Australian Integrated Marine Observing System (IMOS) Acoustic Observatories1 Summary The Acoustic Observatories sub‐facility archives ocean noise from around Australia at three primary locations: 32o S off the east and west Australian coasts (since 2008 in Western Australia and 2010 in eastern Australia); and south of Portland, Victoria (since 2009) and makes the data publicly available. The facility has been “paused” in 2017‐218 pending Australian Commonwealth Government funding decisions. The moorings are, or were located near or on, the continental shelf break. A passive acoustic mooring comprises a calibrated (2 Hz to 2.5 kHz) autonomous sea noise logger set on the seabed recording 5 minute ocean noise samples at 6 kHz sample rate every 15 minutes for 11‐12 months. In several instances grids of three or four instruments have been set up for tracking purposes, although more recent deployments use one or two instruments only. Currently we have a fourth site being sampled, on the shelf edge west of Kangaroo Island in South Australia. The Western Australian Government funded three years of data collection (2012‐2015) at 15.5o S and 19.5o S on the shelf edge of the northern WA coast. The passive acoustic observatories record sound emitted by natural processes in the ocean, underwater noise sources of biological origin, such as marine mammals, crustaceans or fish, plus man‐made noise sources. Through analysis of these signals it is possible to discriminate and identify different animal species and to assess the relative number of animals present within the range of acoustic observation, which can then be linked to ocean productivity or yearly migratory passage for great whales. Sea noise can be visualised on the IMOS web page, small amounts downloaded or large amounts requested from IMOS. Data is currently supplied in its raw format only, although we are considering making available processed detection times of various whales for selected sites. Please note that if requesting data from IMOS the best format to ask for is the raw data (*.DAT files), IMOS will offer *.wav file versions of the data but without a list of magic numbers the *.wav data is uncalibrated, whereas the raw data is calibrated. Instrumentation and Data The passive acoustic observatories comprise a sea noise logger placed on the ocean floor attached to a mooring which on command, releases floats to the surface. The sea noise loggers were designed and built at Curtin University and are well proven fully calibrated instruments (see McCauley et al. 2017). Each mooring also has ancillary temperature loggers attached with one on the seafloor and one between 30‐50 m above the seafloor. Sea noise data sets can be viewed via the AODN web portal: https://acoustic.aodn.org.au/acoustic/ as time stacked spectrograms (~ 15 days, x‐scale) on a logarithmic frequency scale (y‐scale, 5 Hz to 3 kHz) with intensity colour coded. Choose a deployment then scroll through in time. By left clicking a location on the spectrogram an image of the nearest sample's spectrogram and its waveform are displayed (while this feature should work, it may not work for some browser types). The time‐stacked spectrograms are useful for easily visualising when major sources are present as the dominant sources have a unique frequency content, so fill certain bands of energy in the spectrograms. For example a section from the Perth Canyon 1 Provided by Dr. Robert McCauley, Curtin University, Australia – 21 February 2018 version 1 with nearby pygmy blue whale calling is shown on the figure below, with a full pygmy blue whale song type on the left and two days of stacked sea noise on the right. Figure showing on the left a pygmy blue whale three part song and on the right two days of sea noise containing thousands of pygmy blue whale calls. The frequency alignment can be easily seen here. A vessel passes during the middle time period. The archived sea noise contains information on whatever noise sources were active at the time, within the listening range of the receiver. What the listening range is for the receiver depends on a variety of factors, including the source type, its intensity, location in the water column, combined with the sound transmission environment along the path to receiver and the local ambient noise field. Typical detection ranges for the locations sampled are 50‐150 km for blue whales in the deep ocean and perhaps 20‐40 km back onto the continental shelf, or 10‐30 km for humpbacks on the shelf. Processing sea noise is not an easy task and to do it properly requires some degree of understanding of underwater acoustics and signal processing. There are tools developed for this (see links below) and to simplify interpretations we are intending to load onto the