Traits of brown trout prey in relation to habitat characteristics and benthic invertebrate communities

A trait database based on habitat, behavioural and morphological characteristics of invertebrates enabled invertebrates known to have differential vulnerability to salmonid predation to be discriminated. Among all of these traits the most important were associated with invertebrate spatial proximity to salmonids (macrohabitat preference), potential conspicuousness and accessibility (e.g. flow exposure, mobility, tendency to drift, trajectory and agility) and handling efficiency (body flexibility and size). Data on benthos assemblages and brown trout salmo trutta diets collected over two consecutive summers in different macrohabitat types (e.g. pools, runs and riffles) of two Pyrenean streams demonstrated that: (i) decrease in torrential flow characteristics of habitats was associated with changes in invertebrate body shape and movement, and a decrease in body flexibility and tendency to drift; (ii) brown trout tended to feed on the most vulnerable invertebrates (e.g. those living in exposed microhabitats and with a high tendency to drift, agility and aggregation), except in faster flowing habitats. The results suggest that the potential vulnerability of benthic invertebrates increased with increases in flow (torrential) characteristics. Also, the database compiled on traits explained differences among brown trout diets, especially in slow flowing areas.