Wild adult hybrids of Salmo salar L. and Salmo trutta L.

Ovarian development was impaired in three adult Salmo salar L. × S. trutta L. hybrids identified among adult salmonids in Scottish fisheries. Species-specific variation at enzyme loci indicated that the fish were F1 hybrids and mitochondrial DNA analysis showed them to be the progeny of S. salar females.     

Genetic variation for tolerance to acidic water in salmonids

Water pH is an important factor affecting the general water quality as well as quality traits in fishes, and the magnitude of the effect varies among species. The massive and negative effect of acidification of rivers and lakes became evident during the 1960s and 1970s and caused the depletion of fish stocks in several countries in the northern hemisphere. Significant variation in tolerance to acidic water has been documented among salmonid species, and large genetic variation has been identified among strains of brown trout Salmo trutta, brook trout Salvelinus fontinalis and Atlantic salmon Salmo salar. For S. trutta, S. fontinalis and S. salar, there is considerable additive genetic variation in tolerance to acidic water, with heritabilities (h(2)) ranging from 0·09 to 0·27 for dead eyed-eggs (the period most sensitive to low pH). The main reasons for depletion of freshwater fish stocks are discussed.     

Effect of water temperature and density of juvenile salmonids on growth of young-of-the-year Atlantic salmon Salmo salar

A von Bertalanffy growth model for young-of the-year Atlantic salmon Salmo salar in a small French coastal stream was fitted using water temperatures and densities of juvenile salmonids (S. salar and brown trout Salmo trutta) as covariates influencing daily growth rate. The Bayesian framework was used as a template to integrate prior information from external data sets. The relative influence of the covariates on parr growth was quantified and results showed that growth of S. salar juveniles depended on both water temperatures and densities, but that most of the spatiotemporal variability of growth resulted from local spatiotemporal variations of 0+ age salmonid (S. salar and S. trutta) densities. Further analysis revealed that the fluctuations in young-of-the-year salmonid densities are likely to dominate the effects of potential future warming of water temperature due to climate change. It is concluded that factors that could affect salmonid densities might well have a greater effect on S. salar population dynamics than factors influencing water temperatures.     

A comparison of five of the methods commonly used to measure protein concentrations in fish sera

The concentration of protein in the sera of rainbow trout, Salmo gairdneri , brown trout S. trutta and Atlantic salmon S. salar has been measured by six standard techniques viz refractometry, copper sulphate specific gravity, automated and manual biuret, optical density and Lowry et al. phenol reagent and the results compared. Good correlation was obtained in most cases and interconversion formulae are given between each method in the three salmonid species. The concentrations obtained with the refractometer and optical density methods were approximately one and a half times those obtained with the others.     

Host specificity and geographical distribution of Eubothrium in European salmonid fish

The host specificity and distribution of Eubothrium crassum (Bloch, 1779) and Eubothrium salvelini (Schrank, 1790), morphologically fairly similar pseudophyllidean tapeworms parasitizing salmonid fish, were critically assessed on the basis of morphological and genetic evaluation of extensive material collected from different definitive hosts and geographical regions in Europe. Eubothrium crassum occurs in fish of the genera Salmo, i.e. salmon (S. salar - both freshwater and marine), sea trout (S. trutta trutta), brown trout (S. trutta fario), and lake trout (S. trutta lacustris), and also in Danubian salmon (Hucho hucho) and vendace (Coregonus albula). Eubothrium salvelini parasitizes Arctic char (Salvelinus alpinus) and brook trout (Salvelinus fontinalis) in Europe, and also whitefish (Coregonus wartmanni). Rainbow trout (Oncorhynchus mykiss), which is not a native European fish species, was found to be a suitable definitive host for both Eubothrium species, which may occur simultaneously in the same fish. Previous records of E. crassum in Arctic char and brook trout, and those of E. salvelini in fish of the genus Salmo were most probably misidentifications. Most studies of Eubothrium have involved salmonids from the northern part of Europe, with few records from southern and south-eastern Europe. This study also confirmed the reliability of the morphology of the apical disc for the discrimination of E. crassum and E. salvelini.     

High incidence of Atlantic salmon × brown trout hybrids in a Lake District stream

Hypervariable minisatellite DNA single-locus profiling and mitochondrial DNA analysis revealed that 18.48% of juvenile Atlantic salmon Salmo salar in Troutbeck, a stream in the R. Leven catchment of the English Lake District, were hybrids between Atlantic salmon and brown trout S. trutta , and that hybridization was bidirectional.     

