Compensatory releases reduce genetic differentiation among Atlantic salmon populations in the Baltic Sea: evidence from the River Ume‐Vindelälven

About 90% of salmon smolts in the Baltic Sea derive from hatchery enhancement programmes designed to compensate for loss of catches and genetic resources due to the damming of salmon rivers. The potential threat of genetic homogenization from extensive hatchery releases, however, has not been thoroughly investigated. We provide evidence that straying from deliberate releases poses a threat to indigenous populations by identifying the origin of 127 fin‐clipped (hatchery) salmon caught in the River Ume‐Vindelälven during 1997 to 2000, using mtDNA and six microsatellite loci. The analysis of eight potential donor stocks revealed that compensatory releases from the R. Ångerman and R. Luleälven hatcheries have resulted in a significant amount of straying to the river Ume‐Vindelälven (at least 10 and 12 migrants per year). As predicted to due to increased migration, the analysis of temporal samples from the wild population of R. Vindelälven showed a decreasing trend in genetic differentiation estimates (measured as Fst) relative to hatchery the hatchery strains of R. Ångermanälven and R. Luleälven. Our results suggest that gene flow from compensatory releases poses a serious threat to the genetic makeup of the existing wild populations in the Baltic.     

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.     

Assessing seasonal changes in stock composition of Atlantic salmon catches in the Baltic Sea with genetic stock identification

The feasibility of using genetic stock identification to analyse seasonal changes in stock compositions of Atlantic salmon catches in the Baltic Sea was examined. The analysis employed seven variable allozyme loci from most of the potentially contributing stocks (16) from Finland and Sweden. Catch samples were collected from Finnish salmon fisheries in the eastern Bothnian Sea during the 1992 fishing season. Simulation studies were used to evaluate the feasibility of identifying Baltic salmon stocks with allozyme data. Special attention was paid to analysing the wild production of salmon stocks. Clear seasonal differences in stock composition were found. The estimates were compared with smolt production and Carlin-tag data. The proportions of the Neva and Oulujoki river stocks could be estimated as individual stocks, whereas the contributions of the remaining stocks were estimated as four composite stock groups. One of the groups consisted of wild stocks from the rivers Kalixälven and Simojoki. Identification of this group, which could be used as an index of wild production in the catches, requires catch sample sizes >300 salmon if <15% error is required.     

Overview of salmon stock enhancement in southeast Alaska and compatibility with maintenance of hatchery and wild stocks

Modern salmon hatcheries in Southeast Alaska were established in the 1970s when wild runs were at record low levels. Enhancement programs were designed to help rehabilitate depressed fisheries and to protect wild salmon stocks through detailed planning and permitting processes that included focused policies on genetics, pathology, and management. Hatcheries were located away from significant wild stocks, local sources were used to develop hatchery broodstocks, and juveniles are marked so management can target fisheries on hatchery fish. Initially conceived as a state-run system, the Southeast Alaska (SEAK) program has evolved into a private, non-profit concept centered around regional aquaculture associations run by fishermen and other stakeholders that pay for hatchery operations through landing fees and sale of fish. Today there are 15 production hatcheries and 2 research hatcheries in SEAK that between 2005 and 2009 released from 474 to 580 million (average 517 million) juvenile salmon per year. During this same period commercial harvest of salmon in the region ranged from 28 to 71 million salmon per year (average 49 million). Contributions of hatchery-origin fish to this harvest respectively averaged 2%, 9%, 19%, 20%, and 78% for pink, sockeye, Chinook, coho, and chum salmon. Both hatchery and wild salmon stocks throughout much of Alaska have experienced high marine survivals since the 1980s and 1990s resulting in record harvests over the past two decades. Although some interactions between hatchery salmon and wild salmon are unavoidable including increasing concerns over straying of hatchery fish into wild salmon streams, obvious adverse impacts from hatcheries on production of wild salmon populations in this region are not readily evident.     

