Dietary breadth and trophic position of introduced European catfish Silurus glanis in the River Tarn (Garonne River basin), southwest France

Although being a widely introduced and successfully established species, the European catfish Silurus glanis L. (the world’s third largest freshwater fish) remains poorly studied in its introduced areas. Here we studied the trophic ecology of non-native European catfish in a large river system in south-western France using stomach content and stable isotope analyses (SIA). We used fin samples for SIA of catfish and hence tested the validity of using fin tissue as a proxy for muscle in SIA. The mean δN and δC values analysed from fin tissues did not differ from those analysed from muscle tissue and reflected strong and consistent relationships (r = 0.95 for carbon and r = 0.98 for nitrogen). The δN values varied almost 5‰ among the analysed catfish individuals, while δC values varied >5‰. Total length of these catfish ranged from 200 to 2240 mm and was correlated with δ N and especially with δC values. Although catfish length and δN values were positively correlated, the mean trophic positions of catfish increased only slightly from smaller individuals to larger ones (4.3 to 4.7). However, larger catfish were considerably C-enriched in their δC values compared to smaller individuals and had up to 4‰ higher δC values than their expected aquatic prey. This might indicate frequent consumption of mammals and/or non-aquatic birds by the larger sized individuals, which were found in the catfish stomachs.