A cross-modal effect of noise: the disappearance of the alarm reaction of a freshwater fish

Anthropogenic noise pollution is recognized as a major global stressor of animals. While many studies have assessed the unimodal impacts of noise pollution with a focus on intraspecific acoustic communication, little is known about noise pollution on the perception of visual and chemical information. The ‘distracted prey hypothesis’ posits that processing noise interferes with processing other information in the brain. Here, we found evidence for such a cross-modal effect of noise on the antipredator behaviour of a freshwater prey fish, the fathead minnow, Pimephales promelas. In laboratory trials, exposure to noise from a motorboat caused the total absence of the classical fright reaction of minnows to conspecific alarm cues, whereas an ambient noise control had no such impact. In natural habitats, the impairment of such antipredator behaviour due to noise pollution could have major fitness consequences. We discuss how our findings translate to animal ecology and the need for future studies that target specific management decisions regarding noise pollution.

Behaviour and neural responses in crucian carp to skin odours from cross-order species

Many teleost species respond with fright reactions to olfactory cues from injured skin of conspecifics, but they may also display responses to skin extracts of heterospecific fish. In the present study, we exposed crucian carp to skin extracts of conspecifics and three cross-order species of fish (brown trout, pike, and perch). Behavioural experiments showed that conspecific skin extracts induced fright reactions in crucian carp; extracts of brown trout induced such behaviour less frequently, while extracts of perch and pike were poor inducers of fright responses. The olfactory bulb is chemo-topically organized, and different sub-sets of neurons respond to functionally related odorants that mediate distinct behaviours. Accordingly, behavioural responses to an alarm signal should be reflected by activation of the neurons mediating fright reaction. Extracellular recordings from single units in the olfactory bulb showed that the relay neurons activated by conspecific skin extracts were also activated by extracts of brown trout, whereas extracts of perch and pike less frequently activated these units. Thus, the difference in behavioural responses matched the differences in the neural responses, indicating that skin extracts of heterospecific fish are more likely to induce fright behaviour when the responding sub-set of olfactory neurons is similar to the sub-set responding to conspecific extracts. Our results suggest that responses to injured heterospecific fish rely on chemical resemblance between odorants from heterospecific and conspecific skin, and need not be based on any form of associative learning.

Sensitivity of northern redbelly dace, Phoxinus eos, to chemical alarm cues

The northern redbelly dace, Phoxinus eos (Cope, 1862), is subject to predation by brook trout, Salvelinus fontinalis (Mitchill, 1814), in Canadian Shield lakes, particularly when individuals migrate to the pelagic zone at sunset to feed on zooplankton and fish shoals break up into single individuals. The objectives of the present study were to (i) determine whether northern redbelly dace react to skin extracts from conspecifics and thus potentially use chemical alarms to detect predators in nature, (ii) characterize the fright reaction in northern redbelly dace under different concentrations of alarm substance, and (iii) estimate the active space of the alarm substance in this species. Northern redbelly dace responded to skin extracts of conspecifics with a series of antipredator behaviours. The dace moved closer to the substrate and away from the area where the alarm substance was injected, increased both the cohesion and polarization of their schools, and performed more dashing and freezing behaviours. The observed responses were closely correlated with the concentration of the skin extract solution, suggesting that individuals could use this sensitivity to different concentrations of alarm substance to estimate the risk of predation in nature. The dilution experiment allowed us to estimate that 1 cm2 of northern redbelly dace skin contains enough alarm substance to generate a response of individuals in 110 558 L of water, which corresponds to a cube with sides of approximately 4.8 m. Indirect evidence suggests that the northern redbelly dace could use chemical alarm cues to assess the risk of predation by brook trout in nature.