Diurnal variation in freshwater ecoacoustics: Implications for site‐level sampling design

Abstract

Ecoacoustic methods are increasingly used to monitor the state of populations and ecosystems. In freshwater environments, they present the clear advantages of being non‐invasive, reducing bias, and providing continuous observations instead of only limited sampling snapshots in time. However, similar to standard bioassessment methods, temporal variation and choice of indicators can greatly influence ecoacoustic assessments, highlighting the importance of sampling and analysis design. In this study, we quantified diurnal variation in underwater sound and its effect on sampling regimes for two waterholes in the Einasleigh River, Northern Australia. Recording continuously for 6 days, and subsampling 5 s every 10 min, we found 22 distinct sounds that were emitted by fish, Hemiptera and Coleoptera as well as another 22 of abiotic or unknown origin. Through rarefaction analyses, we found that subsampling the data to 60% of the recorded sound events resulted in capture of most of the 44 identified sound types. Temporal heterogeneity—patchy sound events through time—needs to be considered when maximising detected sound events. Reducing the sampling interval from every 10 min to half‐hourly or hourly had a much greater effect on capturing all sound types compared to the number of days recorded or the length of the recording. Overall, only 10–20% of the sound events need to be annotated for most sound types to be described; for example, restricting analysis of the days recorded to only three and the recording interval to 0.5–1 s. Acoustic indices were dominated by three main event types—a diurnally flowing creek, a nocturnal chorus of Hemiptera, as well as a dawn chorus of terapontid fishes. We conclude with two key messages: First, a select group of informative signals can be monitored using very simple methods—namely, converting an audio stream into indices using freely available software. Second, however, to detect less acoustically dominant sound events, manual annotation or single call processing will still be needed. While these findings are encouraging, similar analysis will need to be conducted within other freshwater ecosystems before general conclusions about optimal sampling regimes can be drawn.

Passive acoustic monitoring as a potential tool to survey animal and ecosystem processes in freshwater environments

“…In this special issue, Linke, Decker, et al. () reveal that in order to capture most acoustic diversity in a freshwater pool, it is more efficient on average to sample short recordings at many times of the day and over several days than longer recordings at fewer times and days.…”

“…A diversity of species produces sounds. This acoustic diversity can be used to extract information on species richness (Buxton, Agnihotri, Robin, Goel, & Balakrishnan, ; Pieretti et al., ; Pijanowski, Farina, Gage, Dumyahn, & Krause, ; Sueur, Pavoine, et al., ); however, the relationships between acoustic and species diversity are not always straightforward (Linke, Decker, et al., ). Soundscape composition can be quantified using acoustic indices.…”

“…Acoustic indices have also been applied in freshwater streams with some success; for example, Linke, Decker, et al. () were able to detect onset and end of major acoustic events such as fish and insect choruses in freshwater environments. Acoustic indices allow discrimination of different temporal or ecological conditions in freshwater environments.…”

Six steps towards operationalising freshwater ecoacoustic monitoring

“…a hydrophone) is used for the detections. While ecoacoustic analysis using indices or spectrograms is still feasible (Gottesman et al, ; Linke, Decker, Gifford & Desjonquères, ), even when not all sounds are resolved, interpretation can sometimes be difficult. We, therefore, concur with Rountree, Bolgan and Juanes (): in many cases, it is imperative to know and catalogue the source of the sounds.…”

“…Ecoacoustics can detect and monitor: (a) water‐dependent birds and amphibians (Dema et al, ; Dutilleux & Curé, ; Indraswari et al, ; Linke & Deretic, ); (b) activity of aquatic insects and other invertebrates (Desjonquères, Rybak, et al, ; Gottesman et al, ; Linke, Decker, et al, ); (c) communication between fish (Grabowski et al, ; Hanache et al, ; Higgs & Humphrey, ; Linke, Decker, et al, ; Roca et al, ; Rountree & Juanes, ); (d) biophysical processes such as sediment transport and gas exchange (Gottesman et al, ; Linke, Decker, et al, ); (e) anthropogenic disturbance, for example boat and engine noises (Desjonquères, Rybak, et al, ; Higgs & Humphrey, ) [Colour figure can be viewed at wileyonlinelibrary.com]…”