Early warning, rapid eradication of alien fish and ecological recovery in an Alpine lake

Invasive species guidelines recommend the early detection and rapid eradication of introduced species before they establish and spread. We adopted this approach to eradicate a recently introduced trout population from Timmelsjochsee, a small high mountain lake in the Austrian Alps, aiming to protect a rare melanic population of Daphnia longispina (“zschokkei ” morphotype) inhabiting the lake. Eradicating introduced trout from originally fishless mountain lakes by intensive gillnetting is effective but typica lly time - and resource -intensive. This case study demonstrates that eradication can be achieved with minimal investment if carried out promptly, before the fish can establish. We deployed six multi-mesh gillnets over one year and monitored the native prey community (i.e., invertebrates and amphibians) throughout the process. Within a year, it appeared that all fish had been removed: seven adult Arctic charr (Salvelinus alpinus) and six adult rainbow trout (Oncorhynchus mykiss). Their similar size indicated that they had not yet established a reproductive population. The entire operation, including fish removal and field surveys, required eight days of fieldwork. During the eradication, the zooplankton community and the D. longispina (“zschokkei ”) population did not show any obvious changes in density or composition. However, some changes in the littoral invertebrate community and in the common frog ( Rana temporaria) population may indicate ecological recovery. The density of D. longispina (“zschokkei”) suggests that fish eradication occurred at an early stage during which the impact of such a small fish population was still minimal. This study provides an example of early detection and rapid eradication as a cost -effective restoration method. It underscores th e importance of monitoring fish introductions into high mountain lakes and taking immediate action to protect fragile ecosystems before costs and impacts escalate. Key words: alien species, restoration ecology, fish eradication, resource optimization, sustainable management