Population and size-specific distribution of Atlantic salmon Salmo salar in the Baltic Sea over five decades

Population-specific assessment and management of anadromous fish at sea requires detailed information about the distribution at sea over ontogeny for each population. However, despite a long history of mixed-stock sea fisheries on Atlantic salmon, Salmo salar, migration studies showing that some salmon populations feed in different regions of the Baltic Sea and variation in dynamics occurs among populations feeding in the Baltic Sea, such information is often lacking. Also, current assessment of Baltic salmon assumes equal distribution at sea and therefore equal responses to changes in off-shore sea fisheries. Here, we test for differences in distribution at sea among and within ten Atlantic salmon Salmo salar populations originating from ten river-specific hatcheries along the Swedish Baltic Sea coast, using individual data from >125,000 tagged salmon, recaptured over five decades. We show strong population and size-specific differences in distribution at sea, varying between year classes and between individuals within year classes. This suggests that Atlantic salmon in the Baltic Sea experience great variation in environmental conditions and exploitation rates over ontogeny depending on origin and that current assessment assumptions about equal exploitation rates in the offshore fisheries and a shared environment at sea are not valid. Thus, our results provide additional arguments and necessary information for implementing population-specific management of salmon, also when targeting life stages at sea.     

Genetic assessment of Atlantic salmon Salmo salar and sea trout Salmo trutta stocking in a Baltic Sea river

Microsatellite DNA variation was used to assess the outcome of stocking Atlantic salmon Salmo salar and migratory trout Salmo trutta in River Sävarå, N Sweden. No information on pre‐stocking genetic composition of S. salar and S. trutta in River Sävarå was available. In 2 year‐classes of S. salar smolt, microsatellite data indicated that post‐stocking genetic composition differed markedly (F_ST= 0·048) from the main donor strain, Byskeälven S. salar, and from other Gulf of Bothnia S. salar stocks (F_ST 0·047 and 0·132). The STRUCTURE programme failed to detect any substructuring within Sävarå salmon. It was concluded that only minor introgression estimated to a proportion of 0·11 (95% CI 0·07–0·16) has occurred in S. salar. Salmo trutta showed overall low differentiation among populations with maximum F_ST of 0·03 making analysis more cumbersome than in S. salar. Still, the SävaråS. trutta deviated significantly from potential donor populations, and STRUCTURE software supported that majority of trout in Sävarå formed a distinct genetic population. Admixture was more extensive in S. trutta and estimated to 0·17 (95% CI 0·10–0·25).     

Divergent trends in life‐history traits between Atlantic salmon Salmo salar of wild and hatchery origin in the Baltic Sea

Four Atlantic salmon Salmo salar stocks in the Baltic Sea, varying in their breeding history, were studied for changes in life‐history traits over the years 1972–1995. Total length (L_T) at age of captured (L_TC) fish had increased throughout the study period, partly due to increased temperature and increased L_T at release, (L_TR) but also due to remaining cohort effects that could represent unaccounted environmental or genetic change. Simultaneously, maturation probabilities controlled for water temperature, L_TC and L_TR had increased in all stocks. The least change was observed in the River Tornionjoki S. salar that was subject only to supportive stockings originating from wild parents. These results suggest a long‐term divergence between semi‐natural and broodstock‐based S. salar stocks. Increased L_T at age explained advanced maturation only marginally, and it remains an open question to what extent the generally increased probabilities to mature at early age reflected underlying genetic changes.     

Identification of the origin of an Atlantic salmon (Salmo salar L.) population in a recently recolonized river in the Baltic Sea

The founder event in a recently recolonized salmon population in the Baltic Sea (Gulf of Finland) was investigated. To identify the origin of the founders, four wild populations and two hatchery stocks were analysed using six microsatellite loci. The results of assignment tests and factorial correspondence analysis suggest that the initial recolonizers of the river Selja originated from the geographically nearest (7 km) wild population (river Kunda) but as the result of stocking activities, interbreeding between recolonizers and hatchery individuals has occurred in subsequent years. Although the hatchery releases are outnumbering the wild salmon recruitment in the Baltic Sea at present, our results suggest that the native populations may still have an important role in colonization processes of the former salmon rivers.     

