# A Collection of Best Practices for the Collection and Analysis of Bioacoustic Data
## 3 Population Structure  
A second realm in which acoustic methods are making useful contributions concerns the study of population structure. Population structure includes analyses of age structure, demography, how individuals in a population are distributed across given areas, and how multiple populations might isolate or mix through interbreeding [94]. A key component to population structure is gene flow among populations [95]. Traditionally, studies of population structure have relied upon direct observations of animal movement, combined with genetic analyses to document patterns of interbreeding. While there is scientific merit to genetic analyses, these methods are not suitable in all cases. For instance, species that are rare or live in remote habitats may not be accessible for collecting genetic samples. In other cases, a species may be highly mobile or cryptic, and therefore difficult to observe or trap. For each of these cases, alternate methods are needed for population sampling. With advances in acoustic methods, researchers have turned to passive acoustic surveys to assess population structures for soniferous species when traditional observational methods or genetic analyses are unavailable or inconclusive (Figure 2) [36; 96; 97; 98]. Many potential pitfalls in acoustic analyses of population structure parallel those that apply to acoustic species classification. First, as pointed out in the previous section, species need to be identified reliably based on their signals. Furthermore, just as species classifiers rely on data that are visually validated, most methods of testing for population structure require that the recordings come from source populations that have been validated using other methods such as visual observations or genetic analysis. If the populations being tested are geographically segregated, the location of the recorders can provide this validation if it can be shown that there is enough distance between recorders that sounds from the adjacent population will not be recorded. However, if the populations being tested are sympatric, it is important to use independent means (e.g., visual scans) to identify the focal population or group being recorded and to ensure that individuals from other populations are not present or nearby during the recording (see, for example, [31; 98]). In addition to these considerations, the parameters used in the recording and analysis of  
Figure 2: Linking sound structure to population structure. (**a**) Animals are recorded, with or without consideration for non-acoustic socio-environmental variables. (**b**) A range of acoustic features are extracted from each sound sample, and samples are clustered along these features. (**c**) Acoustic structure is mapped to potential features of population structure (e.g., learning from nearby conspecifics, dialects). Numbers refer to different individuals (**d**) Population structure is summarized and visualized, providing a statistical prior for inference about non-acoustic socio-environmental variables (see point a). acoustic data should be consistent across the populations being tested to avoid introducing variability as an artifact of the experimental design. As with species classification, some parameters to hold consistent, or at least attempt to control for, include sampling rate, recorder sensitivity, signal-to-noise ratio (SNR), time and seasonality of recordings, behavioral states of the animals, call classification categories, FFT sample size, and length of the spectrogram window. A recent passive acoustic survey of [PERSON]'s killer whales (_Orcinus oroa_) within the Gulf of Alaska aimed to characterize aspects of this population's structure [68], following a genetic study in this species that suggested the presence in that region of as many as five potential subpopulations [99]. [PERSON] and colleagues collected and compared 1575 calls from [PERSON]'s killer whales over a nine-year period, from 49-773 encounters per population per year.