Longlines for Sampling, Reduction and Eradication of Large Alien Invasive Predatory Species: The Case of European Catfish ^†

Abstract

Introduction: Longlines are traditional fishing gear and are widely used in marine systems for both quantitative and qualitative sampling of large predators like tuna, swordfish, sailfish, marlin or sharks. This highly selective method has been applied for the sampling of European catfish (Silurus glanis), the largest freshwater fish in Europe. This apex predator is a highly valuable fish species in its native localities of Central and Eastern Europe (commercial fishing, anglers’ trophy fishing, biomanipulation purposes) but a dangerous alien invasive species in South and Western Europe. Objective: The efficiency and selectivity of longlines for European catfish sampling were compared with more traditional fishing methods like gillnets and electrofishing. Methodology: European catfish were sampled in native areas (Římov, Žlutice, Vrchlice, Hubenov, Lipno, Želivka reservoirs; Czech Republic) and areas of invasion (Lakes Campagna, Avigliana Grande, Maggiore; Italy; Belver, Meimoa, Cedillo reservoirs; Portugal; Iznajar, Mequinenza reservoirs; Spain) following the protocols of best catfish (Silurus glanis) capture methodologies in small and large lakes and reservoirs established for the current needs of the European Commission within the LIFE PREDATOR project. Results: The longline efficiency expressed as the proportion of individual hooks catching catfish (live baits exposed overnight) was from low to medium (5–25%; Czech reservoirs, Italian lakes, Meimoa reservoir) to very high (up to over 50%; Portuguese and Spanish reservoirs). In many water bodies, specifically in areas of invasion, catfish represented 100% of the longline catch (Campagna, Maggiore, Cedillo, Iznajar, Mequinenza). In the rest of the water bodies, the by-catch ranged from 2 to 20% with the highest values occurring in its native localities (due to the presence of large individuals of Northern pike, Esox lucius, another predatory fish in these systems). In areas of invasion, the inconsiderable by-catch was composed of other non-native predatory fish species like pikeperch (Sander lucioperca), European perch (Perca fluviatilis) or channel catfish (Ictalurus punctatus). Both the efficiency and selectivity of longlines were higher compared to gillnets and electrofishing, and longlines also caught larger catfish individuals. Conclusions: Longlines represent an efficient and extremely selective method for European catfish sampling, reduction and even eradication, especially in areas of invasion. Their usage, however, requires specific equipment, skills and also baits.