Genetic variation in Atlantic salmon, Salmo salar L., within the Tamar catchment in south-west England

Electrophoretic analysis of enzymes in 383 juvenile Atlantic salmon, Salmo salar L., within the Tamar catchment (south-west England) revealed significant genetic differences at the IDHP-3* locus between the three tributaries studied. Aspects of temporal and spatial variation, and management policy within catchments, are discussed.     

Diet,feeding patterns,and prey selection of subyearling Atlantic salmon (Salmo salar) and subyearling chinook salmon (Oncorhynchus tshawytscha) in a tributary of Lake Ontario

Since juvenile Atlantic salmon (Salmo salar) and Chinook salmon (Oncorhynchus tshawytscha) occupy a similar habitat in Lake Ontario tributaries, we sought to determine the degree of diet similarity between these species in order to assess the potential for interspecific competition. Atlantic salmon, an historically important but currently extirpated component of the Lake Ontario fish community, are the focus of a bi‐national restoration effort. Presently this effort includes the release of hatchery produced juvenile Atlantic salmon in Lake Ontario tributaries. These same tributaries support substantial numbers of naturally reproduced juvenile Pacific salmonids including Chinook salmon. Subyearling Atlantic salmon and subyearling Chinook salmon had significantly different diets during each of the three time periods examined. Atlantic salmon fed slightly more from the benthos than from the drift and consumed mainly chirononmids (47.0%) and ephemeropterans (21.1%). The diet of subyearling Chinook salmon was more closely associated with the drift and consisted mainly of chironomids (60.2%) and terrestrial invertebrates (16.0%). Low diet similarity between subyearling Atlantic salmon and subyearling Chinook salmon likely minimizes competitive interactions for food between these species in Lake Ontario tributaries. However, the availability of small prey such as chironomids which comprise over 50% of the diet of each species, soon after emergence, could constitute a short term resource limitation. To our knowledge this is the first study of interspecific diet associations between these two important salmonid species.     

Predation on adult Atlantic salmon, Salmo salar L., by otters, Lutra lutra (L.), within the River Dee system, Aberdeenshire, Scotland

Predation on adult salmon, Salmo salur L., by otters, Lurra lutra (L.), varied seasonally on the R. Dee, Aberdeenshire, Scotland, being highest during the spawning season in winter. Predation is described for some tributaries of the river. Male fish were caught by otters more often than females, and it is suggested that they were most vulnerable during their extensive excursions up and down stream, particularly as they negotiated shallow riffles. Otters appeared to prey upon healthy fish rather than those infested with Saprolegnia sp. but there was no difference in the size, freshwater- or sea-age offish killed by otterscompared with 'kelts' which had died non-violently. At least some of the otters obtained most of their daily food requirements by catching a single salmon per night.
Considerable numbers of adult salmon may be killed by otters during the spawning period but it is suggested that, because most are males, this is unlikely to affect the breeding success of the salmon population. Most predation occurs outside the fishing season and so is unlikely to reduce the numbers of salmon caught by anglers.     

Interspecific habitat associations of juvenile salmonids in Lake Ontario tributaries: implications for Atlantic salmon restoration

Diel variation in habitat use of subyearling Chinook salmon (Oncorhynchus tshawytscha), subyearling coho salmon (O. kisutch), yearling steelhead (O. mykiss), and yearling Atlantic salmon (Salmo salar) was examined during the spring in two tributaries of Lake Ontario. A total of 1318 habitat observations were made on juvenile salmonids including 367 on steelhead, 351 on Chinook salmon, 333 on Atlantic salmon, and 261 on coho salmon. Steelhead exhibited the most diel variation in habitat use and Chinook the least. Juvenile salmonids were generally associated with more cover and larger substrate during the day in both streams. Interspecific differences in habitat use in both streams occurred with Atlantic salmon (fast velocities) and coho salmon (pools) using the least similar habitat. Chinook salmon and Atlantic salmon used similar habitat in both streams. These findings should help guide future management actions specific to habitat protection and restoration of Atlantic salmon in Lake Ontario tributaries.     

