Bridge under troubled water: Turbulence and niche partitioning in fish foraging.

The coexistence of competing species relies on niche partitioning. Competitive exclusion is likely inevitable at high niche overlap, but such divide between competitors may be bridged if environmental circumstances displace competitor niches to enhance partitioning. Foraging-niche dimension can be influenced by environmental characteristics, and if competitors react differently to such conditions, coexistence can be facilitated. We here experimentally approach the partitioning effects of environmental conditions by evaluating the influence of water turbulence on foraging-niche responses in two competing fish species, Eurasian perch and roach , selecting from planktonic and benthic prey. In the absence of turbulence, both fish species showed high selectivity for benthic chironomid larvae. fed almost exclusively on zoobenthos, whereas complemented the benthic diet with zooplankton (mainly copepods). In turbulent water, on the other hand, the foraging-niche widths of both and increased, while their diet overlap simultaneously decreased, caused by 20% of the individuals turning to planktonic (mainly bosminids) prey, and by increasing foraging on littoral/benthic food sources. We show that moderate physical disturbance of environments, such as turbulence, can enhance niche partitioning and thereby coexistence of competing foragers. Turbulence affects prey but not fish swimming capacities, with consequences for prey-specific distributions and encounter rates with fish of different foraging strategies (pause-travel and cruise ). Water turbulence and prey community structure should hereby affect competitive interaction strengths among fish species, with consequences for coexistence probability as well as community and system compositions.