Organic pollution induces domestication-like characteristics in feral populations of brown trout (Salmo trutta)

Sewage pollutants may impair growth or survival of the freshwater biota, though animals might benefit from the extra food availability as production increases. We examined biochemical (muscle glycogen), morphological (condition factor and hepatosomatic index), and diet biomarkers in brown trout for evaluating the effects of chronic exposure to organic pollution. Trout were collected at three locations: ST1 downstream of a trout farm, ST2 affected by the effluents of a wastewater treatment plant and ST3, the reference site. Individuals at polluted sites showed high hepatosomatic index, although no differences were found between ST2 and ST3 for the condition factor. A significant reduction was detected in the levels of muscle glycogen of individuals captured at polluted sites. Moreover, trout diet in these rivers was dominated quantitatively by Chironomidae and Simuliidae, in contrast with the diverse diet of individuals at ST3. Remarkably, individuals at polluted sites showed high stomach fullness and energy gut values, which might be considered as a case of hyperphagia. Our findings suggest that food surplus in organic enriched sites, in the form of high densities of macroinvertebrates, provide an environment similar to that of domesticated animals, where individuals might adopt less energetically costly behavioural strategies to ingest more food.