First principal response curves (PRC) showing effects of season and study site on otter diet composition expressed as relative frequency of occurrence (RFO; a, c, e) and relative biomass (BIO; b, d, f). In every line, the reference site changes (a, b: reference Ysper; c, d: reference Ois; e, f: reference Piesting). Dotted yellow lines: Ysper; solid black lines: Ois; dashed red line: Piesting. Prey category weights (b_k ), shown on the right y-axis, can be interpreted as the affinity of each category to the Principal response curves and have the same numerical scale as the left y-axis (c_dt ). ANU: amphibians; AVE: birds; COT: bullhead; CRU: crayfish; CYP: cyprinids; ESO: pike; INS: insects; MAM: mammals; PER: perch; REP: reptiles; SAL: salmonids; THY: grayling.