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doi: 10.1016/S1054-3139(97)80001-Xpmid: N/A
The effect of deliberate or inadvertent transfers of cultured Atlantic salmon (Salmo salar L.) on wild conspecifics depends on the nature and extent of biologically important genetic diversity among wild and transferred fish. Tagging and genetic studies show that salmon are divided into local, reproductively discrete populations associated with individual river systems or tributaries within systems. These are likely to be linked by historical patterns of gene flow into larger aggregations, which can be conceptualized as metapopulations, within which an evolutionary dynamic of local population formation, genetic exchange and extinction probably occurs. Diversity among populations has been documented within and between rivers, between North American and European population groups, and between Baltic and Atlantic sub-groups within Europe. Diversity is in most cases associated with differences in biological performance relevant to survival and recruitment. As such, transfers have the potential to genetically alter native populations, reduce local adaptation, and negatively affect population viability and character.
Bourke, E. A.; Coughlan, J.; Jansson, H.; Galvin, P.; Cross, T. F.
doi: 10.1016/S1054-3139(97)80002-1pmid: N/A
A comprehensive understanding of the population structure of Atlantic salmon (Salmo salar L.) throughout the species range would help to determine the impact that cultured fish could have on wild populations. To help achieve this aim, Atlantic salmon samples were obtained from 14 locations throughout Europe (including Iceland) and screened for variation at 32 allozyme loci. A sample was also obtained from Canada to serve as an out-group. Seventeen allozyme loci were found to be variable in one or more of the populations studied and three, sAAT-4*, IDDH-2*, and mMEP-2* were variable across the range. This is the widest-ranging study to include ESTD-2*, FBALD-3*, and TPI-3*, which combined contributed 28% to the total genetic diversity detected. Genotype frequencies complied with Hardy-Weinberg expected proportions. Over all loci, highly significant heterogeneity was observed between samples. Alternate alleles segregating at ESTD-2* were found to be largely exclusive to Europe or North America. A neighbour-joining dendrogram was constructed to visualize relationships between populations and was consistent with previous findings that revealed Baltic and European clusters, with the Canadian population being the most genetically distinct. A significant association was observed between geographic and genetic distance, which suggests the potential for local adaptation, thus highlighting the need for conservation of wild populations.
Danielsdottir, A. K.; Marteinsdottir, G.; Arnason, F.; Gudjonsson, S.
doi: 10.1016/S1054-3139(97)80003-3pmid: N/A
The genetic variation within and between Atlantic salmon (Salmo salar L.) populations in Iceland was studied. A total of 1519 individuals from 32 rivers and three fish farms were examined. Nineteen isoenzyme systems were analysed, representing 49 loci, of which the number of polymorphic loci varied from two (4.1%) in the river Sunnudalsa to five (10.4%) in the farmed fish (Isno). Mean polymorphism (P) was 6.5%. Seven variable enzyme loci were observed in this study: AAT-4*, GLUDH-2*, GPI-3*, IDDH-2*, IDHP-3*, MDH-3* and MEP-2*.Average observed heterozygosity (H) varied from 0.022 (s.e.=0.014) to 0.048 (s.e.=0.023) and the mean H was 0.030. Heterogeneity of allele frequencies of sample years and age groups within and between the rivers and drainages in Iceland is presented. Wright's FST value was 0.062 and the total gene diversity (HT) was 0.048. Nei's genetic distance value (D) between salmon from the 32 rivers varied from 0.000 to 0.006, with a mean value of 0.0017. Genetic distance values between the salmon from the rivers and the farmed salmon varied from 0.000 to 0.008 (mean D=0.0032). The majority of salmon rivers in Iceland contain genetically distinct populations and in larger river systems there may be more than one population. Salmon populations within the same region showed lower values of genetic distance than between populations in different regions. These findings have important management implications. Every effort should be taken to avoid genetic mixing and consequent breakdown of stock differentiation causing destruction of the local adaptations in wild stocks. Salmon enhancement should, therefore, be conducted on a river stock basis, as has been the practice in Iceland for the last 10 to 15 years. Ocean ranching stations should use stocks from the local region, the size of the operation should be carefully considered and measures should be taken to minimize straying. Cage farming of salmon now only occurs in a few locations in Iceland and in most cases these are at a distance from salmon rivers. However, use of local stocks and sterile fish should be seriously considered.
