Most studies of the effects of ocean noise from natural and human sources broadcast acoustic signals and monitor for any resulting short-term effects on marine life, or they work backward from observed effects to determine sound sources. IQOE is based upon the recognition that if added noise already harms animals and affects marine ecosystems negatively, then measuring effects of reducing noise should be a key goal of marine bioacoustics.
Beginning in January 2020, travel restrictions and economic slowdowns occurring globally in response to the COVID-19 pandemic—and intensified by an oil price war—put the brakes on much human activity in the ocean. Drastic decreases were seen in shipping, tourism and recreation, fishing and aquaculture, energy exploration and extraction, naval and coast guard exercises, offshore construction, and port and channel dredging.
The global pandemic has thus created a chance for scientists to determine how a sudden decrease in human activities—already attested to by atmospheric and terrestrial measures—and the subsequent slow recovery of the global economy affect sound levels in the ocean.
Other unintentional historical events provide analogies to the current drop in human activities on and in the world’s oceans. The start (1945) and stop (1980) of aboveground nuclear testing provided traces of carbon-14 and tritium, whose movements and decay have provided major insights into ocean physics, chemistry, and biology. And after the terrorist attacks in New York City and Arlington, Va., on September 11, 2001, the cancellation of hundreds of civilian airline flights allowed scientists to study the effects of jet contrails (or their absence) on weather patterns.
Listening to Quieter Oceans
A previous opportunity to study the effects of reduced ocean noise arose from the slowdown of shipping following the events of September 11, 2001. For example, a group of biologists studied the levels of stress hormones in endangered North Atlantic right whales in the Bay of Fundy. They found that during the four years after 2001, stress hormone levels increased in mid-September as the whales prepared to migrate from their northern feeding and mating area to warmer southern waters where they calve. However, immediately after September 11, 2001, ocean noise levels from shipping decreased, and levels of stress hormone dropped [Rolland et al., 2012]. This study suggests that living in an industrialized ocean chronically stresses these whales and that the reduction in noise reduced their stress.
Fortuitously for the ocean research community, a large number of nonmilitary hydrophones were already deployed worldwide before the pandemic, so there is a significant opportunity to study its ramifications on ocean sound globally. IQOE is working with the ocean observing community to identify a global network of civilian-operated, passive acoustic hydrophones (i.e., those that record ambient sounds rather than generating sounds of their own) useful for this purpose. This network could also serve more broadly as a prototype global hydrophone network for scientific and monitoring purposes.
As of February 8, 2021, we had identified 231 hydrophones that could contribute to a global analysis of the effects of the pandemic on ocean sound (below). Most identified so far are located in waters of the United States and Canada, but increasing numbers are being added elsewhere, particularly in European waters. Meanwhile, more acoustic instrumentation and measurements are clearly needed across the Southern Hemisphere.
It is important that acoustic monitoring by many existing hydrophones continue at least through 2021 to make sure researchers can observe a complete return to baseline conditions if the pandemic subsides by then. Such consistency is a challenge, however, because the pandemic has interrupted the placement and servicing of hydrophones and other parts of ocean observing systems in most of the world.
Because most hydrophone installations are autonomous recorders that are not equipped for telemetry (Figure 1), most hydrophone data are not available in real time; further, the pandemic has delayed the recovery of some data. The schedule of data recovery dictated by pandemic-related disruptions will affect the timing of the global analysis of hydrophone data for assessing large-scale impacts. However, researchers have already released some preliminary real-time results detailing regional ocean quieting near Vancouver, Canada [Thomson and Barclay, 2020]. Extending these observations to include measurements on a global scale should be exciting.
The existing hydrophone network covers coastal areas, which are likely those most influenced by local changes in human activity, and it also includes deep-ocean stations that can measure effects of low-frequency sound sources over large areas. The more areas that can be sampled, the more accurate our view of the global effect of COVID-19 on ocean sound will be. As such, IQOE continues to welcome expansion of the global passive acoustic network.
A Broader Scope for Ocean Acoustics
Beyond analyzing effects of pandemic-related decreases in human activity on ocean sound and potential follow-on effects on ocean ecosystems, the fledgling hydrophone network will continue contributing to the Global Ocean Observing System (GOOS). This system is a worldwide collaboration of observing assets, including Argo floats, sea level monitoring stations, fixed and drifting buoys, and others.
Timely maturation of a global marine passive acoustic network is helping to meet the GOOS goal of monitoring ocean sound. Such capabilities will allow researchers to further analyze acoustic data to shed light on the distribution of ocean sound and its effects in Anthropocene seas [Duarte et al., 2021]. These efforts will go a long way toward improving our limited understanding of the effects that humans have on marine life and ecosystems.