they were detected after 22 days. However, using counter-current electrophoresis, precipitins were detected after 8 days by the intraperitoneal route and after 9 days by the intramuscular. Precipitins to horse serum given intraperitoneally were demonstrated after 22 days by both gel diffusion and counter-current electrophoresis. Fish given 2 intraperitoneal injections of haemocyanin in adjuvant reached maximum CFA titres after 55 days; a 3rd injection on day 56 did not produce a marked increase in titre. Fish given intramuscular injections of haemocyanin in adjuvant showed maximum CFA titres at day 43. After a 3rd injection on day 56, maximum CFA titres were reached between days 92 and 106. Intramuscular injections gave significantly higher titres than those given by the intraperitoneal route. Some fish which showed no precipitins by gel diffusion were positive by counter-current electrophoresis. Precipitating antibodies to haemocyanin migrating in the beta2-gamma1 region were detected by immuno-electrophoresis.     

Population genetic structure of brown trout (Salmo trutta L.) within a northern boreal forest stream

Brown trout (Salmo trutta L.) population genetic structure and its temporal stability were studied within a small forest stream in central Sweden using five microsatellite loci. Both resident and migrating brown trout are present in the watershed. Tissue samples were collected from seven sections of the stream during two consecutive years. No differences were found in multilocus FST estimates between years within sections except in one case. Moreover, differences between age cohorts within sample sections were rare. The low interannual variation and the low heterogeny between cohorts is interpreted as indications of temporal stability. Pairwise multilocus FST estimates increased with increasing geographic distance, indicating isolation by distance. It is argued that the brown trout of the F?rs?n stream represent a population complex. The structure is probably maintained by precise natal homing and a limited, but important, amount of gene flow between closely situated sections within the stream supporting a stepping-stone model of gene flow.     

Effects of wastewater on fish health: an integrated approach to biomarker responses in brown trout (Salmo trutta L.)

The impact of wastewater effluent from a sewage treatment works (STW) on the health of brown trout held in cages and wild brown trout in a river was investigated. Biochemical, histological, and organismal responses as well as parasite abundances were monitored and then analyzed using multivariate analyses. Stress responses in trout induced by the water quality of the river upstream of the STW were enhanced by the discharge of the STW. For caged fish, the serum chemistry values alkaline phosphatase, blood urea nitrogen, and creatinine, as well as histological indices of gills and liver, were most effective at distinguishing among fish held in river water, a mixture of river water and wastewater, and tap water. For wild fish, total protein, histological liver alterations and abundance of two parasites (sessile peritrichia, Sphaerospora sp.) were the most indicative parameters for discriminating the health of fish between sites upstream and downstream of the STW. Considering the relationships between the measured parameters concurrently, the multivariate discriminant analysis is an effective method to evaluate which combination of parameters provide the best discrimination between the treatment groups. In contrast to the calculation of group differences based on individual responses, the integrated responses of parameters representing different biological levels lead to a more comprehensive assessment of organismal health and a more accurate distinction in differences between treatment groups.     

Piscivory by brown trout Salmo trutta L. and Arctic charr Salvelinus alpinus (L.) in Norwegian lakes

Size and frequency of occurrence of prey of brown trout Salmo trutta L. and Arctic charr Salvelinus alpinus (L.) were recorded in 13 Norwegian lakes during 1973–1990. Piscivores usually comprised less than 5% of the total population. Arctic charr were less piscivorous than brown trout. Trout and charr became piscivorous at 13 and 16 cm length, respectively. These size thresholds were similar to those of other facultative piscivorous freshwater fish species. When present, three-spined sticklebacks, Gasterosteus aculeatus (L.), were preferred by all length groups of piscivorous brown trout and Arctic charr. Length of prey increased with increasing predator length, and the mean body length of prey was about 33 and 25% of predator length for trout and charr, respectively. Yearlings of charr were not recorded as prey.     

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.     

The movements of brown trout, Salmo trutta (L.) in Loch Leven, Kinross, Scotland

Juvenile trout enter Loch Leven during autumn and winter from the nursery streams, spend their adolescent phase offshore until reaching a length of 0.30 cm, and then move to the littoral areas in early summer. There are two types of littoral area:'favourable'areas from which movement of individual fish in the summer is very restricted, and'unfavourable'areas used briefly in early summer from which movement away is pronounced. Trout are absent from the littoral areas in winter. In subsequent summers homing to previous feeding areas is characteristic of fish from'favourable'areas, with a tendency for older fish to move to the south east area of the loch. Spawning runs occur simultaneously into the main inflows, chiefly between October and December. Emigration into the outflow occurs, but is probably unimportant.