McGinnity, P.; Stone, C.; Taggart, J. B.; Cooke, D.; Cotter, D.; Hynes, R.; McCamley, C.; Cross, T.; Ferguson, A.
doi: 10.1016/S1054-3139(97)80004-5pmid: N/A
Since Atlantic salmon (Salmo salar L.) used for farming are usually genetically different from local wild populations, breeding of escaped farmed salmon potentially results in genetic changes in wild populations. To determine the likelihood and impact of such genetic change, an experiment was undertaken, in a natural spawning tributary of the Burrishoole system in western Ireland, to compare the performance of wild, farmed, and hybrid Atlantic salmon progeny. Juveniles were assigned to family and group parentage by DNA profiling based on composite genotypes at seven minisatellite loci. Survival of the progeny of farmed salmon to the smolt stage was significantly lower than that of wild salmon, with increased mortality being greatest in the period from the eyed egg to the first summer. However, progeny of farmed salmon grew fastest and competitively displaced the smaller native fish downstream. The offspring of farmed salmon showed a reduced incidence of male parr maturity compared with native fish. The latter also showed a greater tendency to migrate as autumn pre-smolts. Growth and performance of hybrids were generally either intermediate or not significantly different from the wild fish. The demonstration that farmed and hybrid progeny can survive in the wild to the smolt stage, taken together with unpublished data that show that these smolts can survive at sea and home to their river of origin, indicates that escaped farmed salmon can produce long-term genetic changes in natural populations. These changes affect both single-locus and high-heritability quantitative traits, e.g. growth, sea age of maturity. While some of these changes may be advantageous from an angling management perspective, they are likely, in specific circumstances, to reduce population fitness and productivity. Full assessment of these changes will require details of marine survival, homing and reproductive performance of the adults together with information on the F2 generation.
doi: 10.1016/S1054-3139(97)80005-7pmid: N/A
The farming of Atlantic salmon has become an important industry in several countries, and breeding programmes have been implemented to improve genetic performance and adaptation to farm environments. Founder stocks used in the Norwegian salmon breeding programme have originated solely from Norwegian rivers and no extrinsic genes have been introduced. The majority (more than 90%) of the additive genetic variation in Norwegian populations of Atlantic salmon has been found within, and not between, river strains. The Norwegian breeding programme comprises four sub-populations. In the event of reduced additive genetic variability due to random drift in the closed breeding populations, crosses between the sub-populations could be made to re-establish the variability. Selection in itself is not expected to reduce the additive genetic variability as long as inbreeding is avoided. To date, seven different traits (body weight at slaughter, age of sexual maturation, survival in challenge tests with furunculosis and ISA, flesh colour, total fat content, and amount of fat tissues) have been included in the breeding goal. The selection response obtained is about 10% per generation for each of these traits. In the future, more traits are likely to be included. The results of and prospects for selective breeding and the use of modern DNA technology to improve genetic performance in aquatic species are discussed.
Koljonen, M. -L.; Pella, J. J.
doi: 10.1016/S1054-3139(97)80006-9pmid: N/A
Information about the occurrence and proportions of endangered wild Atlantic salmon stocks in Baltic Sea catches is invaluable to the fishery managers responsible for sustaining these populations. The usefulness of stock mixture analysis (SMA) from characters of sampled fish was evaluated by applying the method in an estimation of actual and simulated catch compositions. Samples for seven variable genetic loci, expressed by allozymes, and smolt age from 17 stocks from Finland and Sweden which are potential contributors to the fishery were used with similar samples collected in 1993 from catches in a Finnish trapnet fishery along the east coast of the Bothnian Sea. Identified by their character similarities, three stock management units and their estimated catch contributions were: resident Neva stock (29%), wild baseline stocks (28%), and migratory hatchery stocks (43%). The contributions of the units varied during the fishing season. With allozyme data alone, the management units were not so distinctive. Nevertheless, three major wild stocks (Tornionjoki, Kalixälven, and Simojoki), which accounted for 68% of the total wild smolt production of the area, were similar in their allozymes and distinctive from the other stocks. The increase in reliability of wild stock composition estimates when smolt age was added to the allozyme loci was evaluated by simulation. Smolt age improved the precision of composition estimates for both the combined wild baseline stocks and their aforementioned subset comprising three major stocks; it controlled bias better for the combined wild baseline stocks than their subset.
