Contour plots based on GAMs for A native crucian carp (Carassius carassius) B invasive gibel carp (C. gibelio) C common bream (Abramis brama) and D roach (Rutilus rutilus) for all study period, where the number of reported record catches in a given area was used as the response variable and their GPS locations as the explanatory variable. Each line (or contour) represents the reported catches in that region within the country. The proximity of the lines indicates the steepness of the gradient. Colours show the abundance of records with red indicating the highest abundance.