Migration of hatchery‐reared Atlantic salmon and wild anadromous brown trout post‐smolts in a Norwegian fjord system

Hatchery‐reared Atlantic salmon Salmo salar ( n  = 25) and wild anadromous brown trout (sea trout) Salmo trutta ( n  = 15) smolts were tagged with coded acoustic transmitters and released at the mouth of the River Eira on the west coast of Norway. Data logging receivers recorded the fish during their outward migration at 9, 32, 48 and 77 km from the release site. Seventeen Atlantic salmon (68%) and eight sea trout (53%) were recorded after release. Mean migratory speeds between different receiver sites ranged from 0·49 to 1·82 body lengths (total length) per second (bl s⁻¹) for Atlantic salmon and 0·11–2·60 bl s⁻¹ for sea trout. Atlantic salmon were recorded 9, 48 and 77 km from the river mouth on average 28, 65 and 83 h after release, respectively. Sea trout were recorded 9 km from the release site 438 h after release. Only four (23%) sea trout were detected in the outer part of the fjord system, while the rest of the fish seemed to stay in the inner fjord system. The Atlantic salmon stayed for a longer time in the inner part than in the outer parts of the fjord system, but distinct from sea trout, migrated through the whole fjord system into the ocean.     

Enhancement of pH‐resistant iron‐binding activity in supernatants of rainbow trout blood leucocytes by in vitro treatment with phorbol‐12‐myristate‐13‐acetate

Leucocyte lysates from rainbow trout Oncorhynchus mykiss showed an iron‐binding activity that was retained even if the samples were exposed to an acid pH (4·5). Iron‐binding activity of leucocyte supernatants was enhanced by the presence of 1 μg ml⁻¹ phorbol‐12‐myristate‐13‐acetate in the cell medium.     

Competition between brown trout and Atlantic salmon parr over pool refuges during rapid dewatering

Variations in distributions and behaviours of Atlantic salmon Salmo salar in allopatry (homogeneous) and in sympatry with brown trout Salmo trutta (mixed) were observed before, during and after 2 day periods of dewatering in a large glass-sided indoor stream at densities typical of Scottish upland streams. Brown trout utilized pools more than Atlantic salmon at normal flows and in both species the majority of fishes moved into pools during dewatering. There was no significant effect of brown trout, which was the more dominant species, on the overall ability of Atlantic salmon to use pool habitat as a refuge during dewatering. Within mixed and homogeneous groups, average feeding levels decreased during dewatering. The highest ranking fish, which was always a brown trout in mixed groups, predominantly monopolized the pool and other individuals in pools adopted a more cryptic, stationary behaviour. Dewatering effectively increased local population density with the result that dominance status became much more important in maintaining food intake, and polarization between the top ranking fish and others increased. During the first day of dewatering, there was extreme behavioural polarization such that the dominant fish exhibited most aggression and least feeding within the group. Among dominant fish on the second day of dewatering, aggression had largely abated and feeding had returned to pretreatment levels despite the reduced average feeding within the group. The main difference between mixed and homogeneous groups was in the behaviour of the most dominant Atlantic salmon, which was near-despotic in allopatry and subordinate to brown trout in sympatry.     

Comparing upriver spawning migration of Atlantic salmon Salmo salar and sea trout Salmo trutta

Radio tagged wild Atlantic salmon Salmo salar(n = 30) and sea trout Salmo trutta(n = 19) were simultaneously released from a sea pen outside the mouth of the River Lærdalselva and their migration to spawning areas was recorded. The distance from the river mouth to a position held at spawning ranged from 2 to 24 km and did not differ between the species (mean ± s .d . 15·9 ± 4·3 and 14·9 ± 5·2 km for Atlantic salmon and sea trout, respectively). The duration of the migration phase, however, was significantly shorter for Atlantic salmon than for sea trout (8–12 days, respectively). All Atlantic salmon migrated straight to an area near the spawning ground, whereas 50% of the sea trout had a stepwise progression with one or more periods with erratic movements before reaching the spawning area. After the migration phase, a distinct search phase with repeated movements up‐ and downstream at or close to the position held at spawning was identified for the majority of the fishes (75%, both species). This search phase was significantly shorter for Atlantic salmon than for sea trout (mean 13–31 days, respectively). Mean ± s .d . length of the river stretch used during the search phase was larger for sea trout (3·3 ± 2·5 km) than for Atlantic salmon (1·2 ± 0·9 km). A distinct holding phase, with no movements until spawning, was also observed in the majority of the Atlantic salmon (80%, mean duration 22 days) and sea trout (65%, mean duration 12 days). For both species, a weak, non‐significant trend was observed in the relationship between time spent on the migration phase, and time spent on the search (r² = 0·43) and holding phase (r² = 0·24). There was a highly significant decrease, however, in the duration of the holding phase with an increase in the time spent on the search phase (r² = 0·67).     