doi: 10.1016/S1054-3139(97)80007-0pmid: N/A
Cultured Atlantic salmon (Salmo salar L.) may be introduced into natural systems intentionally or accidentally. As smolts or post-smolts, they move to the feeding areas of wild salmon in the North Atlantic Ocean. As maturing fish, they return to the area of release and enter rivers to spawn. Lack of juvenile river experience is the prime reason why cultured salmon often enter fresh water later in the season than wild fish. During spawning, cultured female salmon from fish farms make fewer nests, tend to breed for a shorter period of time, are poorer at nest covering, and retain greater amounts of unspawned eggs than wild females. Cultured male salmon from fish farms exhibit less combat and display behaviour, have greaterdifficulty in acquiring access to mates, show less quivering and courting behaviour, and have lower reproductive success than wild males. However, cultured male salmon are more involved in prolonged, reciprocal fights than wild males and are, therefore, more often wounded. The reproductive success of cultured salmon increases with the time the fish have lived in nature before maturing sexually; for cultured females released in nature at the smolt stage, reproductive success is similar to that of wild females. The relative reproductive success of cultured males is smaller than that of corresponding females. Within both sexes of cultured and wild salmon, competitive spawning ability increases with body size. As a phenotypic response to increased growth rate during the first year of life, cultured salmon tend to have smaller sized but more numerous eggs than wild fish of the same size. Offspring of cultured salmon are more generally aggressive, more risk prone, and have a higher growth rate than wild offspring. Consequently, their survival rate in nature may be lower.
Berejikian, B. A.; Tezak, E. P.; Schroder, S. L.; Knudsen, C. M.; Hard, J. J.
doi: 10.1016/S1054-3139(97)80008-2pmid: N/A
Captive rearing is an evolving strategy for restoring depleted salmon populations; it involves capturing wild juvenile salmon from natural streams, rearing them in captivity to adulthood, and then releasing them as adults back into their natal streams to spawn naturally. The conservation benefit of captive rearing is that it bypasses the typically high smolt-to-adult mortality experienced by wild populations, but its success as a restoration strategy depends upon the ability of captively reared salmon to spawn and reproduce in natural streams. In an experimental channel, wild males dominated captively reared males of similar size in 86% of spawning events. Both wild and captively reared females attacked captively reared males more frequently than wild males, indicating a preference for wild over captively reared males, although the interplay between male dominance and female mate choice was unclear. Wild females established nesting territories earlier and constructed more nests per individual than captively reared females of similar size, suggesting a competitive advantage for wild females. Nevertheless, captively reared coho salmon demonstrated the full range of behaviors shown by wild coho salmon of both sexes and the ability to spawn naturally.
doi: 10.1016/S1054-3139(97)80009-4pmid: N/A
The genetic response of Atlantic salmon to culture is important in predicting the success of these fish in nature and their impacts on wild populations through competition and interbreeding. We compared a seventh-generation strain of farmed Atlantic salmon from Sunndalsøra, Norway, with its principal founder population from the wild, the River Namsen. The fish were reared from eggs in a common environment and assessed for the extent of genetic divergence in several fitness-related traits. Morphology had diverged, as farmed juveniles showed more robust bodies and smaller rayed fins than the wild juveniles. Ecologically important aspects of behaviour also differed. Farmed juveniles were more aggressive in a tank environment typical of culture facilities, while wild juveniles dominated in a stream-like environment. Farmed juveniles were also more risk-prone, reappearing from cover sooner after a simulated predator attack. It was not surprising that growth performance was higher in farmed than wild juveniles, as the former had been subjected to intentional selection for this trait. Correlated responses to this selection may also explain the higher rate of smolting and lower rate of male parr maturity in the farmed than the wild salmon. Competition with wild juveniles, however, negatively affected the growth of the farmed juveniles, particularly under semi-natural conditions. Our results indicate that farming of Atlantic salmon generates rapid genetic change, as a result of both intentional and unintentional selection in culture, that alters important fitness-related traits.
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