Parasite fauna of the landlocked Atlantic salmon Salmo salar L. in the basin of Namsen river

The 26 specimens of the landlocked Atlantic salmon Salmo salar L. from Namsen river and its tributary Melingselva river (central Norway) were studied by the method of complete parasitological examination in the period of 1997-1998. The 15 parasite species have been recovered. Ration of parasite with a direct and complex cycle is approximately equal, 6 and 9 respectively. Larval stages of parasites predominate among the parasites with a complex cycle. The parasite fauna fond in the landlocked Atlantic salmon is most similar to that in youngsters of the lake salmon (Salmo salar morpha sebago Girard) and the migrating Atlantic salmon (Salmo salar L.).     

Otolith shape discriminates between juvenile Atlantic salmon, Salmo salar L., and brown trout, Salmo trutta L.

Otoliths of Atlantic salmon, Salmo salar L., are more slender than the otoliths of brown trout, Salmo trutta L. Discriminant analysis on otolith measurements of juvenile Atlantic salmon and brown trout from four river systems revealed a discriminant function which distinguished more than 94% of the cases. This function was tested by using data from a fifth river with cohabiting Atlantic salmon and brown trout: all Atlantic salmon and 91 % of the brown trout were correctly classified.     

Genetic and biochemical aspects of lactate dehydrogenase isozymes in the salmonid eye

Polyacrylamide gel electrophoresis reveals similar differences between the retinal-specific lactate dehydrogenase (LDH) isozymes of the salmon (Salmo salar L.) and the sea-trout (Salmo trutta forma trutta L.) as those previously described for the salmon and the brown trout (Salmo trutta L.). F₁ salmon × sea-trout hybrids give a classic hybrid isozyme pattern, but the F₂ hybrids all possess the parental sea-trout type pattern. Loss of part of the salmon genome in these latter hybrids is the most likely explanation. It was observed that when the individual eye isozymes of the salmon, the sea-trout, and the rainbow trout (Salmo gairdneri Richardson) were eluted from preparative polyacrylamide gels and re-electrophoresed, an apparent interconversion of certain isozyme bands occurred. This phenomenon was also evident using starch gel. However, the major cathodally migrating isozyme in each case (presumably the E4 isozyme) re-electrophoresed pure. The reasons for these interconversions are, as yet, unclear. Attempts to produce in vitro hybridization between the various isolated individual isozymes were unsuccessful. K_m pyruvate values for the different salmon isozymes were of the order expected from results already published for other teleosts.     

Population structure of Atlantic salmon from the Conne River, Newfoundland as determined from microsatellite DNA

Variation at four microsatellite loci was examined for three populations of Atlantic salmon Salmo salar from the Conne River, Newfoundland. Samples of wild parr were collected from the mainstem Conne River during 4 years, and from tributaries Twillick Brook and Bernard Brook during 2 years. No significant temporal variation was observed in allele frequencies at the Ssal4, Ssal97, Ssa202, and Ssa289 loci. No difference in allele frequencies was observed between parr from Bernard and Twillick brooks at any locus, but allele frequencies of mainstem Conne River parr were significantly different from those of the tributaries at Ssal4 and Ssa202, indicative of differentiation among local populations. Atlantic salmon from the Conne River system were well differentiated from those in Nova Scotia, Canada and from those in Europe.     

Use of lesion filtrates for the transmission of UDN (ulcerative dermal necrosis) in salmonids

Spawning Atlantic salmon, Salmo salar (L.), brown trout, Salmo trutta (L.) and rainbow trout, Salmo gairdneri (Richardson) were held in tanks for four weeks and then injected with filtrates from lesions of salmon naturally infected with UDN. In some cases prednisolone acetate was included in the inoculum. Water temperatures were recorded daily and the time of appearance of the lesions was noted. Passage experiments using extracts of these lesions were carried out.     

Dissolved oxygen concentration and emergence of sea trout fry from natural redds in tributaries of the River Rhine

A total of 4289 sea trout Salmo trutta alevins swam up from six of eleven covered redds. They emerged only from those redds where the mean dissolved oxygen level was >6·9 mg l⁻¹ and the Sorting coe.cient, an indicator of intragravel void space, was <5. These results are discussed in relation to field studies using artificially buried salmonid eggs but also in relation to the ongoing programme for the reintroduction of Atlantic salmon in tributaries of the River Rhine.     