Size and age of maturity of Atlantic salmon correlate with the North Atlantic Oscillation Index (NAOI)

Sea‐age at maturity of Atlantic salmon Salmo salar decreased with increasing values of the seasonal NAOI from February to April. Body mass increment from smolts to adults of one‐sea‐winter Atlantic salmon increased with increasing NAOI in May at the time when the juveniles moved to sea.     

Effects of body size on sympatric shelter use in over‐wintering juvenile salmonids

The study tests two hypotheses: (1) the degree of shelter dominance in Atlantic salmon Salmo salar and brown trout Salmo trutta increases progressively with increasing size differential between heterospecific fish in a pair and (2) shelter dominance, standardized to size differential, correlates with aggression. The results support the first but not the second hypothesis, suggesting that the fitness consequences of high growth performance during the summer are likely to become evident during winter. At this time of year, when mortality is high among both Atlantic salmon and brown trout, shelter dominance may increase the chances of survival.     

Production of juvenile salmonids in small Norwegian streams is affected by agricultural land use

1. We estimated the biomass and production of juvenile anadromous brown trout (Salmo trutta) and Atlantic salmon (Salmo salar) (parr) in 12 streams in the Skagerrak area of Norway to identify controlling environmental factors, such as land‐use and water chemistry. 2. Production estimates correlated positively with fish density in early summer, but not with the size of the catchment. The summer biomass of age‐0 brown trout and Atlantic salmon was smaller than that of age‐1 and constituted 27.4 and 25.7%, respectively, of the total biomass of the two groups. 3. Mean production of brown trout from July to September varied between streams, but in most cases it was below 2 g 100 m^?2 day^?1. Yearly cohort production from age‐0 in July to age‐1 in July was 10 g m^?2 or less, with mean annual production of 1.32 g 100 m^?2 day^?1, equivalent to 4.8 g m^?2 year^?1. The corresponding annual cohort production of Atlantic salmon was 0.38 g 100 m^?2 day^?1 or 1.4 g m^?2 year^?1. Annual production to biomass ratio (P/B) for brown trout of the same cohort in the various streams was between 1.47 and 4.37; the overall mean (±SD) for all streams was 2.25 ± 0.94. Mean turnover rate of Atlantic salmon was 2.73 ± 0.24. 4. Production of 0+ brown trout during the summer correlated significantly with the percentage of agricultural land and forest/bogs in the catchment, with maxima at 20 and 75%, respectively. Age‐0 brown trout production also correlated with concentration of nitrogen and calcium in the water, with maxima at 2.4 and 14 mg L^?1, respectively. 5. The results support the hypothesis that brown trout parr production reflects the quality of their habitat, as indicated by the dome‐shaped relationship between percentage of agricultural land and the concentration of nitrogen and calcium in the water.     

Clonal Atlantic salmon × brown trout hybrids produced by gynogenesis

Clonal full-sib progeny groups of Atlantic salmon Salmo salar × brown trout Salmo trutta hybrids were produced by gynogenesis. Eggs obtained from two 3-year-old Atlantic salmon (female) × brown trout (male) F₁ hybrids were activated with UV-irradiated rainbow trout Oncorhynchus mykiss sperm. Fecundity, percentage egg activation and percentage survival to completion of yolk-sac absorption were similar for the two females, and averaged 800 eggs kg⁻¹, 90 and 65%, respectively. Flow cytometric and protein electrophoretic analyses confirmed the progeny to be diploid hybrids. Isogenicity within progeny groups and to the maternal parent was indicated by identical DNA fingerprint patterns detected with multilocus oligonucleotide probes–GATA₍₅₎ and ACTG_(n). Isogenicity was also observed in the gynogenetic progeny of a third female spawned the following year. It appeared that a large portion of the oocytes in females of this hybrid underwent a premeiotic chromosome doubling, or possibly a complete suppression of meiosis. The result was ovulation of diploid eggs, each possessing a full set of both Atlantic salmon and brown trout chromosomes identical to those in the maternal somatic cells. Lines of clonal hybrids could therefore be perpetuated by gynogenesis and would have potential both as experimental animals and in commercial aquaculture.     