Microsatellite DNA markers for the study of Atlantic salmon (Salmo salar) kinship,population structure,and mixed‐fishery analyses

Eleven microsatellite DNA loci were identified and characterized for Atlantic salmon (Salmo salar) collected from the Penobscot River, Maine, USA and the River Nith, Scotland, UK. The markers revealed high levels of genetic diversity (seven to 48 alleles per locus), heterozygosity (to 100%), and allelic heterogeneity (all comparisons). Considerable differentiation was observed as the genetic distance (chord) between the two collections was 0.680 and the pairwise F_ST, 0.12, was highly significant. These findings are consistent with patterns of continental‐level differentiation observed previously using an alternate suite of microsatellite loci. Locus‐by‐locus analyses of molecular variance suggested that most markers were suitable for delineating kinships and population genetic structure.     

The effect of intragastric transmitters on feeding behaviour of Atlantic salmon, Salmo salar; parr during autumn

The proportion of Atlantic salmon parr (146 265 mm total length) that fed in laboratory tanks during autumn was 0% in fish filled with intragastric transmitters compared with 78 100% in controls.     

Matrilinear phylogeography of Atlantic salmon (Salmo salar L.) in Europe and postglacial colonization of the Baltic Sea area

Sixty‐four samples from 46 salmon populations totalling 2369 specimens were used for polymerase chain reaction–restriction fragment length polymorphism (PCR–RFLP) analysis of the mitochondrial ND1 region. The final analyses included 3095 specimens from 60 populations in Northern Europe. A subsample was analysed by RFLP of ND3/4/5/6. Representative RFLP haplotypes from different parts of the distribution area were sequenced and the phylogeny of European haplotypes and their relations to the North American lineage was described. The four common European haplotypes derive from the ancestral ND1‐BBBA (rooting the European clade to the North American) by one‐step substitutions: AAAA < AABA < BBBA > BBBB. The Swedish west‐coast populations differ from the geographically close southern Baltic, indicating absence of inward and limited outward gene flow through the Danish straits during the last 8000 years. Within the Baltic Sea, only three ND1 haplotypes were detected and there was no variation for ND3/4/5/6. In the whole southern Baltic and in lakes Vänern, Ladoga and Onega the haplotype AABA dominated. Proposed postglacial colonization routes to the Baltic Sea are discussed in relation to the haplotype distribution pattern.     

Plasma catecholamines and accumulation of adrenaline in the atrial cardiac tissue of aquacultured Atlantic salmon (Salmo salar) during stress

The effects of stress on plasma catecholamines (CA) and capacity for tissue accumulation of CA were studied in cardiac and skeletal muscle of cultured Atlantic salmon ( Salmo salar L.). Adrenaline (A) and noradrenaline (NA) were quantified by high performance liquid chromatography.
Plasma A and NA levels were 56±10 nmoll⁻¹ and 77±17 nmoll⁻¹ (± s.e.m. ), respectively, in a control group living under normal rearing conditions in a fish farm. Following a ±3 h period of pre-slaughter crowding and handling in the fish farm, plasma A reached 221 ± 72 nmol1⁻¹ with no increase in plasma NA. An 0.5 h period of struggling out of water led to even higher level of plasma A (480 ± 89 nmol1⁻¹), without change in NA.
Skeletal muscle was low in CA (A, 0.07 ± 0.02 and NA, 0.06 ± 0.01 nmol g⁻¹ wet wt). Tissue CA was higher in the atrium (A, 0.47 ± 0.04 and NA 0.94 ± 0.10 nmol g⁻¹) than in the ventricle (A, 0.25 ± 0.03 and NA, 0.30 ± 0.02 nmol g⁻¹). The 0.5 h period out of water and the 0.5-3 h period of pre-slaughter crowding led to accumulation of A, but not NA, in the atrium.
These data show that A, released during stress, accumulates in an undegraded form in the atrial tissue of the Atlantic salmon. This suggests a potent uptake mechanism for A in the atria presumably in the sympathetic nerve terminals. The acumulation of A in the atrium appears to reflect the period of high plasma A during stress.     