Differential Reproductive Success of Sympatric, Naturally Spawning Hatchery and Wild Steelhead, Oncorhynchus mykiss

Hatchery propagation of salmonids has been practiced in western North America for over a century. However, recent declines in wild salmon abundance and efforts to mitigate these declines through hatcheries have greatly increased the relative abundance of fish produced in hatcheries. The over-harvest of wild salmon by fishing mixed hatchery and wild stocks has been of concern for many years but genetic interactions between populations, such as hybridization, introgression and outbreeding depression, may also compromise the sustainability of wild populations. Our goal was to examine whether a newly established hatchery population of steelhead trout successfully reproduced in the wild and to compare their rate of reproductive success to that of sympatrically spawning native steelhead. We used eight microsatellite loci to create allele frequency profiles for baseline hatchery and wild populations and assigned the smolt (age 2) offspring of this parental generation to a population of origin. Adults originating from a generalized hatchery stock artificially selected for early return and spawning date were successful at reproducing in Forks Creek, Washington. Although hatchery females (N = 90 and 73 in the two consecutive years of the study) produced offspring that survived to emigrate as smolts, they produced only 4.4–7.0% the number produced per wild female (N = 11 and 10). This deficit in reproductive success implies that the proportion of hatchery genes in the mixed population may diminish since deliberate releases into the river have ceased. This hypothesis is being tested in a long-term study at Forks Creek.     

Straying of hatchery salmon in Prince William Sound,Alaska

The straying of hatchery salmon may harm wild salmon populations through a variety of ecological and genetic mechanisms. Surveys of pink (Oncorhynchus gorbuscha), chum (O. keta) and sockeye (O. nerka) salmon in wild salmon spawning locations in Prince William Sound (PWS), Alaska since 1997 show a wide range of hatchery straying. The analysis of thermally marked otoliths collected from carcasses indicate that 0–98% of pink salmon, 0–63% of chum salmon and 0–93% of sockeye salmon in spawning areas are hatchery fish, producing an unknown number of hatchery-wild hybrids. Most spawning locations sampled (77%) had hatchery pink salmon from three or more hatcheries, and 51% had annual escapements consisting of more than 10% hatchery pink salmon during at least one of the years surveyed. An exponential decay model of the percentage of hatchery pink salmon strays with distance from hatcheries indicated that streams throughout PWS contain more than 10% hatchery pink salmon. The prevalence of hatchery pink salmon strays in streams increased throughout the spawning season, while the prevalence of hatchery chum salmon decreased. The level of hatchery salmon strays in many areas of PWS are beyond all proposed thresholds (2–10%), which confounds wild salmon escapement goals and may harm the productivity, genetic diversity and fitness of wild salmon in this region     

Analysis of gene associated tandem repeat markers in Atlantic salmon (<Emphasis Type="Italic">Salmo salar L.</Emphasis>) populations: implications for restoration and conservation in the Baltic Sea

Patterns of genetic diversity and differentiation among five wild and four hatchery populations of Atlantic salmon in the Baltic Sea were assessed based on eight assumedly neutral microsatellite loci and six gene-associated markers, including four expressed sequence tag (EST) linked and two major histocompatibility complex (MHC) linked tandem repeat markers (micro- and mini-satellites). The coalescent simulations based on the method of Beaumont and Nichols (1996, Proc. R. Soc. Lond. Ser. B – Biol. Sci., 263, 1619–1626) indicated that two loci (MHCIIα and Ssa171, with the lowest and highest overall F_ST estimates, respectively) exhibited significant departures (P<0.05) from the neutral expectations. Another coalescent-based test for selective neutrality (Vitalis et al. 2001, Genetics, 158, 1811–1823) further supported the outlier status of the Ssa171 microsatellite locus but not of the MHCIIα linked minisatellite. In addition, actin related protein linked microsatellite locus was identified with this test as an outlier in six pairwise population comparisons. All genetic diversity estimates revealed more genetic variation in hatchery stocks than in the small wild salmon populations from the Gulf of Finland. However, the wild populations possessed alleles at gene-associated markers (e.g. MHCI and IGF) not found in the hatchery stocks, which together with moderate genetic differentiation and distinctive environmental conditions justifies the special conservation measures for the last remaining native salmon populations in the Gulf of Finland.     

Restitution and genetic differentiation of salmon populations in the southern Baltic genotyped with the Atlantic salmon 7K SNP array

Background

Native populations of Atlantic salmon in Poland, from the southern Baltic region, became extinct in the 1980s. Attempts to restitute salmon populations in Poland have been based on a Latvian salmon population from the Daugava river. Releases of hatchery reared smolts started in 1986, but to date, only one population with confirmed natural reproduction has been observed in the Slupia river. Our aim was to investigate the genetic differentiation of salmon populations in the southern Baltic using a 7K SNP (single nucleotide polymorphism) array in order to assess the impact of salmon restitution in Poland.

Methods

One hundred and forty salmon samples were collected from: the Polish Slupia river including wild salmon and individuals from two hatcheries, the Swedish Morrum river and the Lithuanian Neman river. All samples were genotyped using an Atlantic salmon 7K SNP array. A set of 3218 diagnostic SNPs was used for genetic analyses.