AODN portal time stamps of detection algorithm outputs with identification of when different whale types occurred, initially along the eastern Australian coast using the NSW receiver data. The archived sea noise contains an invaluable record of the vocalising and physical noise sources. Applications of Data Examples of applications of the data include: An air craft crashing into the ocean may generate underwater sound by its impact at the surface or by debris imploding as it sinks. A signal was detected on two IMOS receivers coincident with the best estimate of the time of the loss of Malaysian aircraft MH370. After identification on the Perth Canyon IMOS receiver this signal was found in the Comprehensive Test Ban Treaty Organisation (CTBTO) hydrophone sensors at the Cape Leeuwin (HA01) site which gave a bearing consistent with the believed crash area. The signal falls almost precisely at the believed crash time but is not located along the search path given by analysis of the aircraft satellite communications (so called "7th arc"). Using data from the IMOS Scott Reef, IMOS Perth Canyon and CTBTO hydro‐acoustic stations the source location is some 8000 km from the Perth Canyon, slightly west of the Maldives assuming the same source was received at each site. The signal was later found on the CTBTO Diego Garcia hydrophones where it had arrived by horizontally refracting around the Diego Garcia Island group. This is a good example of long range ducting of signals in the open ocean due to the "deep sound channel" created 2 by a minimum sound speed at around 1000 m depth in the deep ocean which traps sound energy over a narrow frequency band (5 ‐100 Hz) by refraction. Sound energy over this frequency band suffers practically no absorption losses so can travel ocean scale distances losing energy only by 2‐dimensional spreading. Currently we believe this signal was generated by a small earthquake and was not related to the loss of MH370 although no matching earth tremor signals have yet been identified on the earthquake seismic network. References are Duncan et al. (2014 a & b). An example of the types of sources commonly found in the IMOS passive acoustic observatories was published in Erbe et al. (2015). Using four years of data this paper summarises the major contributors to sea noise in the Perth Canyon, these being whales, fish, wind, rain and ships. Statistics of the sea noise are given along with examples of the contribution of these sources to sea noise, their spectral characteristics, temporal patterns and how the sea noise can be used to define physical and biological parameters. A comprehensive analysis of fish chorusing behaviour from the Perth Canyon has been carried out, with the aim of identifying the chorus source (McCauley and Cato, 2016). Fish chorusing behaviour in the oceans is common but hardly reported or known of in mainstream biological sciences. The paper by Erbe et al. (2015) mentioned above presents a summary of the Perth Canyon fish choruses while the McCauley and Cato (2016) paper gives details of this chorus and suggest the most likely source is fish of the family Myctohidae. The detailed analysis suggest the chorus, which encompasses spatial scales in excess of the tens of nautical miles sampling program carried out and which occurs nightly and year round, is associated with the rise of the deep scattering layer and is found in regions of known highest secondary productivity in the Perth Canyon. This chorus is currently believed to be produced by small fishes foraging high in the water column. A figure showing the trends in this chorus over 4.3 years of IMOS data and its regular, nightly and seasonal occurrence is shown below. Figure showing the trend in fish chorus level activity in the Perth Canyon over a 4.3 year period using IMOS data. The top plot shows the level across an evening in the fish chorus frequency band stacked over 1,574 days with each evenings times zeroed to time of local sunset and intensity as the colour scale. The heavy black line is the time of sunrise. The lower plot shows the level in the chorus frequency band each evening over 0.4 to 5 hours post sunset. Several PhD students are making use of IMOS data. This includes further work on fish chorusing patterns, fin whale patterns around Australia or studies onto how sea noise derived indexes of whale calling can be turned into density estimates of the number of whales present in the listening range of 3 the sea noise logger. These are works in progress. The 2014 Perth Canyon IMOS sea noise data is being used by one of the students to correlate against a high resolution tag attached to a pygmy blue whale by the Centre for Whale Research (WA), which gave eight days of fine scale whale diving behaviour.