The present study is the first to explore the behavioral response of a prey fish to its predator as well as the effect of the body size of predators on the behavioral response of the prey. To date, research on the relative sizes of prey and predator has attracted increasing attention, but studies have mainly considered the survival indicators of prey fish. Thus, examining behavioral responses of prey fish to its predator and the possible impact of relative body size difference between them on these responses is very important. Therefore, the behavior of prey to its predator will likely be quite different if the relative body size difference between them changes. Thus, body size effect has attracted the attention in research and is usually regarded as one of the critical characteristics correlated with prey survival during predatory encounters ( Holmes & McCormick, 2010 Rodgers, Downing & Morrell, 2015). Furthermore, it has been found that whether a prey can escape from its predator largely depends on relative body size difference between them which is mainly imposed by morphological restriction of the predator’s mouth width (also known as gape size limitation) ( Dörner & Wagner, 2003). The difference might be due to differences in morphological, ecological characteristics, nutritional status and anti-predator strategy of the prey as well as the hunting strategies of different predators ( Peacor, 2002 Oplinger, Wahl & Nannini, 2011 Lönnstedt et al., 2012). However, in fish species such as bluegill sunfish ( Lepomis macrochirus), a predator stimulus showed the opposite effects, i.e., no change in spatial position or refuge seeking ( Urban, 2007 Oplinger, Wahl & Nannini, 2011 Albecker & Vance-Chalcraft, 2015). For example, it has long been found that the chemical and visual cues of predators elicited alterations in space use and decreased foraging and exploration swimming activity in juveniles of fish species such as sticklebacks ( Gasterosteus aculeatus), perch ( Perca fluviatilis) and rainbow trout ( Oncorhynchus mykiss) ( Milinski & Heller, 1978 Fraser & Gilliam, 1992 Diehl & Eklöv, 1995 Brown & Smith, 1998 Lehtiniemi, 2005 Kopack et al., 2015). Predation is one of the central factors governing patterns in natural systems ( Sih, Englund & Wooster, 1998), and the prey behavior is expected to change as a result of predation ( Diehl & Eklöv, 1995 Ryan et al., 2012 Fu et al., 2015a Fu et al., 2015b). These findings suggested that (1) an anti-predator strategy in black carp might involve maintaining a safe distance, decreasing activity and possibly increased vigilance and that (2) the behaviors of prey response to predators were not influenced by their relative size difference. However, there was no significant difference in either distance or spontaneous activities between two predator treatments. The prey fish spent substantially less time moving and exhibited an overall shorter total distance of movement after the size-matched or large predator was introduced, which possibly occurred due to increased vigilance or efforts to reduce the possibility of detection by potential predators. The distance between prey and predator fish was approximately 200% of the distance between two prey fish, which suggested that black carp can distinguish their conspecifics from heterospecifics and probably recognize the snakehead as a potential predator. Their spontaneous activities were videorecorded in a central circular arena surrounded by a ring holding the stimulus fish. To investigate the effects of predators on the behavior of prey fish and to test whether the possible effects varied with predator size, black carp (Mylopharyngodon piceus) and snakehead (Channa argus) (a size-matched predator treatment with a similar body size to prey fish and a larger predator treatment with approximately 2.7 times of the body mass of prey fish) were selected to function as prey and predator, respectively. Predation is one of the key factors governing patterns in natural systems, and adjustments of prey behaviors in response to a predator stimulus can have important ecological implications for wild fish.
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