Coastal bay Passive Acoustic Monitoring

This study aimed to quantify the ambient noise levels of a relatively undeveloped coastal bay in Florida, USA. An increasing level of vessel activity is projected for the area, due to both recreational and commercial coastal developments, and underwater noise monitoring was selected as a cost-efficient method for surveying the differences in both natural and anthropogenic soundscapes throughout the area of interest.

Methodology

Static autonomous acoustic recorders were deployed from a small boat for periods of between two weeks and three months and programmed to record full spectrum acoustic data from approximately eight location, which were believed to vary in human activity and the presence of cetaceans and other biological sound producers. This method allowed simultaneous recording of low-frequency sounds from fish and invertebrates as well as the high-frequency echolocation clicks of bottlenose dolphins (the only marine mammal in the area).

Challenges

Both spatial and temporal monitoring of the soundscape was needed to research natural variations in biological soundscape and sounds made by particular fish and invertebrates.

A hidden method of data collection was necessary to improve the likelihood of obtaining unbiased data in an area where recreational users were suspicious of research activities.

Access was limited due to commercial fishing, distance to sites, shallow water, inclement winter weather conditions and available resources, making boat-based survey work impractical.

Outcomes

Locations rarely or never used by cetaceans within the wider area of interest were identified based on lack of vocalisations.  Similarly, areas frequently used were identified and a basic understanding of the behaviours occurring there was gained. New sounds produced by other vocalising animals were recorded and added to an ongoing database of biological sounds in Florida’s marine environment.

Recordings conducted over 24 hour periods throughout all four seasons allowed a complete understanding of both the temporal and spatial variation in the biological soundscape. Baseline data on the range of ambient underwater noise levels were collected, both in response to natural variations by site and additional noise from anthropogenic sources, such as commercial fishing and recreational usage.

The study verified that this area is relatively undeveloped and that the main sources of noise are biological. Certain locations within the site are much more biologically active, and our research made efforts to identify development opportunities outside those areas possible.

Benefits of Passive Acoustic Monitoring

The low frequency nature of fish vocalisations requires precise equipment to accurately record and quantify them. A high level of analysis expertise is also necessary to differentiate these sounds from other abiotic ones. Using Passive Acoustic Monitoring, both temporal and spatial coverage of a reasonably large area (540 km2) were sampled at a much lower cost than that of any other survey method. This monitoring protocol also allows for samples to be collected 24 hours a day throughout all four seasons, with one team able to deploy equipment to record samples simultaneously throughout the study area. This considerably reduces staff time and field-related expenses, as well as allowing recordings to be collected from a diverse range of locations, including those used frequently by commercial fisheries.