Temperature requirements of Atlantic salmon Salmo salar, brown trout Salmo trutta and Arctic charr Salvelinus alpinus: predicting the effects of climate change

Atlantic salmon Salmo salar, brown trout Salmo trutta (including the anadromous form, sea trout) and Arctic charr Salvelinus alpinus (including anadromous fish) provide important commercial and sports fisheries in Western Europe. As water temperature increases as a result of climate change, quantitative information on the thermal requirements of these three species is essential so that potential problems can be anticipated by those responsible for the conservation and sustainable management of the fisheries and the maintenance of biodiversity in freshwater ecosystems. Part I compares the temperature limits for survival, feeding and growth. Salmo salar has the highest temperature tolerance, followed by S. trutta and finally S. alpinus. For all three species, the temperature tolerance for alevins is slightly lower than that for parr and smolts, and the eggs have the lowest tolerance; this being the most vulnerable life stage to any temperature increase, especially for eggs of S. alpinus in shallow water. There was little evidence to support local thermal adaptation, except in very cold rivers (mean annual temperature <6·5° C). Part II illustrates the importance of developing predictive models, using data from a long-term study (1967-2000) of a juvenile anadromous S. trutta population. Individual-based models predicted the emergence period for the fry. Mean values over 34 years revealed a large variation in the timing of emergence with c. 2 months between extreme values. The emergence time correlated significantly with the North Atlantic Oscillation Index, indicating that interannual variations in emergence were linked to more general changes in climate. Mean stream temperatures increased significantly in winter and spring at a rate of 0·37° C per decade, but not in summer and autumn, and led to an increase in the mean mass of pre-smolts. A growth model for S. trutta was validated by growth data from the long-term study and predicted growth under possible future conditions. Small increases (<2·5° C) in winter and spring would be beneficial for growth with 1 year-old smolts being more common. Water temperatures would have to increase by c. 4° C in winter and spring, and 3° C in summer and autumn before they had a marked negative effect on trout growth.     

Hybridization between Atlantic Salmon Salmo salar L. and Brown Trout S. trutta L. upon Artificial Propagation

Samples of Salmo salar and S. trutta were examined in 12 Russian fish hatcheries. With protein markers, hybrids of the two species were found in three hatcheries of the Baltic Sea basin. Some fishes had a phenotype intermediate between theS. salar and S. trutta phenotypes by morphological traits, but did not differ genetically from one of the parental species. Possible consequences of hybridization and ways to prevent it are discussed.     

Migratory timing, marine survival and growth of anadromous brown trout Salmo trutta in the River Imsa, Norway

The aim of the paper was to study sea migration, growth and survival of brown trout Salmo trutta of the River Imsa, 1976–2005. The migratory S. trutta were individually tagged and fish leaving or entering the river were monitored daily in traps located near the river mouth. The mean annual duration of the sea sojourn was 6–9 months for first-time migrants moving to sea between January and June. It was 8–18 months for those migrating to sea between July and December. Veteran migrants stayed 12 months or less at sea and most returned to the river in August. Early ascending fish stayed the longest in fresh water because most returned to sea in April to May. The day number of 50% cumulative smolt descent correlated negatively with mean water temperature in February to March and the February North Atlantic Oscillation index (NAOI). Mean annual sea growth during the first 2 years after smolting was higher for S. trutta spending the winter at sea than those wintering in the River Imsa. First year's sea growth was lower for S. trutta descending in spring than autumn. For first-time migrants, it correlated negatively with the February NAOI of the smolt year. Sea survival was higher for spring than autumn descending first-time migratory S. trutta with a maximum in May (14·9%). Number of anadromous S. trutta returning to the river increased linearly with the size of the cohort moving to sea, with no evidence of density-dependent sea mortality. Sea survival of S. trutta smolts moving to sea between January and June correlated positively both with the annual number of Atlantic Salmo salar smolts, the specific growth rate at sea, and time of seaward migration in spring. This is the first study indicating how environmental factors at the time of seaward migration influence the sea survival of S. trutta .     

