| Summary: | The aim of this study was to evaluate the newly-developed juvenile fish shelter (JFS) for its ability to increase prey survival and to improve species diversity in a freshwater ecosystem. An experiment was performed in an outdoor large-scale mesocosm three times from 2011 to 2012 by comparing the responses to adjustment as a function of the volume of JFS in the control and experimental groups. Analysis results of the environmental monitoring over three periods indicated only minor differences in the physicochemical characteristics of the water quality and phyto- and zoo-plankton biomass, thereby enabling a comparative analysis of the feeding ecology. However, the water temperature exhibited large fluctuations, ranging from 16.4 to 27.6 °C, and high water temperature conditions (Period 1, 25.6 ± 2.0 °C) enhanced the predation activity of the piscivorous fish Coreoperca herzi (C. herzi, size 89 ± 4 mm). Statistically, the survival rates of the prey fish, Rhynchocypris oxycephalus (R. oxycephalus, size 29 ± 1 mm), with JFSs were greater by 35.9%–46.7%, and improved as the patch volume of JFS increased. Based on both experimental observations and statistical analysis, the JFS developed in this study could reduce the chances of predator–prey encounters, minimize prey vulnerability, and thereby increase prey survival rates.
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