How much did the COVID-19 pandemic quiet the oceans?

The Skimmer on Marine Ecosystems and Management

Editor’s note: Anthropogenic noise in the ocean – from ships, sonar, construction, oil wells, windfarms, seismic surveys, and other activities – harms marine animals ranging from marine mammals to fish to invertebrates. Ocean noise has been documented to:

As the Skimmer is covering various way that the COVID-19 pandemic has impacted marine ecosystems and communities, a likely reduction in ocean noise is one possible bright spot. As we collected news and research articles on this topic, however, almost all reports that we found related to ocean noise and marine mammals off the West Coasts of the U.S. and Canada in the first half of 2020. To help broaden our understanding, we asked scientists from Applied Ocean Sciences, a collective of ocean consultants with expertise in ocean acoustics, to share what they have learned about noise trajectories over a longer timescale and in other areas of the world. Below is our Skimmer-style summary of news and research articles and an interview with Chris Verlinden, a senior scientist and chief technology officer at Applied Ocean Solutions.

Part 1: “An unprecedented pause in ocean noise that probably hasn’t been experienced in decades”: A summary of ocean noise news from the COVID-19 pandemic

Part 2: “In many parts of the ocean a whale might be audible twice as far away in 2020 as in 2019”: Interview with Chris Verlinden of Applied Ocean Sciences

Skimmer: We know the pandemic led to some dramatic decreases in ocean noise in the first half of 2020, but many noise-producing ocean uses have picked back up (e.g., shipping, fishing) since then. What can you tell us about changes in the ocean soundscape from before the pandemic to now – one year in?

Verlinden: Great question. The financial shutdown associated with the COVID-19 pandemic led to a dramatic decrease in certain types of shipping activity all over the world. It is important to note that this decrease was not uniform geographically or across vessel types. Cruise ship traffic all but disappeared during the height of the pandemic and has not yet returned to its pre-pandemic numbers. Similar reductions occurred in some passenger ferry routes where the number of vessels required to fill the needs of the community decreased during the height of the pandemic. Ferry routes, at least in the U.S., have now largely returned to pre-pandemic levels. Recreational fishing traffic (charters, sport fishing, deep sea fishing, etc.) decreased dramatically in the U.S. early in the pandemic, while most commercial fishing traffic continued at its pre-pandemic levels. Traffic from military vessels remained fairly constant, as did survey and research vessel traffic. Tanker traffic experienced a drop from the second half of March into April of last year but then steadily rose to pre-pandemic rates by the end of the year. Container vessel traffic experienced a similar trend in March and April 2020 but is only now starting to increase. Interestingly, bulk carrier traffic along certain areas on the U.S. West Coast decreased dramatically in March and April 2020, more than container traffic. This traffic included cargo such as gravel, chemicals, and bulk agricultural goods such as wheat and lumber. This made certain areas, such as the mouth of the Colombia River, substantially less trafficked and thus quieter than usual.

These trends by vessel type were roughly similar across the U.S. So in areas where much of the vessel traffic was cruise ships (such as parts of south central or southeast Alaska), ocean noise decreased significantly during the COVID-19 pandemic. Below are several plots of the number of vessels underway in a few different areas of U.S. waters during March-April 2019 and 2020. You can see that it varies dramatically with regard to geography. Most of this traffic is expected to return to usual pre-pandemic levels, with the possible exception of the cruise ship industry.

 

So in a nutshell, changes to shipping traffic depends on the type of traffic and location. But for the most part, shipping levels dropped dramatically in most places in the U.S. in the latter half of March 2020 into April and have increased to nearly pre-pandemic levels in most places for most types of vessels.

Now let’s talk about the acoustic implications. It is important to note that the acoustic impact of a ship depends on a lot of variables such as the vessel type and speed as well as local oceanographic characteristics, bathymetry, and ocean bottom properties. Sound propagates differently in deep water versus shallow water and in cold water versus more temperate waters. For these reasons, the changes in ambient ocean noise from ship traffic do not mirror the changes in shipping density. Also, there are other sources of noise in the ocean including wind, waves, industrial activity, seismic disturbances, ice, and biologics such as whales, fish, and shrimp. All these things taken together are referred to as the soundscape. So, the changes in the global soundscape are a more nuanced discussion than the changes in shipping activity. Below are some figures that illustrate this. The figures on top show global ship densities (from Spire Inc. global AIS data) in 2019 (top) and 2020 (bottom). You can see a subtle reduction in density in the major lanes in 2020.

