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Metapopulation dynamics over the course of an invasion are usually difficult to grasp because they require large and reliable data collection, often unavailable. The invasion of the fish-free freshwater ecosystems of the remote sub-Antarctic Kerguelen Islands following man-made introductions of brown trout (Salmo trutta) in the 1950's is an exception to this rule. Benefiting from a full long term environmental research monitoring of the invasion, we built a Bayesian dynamic metapopulation model to analyze the invasion dynamics of 85 river systems over 51 years. The model accounted for patch size (river length and connections to lakes), alternative dispersal pathways between rivers, temporal trends in dynamics, and uncertainty in colonization date. The results show that the model correctly represents the observed pattern of invasion, especially if we assume a coastal dispersal pathway between patches. Landscape attributes such as patch size influenced the colonization function, but had no effect on propagule pressure. Independently from patch size and distance between patches, propagule pressure and colonization function were not constant through time. Propagule pressure increased over the course of colonization, whereas the colonization function decreased, conditional on propagule pressure. The resulting pattern of this antagonistic interplay is an initial rapid invasion phase followed by a strong decrease in the invasion rate. These temporal trends may be due to either adaptive processes or environmental gradients encountered along the colonization front. It was not possible to distinguish these two hypotheses. Because invasibility of Kerguelen Is. freshwater ecosystems is very high due to the lack of a pre-existing fish fauna and minimal human interference, our estimates of invasion dynamics represent a blueprint for the potential of brown trout invasiveness in pristine environments. Our conclusions shed light on the future of polar regions where, because of climate change, fish-free ecosystems become increasingly accessible to invasion by fish species.

The Subantarctic Kerguelen Islands (49°S, 70°E) contain freshwater ecosystems among the most isolated in the world. Concentrations of polychlorinated biphenyls (PCBs) were assessed in the muscle of 48 brook trout and 38 brown trout caught during summer and spring 2006 in the rivers, lakes and ponds of Kerguelen. The sum of 29 PCBs averaged 404 and 358 ng g(-1) lipid, and dioxin-like PCB was 19 and 69 ng g(-1) lipid, in brook and brown trout, respectively. The values showed a high variability and some fish accumulated PCBs at levels similar to those of fish from impacted areas. While inter-sex differences were limited, the season and the morphotype appeared to have the most influence. Fish captured in summer had muscle PCB concentrations about three times higher than those caught in spring and the 'river' morphotype of brook trout showed the highest PCB levels., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Interspecific hybridization may occur in situations of recent contact between a colonizer and a resident species, being more intense in the colonization front. Atlantic salmon Salmo salar and brown trout S. trutta have been sympatric species since their origin and they share spatial and temporal spawning niches, exhibiting low levels of bidirectional interspecific hybridization and introgression throughout their distribution range. Different causes have been identified for increased hybridization, from escapes or deliberate releases of domesticated fish to sneaking male behavior. We have examined hybridization rates and direction in different situations of advance of one of these species into a territory formerly inhabited by the other (247 samples were analyzed in northern Spain and 487 in Kerguelen Islands). In all cases, hybrids found in the colonization front were offspring of colonizer females and resident males. We hypothesize that these findings are the result of adaptive relaxed mate choice of colonizing females, regardless of the relative abundance of each species.

By genetic analysis of 1625 samples from 10 south European rivers, we have found that Atlantic salmon Salmo salar hybridize with sympatric brown trout S. trutta in the wild and provide the female in most heterospecific crosses. Hybrids exhibit reduced fertility and could be considered a wasted reproductive effort by salmon females. In 7 experiments involving salmon females, large brown trout males, and small salmon male sneakers, reproductive success of Atlantic salmon females mating with brown trout males was not significantly different from that of 5 experiments of females mating with conspecific males because small Atlantic salmon sneakers fertilized most ova (mean 93%) in salmon x trout matings. Although egg retention tended to be higher in heterospecific than in conspecific crosses (mean 5.7% vs. 20.5% respectively), mean offspring survival was 24.4% and 30.3%, respectively (t = 1.5 x 10(-8), not significant). Brown trout males taking on a courting role may benefit late-maturing females in absence or scarcity of anadromous salmon males because they play a protective role against disturbances from other fishes (including cannibal sneakers).

Hybridization between sympatric species is not uncommon in the wild. Wild allotriploids (individuals with two chromosome sets from a species + one chromosome set from another species) are generally the result of a backcross between interspecific hybrids that produce unreduced gametes and one of the parental species. In animals, allotriploids are commonly sterile, except for some vertebrate species complexes in which allotriploids reproduce by parthenogenesis, gynogenesis and/or hybridogenesis, producing generally clonal or hemiclonal gametes; nuclear DNA introgression between hybridizing species is considered to be extremely rare. Employing species-specific molecular markers, we show genetic introgression between the chromosomally well-differentiated salmonids Atlantic salmon (2n = 58) and brown trout (2n = 80) through spontaneous bisexual reproduction of allotriploids leading to salmon-like offspring bearing some brown trout genes. Although introgression between these Salmo species can occur via allotriploids, we hypothesize that extinction of parental species can be discarded based on very low survival of allotriploid offspring.

Two brown trout Salmo trutta stocks of different origin (wild Polish, domestic commercial) came into secondary contact after deliberate releases conducted in virgin rivers systems of the Subantarctic Kerguelen Islands (70 degrees E 49 degrees S). Samples obtained in 2001-2003 and a historical sample from 1993 were analysed for genetic variation at seven microsatellite loci and one allozyme locus (LDH-C1*). Bayesian clustering analysis demonstrated that rapid genetic differentiation formed separate genetic units in neighbouring rivers in less than 20 years. These genetic units were characterized by a large proportion of Polish genotypes mixed with some genomes of domestic origin (up to 30%). A different colonization strategy of the naturalized stocks, likely related with differential performance, was identified as a cause of rapid population differentiation in this area.

By screening variable number of tandem repeat (VNTR) loci, multiple paternity within clutches has been found in wild populations of southern European Atlantic salmon (Salmo salar) and brown trout (Salmo trutta). For Atlantic salmon, we determined the relative contribution of alternative male phenotypes to the next generation. Individual males that are morphologically juvenile yet sexually mature fertilized a large proportion of eggs, and they thereby contributed to an increase of genetic variability in wild populations via (1) balancing the sex ratio, (2) increasing outbreeding, and (3) enlarging the effective population size, in part a consequence of (1) and (2). In addition, these precocious males ensured that interspecific spawns involving Atlantic salmon females and brown trout males (a fairly common occurrence in southern Europe where the two species are sympatric) resulted mostly in Atlantic salmon progeny. For brown trout, preliminary genetic results indicated that multiple paternity, when present, was not due to alternative mating strategies by males, but rather to successive fertilizations by adult suitors.