No evidence of kin bias in dispersion of young‐of‐the‐year Atlantic salmon Salmo salar L. in a natural stream

Ninety‐one young‐of‐the‐year Atlantic salmon Salmo salar were captured using a non‐invasive snorkelling technique in a 38 m section of Catamaran Brook, New Brunswick, Canada, to test whether related fish settle closer to one another than unrelated fish. A maximum likelihood estimate of parentage relationships assessed by genotyping eight microsatellite loci revealed five half‐sibling families in the sample of fish. Related juvenile S. salar were not found closer to one another than unrelated fish in three analyses at two spatial scales: a comparison of the relatedness of focal fish to their nearest neighbour and to their four nearest neighbours, and a correlation of the pair‐wise relatedness and distance matrices for all fish in the sample. The lack of a kin‐biased dispersion pattern may be related to the lower density of fish or the scarcity of full‐siblings at the study site compared to laboratory conditions.     

Synergistic effect on mortality in Atlantic salmon,Salmo salar,smolt caused by osmotic stress and presence of predators

Synopsis The hypothesis that an increase in the mortality rate of Atlantic salmon would be produced by the synergistic effect of osmotic stress and of stress due to the presence of predators was tested by putting two groups of smolts (one acclimated to seawater and the other not acclimated) into a tank containing predators. These smolts were tested afterwards in a seawater-challenge test, together with smolts of the control groups for both experimental groups. The mortality rate of the non-acclimated and predator-exposed smolts was 90%, and that of the seawater acclimated and predator-exposed smolts 43%, compared to no mortality among the control groups. Lack of acclimation to seawater did not in itself induce any lethal stress, but in conjunction with predator-stress there was a synergistic effect, leading to an increase in the mortality rate. The hypothesis is therefore considered to be valid.     

Rapid Carbon Turnover During Growth of Atlantic Salmon (Salmo salar) Smolts in Sea Water, and Evidence for Reduced Food Consumption by Growth-Stunts

Wild Atlantic salmon smolts were captured during spring out-migration in the Northwest Miramichi River, New Brunswick, Canada, and placed on an isotopically distinct hatchery diet to determine the relative contributions of growth and metabolic turnover to isotopic change. As expected for an ectothermic species, growth explained a large amount of isotopic variation in changing stable carbon ratios of muscle tissue (average r ²= 0.46 ) for the 3 years of study. Turnover rates of muscle carbon in all 3 years in growing fish (0.24–0.66 month^–1) were higher than previously reported values for other ectothermic species, but there was little evidence for isotopic change in non-growers (average r ²= 0.041, p > 0.1). It is unlikely that non-growers had consumed any of the hatchery diet over a 2-month period, thus preventing them from acquiring the new carbon isotopic signature. This period of food deprivation resulted in nitrogen-15 enrichment in liver relative to muscle (p= 0.003). It is advised that future isotope studies of metabolic turnover rates in ectotherms be conducted on slow-growing animals over a long time period. This would serve to avoid the obscuring effects of growth on isotopic change, and provide stronger comparisons to endothermic tissue turnover rates.     

Ecological determinants and temporal stability of the within-river population structure in Atlantic salmon (Salmo salar L.)

