Abstract

One of the many environmentally deleterious effects of industrialization is the acidification of freshwater, which results in part from precipitative inputs of anthropogenic pollutants. Considerable effort has been allocated to the study and remediation of severely acidified aquatic systems, with the effects of weak levels of acidification (pH 6 – pH 7) on biological communities having received less attention. One effect that has been studied is the chemical mediation of predator-prey interactions in freshwater and marine fishes. Following mechanical damage to the epidermis, as would likely occur during a predation event, many taxa release chemical alarm cues which elicit antipredator responses from conspecifics and other opportunistic receivers subject to similar levels of predation risk and potentially enhance the survival of alarm cue receivers during subsequent interactions with predators. Under weakly acidic conditions (pH < 6.6), these chemical alarm cues are rendered non-functional and do not elicit alarm responses from conspecific or heterospecific receivers. Weak acidification effectively deprives prey fishes of one source of chemosensory information on ambient risk levels.
Here, I describe a series of field experiments designed to evaluate the effects of this environmentally-mediated loss of information on wild populations of juvenile Atlantic salmon (Salmo salar) in four acidic and five neutral streams in the Miramichi River system, New Brunswick, Canada. Acid-impacted Atlantic salmon in these experiments demonstrated a loss of response to conspecific and heterospecific chemical alarm cues, as well as significantly greater responses to remaining (visual) threat cues than salmon under neutral conditions. Fish in neutral streams appeared to demonstrate additive responses to multiple risky cues consistent with dynamic threat-sensitivity and the sensory complementation hypothesis, whereas fish in acidic streams demonstrated non-threat-sensitive responses consistent with the absence of sensory complementation and greater value being assigned to information received through visual cues. Contrary to predictions, juvenile salmon do not appear to suffer increased mortality through predation as a result of this loss of information, nor do they experience negative growth effects resulting from temporal trade-offs between antipredator and foraging behaviours. Rather, acid-impacted Atlantic salmon demonstrated behavioural compensation through increased preference for complex habitats which offer greater abundance of physical refugia and limit line-of-sight for visually foraging predators, effectively mitigating the increased risk of predation associated with limited information.