Consequences of fish introductions on paedomorphic newts (Triturus alpestris and T. helveticus)

Fish stocking is largely recognized as a major thread to amphibian populations. Although it acts at a local level, it is widespread at a world scale. Despite this good knowledge, the effect of fish on alternative phenotypes has only been described in a few local studies. Our aim was to report on the loss of intraspecific heterochrony as a loss to diversity in determining the consequences of fish introductions on paedomorphs (morphs retaining gills at the adult stage) of two European species of newts. We surveyed almost all the main known paedomorphic populations of the Alpine (Triturus alpestris) and palmate (T. helveticus) newts, which all but one initially occupied fishless ponds and lakes in France, Italy, Slovenia, Bosnia, Montenegro, and Greece. Exotic fish were found in almost half of the studied aquatic habitats. Introductions involved several ornamental and large predatory species of fish. At all sites, in which fish were introduced, paedomorphs disappeared and metamorphs declined. The largest known populations of newts, including some endemic subspecies, were concerned. If measures are not taken to stop fish stocking, protect paedomorphs as conservation units, and restore natural habitats, all the largest paedomorphic populations may disappear in the near future.