Results
Reliance on terrestrial prey was higher in sympatric than in allopatric brown trout (F1;161 = 52.67, p < 0.001; Fig. 1). The reliance on terrestrial prey also increased with increasing body size (F1;161 = 98.26, p < 0.001), but it did not differ between the sexes (F1;79 = 0.01, p = 0.911; Fig. 1). Reliance on terrestrial prey increased with increasing abundance (F1;161 = 120.01, p < 0.001) and biomass (F1;161 = 63.06, p < 0.001) of aquatic macroinvertebrates at the sampling site. This indicates that the consumption of aquatic and terrestrial prey by brown trout was not limited by the availability of aquatic prey in the stream, but abundance and biomass of aquatic macroinvertebrates appeared to be controlled by the foraging behaviour of brown trout.
Terrestrial macroinvertebrates contained less total lipids (mean±SD = 148±69 mg/dry mass) than aquatic prey macroinvertebrates (mean±SD = 204±86 mg/g dry mass; F1;51 = 8.93, p = 0.004). Terrestrial macroinvertebrates had similar content of ALA (F1;51 = 1.39, p = 0.245) and significantly lower EPA (F1;51 = 30.27, p < 0.001) than aquatic macroinvertebrates (Fig 1a). Total lipids (F5,55 = 1.70, p = 0.151), and the content of ALA (F5,55 = 0.40, p = 0.845) and EPA (F5,55 = 1.00, p = 0.42) of macroinvertebrates did not differ across the sampling sites. Aquatic macroinvertebrates were thus a richer dietary source of n-3 LC-PUFA, particularly of EPA, than terrestrial macroinvertebrates across all sampling sites (Fig. 1a).
The content of total lipids in brown trout tissues did not differ between sympatric and allopatric populations (F1,80 = 0.06, p = 0.803) and was not significantly related to the reliance on terrestrial prey (F1,80 = 0.02, p = 0.890), fork length (F1,80 = 2.23, p = 0.139), and sex (F1,80 = 0.07, p = 0.789) of individuals. The ALA content in trout tissues was affected by an interaction between the competition mode and reliance on terrestrial prey (F1,79= 5.71, p = 0.019; Fig. 1b), so ALA increased with increasing reliance on terrestrial prey in sympatric (F1,39 = 8.74, p = 0.005), but not in allopatric (F1,38 = 0.73, p = 0.397) brown trout. Sex (F1,79 = 0.52, p = 0.472) and body length (F1,79 = 3.14, p = 0.080) had no significant effect on ALA content in trout tissues. The EPA content was higher in allopatric than in sympatric brown trout (F1,80= 14.29, p < 0.001; Fig. 1c) and decreased with increasing fork length (F1,80 = 146.89, p = 0.001), but was not affected by individual’s sex (F1,80 = 0.28, p = 0.597) and reliance on terrestrial prey (F1,80 = 0.87, p = 0.353). The DHA content in trout tissues was affected by an interaction between the competition mode and reliance on terrestrial prey (F1,78= 7.46, p = 0.007; Fig. 1d), so DHA decreased with increasing reliance on terrestrial prey in sympatric (F1,38 = 7.14, p = 0.011), but not in allopatric (F1,38 = 1.07, p = 0.307) populations. Sex had no effect on DHA in trout tissues (F1,78 = 0.74, p = 0.391), but DHA content decreased with increasing body size (F1,78 = 6.33, p = 0.014).