Results

Genetic structure analyses indicated that the individuals from the investigated populations were clustered into three groups i.e. one clade that included individuals from both hatcheries and the wild population from the Polish Slupia river, which was clearly separated from the other clades. An assignment test showed that there were no stray fish from the Morrum or Neman rivers in the sample analyzed from the Slupia river. Global F_ST over polymorphic loci was high (0.177). A strong genetic differentiation was observed between the Lithuanian and Swedish populations (F_ST = 0.28).

Conclusions

Wild juvenile salmon specimens that were sampled from the Slupia river were the progeny of fish released from hatcheries and, most likely, were not progeny of stray fish from Sweden or Lithuania. Strong genetic differences were observed between the salmon populations from the three studied locations. Our recommendation is that future stocking activities that aim at restituting salmon populations in Poland include stocking material from the Lithuanian Neman river because of its closer geographic proximity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12711-015-0121-9) contains supplementary material, which is available to authorized users.     

Effect of hatchery environment on cranial morphology and developmental stability of Atlantic salmon (Salmo salar L.) from North‐West Russia

The study addresses the effect of hatchery rearing on morphological variation and developmental stability of Atlantic salmon parr from North‐West Russia. Totally, we collected nine samples. Four wild samples were collected from each of the rivers Kola, Umba, Keret’ and Shuia. Five samples of hatchery‐reared parr were the first‐generation progeny of wild adults from these rivers reared separately at the four hatcheries (one hatchery was represented by two samples). Ten meristic and 48 morphometric cranial characters were analysed. We studied the morphological divergence between wild and hatchery fishes of the same river of origin. To analyze developmental stability we used fluctuating asymmetry (random deviations from perfect bilateral symmetry). It was found that hatchery‐reared parr significantly differ from wild parr in both meristic characters and the shape of cranial bones. Different hatcheries caused similar effect on morphological variation in all populations. Fluctuating asymmetry in morphometric characters was significantly higher in hatchery fish than in wild from the Shuia River, indicating a higher level of developmental instability. However, wild parr from the Keret’ River had significantly higher fluctuating asymmetry than cultivated parr of the same origin, possible due to a high infection pressure of the parasite Gyrodactylus salaris Malmberg which has led to significant decline of the wild salmon population in this river, or from genetic changes caused by cultivation. The obtained results indicate a notable effect of hatchery environment on Atlantic salmon’s phenotype.     

Lack of molecular genetic divergence between sea-ranched and wild sea trout (Salmo trutta)

The supportive breeding programme for sea trout (Salmo trutta) in the River Dalälven, Sweden, is based on a sea‐ranched hatchery stock of local origin that has been kept ‘closed’ to the immigration of wild genes since the late 1960s (about seven generations). In spite of an apparent potential for substantial uni directional gene flow from sea‐ranched to wild (naturally produced) trout, phenotypic differences with a presumed genetic basis have previously been observed between the two ‘stocks’. Likewise, two previous studies of allozyme and mitochondrial DNA variation based on a single year of sampling have indicated genetic differentiation. In the present study we used microsatellite and allozyme data collected over four consecutive years, and tested for the existence of overall genetic stock divergence while accounting for temporal heterogeneity. Statistical analyses of allele frequency variation (F‐statistics) and multilocus genotypes (assignment tests) revealed that wild and sea‐ranched trout were significantly different in three of four years, whereas no overall genetic divergence could be found when temporal heterogeneity among years within stocks was accounted for. On the basis of estimates of effective population size in the two stocks, and of F_ST between them, we also assessed the level of gene flow from sea‐ranched to wild trout to be ≈ 80% per generation (with a lower confidence limit of ≈ 20%). The results suggest that the reproductive success of hatchery and naturally produced trout may be quite similar in the wild, and that the genetic characteristics of the wild stock are largely determined by introgressed genes from sea‐ranched fish.     