Infestations by Gyrodactylus sp. of Atlantic salmon, Salmo salar L., in Norwegian rivers

Young Atlantic salmon, Salmo salar , brown trout, Salmo trutta , and Arctic char, Salvelinus alpinus , from eight rivers in North and Mid Norway were examined for Gyrodactylus. The fish were collected from 1975 to 1980. A Gyrodactylus salaris type was observed infecting salmon from six of these rivers. No trout or char were infected. A high frequency and intensity of infection of salmon were observed in all but one of the rivers surveyed. In the Saltdalselva, only one specimen of Gyrodactylus infecting one fish was observed. No subsequent mortality of salmon was observed in this river while there were signs of a high mortality of salmon in the other rivers. The salmon parr were more frequently attacked than the fry, the mortality of salmon seemed to have happened in most of the rivers one year after the first observations of Gyrodactylus were made. The mortality of salmon and the high infection rate of Gyrodactylus in these rivers appears unique, and as far as known to the authors there are no other described cases of mortality due to Gyodactylus in Atlantic salmon in natural waters. The reasons for the outbreak of Gyrodactylus in these rivers are not known. Two theories are discussed: one that the fish were weakened by environmental factors and the other that Gyrodactylus was introduced from some of the infected salmon hatcheries in Scandinavia.     

Identification of juvenile natural hybrids between Atlantic salmon (Salmo solar L.) and trout(Salmo trutta L.)

The identification of two wild salmonid smolts, suspected to be hybrids between Atlantic salmon ( Salmo salar ) and trout ( S trutta ) on morphological grounds, was confirmed by electrophoretic separation of serum proteins. A third, trout-like, hybrid was identified in a sample of parr. Hybrids showed presence of both parent types of esterase, prealbumin and transferrin.     

Interaction for food and space between populations of Galaxias vulgaris Stokell and juvenile Salmo trutta L. in a New Zealand stream

The trophic and spatial interrelationships between a native ( Galaxias vulguris Stokell, Galaxiidae) and an exotic ( Salmo trutta L., Salmonidae) fish species were investigated over a 24-h period in a New Zealand stream. Interspecific overlap in feeding was greatest at dusk and dawn, as G. vulgaris fed primarily from dusk to post-dawn and S. trutta fed primarily from pre-dawn to post-dusk. Both species fed mainly on benthic and drifting aquatic invertebrates, with larval Deleatidium (Ephemeroptera). Hydora (Coleoptera) and Chironomidae (Diptera) being their preferred prey, although Trichoptera imagos were also preferred by S. trutta . Both species were found primarily in runs and riffles with G. vulgaris occupying slightly shallower (≤0.3 m) and faster (0.3–0.7 m s¹) waters than did S. trutta (≤0.5 m deep and 0.2–0.4 m s⁻¹ water velocity). We suggest that these rather subtle interspecifc differences in die1 feeding periodicities, diets and microdistributions play a part in lessening the interaction between co-occurring populations of G. wlguris and S. trutta .     

Interspecific hybridization, a matter of pioneering? Insights from Atlantic salmon and brown trout

Interspecific hybridization may occur in situations of recent contact between a colonizer and a resident species, being more intense in the colonization front. Atlantic salmon Salmo salar and brown trout S. trutta have been sympatric species since their origin and they share spatial and temporal spawning niches, exhibiting low levels of bidirectional interspecific hybridization and introgression throughout their distribution range. Different causes have been identified for increased hybridization, from escapes or deliberate releases of domesticated fish to sneaking male behavior. We have examined hybridization rates and direction in different situations of advance of one of these species into a territory formerly inhabited by the other (247 samples were analyzed in northern Spain and 487 in Kerguelen Islands). In all cases, hybrids found in the colonization front were offspring of colonizer females and resident males. We hypothesize that these findings are the result of adaptive relaxed mate choice of colonizing females, regardless of the relative abundance of each species.     

Deep water observations of Lophius piscatorius in the north‐eastern Atlantic Ocean by means of a remotely operated vehicle

To determine whether the current classification of the flathead trout Salmo (Platysalmo) platycephalus , endemic to the upper reaches of the Zamanti River system, Turkey, based solely on morphology, is in congruence with molecular phylogeny, the nucleotide sequence variation in mitochondrial (control region and cytochrome b gene) and nuclear (internal transcribed spacer of rRNA genes) DNA for the flathead trout and various representatives of the genus Salmo was studied. On the basis of pair‐wise genetic distance estimates, the highest differences were found to exist between the flathead trout and S. salar , S. ohridana and S. obtusirostris , whereas the differences between the flathead trout and S. trutta were minimal. All the analyses performed firmly positioned the flathead trout within the Adriatic phylogeographic lineage of S. trutta ; however, the exact position of the flathead trout within the Adriatic cluster was irresolvable. Accordingly, classifying the flathead trout as a subgenus of Salmo is unjustifiable and its reclassification in a lower taxonomic category is suggested by the present study.     

