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Biological sources are significant contributors to aquatic soundscapes. Soniferous fish can dominate the soundscape in certain locations, at specific times and frequencies, particularly during the production of choruses. Passive acoustic monitoring of fish choruses can provide important ecological information about soniferous fish populations. This study presents the Australian Fish Chorus Catalogue, an inventory of fish choruses detected from 83 locations in Australian estuarine and marine waters. The Australian Fish Chorus Catalogue contains data on fish chorus occurrence and the spectral and temporal measurements, spectrographic images, and audio examples of 301 fish choruses. This catalogue has been developed to establish the foundations of an ongoing effort to document, quantify, compare, and track Australian fish choruses. We hope this open-access data depository will be used as a reference for future research and will facilitate an increase in understanding of fish choruses, which can then be applied to the monitoring and management of fish populations and their respective ecosystems. [ABSTRACT FROM AUTHOR]

Multibeam imaging SONARs have been used for a range of measurement applications, such as measurements of fish lengths. This study aimed to quantify the accuracy of imaging SONAR systems, that varied in frequency and beam geometry, to measure the length of synthetic targets positioned perpendicularly.Blueprint Oculus imaging SONAR systems, with four different (centre) frequencies (750 kHz, 1.2 MHz, 2.1 MHz and 3 MHz), were used to measure the length of three targets of nominal lengths: 10 cm, 20 cm and at ranges between 1 and 15.5 m. The effect of beam geometry on measurement error was then examined using regression analysis.This study found that there was an overestimation of the actual length of the target for all measurements that was inversely proportional to the horizontal beamwidth. The measurement error can be reduced by normalising for beamwidth. However, the variation of measurements (i.e. the precision) was found to also increase with range, which was attributed to the increasing beam separation.It is important that the effect of beamwidth on the accuracy of target length measurements, by imaging SONARs is acknowledged in future studies. One approach to mitigating this problem, is to limit the range at which length measurements are made to a beamwidth that produces an estimated level of error that is acceptable for that study. [ABSTRACT FROM AUTHOR]

Limiting environmental impacts of marine industrial operations and mitigating hazardous encounters between humans and marine fauna have become increasingly important as anthropogenic activity expands. To this end, significant effort has been made to develop sonar imaging of fauna and to increase detection and identification ranges. A Tritech Gemini imaging sonar was used to observe sharks of 1.4 to 2.7 m length, at ranges from 1 to 50 m, in various water depths ≤15 m. Within 5 m, shark shape, length and swimming action were readily discernible. However, as range increased, knowledge of movement patterns was required to discriminate a 'shark-like' object, before the shark became purely an acoustic target at greater ranges, where visual confirmation of the target was necessary for identification. Once the seafloor is ensonified by the acoustic beam, seafloor backscatter can dominate the image and mask shark detection. The results presented concur with other active acoustic detection studies that, for a given frequency and noise level, maximum detection and identification ranges are reliant on system source level, beam pattern, bathymetry, and target size and acoustic reflectivity. [ABSTRACT FROM AUTHOR]

Multi-beam (swath) sonar systems provide the capability to ensonify an entire aggregation of fish in a single pass. However, estimation of abundance and discrimination between species via the use of target strength are considerably more complex than using traditional echosounders, because they ensonify targets at a much wider range of incidence angles. The beam pattern and along beam resolution of multi-beam swaths can produce individual sample volumes that are of similar magnitude to an individual fish (particularly for large fish, say >1m in length). If individual fish can be resolved, (either as a single fish within a sample, or as multiple contiguous samples that delineate a single fish), and if one assumes that this situation applies to the whole school, acoustic packing density can be determined by dividing the volume of the school by the number of detected acoustic targets. This estimate is proportional to the actual packing density of the fish, defined as the number of fish per unit volume of water. Acoustic backscatter of fish from a number of schools comprising different species were collected off Perth, in 2005 and 2007, using a Reson Seabat 8125 and 7125 respectively. Nearest neighbour distances of between 1 and 3 body lengths were observed and packing density of acoustic targets showed distinct variation between some species. However, schools of the same species also displayed different acoustic packing densities at different stages of their growth and development. Such differences were more difficult to observe in schools of fewer fish because the variations in packing density had less impact on the overall volume of the smaller schools associated with fewer fish. Therefore discrimination between species was only deemed possible when surveying two species of different sized fish at the same time. Video ground truth data is recommended to confirm species composition whatever the type of school observed. [ABSTRACT FROM AUTHOR]

The concern about the effects of the noise of human activities on marine mammals, particularly whales, has led to a substantial amount of research but there is still much that is not understood, particularly in terms of the behavioural responses to noise and the longer term biological consequences of these responses. There are many challenges in conducting experiments that adequately assess behavioural reactions of whales to noise. These include the need to obtain an adequate sample size with the necessary controls and to measure the range of variables likely to affect the observed response. Analysis is also complex. Well designed experiments are complex and logistically difficult, and thus expensive. This paper discusses the challenges involved and how these are being met in a major series of experiments in Australian waters on the response of humpback whales to the noise of seismic airgun arrays. The project is known as BRAHSS (Behavioural Response of Australian Humpback whales to Seismic Surveys) and aims to provide the information that will allow seismic surveys to be conducted efficiently with minimal impact on whales. It also includes a study of the response to ramp-up in sound level which is widely used at the start of operations, but for which there is little information to show that it is effective. BRAHSS also aims to infer the longer term biological significance of the responses from the results and the knowledge of normal behaviour. The results are expected to have relevance to other sources and species. [ABSTRACT FROM AUTHOR]