A gene diversity analysis was performed using microsatellite loci in order to (i) describe the extent and pattern of population structure in Atlantic salmon (Salmo salar L.) within a river system; (ii) establish the importance of quantifying the signal:noise ratio in accurately estimating population structure; and (iii) assess the potential usefulness of two evolutionary models in explaining within-river population structure from the ecological and habitat characteristics of Atlantic salmon. We found weak, yet highly significant microscale spatial patterning after accounting for variance among temporal replicates within sites. Lower genetic distances were observed among temporal samples at four sampling sites whereas no evidence for temporal stability was observed at the other three locations. The component of genetic variance attributable to either temporal instability and/or random sampling errors was almost three times more important than the pure spatial component. This indicates that not considering signal:noise ratio may lead to an important overestimation of genetic substructuring in situations of weak genetic differentiation. This study also illustrates the usefulness of the member-vagrant hypothesis to generate a priori predictions regarding the number of subpopulations that should compose a species, given its life-history characteristics and habitat structure. On the other hand, a metapopulation model appears better suited to explain the extent of genetic divergence among subpopulations, as well as its temporal persistence, given the reality of habitat patchiness and environment instability. We thus conclude that the combined use of both models may offer a promising avenue for studies aiming to understand the dynamics of genetic structure of species found in unstable environments.     

Mitochondrial DNA and allozyme analysis of Atlantic salmon, Salmo salar L., in England and Wales

Allozyme variation was examined in 429 Atlantic salmon, from seven sampling sites in England and Wales. Contingency tests for genetic homogeneity using three diagnostic loci revealed no significant differences among populations from the north-east and north-west of the U.K., but detected significant aspartate aminotransferase (AAT) differences between these samples and those from the R. Itchen in Hampshire (southern England). Mitochondrial DNA variation was analysed in 40 salmon from five spawning sites in the R. Itchen. Eight restriction endonucleases were used, of which three (AVAII, HAE III and HINFI) revealed polymorphisms. Six matriarchal lineages were discriminated. Two of the six mtDNA types observed were site-specific. These results suggest a reasonable expectation of discriminating spawning aggregations of Atlantic salmon.     

The environmental requirements of Atlantic salmon, Salmo salar L., during their passage through the Thames Estuary, 1982–1989

An examination of data on both water quality in the Thames Estuary and on the count of salmon. Salmo salar L . trapped in fresh water above the head of the tide in 1982–1989, was carried out to establish statistical correlations. The annual and monthly return of salmon as 1-sea-winter fish (grilse) in June to September was negatively correlated with water temperature, a nil catch being associated with a maximum value of 24.2° C, coupled with lower values maintained over substantial lengths of the estuary (e.g. 21.5° C over a distance of no more than 50 km). The annual return was negatively related to the extent to which the estuary was predicted to be lethal from the combination of low concentration of dissolved oxygen (DO) and high temperature, a reduction in DO of 1 mg 1 ¹ being equivalent to an increase in temperature of 4° C. The annual return for the whole year was directly related to the return in July to September. Depending upon the year, the monthly returns were related to both DO and temperature; they were reduced to a tenth at a 95 percentile DO of 2.7 mg 1 ¹, whilst the weekly catches were reduced to zero at 2.4 mg 1⁻¹. Weekly catches increased with river flow and daily catches increased with both river flow and tidal height. The few mortalities observed in the estuary in July are generally related to the quality of the water as predicted from the combination of high temperature and low DO.     

Maladaptation and phenotypic mismatch in hatchery‐reared Atlantic salmon Salmo salar released in the wild

Changes in body shape, fluctuating asymmetry (FA) and crypsis were compared among Atlantic salmon Salmo salar fry kept as controls in captivity and those released and subsequently recaptured in the wild according to a before‐after‐control‐impact (BACI) design. Hatchery fish that survived in the wild became more cryptic and displayed a much lower incidence of fin erosion and of asymmetric individuals than control fish kept in captivity. Significant differences in body shape were also apparent, and survivors had longer heads, thicker caudal peduncles and a more streamlined body shape than hatchery controls as early as 20 days following stocking, most likely as a result of phenotypic plasticity and non‐random, selective mortality of maladapted phenotypes. Hatchery‐reared fish typically perform poorly in the wild and the results of this study indicate that this may be due to phenotypic mismatch, i.e. because hatcheries generate fish that are phenotypically mismatched to the natural environment.     

