Terrestrial prey, an important part in Salmo trutta diet or a stochastic event?

Abstract

The brown trout is a valuable species for recreational fishing and its diet is of great interest. It feeds on various aquatic invertebrates, but also consumes terrestrial prey, whose contribution is often unjustly underestimated. The importance of terrestrial prey in diets of two trout populations was assessed at two sites (April-October) with different quantity of available aquatic prey (Site 1 high, Site 2 low quantity). Four indices of importance were used: frequency of occurrence (FO), number of prey items (AN), weight (AW), and index of relative importance (IRI). Preference for terrestrial prey was also estimated based on the electivity index (EI), where the availability of aquatic and terrestrial prey was expressed as their percentage in the drift samples. Significantly less terrestrial prey was available at Site 1 (0.39% ± 0.57%) in contrast to Site 2 (13.21% ± 10.38%), terrestrial prey was significantly more abundant in the trout diet than in the drift samples at both sites (t test). The EI was positive for terrestrial prey, additionally FO showed that this prey was present in 36% ± 16.65% (site 1) and in 87.55% ± 8.94% individuals (site 2), indicating a high preference for this type of prey. The importance (IRI) of terrestrial prey was higher than expected at Site 1 (1.39% ± 1.53%; maximum 4.7% in September) and high at Site 2 (58.6% ± 23.56%; maximum 84.67% in September). Terrestrial prey became more present in the drift and more important in the diet toward the end of the study period, while the importance of aquatic prey decreased based on all four indices at both sites. The correlation between the increasing importance of terrestrial prey (AN, IRI) in the diet and the presence of aquatic prey in the drift (%) towards the end of the season was not significant (Pearson). Thus, the positive trend in the importance of terrestrial prey may be due to their increasing availability and simultaneous decline of large prey in the aquatic community due to the emergence of adults. Terrestrial prey is generally larger (i.e., yielding more energy) than most aquatic prey and easily captured due to its low mobility in the aquatic environment. In the habitat with low availability of aquatic prey, terrestrial prey played an important role in brown trout diet. In addition, brown trout also preferred it in the habitat rich in aquatic prey. This study shows that terrestrial prey is significant brown trout feeding resource regardless of its general availability in the environment.