High mountain lakes are originally fishless, although many have had introductions of non-native fish species, predominantly trout, and recently also minnows introduced by fishermen that use them as live bait. The extent of these introductions is general and substantial often involving many lakes over mountain ranges. Predation on native fauna by introduced fish involves profound ecological changes since fish occupy a higher trophic level that was previously inexistent. Fish predation produces a drastic reduction or elimination of autochthonous animal groups, such as amphibians and large macroinvertebrates in the littoral, and crustaceans in the plankton. These strong effects raise concerns for the conservation of high mountain lakes. In terms of individual species, those adapted to live in larger lakes have suffered a higher decrease in the size of their metapopulation. This ecological problem is discussed from a European perspective providing examples from two study areas: the Pyrenees and the Western Italian Alps. Species-specific studies are urgently needed to evaluate the conservation status of the more impacted species, together with conservation measures at continental and regional scales, through regulation, and at local scale, through restoration actions, aimed to stop further invasive species expansions and to restore the present situation. At different high mountain areas of the world, there have been restoration projects aiming to return lakes to their native fish-free status. In these areas autochthonous species that disappeared with the introduction of fish are progressively recovering their initial distribution when nearby fish-free lakes and ponds are available.

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Historical factors (colonization scenarios, demographic oscillations) and contemporary processes (population connectivity, current population size) largely contribute to shaping species' presentday genetic diversity and structure. In this study, we use a combination of mitochondrial and nuclear DNA markers to understand the role of Quaternary climatic oscillations and present-day gene flow dynamics in determining the genetic diversity and structure of the newt Calotriton asper (Al. Dugès, 1852), endemic to the Pyrenees. Mitochondrial DNA did not show a clear phylogeographic pattern and presented low levels of variation. In contrast, microsatellites revealed five major genetic lineages with admixture patterns at their boundaries. Approximate Bayesian computation analyses and linear models indicated that the five lineages likely underwent separate evolutionary histories and can be tracked back to distinct glacial refugia. Lineage differentiation started around the Last Glacial Maximum at three focal areas (western, central and eastern Pyrenees) and extended through the end of the Last Glacial Period in the central Pyrenees, where it led to the formation of two more lineages. Our data revealed no evidence of recent dispersal between lineages, whereas borders likely represent zones of secondary contact following expansion from multiple refugia. Finally, we did not find genetic evidence of sex-biased dispersal. This work highlights the importance of integrating past evolutionary processes and present-day gene flow and dispersal dynamics, together with multilocus approaches, to gain insights into what shaped the current genetic attributes of amphibians living in montane habitats.

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In Lake Grande de Peñalara, an originally fishless small high mountain lake in the Central Iberian Peninsula, the brook trout was introduced in the 70s, eradicating 30 years later using gillnets. We investigated the time-course and changes in macroinvertebrates and zooplankton communities, before and after the eradication, by studying their richness and several biological and ecological traits of macroinvertebrates providing advantages or disadvantages against fish predation and colonising the different habitats. Macroinvertebrates richness increased from 13 taxa coexisting with fish, up to a maximum of 27 taxa after the eradication. Rare groups usually affected by fish predation, e.g. swimmers in surface and open waters, showed high dispersal and recolonization capabilities, while those with burrowing, interstitial or crawler habits maintained their presence even with fish given their advantage to hide from direct fish sight. Taxa with affinities for rare habitats within the lake (e.g. macrophyte beds) appeared occasionally 4-6 years after eradication. Contrastingly, zooplankton assemblage did not significantly change in richness in the next 10 years after eradication. No new species of cladocerans or copepods appeared after fish removal, but 4 new rotifers appeared and 5 taxa disappeared. This was apparently more related to a change in water quality or trophic status as a consequence of the fish removal than to the direct effect of fish removal on rotifers. In contrast, zooplankters were significantly smaller, on average, before fish eradication than later, indicating that the community responded to the change of predation pressure.