Genomewide introgressive hybridization patterns in wild Atlantic salmon influenced by inadvertent gene flow from hatchery releases

Many salmonid fish populations are threatened by genetic homogenization, primarily due to introgressive hybridization with hatchery‐reared conspecifics. By applying genomewide analysis using two molecular marker types (1986 SNPs and 17 microsatellites), we assessed the genetic impacts of inadvertent gene flow via straying from hatchery releases on wild populations of Atlantic salmon in the Gulf of Finland, Baltic Sea, over 16 years (1996–2012). Both microsatellites and SNPs revealed congruent population genetic structuring, indicating that introgression changed the genetic make‐up of wild populations by increasing genetic diversity and reducing genetic divergence. However, the degree of genetic introgression varied among studied populations, being higher in the eastern part and lower in the western part of Estonia, which most likely reflects the history of past stocking activities. Using kernel smoothing and permutation testing, we detected considerable heterogeneity in introgression patterns across the genome, with a large number of regions exhibiting nonrandom introgression widely dispersed across the genome. We also observed substantial variation in nonrandom introgression patterns within populations, as the majority of genomic regions showing elevated or reduced introgression were not consistently detected among temporal samples. This suggests that recombination, selection and stochastic processes may contribute to complex nonrandom introgression patterns. Our results suggest that (i) some genomic regions in Atlantic salmon are more vulnerable to introgressive hybridization, while others show greater resistance to unidirectional gene flow; and (ii) the hybridization of previously separated populations leads to complex and dynamic nonrandom introgression patterns that most likely have functional consequences for indigenous populations.     

Some consequences of Pacific salmon hatchery production in Kamchatka: changes in age structure and contributions to natural spawning populations

Aggregate hatchery production of Pacific salmon in the Kamchatka region of the Russian Federation is very low (< 0.5% of total harvest, with five hatcheries releasing approximately 41 M juvenile salmon annually), but contributions in certain rivers can be substantial. Enhancement programs in these rivers may strongly influence fitness and production of wild salmon. In this paper we document significant divergence in demographic traits in hatchery salmon populations in the Bolshaya River and we estimate the proportion of hatchery chum salmon in the total run in the Paratunka River to demonstrate the magnitude of enhancement in this system. We observed a reduction in the expression of life history types in hatchery populations (ranging from 1 to 9 types) compared to wild populations (17 types) of sockeye salmon in the Bolshaya River. We found similar trends in Chinook salmon in the same river system. This reduced life history diversity may make these fish less resilient to changes in habitat and climate. We estimate hatchery chum salmon currently contribute 17-45% to the natural spawning population in the Paratunka River. As hatchery fish increase in numbers at natural spawning sites, this hatchery production may affect wild salmon production. It is important to investigate the risk of introgression between hatchery and wild salmon that can lead to reduction in salmon fitness in Kamchatka rivers, as well as the potential of ecological interactions that can have consequences on status of wild salmon and overall salmon production in this region.     

The effect of sexual maturation on the spatial distribution of Baltic salmon

This study examines the migratory habits of tagged mature and immature Atlantic salmon parr Salmo salar released from the Norrfors hatchery on the River Umeälven, Sweden from 1975 to 1977 and 1988 to 1990. Tags were recaptured in the Baltic Main Basin, the Gulf of Bothnia, and in the Umeälven. Ninety-three per cent of previously mature males were recovered in the Umeälven compared with 23% of the previously immature smolts during the calendar year of release. In the second year after release (grilse year), the proportion of early maturing males recovered in the Umeälven was significantly greater than the proportion of previously immature smolts recovered in the Umeälven. Likewise, the proportion of previously mature males recovered in the Main Basin was significantly less than the proportion of previously immature smolts recovered in the Main Basin in the second calendar year after release. Previously mature males rematured after fewer years at sea, on average, than the previously immature smolts. Following the second calendar year after release, the proportions of previously mature males and previously immature smolts were not significantly different throughout the Baltic Sea.     

Wild and hatchery reproduction of pink and chum salmon and their catches in the Sakhalin-Kuril region,Russia

In the Sakhalin-Kuril region hatchery culture of pink and chum salmon is of great importance compared to other regions of the Russian Far East. During the last 30 years the number of hatcheries increased two-fold, and significant advances were made in hatchery technologies. As a result, chum salmon capture in regions where hatcheries operate (southwestern and eastern Sakhalin coasts, and Iturup Island) was 9 times as high during 2006–2010 than during 1986–1990, whereas wild chum salmon harvest markedly declined. Recent dynamics in pink salmon catch appear to track trends in natural spawning in monitored index rivers, suggesting natural-origin pink salmon play a dominant role in supporting the commercial fishery. It remains uncertain as to whether hatcheries have substantially supplemented commercial catch of pink salmon in this region, and I recommend continued research (including implementing mass marking and recovery programs) before decisions are made regarding increasing pink salmon hatchery production. Location of hatcheries in spawning river basins poses problems for structuring a management system that treats hatchery and wild populations separately. Debate continues regarding the existence and importance of density-dependent processes operating in the ocean environment and the role hatcheries play in these processes. Loss of critical spawning habitat for chum salmon in the Sakhalin-Kuril region has lead to significant declines in their abundance. I conclude by recommending increases in releases of hatchery chum salmon numbers in the region to help recover depressed wild populations and provide greater commercial fishing benefits in the region.     