Identification of the sex chromosomes of brown trout (Salmo trutta) and their comparison with the corresponding chromosomes in Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss)

Males are the heterogametic sex in salmonid fishes. In brown trout (Salmo trutta) the sex-determining locus, SEX, has been mapped to the end of linkage group BT-28, which corresponds to linkage group AS-8 and chromosome SSA15 in Atlantic salmon (Salmo salar). We set out to identify the sex chromosomes in brown trout. We isolated Atlantic salmon BAC clones containing microsatellite markers that are on BT-28 and also on AS-8, and used these BACs as probes for fluorescent in situ hybridization (FISH) analysis. SEX is located on the short arm of a small subtelocentric/acrocentric chromosome in brown trout, which is consistent with linkage analysis. The acrocentric chromosome SSA15 in Atlantic salmon appears to have arisen by a centric fusion of 2 small acrocentric chromosomes in the common ancestor of Salmo sp. We speculate that the fusion process that produced Atlantic salmon chromosome SSA15 disrupted the ancestral sex-determining locus in the Atlantic salmon lineage, providing the impetus either for the relocation of SEX or selection pressure for a novel sex-determining gene to arise in this species. Thus, the sex-determining genes may differ in Atlantic salmon and brown trout.     

Evaluating the effect of clustering when monitoring the abundance of sea lice populations on farmed Atlantic salmon

Using cluster random sampling theory and empirical estimates of the intra‐class correlations for sea lice Lepeophtheirus salmonis abundances, methods on how best to sample Atlantic salmon Salmo salar from cages on farms were derived. Estimates of intra‐class correlations for the abundance of the chalimus and mobile sea lice stages on Atlantic salmon in Scottish farms are given. These correlations were higher for mobile stages than for chalimus, and they had a substantive effect on increasing the number of cages and fish to be sampled for all sea lice stages. An important finding is that it is better to have a procedure that randomly samples a large number of cages using a small number of fish from each. This finding and the cluster random sampling approach have relevance to the monitoring of all marine species being farmed in cages or tanks.     

Static habitat partitioning and dynamic selection by sympatric young Atlantic salmon and brown trout in south-west England streams

Direct underwater observation of micro‐habitat use by 1838 young Atlantic salmon Salmo salar [mean L_T 7·9 ± 3.1(s.d.) cm, range 3·19] and 1227 brown trout Salmo trutta (L_T 10·9 ± 5·0 cm, range 3·56) showed both species were selective in habitat use, with differences between species and fish size. Atlantic salmon and brown trout selected relatively narrow ranges for the two micro‐habitat variables snout water velocity and height above bottom, but with differences between size‐classes. The smaller fishes <7 cm held positions in slower water closer to the bottom. On a larger scale, the Atlantic salmon more often used shallower stream areas, compared with brown trout. The larger parr preferred the deeper stream areas. Atlantic salmon used higher and slightly more variable mean water velocities than brown trout. Substrata used by the two species were similar. Finer substrata, although variable, were selected at the snout position, and differences were pronounced between size‐classes. On a meso‐habitat scale, brown trout were more frequently observed in slow pool‐glide habitats, while young Atlantic salmon favoured the faster high‐gradient meso‐habitats. Small juveniles <7 cm of both species were observed most frequently in riffle‐chute habitats. Atlantic salmon and brown trout segregated with respect to use of habitat, but considerable niche overlap between species indicated competitive interactions. In particular, for small fishes <7 cm of the two species, there was almost complete niche overlap for use of water depth, while they segregated with respect to water velocity. Habitat suitability indices developed for both species for mean water velocity and water depth, tended to have their optimum at lower values compared with previous studies in larger streams, with Atlantic salmon parr in the small streams occupying the same habitat as favoured by brown trout in larger streams. The data indicate both species may be flexible in their habitat selection depending on habitat availability. Species‐specific habitat overlap between streams may be complete. However, between‐species habitat partitioning remains similar.     

Lack of species-specific primer effects of odours from female Atlantic salmon, Salmo salar, and brown trout, Salmo trutta

We exposed, in two successive spawning seasons, individually placed precocious male Atlantic salmon ( Salmo salar ) and brown trout ( Salmo trutta ) parr to odour stimuli (ovarian fluid and urine mix) from ovulated conspecific or heterospecific anadromous females. Atlantic salmon parr had significantly higher plasma concentrations of the hormones 17α,20β-dihydroxy-4-pregnen-3-one (17,20β-P), 11-ketotestosterone (11-KT) and testosterone (T) after exposure to odours from conspecific females or from brown trout females compared to parr exposed to a control solution (0.9% NaCl). We did not observe any significant differences between the hormone levels in salmon parr exposed to the two female odours. The salmon parr exposed to conspecific odours had significantly higher volumes of strippable milt compared to the controls, but we did not find any significant differences when comparing the effect of the two female odours. Brown trout parr had significantly higher plasma 17,20β-P levels following exposure to heterospecific female odours compared to control males, but there was no significant difference between males exposed to the different female odours. We did not observe any significant differences in plasma levels of T and 11-KT and in milt volumes between exposed and control trout. Taken together, the results from both tested species indicate that the potency of heterospecific stimuli in stimulating increased plasma sex steroid hormone levels in male parr was as strong as stimuli from conspecific females. The results are discussed in connection to observed hybridisation between the two sympatric species.     

