Establishment of Regional Action Plan for conservation of critically endangered crucian carp in Czechia

The crucian carp ( Carassius carassius ) has declined markedly in recent decades due to the ongoing invasion of its congeneric species complex, the gibel carp ( Carassius gibelio ) and widespread habitat degradation. In Europe, the crucian carp occurs in two highly divergent phylogenetic lineages: the northern lineage distributed across northern and central-eastern Europe and occurs in the Elbe (Labe) and Oder (Odra) River Basins and the Danubian lineage restricted to the Danube (Dunaj) River Basin. Both lineages are naturally present in Czechia. However, their close spatial proximity in some areas, reflecting the natural contact areas between the Elbe and Danube basins, combined with intensive aquaculture practices and likely translocations by recreational anglers, may have led to local mixing of lineages or their introduction into non-native areas. Consequently, the prevailing conservation paradigm on effective conservation of the crucian carp requires maintaining these lineages as distinct conservation units. To address this need, a Regional Action Plan (RAP) was developed to preserve the current distribution of phylogenetic lineages of the crucian carp in the Vysočina Region, where both lineages intersect. A citizen-science project (“Save the crucian carp”) was implemented to locate the remaining populations in the region. Populations identified through citizen-science tips were subsequently selected for systematic monitoring and samples were collected for genetic analyses. Based on lineage assignment (northern or Danubian), suitable populations were selected for conservation management. As part of the RAP, we initiated the repatriation of genetically verified crucian carp populations. Source populations representing both lineages were identified and individuals were transferred to newly-established sites located within their respective watersheds and in close geographical proximity. In total, five northern and eight Danubian populations were identified and three northern and four Danubian populations were successfully repatriated. This study provides a comprehensive conservation framework for a declining wetland fish species. Although regionally focused to the Bohemian-Moravian Highlands, situated at the boundary between the North Sea and Black Sea drainage basins, the approach is broadly applicable to other declining wetland species with similar ecological requirements and genetic structure. Our findings underscore the necessity of integrated, multi-level assessment of species threats and highlight the importance of evidence-based conservation management. Graphical abstract Graphical abstract showing the area where the Regional Action Plan was designed, along with a simplified workflow of suitable population identification.