Response across a gradient: behavioural reactions of newly settled fish to predation cues

How individuals respond to predation threats will have a large influence on which individuals survive within a population. The magnitude and timing of these responses will be particularly important during periods of high predation susceptibility, such as that experienced by coral reef fishes immediately following settlement to the reef environment. Although reef fish are known to adopt certain antipredator behaviours when exposed to chemical alarm scents during this early period, the role of visual cues, and how the response varies with different levels of predation threat (both olfactory and visual), remains unknown. This study examined the behavioural response of a newly settled coral reef fish (Pomacentrus amboinensis) to different levels of an olfactory and visual predation threat. The concentration of a conspecific chemical alarm cue and the visual proximity to a potential predator (Pseudochromis fuscus) were manipulated in separate aquarium experiments. Behavioural responses were found to be threat sensitive in nature, with higher-level threat cues eliciting a more intense response. Although significant changes were observed, responses to visual cues were more inconsistent, while responses to extremely low chemical cue concentrations were marginal, indicating a possible threshold lower limit. These findings demonstrate the ability of newly settled fish to assess the level of predation risk using both visual and chemical cues, and respond appropriately.