Managed metapopulations: do salmon hatchery 'sources' lead to in-river 'sinks' in conservation?

Maintaining viable populations of salmon in the wild is a primary goal for many conservation and recovery programs. The frequency and extent of connectivity among natal sources defines the demographic and genetic boundaries of a population. Yet, the role that immigration of hatchery-produced adults may play in altering population dynamics and fitness of natural populations remains largely unquantified. Quantifying, whether natural populations are self-sustaining, functions as sources (population growth rate in the absence of dispersal, λ>1), or as sinks (λ<1) can be obscured by an inability to identify immigrants. In this study we use a new isotopic approach to demonstrate that a natural spawning population of Chinook salmon, (Oncorhynchus tshawytscha) considered relatively healthy, represents a sink population when the contribution of hatchery immigrants is taken into consideration. We retrieved sulfur isotopes ((34)S/(32)S, referred to as δ(34)S) in adult Chinook salmon otoliths (ear bones) that were deposited during their early life history as juveniles to determine whether individuals were produced in hatcheries or naturally in rivers. Our results show that only 10.3% (CI = 5.5 to 18.1%) of adults spawning in the river had otolith δ(34)S values less than 8.5‰, which is characteristic of naturally produced salmon. When considering the total return to the watershed (total fish in river and hatchery), we estimate that 90.7 to 99.3% (CI) of returning adults were produced in a hatchery (best estimate = 95.9%). When population growth rate of the natural population was modeled to account for the contribution of previously unidentified hatchery immigrants, we found that hatchery-produced fish caused the false appearance of positive population growth. These findings highlight the potential dangers in ignoring source-sink dynamics in recovering natural populations, and question the extent to which declines in natural salmon populations are undetected by monitoring programs.     

Lack of trophic competition among wild and hatchery juvenile chum salmon during early marine residence in Taku Inlet,Southeast Alaska

Early marine trophic interactions of wild and hatchery chum salmon (Oncorhynchus keta) were examined as a potential cause for the decline in harvests of adult wild chum salmon in Taku Inlet, Southeast Alaska. In 2004 and 2005, outmigrating juvenile chum salmon were sampled in nearshore habitats of the inlet (spring) and in epipelagic habitat at Icy Strait (summer) as they approached the Gulf of Alaska. Fish were frozen for energy density determination or preserved for diet analyses, and hatchery stocks were identified from the presence of thermal marks on otoliths. We compared feeding intensity, diets, energy density, and size relationships of wild and hatchery stocks (n = 3123) across locations and weeks. Only hatchery fish feeding intensity was negatively correlated with fish abundance. In both years, hatchery chum salmon were initially larger and had greater energy density than wild fish, but lost condition in early weeks after release as they adapted to feeding on wild prey assemblages. Diets differed between the stocks at all inlet locations, but did not differ for hatchery salmon between littoral and neritic habitats in the outer inlet, where the stocks overlapped most. Both diets and energy density converged by late June. Therefore, if density-dependent interactions affect wild chum salmon, these effects must be very rapid because survivors in Icy Strait showed few differences. Our study also demonstrates that hatchery release strategies used near Taku Inlet successfully promote early spatial segregation and prey partitioning, which reduce the probability of competition between wild and hatchery chum salmon stocks.     

Comparison of maturation timing,egg size and fecundity between hatchery lines of chinook salmon and their wild donor stocks

Increasing concern has been expressed about the genetic effects of cultured salmonid fishes on natural populations. Avoidance of extreme negative outcomes was one reason for the establishment of a genetic management policy for the State of Alaska. However, domestication within the hatchery may still cause divergence from the wild donor population. This divergence could potentially lead to adverse impacts on wild stocks through straying and introgression. This study examines potential domestication in two Alaskan chinook salmon stocks. The Little Port Walter (LPW) Hatchery Chickamin River stock resulted from a small collection of wild broodstock in 1976. The LPW Unuk stock was founded with a larger number of individuals in 1976 and has had subsequent infusion of wild gametes. These lines have been maintained at LPW through ocean ranching of tagged smolts. Comparisons are made between the hatchery lines, progeny of wild chinook collected from the Chickamin and Unuk Rivers, and hybrids between the hatchery and wild groups. Mature ocean‐ranched female chinook salmon returning to the facility were periodically graded for ripeness and spawned. Body size and meristic measurements were collected from these mature spawners. Maturation timing, fecundity, and individual egg size of these fourth generation hatchery fish are compared with that of offspring of wild fish from the same donor stock. Stock of origin is confirmed for all spawners and offspring using microsatellite DNA analysis.     

