Reecent human-mediated translocations of non-native and regionally alien freshwater fish species in Norwegian watercourses

Abstract

We assessed the translocation of freshwater fish in Norway during the period 2013-2021. These introductions include both regionally alien species and non-native species, excluding the pink salmon Oncorhynchus gorbuscha. A network of 12 agencies, organizations and persons was established to obtain information and report on new introductions. Totally, 287 incidents were reported, mainly from lakes. Altogether, this included 21 translocated species, comprising 12 regionally alien species and nine non-native species. The most frequently translocated among the regionally alien species was pike Esox lucius (36 incidents), Eurasian minnow Phoxinus phoxinus (35), rudd Scardinius eryhrophthalmus (31) and roach Rutilus rutilus (14). Non-native species mainly involved carp Cyprinus carpio (38), tench Tinca tinca (28), brook trout Salvelinus fontinalis (20), pumpkinseed Lepomis gibbosus (15) and goldfish Carassius auratus (10). The highest number of introductions were reported from counties in southern and middle parts of Norway. At least 146 incidents, or about 50% of the introductions, have led to the establishment of viable populations. The primary vectors leading to these introductions were intentional release to establish populations for angling (≈ 60%). The species most often introduced for this purpose was pike, tench, brook trout, lake trout Salvelinus namaycush, rainbow trout Oncorhynchus mykiss and carp. Roach and perch Perca fluviatilis may be stocked as forage fish for pike, but probably also other species. Further, introduction of baitfish such as Eurasian minnow and rudd accounted for about 20% of all cases. The extensive unplanned movement of species may disrupt local biodiversity.