A wide variety of aquatic organisms, including juvenile salmonids, assess local predation risks using chemosensory cues. Such chemical cues are typically released from injured conspecifics and their ...detection may lead to species-typical antipredator behaviour, increasing the probability of prey to survive during predator encounters. Studies have demonstrated however, that under weak acidification (pH ~6.0), the response towards these chemical alarm cues is impaired. However, it remains unknown if the loss of response is graded (i.e., the behavioural response decreases with a reduction in pH) or if there is a threshold pH at which prey can no longer detect the alarm cues. We conducted two laboratory experiments to examine the effects of a graded reduction in pH on the behavioural response of juvenile rainbow trout to conspecific chemical alarm cues. The results of our first experiment suggest that at pH 6.6 and above, the alarm cues elicited a strong antipredator response, while alarm cues buffered to pH 6.2 did not (i.e. not different from distilled water). However, alarm cues buffered to pH 6.4 elicited a weak response, suggesting a graded response. We directly tested this in our second experiment using a repeated measures design. The response to alarm cues at varying pH levels did indeed follow a graded loss of function. Together, our results suggest that juvenile rainbow trout exhibit a reduction in the response to conspecific alarm cues proportional to ambient acidity and that the response to these critically important cues is lost at pH below 6.4. As the detection and response to these chemical alarm cues have been shown to confer direct survival benefit to individuals, these results are therefore presented in relation to possible sub-lethal effects of anthropogenic acidification to freshwater fish.
Fish rely on both chemical and visual cues to evaluate predation risk. Decisions with respect to activity partitioning in time (i.e., night vs. day) rely on accurate assessment of predation risk ...relative to energy intake;; predation risk is generally thought to be lower at night at the expense of feeding opportunities. At night, the sensory complement model predicts greater reliance on chemical perception of risk. Under this condition, a lower ability to use vision should result in a more conservative response to chemical cues than during the day. We tested this hypothesis under natural conditions by comparing the alarm response of young-of-the-year Atlantic salmon (
Salmo salar
L., 1758) under summer day and night conditions in salmon nursery streams. We found that salmon responded to the alarm cues to a significantly greater extent at night. This suggests that the sensory complement model may be correct and that nocturnal perception of risk may be generally higher than previously believed for juvenile salmon in the wild. In the absence of a more precise indicator of risk (e.g., vision), a greater reliance on chemosensory risk assessment at night may cause fish to shift to more risk-adverse behaviour.
Even at sublethal concentrations, various anthropogenic pollutants may disrupt the transfer of chemosensory information, often inducing maladaptive behavioral responses. Recent studies of freshwater ...prey fishes have shown impaired abilities to respond to damage‐released chemical alarm cues from conspecifics under weakly acidic conditions (pH ∼ 6.0). Several factors acting individually or collectively may account for such chemosensory impairment. By itself, acidification may chemically disrupt the alarm cues and affect fish olfactory functions. Alternatively, differences in local environmental conditions may affect biochemical composition, quantity of chemical alarm cues produced by epidermal tissue, or both, leading to variations in alarm response. Our goal was to assess whether the ability to produce and detect conspecific chemical alarm cues is similar in individuals reared under neutral versus acidic conditions. We conducted two experiments in which we measured the behavioral response of wild juvenile Atlantic salmon Salmo salar exposed to chemical alarm cues. In particular, we looked for differences in the ability of individual fish to (1) produce alarm cues capable of eliciting consistent antipredator behavior in conspecifics and (2) detect alarm cues upon the fish's introduction into a stream with a pH differing from that of the stream of origin; the latter experiment involved reciprocal transplant of fish between neutral (pH range ∼ 7.0–7.3) and acidic (pH range ∼ 5.9–6.3) sites. Our results demonstrate that the ability to produce and respond to chemical alarm cues is maintained in Atlantic salmon reared under acidic conditions and did not differ from that of fish reared under neutral conditions. Overall, these data suggest that no permanent olfactory damage occurred under reduced pH and, likewise, no significant difference in functional alarm cue production existed between Atlantic salmon reared under neutral and acidic conditions. Short‐term reduction in olfactory sensitivity and degradation of the chemical alarm cues under acidic conditions are the likely mechanisms affecting detection of these important chemicals by prey fish.
Prey can learn to recognize novel predators through the association of aversive stimuli (visual or chemical) paired with a predator cue (i.e. acquired predator recognition). Previous studies have ...demonstrated that a variety of prey can readily learn the identity of a single predator. In this study, we examined whether predator-naïve glowlight tetras,
Hemigrammus erythrozonus, could simultaneously acquire recognition of three novel heterospecific odours as predation threats, and whether this simultaneous learning confers a survival benefit. Tetras were conditioned to the odours of largemouth bass,
Micropterus salmoides, convict cichlids,
Archocentrus nigrofasciatus, and comet goldfish,
Carassius auratus, simultaneously, then subsequently tested 48
h later for learned recognition of each odour individually. Tetras were also tested for the recognition of yellow perch,
Perca flavescens, a novel predator, to assess whether learned recognition is generalized. Tetras significantly increased antipredator behaviour when exposed to each of the conditioned predator odours and the nonpredator goldfish odour, but not to perch odour (i.e. not different from the distilled water control). These results demonstrate acquired recognition and not a generalization to novel predator odours. Subsequent staged encounters with live pumpkinseed sunfish,
Lepomis gibbosus, predators showed that tetras conditioned to either sunfish odour alone or multiple odours increased survival over unconditioned tetras. These results are the first indication that prey possess the ability to learn the cues of multiple predators simultaneously, and that this multiple learning provides a survival benefit.
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