Temporal changes in the distribution of Carassius carassius and the invasive Carassius gibelio in Croatian freshwaters

Anthropogenic modification of freshwater ecosystems has drastically transformed freshwater habitats across Europe, contributing to the decline of limnophilic fish species and promoting the spread of invasive taxa. This study examines the long-term spatial and temporal changes in the distribution of the native crucian carp ( Carassius carassius ) and the invasive gibel carp ( C. gibelio ) in Croatian freshwater ecosystems. We compiled validated presence/absence data covering the period 1883–2025 from historical literature, national monitoring programmes, citizen science platforms, and verified angler records. The results reveal a pronounced shift in Carassius assemblages over time. Historically, C. carassius was widespread in the lowland waters of the Pannonian Ecoregion, showing preserved river–floodplain connectivity and presence in numerous shallow lentic habitats. Since the mid-20 th century, extensive river regulation, floodplain disconnection, and land reclamation have facilitated the rapid expansion of C. gibelio , which now dominates freshwater systems across Croatia. Currently, C. carassius persists only in a small number of isolated localities, primarily oxbows, regulated channels, and slow-flowing lowland river sections with dense macrophyte vegetation. These populations are highly fragmented, occurring mainly at low densities, and are frequently overlooked by standard monitoring, indicating probable local extinctions. These findings highlight the urgent need for targeted conservation measures and improved management to safeguard persisting C. carassius populations in Croatia.