| Abstract: | Coral reefs provide ecosystem services to millions of people and are a habitat for almost one third of the world’s named marine fish species. As the world’s human population and the demand for natural resources expand, the states of most terrestrial and marine ecosystems continues to decline at an alarming rate. In the past century, coral reefs have suffered significant rates of deterioration, through a combination of stressors, including invasive species, climate change related disturbances and pollution. More recently, El Niño events, combined with several major storm events have been connected with extensive bleaching and significant coral mortality, particularly in Australia’s Great Barrier Reef (GBR).
The designation of marine protected areas (MPAs) worldwide, covering nearly 3% of the world’s oceans, is an example of the world’s commitment towards safeguarding the future of marine ecosystems. However, to meet the demands and goals of marine management policies, there is a need for implementation of well-structured management plans. Fundamentals of effective management, conservation, research of ecosystems and natural resources are built on the ability to track patterns in space and time. This is where long-term monitoring and research can provide critical ecological insight.
Passive acoustic monitoring (PAM) is used to investigate environmental systems through the medium of sound. It permits natural physical, biological and anthropogenic sound sources to be monitored over long time frames and offers an additional approach to visual surveys of coral reefs. However, the full management applicability of PAM remains undetermined.
The aims of this research were to categorise biological reef sounds, identify and investigate temporal patterns of fish choruses, quantify the contribution of anthropogenic (ship) noise and determine how large disturbance events may influence soundscapes over time on Australia’s Great Barrier Reef.
Lizard Island, situated in the GBR, was chosen as the primary location to conduct field work. The GBR supports over 1500 species of fish, 400 species of hard corals and an enormous variety of other life forms, making it an ideal location for investigating how PAM may have potential as a long-term coral reef monitoring tool. Lizard Island has been, and continues to be, a key location for coral reef research. Over the last 30 years, the Island’s renowned research station has helped to support and foster a wealth of marine research projects.
Long-term monitoring in coral reefs has always been a significant challenge due to reefs being logistically difficult and expensive to access, highly heterogeneous and a demanding environment for equipment. This has meant that many surveys have been restricted to fine resolution ‘snapshot’
studies, so that our understanding of larger scale ecosystem dynamics over longer time scales is more limited. Therefore, the combination of logistical support and access to relevant data that Lizard Island research station provides was a deciding factor for choosing this location.
The coral reefs surrounding Lizard Island in the GBR have a diverse soundscape that contains an array of bioacoustic phenomena, particularly biological choruses produced by soniferous fish. Six fish choruses identified around Lizard Island exhibited notable spatial and temporal patterns from 2014 to 2016. Several environmental variables had a significant influence on the timing and received levels of several of the fish choruses. Two out of the six detected choruses exhibited spectral and temporal characteristics similar to choruses produced by planktivorous fish species previously documented at these sites and elsewhere. Three of the six choruses appear to be undocumented and could hold information on the presence, abundance and dispersal patterns of important fish species, which may have long-term management applications. Several choruses displayed high site fidelity, indicating that particular sites may represent important habitat for fish species, such as fish spawning aggregations sites.
The contribution and subsequent impacts that anthropog enic noise may have in marine ecosystems, particularly in coral reefs, has gained substantial attention in the last decade. Vessels (commercial, recreational and research) are the key sources of anthropogenic noise in coral reef soundscapes around Lizard Island. Significant spatial differences in research vessel activity were present around Lizard Island, which led to several vessel traffic hotspots, in areas of high activity. Daily activity of commercial vessels was similar throughout the year, but a notable rise in recreational vessel density occurred during the start of the austral wet season, which coincides with peak biophonic activity in the form of fish choruses. The long-term noise contributions of research vessels were apparent at sites that had high levels of research boat activity, but were not observed at sites with low activity. Noise from commercial vessels raised soundscape sound levels at low frequencies (< 500 Hz) at five of the six field sites. With commercial ship numbers and size continuing to increase, it is expected that vessel noise will be a growing contributor to coral reef soundscapes. Combined with elevated environmental disturbances from climate change, this is likely to be an important additional threat to coral ecosystems.
Lizard Island was exposed to a Category 4 cyclone in March 2015, which resulted in widespread damage to several areas of reef around the Island. The following year, large parts of the GBR experienced severe bleaching; including around Lizard Island which, resulted in substantial mortality of corals. Fish choruses were still present at sites that sustained significant cyclone damage, but a change in chorus energy attributes was observed. Sound recordings taken close to the start of the bleaching event did not reveal any discernible short-term changes to the soundscape. Recordings taken around Lizard Island in November and December 2016 contained five of the six choruses recorded pre-bleaching, also similar to those measured in the previous two years, which implies at least some short-term resilience. However, the long-term impact of these destructive disturbance events and how soundscapes and their key constituents will be affected is yet to be determined. A notable drop in low-frequency noise in 2016, post-bleaching, corresponds with a reduction in vessel activity in the same area, suggesting PAM may provide an indication of potential socio-economic impacts.
These findings highlight the utility of PAM for long-term monitoring and management of coral reefs, which is highly relevant in light of recent global disturbance events, particularly coral bleaching. Comparison of current and historical soundscape recordings made around Lizard Island over 25 years ago revealed long-term site fidelity of fish choruses. This indicates that soundscapes should be strongly considered as a long-term coral reef monitoring tool, with fish choruses showing potential as an ecological indicator of coral reef condition.
Future research should focus on extricating the temporal patterns associated with bioacoustic activity and determining the potential environmental drivers of biological choruses. Utilising and developing automatic signal processing techniques in pattern recognition would help progress this. This could then support technique development for direct identification of vocalising species, which would strongly increase the management applicability of PAM. Research efforts should also be made towards expanding soundscape monitoring across the GBR, with a focus on sampling areas that contain a range of live coral coverage.
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