 

Now let’s look at figures with the average modeled global shipping noise from 2019 (top), 2020 (middle), and the difference (bottom).

 

Our models predict that most of the ocean was quieter in April 2020 than April 2019, an average of about 3 dB quieter. That might not sound like a lot, but decibels are logarithmic so 3 dB represents a doubling of intensity. This means that in many parts of the ocean a whale might be audible twice as far away in 2020 as in 2019. That’s a big deal.

Also, not all areas are impacted the same. Below are plots of what a fictional grid of 16 ships will sound like at the surface of the ocean in the South Pacific compared to the Arctic. The cold water at the surface in the Arctic traps sound near the surface because of a phenomenon known as surface ducting. So if you reduce the number of ships in the Arctic by a couple of ships, the average noise level near the surface will decrease dramatically, while reducing the number of ships in the tropics will not have as big of an impact.

 

 

So, long story short: many areas of the global ocean were quieter in 2020 than 2019 due to the reduction in shipping associated with the COVID-19 pandemic. The average difference was about 3dB, which is significant for marine mammals and other organisms. In most locations, the difference was most pronounced in April 2020. Since that time, vessel traffic and thus the soundscape have been returning to pre-pandemic levels. The acoustic impact is not uniform and does not follow trends in shipping perfectly due to oceanographic and environmental factors. I expect ship noise to return to pre-pandemic levels by the end of 2021 and then continue the trend of increasing. Most literature reports that the ocean gets louder by an average of 3 dB per decade, although some recent research suggests the current trend may be closer to 1-2 dB per decade. I expect this rate of increase to continue for the foreseeable future with our increasingly globalized economy driving more shipping, and by extension, more shipping noise.

That being said, this rate of increase will likely not occur uniformly, and I am most concerned about noise levels in the Arctic. With the cold surface temperatures, surface ducting, and organisms evolved for a very quiet region, the continued increase in vessel activity in the Arctic will make it a lot louder over the next couple decades and have a significant negative impact on the native marine mammal populations.
 

Skimmer: What upcoming data are you most interested in seeing to help us better understand how changes in ocean uses during the pandemic are altering ocean soundscapes and ecosystems?

Verlinden: Model and observational data comparison. The Arctic. Marine mammal stress hormones.

The pandemic gave us a rare opportunity to study the impact of reducing ship traffic on the global soundscape. It is rare that we, as scientists, can see precisely how these knobs are being adjusted. Let’s not waste it. Let’s see what happened during 2020 and how it impacted the marine organisms that rely on sound to survive. Applied Ocean Sciences’ approach has been to model the global ship noise, but that is only part of the story. We need to compare these models to actual observations and adjust our model parameters until we can reproduce the observed data. We can then use those models to estimate and infer the soundscape everywhere. If our models can reproduce the changes that occurred during the COVID-19 pandemic, it will improve our confidence in our ability to model the soundscape everywhere in the world under a variety of circumstances. This includes forecasting the soundscape under projected future shipping and industrial activity levels. I’d particularly like to apply this knowledge to the Arctic to predict soundscape changes over the next 10-50 years.

Finally, I’d love to see what the marine mammal stress physiology community comes out with over the next few years. After the 9/11 terrorist attacks, marine biologists recorded a reduction in stress hormones such as cortisol and aldosterone in marine mammals in the Bay of Fundy. This directly correlated with the reduction in ship traffic that followed those attacks. I am interested and excited to see if this is observed on a global scale during the COVID-19 pandemic and see what we can learn about the impact of ship noise on marine mammal population health using these observations. The COVID-19 pandemic and resulting changes in shipping and ship noise has provided us with an amazing opportunity to understand how ship noise impacts marine mammals so that we can better address the challenges faced by these animals in the future.

For additional background, we encourage you to check out Applied Ocean Science’s excellent StoryMap on COVID-19 and Ship Noise.

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