Physiological responses of topmouth gudgeon, <Emphasis Type="Italic">Pseudorasbora parva</Emphasis>, to predator cues and variation of current velocity

Predators, either through direct or indirect encounter and current velocity, are frequently stressful to fish living in stream waters. In nature, fish may experience both current velocity stress and predation danger simultaneously. Experiments were carried out to clarify to what extent predation risk (with reference to different types of predatory cues) and current velocity can induce physiological stress in a running-water dwelling fish, topmouth gudgeon (Pseudorasbora parva). Fish were exposed to an alarm substance, predator odor, and visual cue, as well to combinations of predation risk and elevated current velocities. Metabolic rate, ventilation rate and fish activity were measured. Results showed that irrespective of the type of encounter, the presence of predator imposed physiological stress on fish. Metabolic rate were 0.983 ± 0.312, 0.641 ± 0.151, 0.572 ± 0.063, and 0.277 ± 0.016 mg O₂ W⁻¹ h⁻¹ following presence of alarm substance, visual cue, predator odor and control, respectively. Dramatic changes in ventilation rate and activity affirmed that alarm substance induced the strongest stress, followed by predator odor and visual cue. Reactions appeared to mirror the extent of fish perceiving danger of predation. Predation risk together with current velocity induces stronger stress, much stronger than if the current velocity works as a single stressor. However, the interaction between predation risk and current velocity did not have a significant effect on metabolic function; magnitude of metabolic response to high current velocity might mask the metabolic response to predator presence. Small fish living in stream habitats that face local predation risk would spend higher energy expenditure that may have negative impacts on growth, and hence their fitness.