Acoustic and visual stimuli combined promote stronger responses to aerial predation in fish

Topic: We investigated the importance of visual and acoustic stimuli in aerial predator avoidance of a small freshwater fish, the sulphur molly Poecilia sulphuraria. Our first aim was to explore the diversity of disturbances to which sulphur mollies are exposed in their natural habitat. We then tested whether fish increased their anti-predator response with the number of predator stimuli by presenting a moving object above the surface (unimodal visual), a playback of an impact sound (unimodal acoustic) or both stimuli simultaneously (bimodal). DATA_SI_BirdActivityField: Bird activity We recorded all sightings of birds in the predefined study area within a 30-min period. For each bird, we determined species (to lowest feasible taxonomical level), entry and exit times, as well as the number of attacks. Bird sightings were categorized as either 'stay' or 'overflight' (i.e. flying through the transect without landing or attacking). We calculated presence time as the mean time birds spent in the study area (excluding overflights) and attacks as the total number of bird attacks launched in a sampling period. DATA_SI_DivingGroupmeans: Lab experiment – predator stimuli We exposed fish to simulated bird attacks presenting them with one of three stimulus types: unimodal visual, unimodal acoustic or bimodal (visual + acoustic). In a laboratory setting, we tested a total of 5 groups (12 individuals each; n = 60). Data provided here is group-pooled, but individual tracks are available upon request. DATA_SI_DivingField: Supplement – predator stimulus validation We assessed fish's dive reaction in response to simulated bird attacks using a focal shoal sampling approach. Six focal shoals were chosen along the sulphidic stream habitat (but in correspondance with sites reported in Culumber et al. 2016). The application of the acoustic stimulus was not feasible under field conditions. We stimulated fish 10–12 times with a minimum of 60 s between exposures and recorded diving responses within a frame of interest that allowed for the detection of individual fish. As the information of a detected threat often propagates through a group, we determined the interval from the point that 50% of fish had initiated diving until 50% had resurfaced. DATA_SI_DivingBodysize: Supplement – lab experiment For the experiment, test fish were visually matched for size. To verify that groups did not significantly differ in size, body sizes were estimated from video images taken during the experiment. To minimize a systematic error introduced through fish’s distribution in the z-axis (i.e. distance to camera), measurements were only taken for individuals that were very close to the tank’s front glass and not occluded by another fish. Monitoring of water paramters (DO, Temp) was carried out prior to a group's first trial and repeated after the last trial. 2021_Lukas_Multisensory.R (see 2021_Lukas_Multisensory_Report for a PDF version) All analyses were performed in R (R Core Team, 2020, version 4.0.2). SIAudio_Stimulus_Impact.wav The acoustic stimulus consisted of an underwater recording of a projectile hitting the water (duration: 1 s)