Hydrophone instruments include high frequency (10 to 200,000 Hz) and low frequency (0.1 to 1600 Hz) models. Hydrophone data at certain locations are used to detect vocalizations, which are examined to study seasonal shifts in cetacean populations. Whale songs are detectable by hydrophones from tens to hundreds of kilometres away from the source. Some species, such as orcas, have resident, transient and offshore sub-populations that are identified by their unique calls.