Characterization of proteases in the skin mucus of Atlantic salmon (Salmo salar) infected with the salmon louse (Lepeophtheirus salmonis) and in whole-body louse homogenate

As part of an investigation of the biochemical interactions between the salmon louse Lepeophtheirus salmonis and Atlantic salmon Salmo salar, we characterized protease activity in the skin mucus of noninfected Atlantic salmon and Atlantic salmon infected with L. salmonis and in an L. salmonis whole-body homogenate. Zymography revealed that mucus from infected salmon contained a series of low-molecular-mass (17-22 kDa) serine proteases that were not present in the mucus of noninfected salmon. Based on molecular mass, inhibition studies, and affinity chromatography, the series of proteases was identified as being trypsin-like. Similar proteases were observed in the L. salmonis homogenate and in mucus from noninfected Atlantic salmon following a 1-hr incubation with live L. salmonis. An antibody raised against Atlantic salmon trypsin failed to recognize any proteases in the mucus of noninfected salmon or infected salmon or in the L. salmonis homogenate. Collectively, these findings suggest that the trypsin-like proteases present in the mucus of infected Atlantic salmon were produced by L. salmonis, possibly to aid in feeding and evasion of host immune responses.     

Studies on ulcerative dermal necrosis of salmonids

Six salmon Salmo salar (L.) were vaccinated with an extract of normal salmon skin, to elicit production of anti-epithelial antibodies. These were produced within 8 weeks and were readily detectable by immunofluorescence, no evidence for the presence of similar antibodies in UDN infected salmon or normal salmon or trout was obtained. Experimental traumatization of the skin of vaccinated salmon with dry ice failed to produce Pemphigus type lesions and it was concluded that UDN was not an auto-immune disease of the pemphigus type.     

Experimental and natural host specificity of Loma salmonae (Microsporidia)

The microsporidian Loma salmonae (Putz, Hoffman & Dunbar, 1965) Morrison & Sprague, 1981 has caused significant gill disease in Pacific salmon Oncorhynchus spp. Host specificity of the parasite was examined experimentally by per os challenge of selected salmonids and non-salmonids with infective chinook salmon O. tshawytscha gill material. Pink Oncorhynchus gorbuscha and chum salmon O. keta, brown Salmo trutta and brook trout Salvelinus fontinalis, and chinook salmon (controls) were positive, whereas Atlantic salmon Salmo salar and Arctic char Salvelinus alpinus were negative. In addition, no non-salmonids were susceptible to experimental exposure. Wild Pacific salmon species in British Columbia, Canada, were examined for L. salmonae during their freshwater life history stages (smolts, prespawning, spawning). All stages were infected, although infections in smolts were only detectable using a L. salmonae-specific PCR test. Many previous Loma spp. described from Oncorhychus spp. are likely L. salmonae based on host, parasite morphology, and site of infection.     

Isolation, characterization, and chromosomal location of the tRNA(Met) genes in Atlantic salmon (Salmo salar) and brown trout (Salmo trutta).

This work describes the isolation, characterization, and physical location of the methionine tRNA in the genome of Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.). An Atlantic salmon genomic library was screened using a tRNA(Met) probe from Xenopus laevis. Two cosmid clones containing the Atlantic salmon tRNA(Met) gene were isolated, subcloned and sequenced. The tRNA(Met) was mapped to metaphase chromosomes by fluorescence in situ hybridization (FISH). Chromosomal data indicated that the tDNA of methionine is tandemly repeated in a single locus in both species. Analysis of genomic DNA by Southern hybridization confirmed the tandem organization of this gene.     

The existence of natural hybrids between the European trout and the Atlantic salmon

Natural hybrids between Atlantic salmon ( Salmo salar L.) and European trout ( S. trutta L.) have been detected in a sample of 4431 fish which appeared to be Atlantic salmon. The methods of analysis for serum protein, serum esterase and serum lactic dehydrogenase are given. The possible occurrence of hybrids having the appearance of European trout and the effect of introgression on salmon stocks are discussed.     