Heart rate as an indicator of metabolic rate and activity in adult Atlantic salmon, Salmo salar

Telemetered heart rate (f_H) was examined as an indicator of activity and oxygen consumption rate (VO₂) in adult, cultivated, Atlantic salmon, Salmo salar L. Heart rate was measured during sustained swimming in a flume for six fish at 10° C [mean weight, 1114 g; mean fork length (f. l.), 50·6 cm] and seven fish at 15° C (mean weight, 1119 g; mean f. l., 50·7 cm) at speeds of up to 2·2 body lengths/s. Semi–logarithmic relationships between heart rate and swimming speed were obtained at both temperatures. Spontaneously swimming fish in still water exhibited characteristic heart rate increases associated with activity. Heart rate and Vo₂ were monitored simultaneously in a 575–1 circular respirometer for six fish (three male, three female) at 4° C (mean weight, 1804 g; mean F. L., 62· cm) and six fish (three male, three female) at 10° C (mean weight, 2045 g; mean f. l., 63·2 cm) during spontaneous but unquantified activity. Linear regressions were obtained by transforming data for both f_H and Vo₂ to log values. At each temperature, slopes of the regressions between f_H and Vo₂ for individual fishes were not significantly different, but in some cases elevations were. All differences in elevation were between male and female fish. There were no significant differences in regression slope or elevation for fish of the same sex at the two temperatures and so regressions were calculated for the sexes, pooling data from 4 and 10° C. There was no significant difference in the mean ± S. D. Vo₂ between the sexes at 4° C (male, 66·0 ± 59·6 mgO₂ kg^?1 h^?1; female, 88·0 ± 60·1 mgO₂ kg^?1 h^?1) or 10° C (male, 166·2 ± 115·4 mgO₂ kg^?1 h^?1; female, 169·2 ± 111–1 mgO₂ kg^?1h^?1). Resting Vo₂ (x?± s. d.) at 4°C was 36·7 ± 8.4 mgO₂ kg^?1 h^?1, and 10° C was 72·8 ± 11·9 mgO₂ kg^?1 h^?1. Maximum Vo₂ (x?± S. D.) at 4° C was 250·6 ± 40·2 mgO₂ kg^?1 h^?1, and at 10° C was 423·6 ± 25·2 mgO₂ kg^?1 h^?1. Heart rate appears to be a useful indicator of metabolic rate over the temperature range examined, for the cultivated fish studied, but it is possible that the relationship for wild fish may differ.     

Prespawning stress in farmed Atlantic salmon Salmo salar: maternal cortisol exposure and hyperthermia during embryonic development affect offspring survival, growth and incidence of malformations

In the current study, mature female Atlantic salmon Salmo salar were given intraperitoneal cortisol implants with the purpose of elucidating potentially detrimental effects on offspring survival, growth, morphological development and also the ability of prenatally stressed offspring to cope with an environmental stressor, typified by a period of mild hyperthermia. Augmented levels of maternal cortisol inflicted a range of progeny somatic parameters, as reflected in increased mortality, reduced fork length and mass, diminished yolk-sac volume, decelerated yolk-sac utilization and, to some extent, enhanced prevalence of morphological malformations. The most pronounced consequences on offspring performance were demonstrated in specimens exposed to both enhanced prenatal cortisol and a subsequent episode of hyperthermia. This accentuates the importance of knowledge on how the maternal endocrinological state during gametogenesis may impinge on offspring characters in farmed Atlantic salmon, and further substantiates the necessity of incorporating maternal effects when evaluating rearing conditions and animal welfare in commercial aquaculture.     

