Alien fish eradication from high mountain lakes by multiple removal methods: Estimating residual abundance and eradication probability in open populations

Diet composition and quality of a Salmo trutta (L.) population stocked in a high mountain lake since the Middle Ages

The introduction of fish into mountain lakes typically leads to profound ecological changes within the food web, but its consequences depend on the dietary preferences of fish and on the resistance of prey organisms against predation. Here we used stable isotopes and fatty acid analyses in combination with the traditional stomach content analysis to examine the diet of an allopatric population of Salmo trutta, which has originally been stocked during the Middle Ages in an alpine lake, and to identify what components of the food web are more affected. The results from stable isotopes and fatty acids indicated that planktonic and benthic food sources, in particular chironomids larvae, were the most important prey items all year round. Airborne terrestrial insects made most of the stomach content during the ice-free period, but their stable isotope and fatty acids values did not match up with those in fish, suggesting a minor role for fish nutrition. Copepods were relevant as fish diet only during the ice-covered period. In contrast to the stable isotope values of the fish muscle tissue, those of the liver, which reflect potentially short-term changes in diet, were significantly different between the ice-covered and ice-free period. Fatty acid analysis revealed that polyunsaturated fatty acids contents of chironomids, copepods, and chydorids contributed similarly to fish diet. Overall, our results suggest that the introduction of this fish species has decreased the lake-to-land resource transfer by reducing the abundance of emerging midges and that the population is food-limited as indicated by its low condition factor. This field study eventually acts as a reference for possible future reintroduction efforts, as this population is one of few existing in Europe with pure Danubian origin.

Minnow introductions in mountain lakes result in lower salmonid densities

Small fish species such as minnows (Phoxinus sp.) are introduced into mountain lakes by anglers that use them as live bait for fishing salmonids that were previously introduced in these naturally fishless habitats. Introduced fish severely impact native biota but minnows are nevertheless released believing to provide forage for game fish, i.e., salmonids. However, our results indicate that minnows negatively interact with game fish, reducing their relative densities. On some occasions, minnows can remain the only present fish following trout extinction. We believe that this information should be urgently disseminated among anglers as a pragmatic argument to also convince anglers less sensitive to conservation issues that carrying minnows to the mountains is not a good idea whatever the main interest is, e.g., nature conservation or outdoor recreation.