Habitat complexity and food item size modify the foraging behaviour of a freshwater

25 26 The functional response describes the relationship between feeding rate and prey density, and 27 is important ecologically as it describes how the foraging behaviour of an animal changes in 28 response to food availability. The effects of habitat complexity and food item size was 29 experimentally tested here on the foraging parameters and the functional responses of the 30 freshwater fish roach Rutilus rutilus (L.). Habitat complexity was varied through the 31 manipulation of substrate and turbidity, and food item size was varied by using fishmeal 32 pellets, in two sizes. As water turbidity and substrate complexity increased, the reaction 33 distance and consumption rate of the fish significantly decreased. Increased food item size 34 significantly decreased consumption rates but had no influence on any other foraging 35 parameter. Analysis of the interactions between substrate complexity, turbidity and food item 36 size revealed food item size had the greatest influence on consumption rate. Turbidity has the 37 least effect on all the foraging parameters tested. Across all experiments, the functional 38 responses were best described by the Type II response, a relatively consistent finding for R. 39 rutilus. These outputs reveal that fish foraging behaviours and functional responses are highly 40 context dependent, varying with environmental parameters and the availability of prey of 41 different sizes. 42