Juvenile fishes in macrophyte beds: influence of food resources, habitat structure and body size

Mean juvenile fish abundance and fish frequency in a large lowland river du ring low discharge largely differed among the unvegetated and three morphol ogically contrasted macrophyte habitats. Single separate models revealed th at juvenile fish distribution was largely influenced by trophic variables. With the exception of Leuciscus cephalus, which responded mainly to physica l variables (depth and substratum), multiple regression models emphasized t he importance of trophic variables for fish distribution. For Blicca bjoerk na, L. cephalus and Lepomis gibbosus, habitat shifts with respect to prey s ize were apparent; small juvenile fishes mainly responded to small zooplank ton abundance, whereas large individuals were more influenced by the abunda nce of large zooplankton. Whatever the species, predictions from multiple r egression models were always better for large individuals, Small juvenile f ishes appeared to be less affected by the habitat variables measured, and e xhibited more uniform spatial distribution. The relative importance of trop hic resources and habitat physical structure among macrophyte types for fis h-habitat relationships is discussed, and the necessity of quantifying habi tat structural complexity is emphasized. (C) 2001 The Fisheries Society of the British Isles.