The European eel (Anguilla anguilla L.) has been a prime example of the panmixia paradigm because of its extraordinary adaptation to the North Atlantic gyral system, semelparous spawning in the Sargasso Sea and long trans-oceanic migration. Recently, this view was challenged by the suggestion of a genetic structure characterized by an isolation-by-distance (IBD) pattern. This is only likely if spawning subpopulations are spatially and/or temporally separated, followed by non-random larval dispersal. A limitation of previous genetic work on eels is the lack of replication over time to test for temporal stability of genetic structure. Here, we hypothesize that temporal genetic variation plays a significant role in explaining the spatial structure reported earlier for this species. We tested this by increasing the texture of geographical sampling and by including temporal replicates. Overall genetic differentiation among samples was low, highly significant and comparable with earlier studies (F ST Z0.0014; p!0.01). On the other hand, and in sharp contrast with current understandings, hierarchical analyses revealed no significant inter-location genetic heterogeneity and hence no IBD. Instead, genetic variation among temporal samples within sites clearly exceeded the geographical component. Our results provide support for the panmixia hypothesis and emphasize the importance of temporal replication when assessing population structure of marine fish species.

We examined the reproductive success of 48 adult brown trout (Salmo trutta L.) which were allowed to reproduce in a stream that was controlled for the absence of other trout. Parentage analyses based on 11 microsatellites permitted us to infer reproductive success and mate choice preferences in situ. We found that pairs with intermediate major histocompatibility complex (MHC) dissimilarity mated more often than expected by chance. It appears that female choice was the driving force behind this observation because, compared with other individuals, males with intermediate MHC dissimilarity produced a larger proportion of offspring, whereas female reproductive output did not show this pattern. Hence, rather than seeking mates with maximal MHC dissimilarity, as found in several species, brown trout seemed to prefer mates of intermediate MHC difference, thus supporting an optimality‐based model for MHC‐dependent mate choice.

Previous studies of genetic structure in the European eel have resulted in seemingly conflicting results, ranging from no detectable heterogeneity to small but statistically significant differences and isolation by distance patterns among eels sampled across the continental range. Differences with respect to sampling design and choice of molecular markers, combined with a lack of power estimates, complicate comparisons of existing results. In this study we have used six microsatellite markers and, for the first time, compared maturing silver eels of known age from southern and northern Europe (Italy and Baltic Sea). In comparison with previous studies, our data may give a better representation of potential spawning stocks because eels were sampled when having begun their migration toward the presumed spawning area in the Sargasso Sea. Despite large sample sizes (404 and 806 individuals) we could not observe any signs of genetic differentiation (average F ST ¼ À0.00003, P ¼ 0.61), and a power analysis showed that the true level of heterogeneity (if existing) must be exceedingly small to have remained undetected (say, F ST o0.0004). A tendency for slightly increased genetic differences between cohorts over time could be seen, but the amount of temporal change was minor and not statistically significant. Our findings reiterate the notion that previous reports of continental genetic differentiation in the European eel may be largely explained by uncontrolled temporal variation between juvenile glass eel samples.

During 1960–2002, the arrival times of all spawning male and female Atlantic salmon (Salmo salar L., 1758) and brown trout (Salmo trutta L., 1758) entering Dalälven River were recorded. To study the role of environmental variation in spawning migration timing, we used long-term temperature (river and sea) and river discharge data. For salmon, the spawning migration peak was strongly correlated with mean monthly sea and river temperatures during spring: salmon arrived earlier when temperatures were higher and later when temperatures were lower. River discharge explained little of the variation in migration timing. Female salmon migration showed a stronger correlation with temperature than male salmon migration, and female salmon arrived ≈18 days earlier than males. Trout showed a larger variation in their spawning migration, but river and sea temperatures and river discharge explained little of the variation. Trout females arrived ≈7 days earlier than males. The sea and river temperatures were highly correlated during the spawning migration, indicating that large climate processes determine the temperature regimes in the Baltic Sea and its tributaries. Time of arrival at the river was not correlated with ovulation date; a female salmon or brown trout arriving late could ovulate almost immediately, whereas a female arriving early could wait to ovulate.