The survival of semi‐wild, wild and hatchery‐reared Atlantic salmon smolts of the Simojoki River in the Baltic Sea

The recapture rate and survival of hatchery‐reared Atlantic salmon Salmo salar stocked as 1 year‐old parr (semi‐wild) with that of hatchery‐reared Atlantic salmon stocked as 2 year‐old smolts and wild smolts of Atlantic salmon in the northern Baltic Sea were compared. This was done through tagging experiments carried out in 1986–1988 and 1992. The recapture rate of the semi‐wild groups varied from 1·0 to 13·1%, being similar in 3 tagging years and lower in 1 year than that of the wild groups (1·7–17·0%). The recapture rate of the semi‐wild groups was similar (in 2 years) or higher (in 2 years) than that of the hatchery‐reared groups stocked as smolts (1·3–6·3%). The survival of semi‐wild smolts during the sea migration was as high as that of wild Atlantic salmon of an equal size and two to three times higher than hatchery‐reared Atlantic salmon stocked as smolts. The survival rate was positively associated with smolt size. The suitability of hatchery‐reared parr and smolts in the management of reduced Atlantic salmon stocks is compared.     

Trophically transmitted parasites in wild Atlantic salmon post‐smolts from Norwegian fjords

The community structure of trophically transmitted intestinal helminths of Atlantic salmon Salmo salar post‐smolts was highly variable among four fjords in Norway. There were no severely pathogenic parasite species. Post‐smolts from the southernmost Trondheimsfjord had a higher diversity of freshwater parasite species compared to the three northern fjords (Tanafjord, Altafjord and Malangen). In contrast, the highest diversity and proportion of marine species was found in the three northern fjords. Post‐smolts were generally more infected with marine parasites in the outer rather than inner parts of all of the fjords. The prevalence of the acanthocephalan Echinorynchus gadi (range: 13–42%) and marine trematodes (range: 14–47%) was higher in post‐smolts in outer zones of the northern fjords than in fish from Trondheimsfjord (0 and 6%, respectively). The within‐fjord variability and north‐south geographical gradient in parasite infection patterns reflected differences in marine feeding of the post‐smolts on potential intermediate hosts such as amphipods ( E. gadi ) and fish larvae (trematodes), which were higher in the northern fjords (range: 27–28 and 67–85%, respectively) than in Trondheimsfjord (5 and 19%, respectively). High intensities of marine parasites suggest that some post‐smolts from northern fjords may have a prolonged fjord‐feeding compared to those from Trondheimsfjord. Parasites of both freshwater and marine origin appear to be suitable as bio‐indicators of feeding and migratory pattern of Atlantic salmon post‐smolts and preadults during their seaward migration.     

Determination of body shape variation in Irish hatchery‐reared and wild Atlantic salmon

Discriminant function analysis was used to distinguish morphologically between samples of parr, smolts and adult Atlantic salmon Salmo salar from several hatchery and river systems in Ireland. The effect of habitat shift was investigated in Atlantic salmon parr. Parr grown from the eyed‐egg stage with a non‐sibling group in a hatchery environment, came to resemble the mean body shape of their host hatchery Atlantic salmon stock more closely than that of a full sibling group grown at their natal hatchery. Wild Atlantic salmon smolts differed in shape from hatchery‐reared smolts. This difference was less pronounced, but still statistically significant when wild adults were compared with hatchery‐reared adults captured in the coastal drift‐net fishery after a year spent at sea. Rearing conditions had a significant impact on the production and growth of fish body shape. This in turn may have affected adaptability and survivorship of ranched Atlantic salmon in the marine environment.     