Fatty acid profiles of wild brown trout and Atlantic salmon juveniles in the Nivelle basin

Samples of Atlantic salmon Salmo salar 0+ year parr and brown trout Salmo trutta 1+ year juveniles, and 2+ and 3+ year sub‐adults were obtained from nine sites of the Nivelle River, France, located at the southern edge of the species distribution area. Fatty acid composition of the different samples were analysed. Within the same river basin, the fatty acid profiles appeared to vary among samples from different locations. Based on cluster analysis, fatty acid signatures of both species appeared to discriminate samples from different locations.     

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.     

Carbohydrate localization on Gyrodactylus salaris and G. derjavini and corresponding carbohydrate binding capacity of their hosts Salmo salar and S. trutta

The congeners Gyrodactylus salaris and G. derjavini are specific ectoparasites of Atlantic salmon Salmo salar and brown trout S. trutta, respectively. To elucidate the involvement of lectin-carbohydrate interactions in this host specificity, carbohydrates on the tegument of the two species and the corresponding lectin activity of their hosts have been studied. Carbohydrate composition on the tegument differed significantly between the two gyrodactylids. Three of four commercially available peroxidase-labelled lectins with primary affinity towards D-mannoside, D-GalNAc and L-fucose bound more strongly to G. derjavini than to G. salaris. Lectins with an affinity towards D-mannoside and D-GalNAc bound significantly stronger to the cephalic lobes on G. derjavini compared to the tegument and sheaths of the hamuli. One brown trout strain and three different salmon strains were tested for lectin activity in skin and plasma. Two Baltic salmon strains and one strain from the Atlantic region were included. Brown trout differed significantly from the salmon strains when skin samples were tested for D-GalNAc activity. Lectins binding to other carbohydrates showed trends for similar host differences. The implications of carbohydrate-lectin interactions for host specificity in gyrodactylids are discussed.     

Morphological differences in the skin of marble trout Salmo marmoratus and of brown trout Salmo trutta

Despite being genetically very closely related, the marble trout Salmo marmoratus and the brown trout Salmo trutta exhibit marked phenotypic differences, particularly with regard to skin pigmentation. Histological analysis of skin from the head and gill cover of differently aged individuals of the two species was carried out in order to characterize differences in skin structure. The basic structure of skin of the individuals studied corresponded with that described for other salmonids, though the head epidermis was somewhat thicker in S. marmoratus than in S. trutta, thickening with age in both species. Numerous secretory goblet cells and sporadic secretory sacciform cells were observed in the upper and middle part of the epidermis in both species. Melanophores were present in both species only in the dermis, and were bigger in S. marmoratus and present at lower average density than in S. trutta, and more or less constant across all age classes. In adult S. marmoratus with fully established marble pigmentation, light areas at low density with small (i.e. aggregated) melanophores were present, while in S. trutta melanophores were more uniformly distributed.     

Studies on the biology of Eubothrium salvelini and E. crassum in resident and migratory Salvelinus alpinus and Salmo trutta and in S. salar in North Norway and the islands of Spitsbergen and Jan Mayen

Samples of Eubothrium salvelini and E. crassum were obtained from resident and migratory Salvelinus alpinus and Salmo trutta and from S. salar at sea from North Norway and the islands of Spitsbergen and Jan Mayen. Scolex dimensions proved an unsuitable character for identifying these two parasite species, and use of alternative characters confirmed that E. salvelini is specific to Salvelinus and E. crassum to Salmo. Distributions of both species are co-extensive with their hosts, but levels of infection in any locality relate to the abundance of zooplankton. E. salvelini exhibits seasonal maturation and incidence cycles in char. It is recognised as being a freshwater parasite, although some individuals can survive the period of marine migration of sea char. Returning migrant char may contain such individuals together with small plerocerciform parasites acquired almost immediately upon their re-entry to freshwater. No evidence for marine infections with E. salvelini was found. The presence of plerocerciform and gravid adult E. crassum in salmon caught at sea confirms the existence of a marine life cycle and a marine race in this species. In salmon moving from one medium to the other the marine and freshwater races replace each other, but in sea trout, which spend shorter periods at sea, complete replacement may not occur and at any one time fish may harbour individuals of both races. Recognition of the existence of two races clarifies much of the confusion surrounding the biology of this species.