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).