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Hypoxic conditions enhance refuge effect of macrophyte zone for small prey fish from piscivorous predators



Body size dependency of the refuge effects of gape limitation and differences in hypoxia tolerance between the predator largemouth bass Micropterus salmoides (Lacépède) and the prey fish, nigorobuna carp Carassius buergeri grandoculis Temminck & Schlegel, were modelled to simulate the relative effect of size and physiological refuges for prey inhabiting the hypoxic macrophyte zone along a lakeshore. The model simulation suggested that a physiological refuge, maintained by the difference in hypoxia tolerance, could protect far smaller prey than a size refuge that is based solely on predator gape size and prey body depth. The dissolved oxygen gradient observed in the macrophyte zone located in Yamanoshita Bay, Lake Biwa, Japan, which was extremely low at the inner part and higher offshore, was determined to provide a physiological refuge that would be effective in terms of space.