Feeding preferences and diet overlap of introduced Atlantic and native Danube lineages of brown trout (Salmo trutta) from Croatia

Abstract

Brown trout has been widely introduced/translocated into suitable environments globally. In Croatia, the Danube (DA) lineage of brown trout is native. However, due to anglers’ activities, brown trout of the Atlantic (AT) lineage were stocked into Croatian streams and rivers. They hybridize with native brown trout causing a loss in native genetic variability. Brown trout of the AT lineage have been determined as invasive, posing a threat to native DA populations. Feeding habits of brown trout are related to insects of both terrestrial and aquatic origin. Introduced AT brown trout compete with native ones for food resources, possibly causing deficiency in available prey. Until now, no research related to feeding competition between AT and DA lineage of brown trout from inland waters has been performed. The purpose of this research is to examine the natural diet of brown trout of AT and DA lineages and to compare their feeding overlapping. In total 10 streams at the Gorski kotar (Bresni p., Mala les., Curak), Žumberak (Kupčina, Slapnica) and Papuk (Jankovac, Brzaja, Veličanka, Toplica and Orljava) Mountains were sampled in May 2017 and 2018, using electrofishing device. After sampling, fish specimens were stored on -20oC. Data of specimens’ haplotype was taken from Kanjuh et al. (2018). In laboratory each specimen was measured for total length (TL, in cm) and weight (W, in g). Stomach content was removed, measured and prey items determined to the lowest possible systematic category. Assessment of the fish diet was based on the frequency of occurrence (F%), numerical frequency (N%) and mass frequency (W%), (Hyslop, 1980). The index of relative importance IRI was calculated for each prey category (Hodgson et al., 1989). Diet overlap was calculated using the index proposed by Schoener (1970) based on IRI. Also, determined prey items were chosen as response variables to analyse the relationships between fish feeding habits from different streams and genetic origin (locations and haplotype used as nominal explanatory variables) using Canonical Correspondence Analysis (CCA) by CANOCO software package. To test the importance of each prey items for AT and DA lineages, the gut content data were analysed using the Poisson generalized linear model with log link (GLM). In total, 92 brown trout specimens were captured and analysed. TL was between 10.0 and 30.3 cm and mass from 10.19 to 323.52 g (TLAT = 15.4 – 30.3 cm; TLDA = 10.0 – 27.6 cm). Four specimens have been found with empty stomach. Sex ratio was 75: 14: 3 (females: males: juveniles). Based on IRI coefficient, the main prey items for brown trout of both lineages were aquatic Coleoptera, Trichoptera, Amphipoda and Diptera and terrestrial Formicidae (Fig.1.). However, brown trout from the Žumberak and Gorski kotar consumed different prey items than specimens from the Papuk Mountain (Fig. 2). Although only 9 specimens of AT haplogroup were detected, according to Schoener index (S), statistically significant diet overlap has been found between AT and DA specimens (S = 0.65, p