Evidence for competition at sea between Norton Sound chum salmon and Asian hatchery chum salmon

Increasing production of hatchery salmon over the past four decades has led to concerns about possible density-dependent effects on wild Pacific salmon populations in the North Pacific Ocean. The concern arises because salmon from distant regions overlap in the ocean, and wild salmon populations having low productivity may compete for food with abundant hatchery populations. We tested the hypothesis that adult length-at-age, age-at-maturation, productivity, and abundance of a Norton Sound, Alaska, chum salmon population were influenced by Asian hatchery chum salmon, which have become exceptionally abundant and surpassed the abundance of wild chum salmon in the North Pacific beginning in the early 1980s. We found that smaller adult length-at-age, delayed age-at-maturation, and reduced productivity and abundance of the Norton Sound salmon population were associated with greater production of Asian hatchery chum salmon since 1965. Modeling of the density-dependent relationship, while controlling for other influential variables, indicated that an increase in adult hatchery chum salmon abundance from 10 million to 80 million adult fish led to a 72% reduction in the abundance of the wild chum salmon population. These findings indicate that competition with hatchery chum salmon contributed to the low productivity and abundance of Norton Sound chum salmon, which includes several stocks that are classified as Stocks of Concern by the State of Alaska. This study provides new evidence indicating that large-scale hatchery production may influence body size, age-at-maturation, productivity and abundance of a distant wild salmon population.     

Consequences to fitness‐related traits of hybridization between farmed and wild Atlantic salmon, Salmo salar

Increasing concern has been expressed about the genetic effects of cultured salmonid fishes on natural populations. Avoidance of extreme negative outcomes was one reason for the establishment of a genetic management policy for the State of Alaska. However, domestication within the hatchery may still cause divergence from the wild donor population. This divergence could potentially lead to adverse impacts on wild stocks through straying and introgression. This study examines potential domestication in two Alaskan chinook salmon stocks. The Little Port Walter (LPW) Hatchery Chickamin River stock resulted from a small collection of wild broodstock in 1976. The LPW Unuk stock was founded with a larger number of individuals in 1976 and has had subsequent infusion of wild gametes. These lines have been maintained at LPW through ocean ranching of tagged smolts. Comparisons are made between the hatchery lines, progeny of wild chinook collected from the Chickamin and Unuk Rivers, and hybrids between the hatchery and wild groups. Mature ocean‐ranched female chinook salmon returning to the facility were periodically graded for ripeness and spawned. Body size and meristic measurements were collected from these mature spawners. Maturation timing, fecundity, and individual egg size of these fourth generation hatchery fish are compared with that of offspring of wild fish from the same donor stock. Stock of origin is confirmed for all spawners and offspring using microsatellite DNA analysis.     

Adaptive and Neutral Genetic Variation and Colonization History of Atlantic Salmon, Salmo salar

Synopsis I combined neutral microsatellite markers with the major histocompatibility complex (MHC) class IIB to study genetic differentiation and colonization history in Atlantic salmon, Salmo salar, in the Baltic Sea and in the north-eastern Atlantic. Baltic salmon populations have lower levels of microsatellite genetic variation, in terms of heterozygosity and allelic richness than Atlantic populations, confirming earlier findings with other genetic markers, suggesting that the Baltic Sea populations have been exposed to genetic bottlenecks, most likely at a founding event. On the other hand, the level of MHC variation was similar in the Baltic and in the north-eastern Atlantic, indicating that positive balancing selection has increased the level of MHC-variation. Both microsatellite and MHC class IIB genetic variation give strong support to the hypothesis that the Baltic salmon are of a biphyletic origin, the southern population in this study is strongly differentiated from both the northern Baltic salmon populations and from the north-eastern Atlantic populations. Salmon may have colonized the northern Baltic Sea either from the south, via the so called “N?rke strait” or from the north, via a proposed historical connection between the White Sea and the northern Baltic. At microsatellites, no significant isolation-by distance was found at either colonization route. At the MHC, populations were significantly isolated by distance when assuming that colonization occurred via the “N?rke strait”.