Summary 1.Although releases of hatchery-produced salmonids to support conspecific wild populations have increased dramatically during recent decades, little information is available about the performance in the wild of hatchery fish and their offspring. Important factors determining the success and genetic outcomes of supportive breeding programmes include (i) the relative reproductive success of released hatchery fish in the wild, and (ii) the extent to which the propagation affects the variance in reproductive success in the population as a whole. 2. We performed two field experiments on brown trout Salmo trutta from the River Dalälven in Sweden, where we examined reproductive success in an experimental stream. In experiment 1 we compared reproductive success between trout from a seventhgeneration hatchery stock of native origin and wild-born trout from the river. In experiment 2, we compared reproductive success between seventh-generation hatchery trout and hatchery-reared trout derived from wild-born parents. Individual reproductive success, based on the number of offspring assigned using microsatellite markers, was assessed on three occasions after reproduction: immediately after hatching and after the first and second growth seasons. 3. In experiment 1 there were no significant differences in reproductive success between seventh-generation hatchery trout and wild-born trout. In experiment 2, males from wildborn parents were more successful than males from the seventh-generation hatchery stock, but this difference was not observed among females. 4. There was some evidence for a positive association between body size and reproductive success among females but not males. For males, the number of mates was significantly associated with reproductive success, but this relationship was not evident among females. 5. The variance in reproductive success was pronounced in both experiments, yielding estimates of the ratio between the genetically effective size and the census size of our experimental populations ranging from 0·12 to 0·59. 6. Synthesis and applications . Our results suggest that the reproductive success in the wild of hatchery-produced and wild-born trout with a common genetic background may be rather similar. These findings, in combination with the pronounced variance in reproductive success observed among breeders, indicate that supportive breeding can be managed to increase not only the census but also the genetically effective size of small, endangered salmonid populations. However, to minimize negative effects of hatchery selection, it is important to give priority to the restoration of natural habitats and thereby increase the reproductive output from individuals in the wild.

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 FST between them, we also assessed the level of gene flow from sea-ranched to wild trout to be approximately 80% per generation (with a lower confidence limit of approximately 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.

There is ample evidence that organisms adapt to their native environment when gene flow is restricted. However, evolution of plastic responses across discrete environments is less well examined. We studied divergence in means and plasticity across wild and hatchery populations of sea-run brown trout (Salmo trutta) in a common garden experiment with two rearing environments (hatchery and a nearly natural experimental stream). Since natural and hatchery environments differ, this arrangement provides an experiment in contemporary adaptation across the two environments. A Q(ST) - F(ST) approach was used to investigate local adaptation in survival and growth over the first summer. We found evidence for divergent selection in survival in 1 year and in body length in both years and rearing environments. In general, the hatchery populations had higher survival and larger body size in both environments. Q(ST) in body size did not differ between the rearing environments, and constitutive divergence in the means was in all cases stronger than divergence in the plastic responses. These results suggest that in this system, constitutive changes in mean trait values are more important for local adaptation than increased plasticity. In addition, ex situ rearing conditions induce changes in trait means that are adaptive in the hatchery, but potentially harmful in the wild, suggesting that hatchery rearing is likely to be a suboptimal management strategy for trout populations facing selection in the stream environment.

Summary1. Many threatened populations of salmonids depend on supplemental releases of hatchery-produced fish. Laboratory studies suggest that altered selection regimes in the hatchery may result in evolutionary changes of traits connected to fitness. Such changes can have profound effects on the performance of the hatchery fish following release in the natural environment, and may also affect the genetic characteristics of locally adapted wild populations. However, surprisingly few studies have looked at the ability of hatchery fish to compete with wild conspecifics under natural conditions. 2. We studied growth, survival and life-history adoption of a wild and a multigeneration sea-ranched strain of brown trout Salmo trutta in a semi-natural stream. The fish were planted in the stream as eyed eggs and their family and strain origins were later revealed by microsatellite markers. 3. In the first experiment, in which the experimental fish originated from a full-sib mating design, there were strong family effects on both growth and survival over the first growth season. In the second experiment, in which the experimental fish originated from a half-sib mating design, there were significant male and female effects on growth parameters but not on survival over the first growth season. 4. When family and male-female effects were accounted for, there were no differences between wild and sea-ranched trout in body size and condition factor after the first growth season, or in survival up to this stage. Nor was there any difference between the groups in the proportions that metamorphosed into the migratory smolt phase at 1 year of age. 5. Synthesis and applications. Our results suggest that wild-born trout of sea-ranched origin can successfully compete with trout of wild origin under semi-natural conditions. This indicates that the impact of hatchery selection on the performance of sea-ranched fish in the wild may not be as pronounced as previously thought. It is suggested that for salmonid populations that depend on supplemental stocking, more effort should be paid to minimizing negative environmental effects during hatchery rearing. The observed differences in fitness characters between families suggest that family effects should be taken into account in stocking programmes because the amount of genetic variation maintained within populations is related to the variance in family performance.