Migratory behaviour and survival rates of wild northern Atlantic salmon Salmo salar post‐smolts: effects of environmental factors

To study smolt behaviour and survival of a northern Atlantic salmon Salmo salar population during river descent, sea entry and fjord migration, 120 wild S. salar were tagged with acoustic tags and registered at four automatic listening station arrays in the mouth of the north Norwegian River Alta and throughout the Alta Fjord. An estimated 75% of the post‐smolts survived from the river mouth, through the estuary and the first 17 km of the fjord. Survival rates in the fjord varied with fork length (L_F), and ranged from 97·0 to 99·5% km^?1. On average, the post‐smolts spent 1·5 days (36 h, range 11–365 h) travelling from the river mouth to the last fjord array, 31 km from the river mouth. The migratory speed was slower (1·8 L_F s^?1) in the first 4 km after sea entry compared with the next 27 km (3·0 L_F s^?1). Post‐smolts entered the fjord more often during the high or ebbing tide (70%). There was no clear diurnal migration pattern within the river and fjord, but most of the post‐smolts entered the fjord at night (66%, 2000–0800 hours), despite the 24 h daylight at this latitude. The tidal cycle, wind‐induced currents and the smolts' own movements seemed to influence migratory speeds and routes in different parts of the fjord. A large variation in migration patterns, both in the river and fjord, might indicate that individuals in stochastic estuarine and marine environments are exposed to highly variable selection regimes, resulting in different responses to environmental factors on both temporal and spatial scales. Post‐smolts in the northern Alta Fjord had similar early marine survival rates to those observed previously in southern fjords; however, fjord residency in the north was shorter.     

Seventy‐five EST‐linked Atlantic salmon (Salmo salar L.) microsatellite markers and their cross‐amplification in five salmonid species

An Atlantic salmon (Salmo salar L.) expressed sequence tag (EST) database consisting of 58 146 ESTs was screened for microsatellite sequences. Subsequent development of 75 polymorphic EST‐associated microsatellite markers in this species is described together with cross‐species amplification results of 133 gene‐associated tandem repeat markers in five salmonid species (Salmo trutta, Oncorhynchus mykiss, Salvelinus aplinus, Thymallus thymallus, Coregonus lavaretus). The number of alleles among EST‐linked microsatellites in Atlantic salmon ranged from two to 41 with an average of 12 alleles per locus. Cross‐species amplification resulted in detection of a total of 111 polymorphic locus‐species combinations (12–32 loci per species).     

Using movements and diet analyses to assess effects of introduced muskellunge (Esox masquinongy) on Atlantic salmon (Salmo salar) in the Saint John River, New Brunswick

The muskellunge was introduced in the Saint John River system from stockings in a headwater lake in the 1970s. They have migrated down the system as far as the river’s first dam, Mactaquac Hydroelectric Facility, at Fredericton and appear to have established several reproducing populations along the river. This exotic invader represents a potential threat to the severely depleted Atlantic salmon stocks in the river. We radio-tracked muskellunge over a 2-year period in the middle reaches. Home ranges extended to ∼100 km in both riverine and lacustrine areas, including 78% of individuals trans-located upstream of the dam making their way back through the dam successfully. Downstream of the dam, home ranges were <25 km. No spawning areas were detected. An isotope analyses of diet indicated that the large sub-adults and adults had established the greatest proportion of their biomass in a more ¹⁵N depleted environment typical of areas farther upstream. Isotope mixing models could not accurately determine the proportion of Atlantic salmon smolts that may have been consumed by muskellunge, but anadromous salmon had ≤7% probabilities of being in the diet. A bioenergetics model suggested ≤5% of the annual food intake by muskellunge occurs during the smolt out-migration period. For the Saint John River, the impacts of growing numbers of muskellunge are multi-faceted creating a complex management challenge. Muskellunge appear to minimally increase predation risk for Atlantic salmon smolts while their increasing numbers are creating a growing recreational fishery and potential threat to the native fish community and ecosystem.