Juvenile salmonid populations in a temperate river system track synoptic trends in climate

Widespread declines among Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) over recent decades have been linked to pollution, exploitation and catchment modification, but climate change is increasingly implicated. We used long‐term, geographically extensive data from the Welsh River Wye, formerly a major salmon river, to examine whether climatically mediated effects on juveniles (>0+) might contribute to population change. Populations of Atlantic salmon and brown trout fell across the Wye catchment, respectively, by 50% and 67% between 1985 and 2004, but could not be explained by pollution because water quality improved during this time. Stream temperatures, estimated from calibrations against weekly air temperature at eight sites, increased by 0.5–0.7 °C in summer and 0.7–1.0 °C in winter, with larger tributaries warming more than shaded headwaters. Rates of winter warming were slightly greater after accounting for the effect of the North Atlantic Oscillation (1.1–1.4 °C). However, warming through time was smaller than measured variations among tributaries, and alone was insufficient to explain variations in salmonid density. Instead, population variations were best explained in multilevel mixed models by a synoptic variate representing a trend towards hotter, drier summers, implying interactions between climate warming, varying discharge and fluctuations in both brown trout and salmon. Taken alongside recent data showing effects of warming on survival at sea, these data suggest that Atlantic salmon might be jeopardized by future climatic effects in both their marine and freshwater stages. Effects on nondiadromous brown trout also imply climatically mediated processes in freshwaters or their catchments. Climate projections for the United Kingdom suggest that altered summer flow and increasing summer temperatures could exacerbate losses further in these species, and we advocate management actions that combine reduced abstraction with enhanced riparian shading.     

Serum agglutinating titres against Renibacterium salmoninarum the causative agent of bacterial kidney disease, in rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L.

Serum agglutinating titres against Renibacterium salmoninarum were determined from clinically infected, sub-clinically infected and non-infected rainbow trout, Salmo gairdneri Richardson, and Atlantic salmon, Salmo salar L. Titres ≥16 occurred in clinically infected rainbow trout, but declined after a few months when disease signs were absent. Non-infected rainbow trout had titres ≤ 16. No correlation between the agglutinating titre and the level of infection was found in farmed Atlantic salmon. Variable titres were found in wild salmon broodstock although there was no evidence of BKD during 4 years of testing.     

Influence of aperiodic summer droughts on leaf litter breakdown and macroinvertebrate assemblages: testing the <Emphasis Type="Italic">drying memory</Emphasis> in a Central Apennines River (Aterno River,Italy)

Management of multiple exploited stocks of anadromous salmonids in large catchments requires understanding of movement and catchment use by the migrating fish and of their harvesting. The spawning migration of sea trout (Salmo trutta) and Atlantic salmon (Salmo salar) was studied in the River Tweed, UK, using acoustic telemetry to complement exploitation rate data and to quantify catchment penetration. Salmon (n = 79) and sea trout (n = 65) were tagged in the tidal-influenced Tweed in summer–autumn. No tagged salmon left the river before spawning, but 3% (2010) and 8% (2011) of pre-spawning sea trout dropped out. Combined tag regurgitation/fish mortality in salmon was 12.5%, while trout mortality was 6% (2010) and 0% (2011). The estimated spawning positions of salmon and sea trout differed; tagged salmon were mostly in the main channel while trout occurred mostly in the upper Tweed and tributaries. Early fish migrated upstream slower than later fish, but sea trout moved through the lower-middle river more quickly than salmon, partly supporting the hypothesis that the lower exploitation rate in autumn of trout (1 vs 3.3% for salmon) there is generated by differences in migration behaviour.     

Isolation of a Loma salmonae variant: biological characteristics and host range

A Salvelinus -infecting variant of Loma salmonae , derived from naturally-infected Chinook salmon Oncorhynchus tshawytscha by serial passage through brook trout Salvelinus fontinalis , has been isolated and amplified. Loma salmonae SV ( Salvelinus -variant) has a high preference for species of Salvelinus (brook trout and Arctic charr S. alpinus ) and low virulence and preference for species of Oncorhynchus (rainbow trout O. mykiss , Chinook salmon, cohoSalmon O. kisutch ) or Salmo (Atlantic salmon Salmo salar ). Although this variant of L. salmonae was different from the original, the differences do not justify describing it as a new species, although definitive determination is pending.     

Diel rhythm of emergence and of first displacement downstream in trout (Salmo trutta), Atlantic salmon (S. salar) and grayling (Thymallus thymallus)

The diel rhythm of emergence from the gravel and displacement downstream has been studied in three salmonoid species: brown trout ( Salmo trutta L.), Atlantic salmon ( S. saiar L.) and grayling ( Thymallus thymallus L.). Grayling emerged in the morning but delayed downstream displacement until the night, while the Salmo species emerged just before downstream displacement chiefly at night.