Performance of juvenile brown trout exposed to fluctuating water level and temperature

Individual daily food intake, mass‐specific growth rate and growth efficiency in groups of juvenile brown trout Salmo trutta were compared in tank experiments with three water level regimes (fluctuating, stable high and low water levels) and two temperature regimes (fluctuating between 10 and 14° C and constant 14° C) to simulate events during hydropeaking in regulated rivers. Fish exposed to high stable water level showed higher food intake and growth rate, and higher or similar growth efficiency than fish exposed to fluctuating or stable low water level. Both groups of slow‐growing and fast‐growing individuals fed less and grew slower at stable low and fluctuating water level than at stable high water level. Furthermore, growth and growth efficiency were lower in brown trout exposed to stable low water level and fluctuating temperature, particularly for groups of fish with slow growth. Temperature did not have any effect at high water level. For groups of fast‐growing fish, there was no difference in growth efficiency between treatments. It is concluded that fluctuating water level and temperature have a potentially detrimental effect on growth in juvenile brown trout and effects are more severe in slow‐ than fast‐growing fish.     

External attachment of data storage tags increases probability of being recaptured in nets compared to internal tagging

Sea running Arctic charr Salvelinus alpinus and anadromous brown trout (sea trout) Salmo trutta (420–2030 g) tagged with externally attached data storage tags (DST) had a higher total recapture rate (39 of 44, 89%) due to entanglement in bag nets at sea (90% of all recaptures), compared with internally tagged fish (12 of 18, 67%) that were mostly trapped when returning to their home river (75% of all recaptures). The internally tagged fishes therefore spent longer time at sea before recapture (median 33 days) than externally tagged fishes (median 8 days), and more DST‐data were collected. Therefore, in areas with high net fishing intensity, external tagging increases the chances of recapture, but less data may then be recorded by the tags due to a generally shorter period of data sampling.     

Diel activity patterns of juvenile Atlantic salmon in rivers with summer water temperature near the temperature‐dependent suppression of diurnal activity

Direct day and night underwater observations of juvenile Atlantic salmon Salmo salar during summer and autumn showed a duality in response to temperature between 7 and 11° C for young‐of‐the‐year (YOY) Atlantic salmon. They were predominantly diurnal in early summer and nocturnal in late summer although water temperatures were similar. Post‐YOY Atlantic salmon did not show a strong response to temperature as they were mostly nocturnal during the study period. It is suggested that the difference in activity patterns between YOY and post‐YOY Atlantic salmon can be explained by size‐dependent trade‐off between growth and predation risk.     

Temporal variation in abundance, return rate and life histories of previously spawned Atlantic salmon in a large subarctic river

The 30 year time series analyses revealed large temporal variation in the return rates and a recent increase in abundance of previous spawning Atlantic salmon Salmo salar in the River Teno, northern Scandinavia. The mean proportion of repeat spawners was 7 and 4% in the total Atlantic salmon catch and 9 and 22% in multi‐sea‐winter (MSW) catch component for females and males, respectively. Previous spawners constituted on the average 7% of the catch in mass but up to 20%(31 t) and 30%(19 t) in 2003 and in 2004, respectively. In 1975–2000, the proportion of previous spawners varied between 1 and 6%(3–12% of MSW Atlantic salmon), whereas in 2001–2004, they accounted for 8–21%(16–35% of MSW Atlantic salmon) of the total Atlantic salmon catch. The number of previous spawners in the catch correlated significantly with the preceding numbers of respective 1–3 sea‐winter (SW) maiden Atlantic salmon 2 years earlier. The recent increase in the numbers of 1S1 and 2S1 (1 or 2 years at sea followed by first spawning and 1 year reconditioning period at sea) alternate spawning Atlantic salmon was a consequence of higher numbers of maiden 1SW and 2SW Atlantic salmon in the catches and increased sea temperatures. Similarly, the return rate of 1SW Atlantic salmon to second spawning has improved in recent years. Most previous spawners ascended and were captured early in the fishing season. The smolt and sea‐age combinations of repeat spawners comprised 68 age groups contributing with the annual mean of 15 age groups to the great diversity of the River Teno Atlantic salmon population complex.     

Rare and fleeting: an example of interspecific recombination in animal mitochondrial DNA

Recombination is thought to occur only rarely in animal mitochondrial DNA (mtDNA). However, detection of mtDNA recombination requires that cells become heteroplasmic through mutation, intramolecular recombination or 'leakage' of paternal mtDNA. Interspecific hybridization increases the probability of detecting mtDNA recombinants due to higher levels of sequence divergence and potentially higher levels of paternal leakage. During a study of historical variation in Atlantic salmon (Salmo salar) mtDNA, an individual with a recombinant haplotype containing sequence from both Atlantic salmon and brown trout (Salmo trutta) was detected. The individual was not an F1 hybrid but it did have an unusual nuclear genotype which suggested that it was a later-generation backcross. No other similar recombinant haplotype was found from the same population or three neighbouring Atlantic salmon populations in 717 individuals collected during 1948-2002. Interspecific recombination may increase mtDNA variability within species and can have implications for phylogenetic studies.