Stocking of water bodies with non-native fish species for fisheries enhancement is a common practice worldwide. However, overall limited benefits in terms of revenues for local communities have so far been reported in some countries, whilst accidental introductions of several non-native fish species have occurred. To evaluate the benefits vs. costs associated with common carp Cyprinus carpio stocking practices, a three-year sampling study (2009-2011) was carried out across twelve adjacent artificial reservoirs of the Kocaeli Peninsula (north-west Anatolia, Turkey). Apart from common carp, 18 fish species in total were recorded, of which 12 native and six non-native, the latter comprising more than half of the total catch and dominated by gibel carp Carassius gibelio. Even though the abundance of common carp, natives and non-natives increased over the study period, common carp was always comparatively less abundant relative to most of the other species. Overall, the present findings indicate that stocking of common carp into the studied reservoirs has not fully met with the proposed objectives, likely a result of less-than-optimal age-0 fish release strategies and limited availability of spawning grounds. On the other hand, invasion especially by gibel carp is thought to be under way and this may ultimately impact on the abundance of the native fish fauna.