title: Bridge under troubled water: Turbulence and niche partitioning in fish foraging.
abstract: 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 perchPerca fluviatilisand roachRutilus rutilus, selecting from planktonic and benthic prey. In the absence of turbulence, both fish species showed high selectivity for benthic chironomid larvae.R. rutilusfed almost exclusively on zoobenthos, whereasP. fluviatiliscomplemented the benthic diet with zooplankton (mainly copepods). In turbulent water, on the other hand, the foraging-niche widths of bothR. rutilusandP. fluviatilisincreased, while their diet overlap simultaneously decreased, caused by 20% of theR. rutilusindividuals turning to planktonic (mainly bosminids) prey, and byP. fluviatilisincreasing 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-travelP. fluviatilisand cruiseR. rutilus). 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.
authors.affiliation: Department of Environmental Sciences University of Helsinki Helsinki Finland.
authors.forename: Zeynep
authors.initials: Z
authors.lastname: Pekcan-Hekim
authors.affiliation: Department of Environmental Sciences University of Helsinki Helsinki Finland.
authors.forename: Noora
authors.initials: N
authors.lastname: Hellén
authors.affiliation: Department of Environmental Sciences University of Helsinki Helsinki Finland.
authors.forename: Laura
authors.initials: L
authors.lastname: Härkönen
authors.affiliation: Department of Biology - Aquatic Ecology Lund University Lund Sweden; Department of Environmental and Life Sciences - Biology Karlstad University Karlstad Sweden.
authors.forename: Per Anders
authors.initials: PA
authors.lastname: Nilsson
authors.affiliation: Department of Environmental Sciences University of Helsinki Helsinki Finland.
authors.forename: Leena
authors.initials: L
authors.lastname: Nurminen
authors.affiliation: Department of Environmental Sciences University of Helsinki Helsinki Finland.
authors.forename: Jukka
authors.initials: J
authors.lastname: Horppila
keywords: competition; diet choice; disturbance; feeding strategy; segregation