Researchers Use Sound to Determine How Many Krill Live Beneath the Icy Surface of the Southern Ocean

Acoustics are key for advancing ecosystem-based management of the regional fishery

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Researchers Use Sound to Determine How Many Krill Live Beneath the Icy Surface of the Southern Ocean
Antarctic krill
Swarms of Antarctic krill such as these seen swimming in the Southern Ocean can contain hundreds of millions of individual krill. Using traditional and novel tools to understand the amount of krill in the East Antarctic section of the Southern Ocean is the key objective of this research expedition.
NOAA NMFS SWFSC Antarctic Marine Living Resources (AMLR) Program Flickr Creative Commons

This is the third instalment of the series, “Krill Chronicles From East Antarctica,” documenting the experiences of scientists onboard the research vessel (RV) Investigator.

For two months at sea, the  focus of Australian Antarctic Program scientists on board the Commonwealth Scientific and Industrial Research Organisation’s Marine National Facility, the RV Investigator, has been an acoustic survey of Antarctic krill, a fundamental species in the Antarctic marine food web and the subject of the largest fishery in Antarctica. Knowing the biomass of krill—in other words, how much is out there—is key to managing the fishery for a healthy and thriving marine ecosystem.

Although the question of “how much is out there?” may seem like the focus of routine research surveys worldwide, the costs and logistical challenges mean there’s really no such thing as a routine research expedition to Antarctica, one of the coldest and most remote regions on Earth.

And this is not merely a theoretical exercise: Scientists and policymakers from the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) will use the updated krill biomass estimate and the predator information collected by the scientific team to inform the revision of krill management conservation measures for East Antarctica. A similar scientific update of a key management measure for krill fishing in the Antarctic Peninsula region is being made by CCAMLR scientists and commissioners at this year’s meeting.

Where the voyage also stands apart is what its research team has been doing to improve the accuracy of krill biomass estimates using a novel camera system that floats with krill swarms to help scientists visualize their physical dynamics, such as the density of individual krill and the direction they like to orient themselves in the water. 

Gavin Macaulay is leading the acoustics research onboard the RV Investigator. An engineer by degree, Macaulay developed an interest in applying underwater acoustic techniques to the study and surveying of aquatic life. His first job was with New Zealand’s Ministry of Agriculture and Fisheries, where he developed echosounders and methods for biomass surveys of the deep-water orange roughy—leading to many voyages in New Zealand’s remote oceanic regions, Europe, South America, and the Middle East. Most recently, Macaulay worked for Norway’s Institute of Marine Research in the Ecosystem Acoustics group, which involved participating in a seven-week voyage to Antarctica to survey krill.

So Kawaguchi is a krill ecologist with the Australian Antarctic Division and serves as chief scientist on the RV Investigator voyage. His research focus on Antarctic krill started when he worked as a scientist for a Japanese fishing company. He went on to work for the Japanese Fisheries Research Agency and then relocated to Australia, where he now leads the Australian Antarctic Division’s krill research program.  

This interview with Macaulay and Kawaguchi has been edited for clarity and length.

Gavin Macaulay
Gavin Macaulay
Courtesy AAD Andreas Klocker
So Kawaguchi
So Kawaguchi
Courtesy AAD Andreas Klocker

Q. What are you trying to accomplish with this survey?

So Kawaguchi: Our expedition’s major objectives are twofold: (1) to determine the krill biomass and coefficient of variation of that biomass estimate for the part of the Southern Ocean off the Mawson coast in East Antarctica and (2) to inform krill fishery management by collecting information that will aid in the design of a monitoring program to understand ecosystem changes.

Q. Why is the research you’re doing so important that it had to go forward during a pandemic?

Kawaguchi: The krill fishery in the Antarctic Peninsula area—the main fishing region—is expanding; catch limits have been reached in the middle of the fishing season for most years in the last decade. The fishing nations are looking to expand their fishing efforts into other regions: In 2016, China started fishing off East Antarctica, and Norway notified CCAMLR of its plans to begin fishing for krill in the East Antarctic region (the Indian Ocean sector) of the Southern Ocean this year.

We know from our recent experience in the southwest Atlantic sector of the Southern Ocean (around the Antarctic Peninsula) that the fishery can increase its catch rapidly. There’s no regular krill monitoring in the Indian Ocean sector, so we don’t have enough information to determine if the precautionary catch limit, which was calculated from a survey conducted in 2006, is still valid. So we had to conduct this survey at the earliest possible timing.

Q. In simple terms, what are the steps needed to turn the raw data you get on the survey into a regional biomass estimate for Antarctic krill?

Gavin Macaulay: We send out sound pulses and measure the strength of the echo that comes back from krill swarms. We divide the echo we get back by the amount expected from an average single krill to get an estimate of how many krill generated each echo. Over the whole area we then estimate the numbers of krill. Because we know the weight of krill, we can then figure out the total weight of krill in the survey area.

A colorful chart captures the output from an acoustic tool called an echosounder, which bounces sound off underwater objects such as fish—or, in this case, krill—to create a visual representation of a krill swarm’s density, structure, and vertical distribution in the water column.
A colorful chart captures the output from an acoustic tool called an echosounder, which bounces sound off underwater objects such as fish—or, in this case, krill—to create a visual representation of a krill swarm’s density, structure, and vertical distribution in the water column.
Gavin Macaulay Australian Antarctic Division/CSIRO

Q: Why has there not been a krill biomass assessment in the area since 2006?  

Kawaguchi: Of course the more frequently you survey, the more information you get. However, depending on the type of question you’re asking, you may not need to do the survey every year; you might be better off doing a one-off detailed process study. Finance and logistics also come in to play: Planning a major survey requires multiple years of planning and requires a huge amount of data that then takes many years to analyze.

On the other hand, routine monitoring could be done more regularly, but it needs to be set up under a clear purpose and hypothesis. If it’s a monitoring for fishery management purposes, then we want to make sure we get information on biomass of the target and the potential error associated with the biomass, and also information on year-to-year variability. If the size of the fishery is extremely small or nil compared to the biomass of the target, then there may not be an urgency to do a biomass survey every year—because it’s highly unlikely that the fishery will pose irreversible impacts on krill and dependent species. In that case a survey every 10 to 20 years may be considered acceptable if the catch limit is set in a precautionary manner—which has been the case for CCAMLR’s krill fishery.

If there is an expectation of the fishery expanding, or the fishery is likely to do so, then there will be a need to ensure its expansion happens in an orderly manner based on best available science. For that we need to know more about the ecosystem structure and the distribution of target species, as well as what the uncertainties are in the system, so that we can improve our management strategy and always walk on the safer side for the fishery management.

If we’re going to assess the biomass only every several years, it becomes more important to understand the year-to-year variability through an annual monitoring program targeted at a small region that represents the variability of the wider region.

Q. How is conducting surveys for Antarctic krill different than conducting surveys for other species like fish? Are there any unique challenges?

Macaulay: Acoustic surveys work in very similar ways regardless of what aquatic organisms are being surveyed. But some of the characteristics of krill make our job easier, while others make it harder.

Q. How so?

Macaulay: Krill form huge dense swarms, so it’s often easy to know if what we are seeing is krill or not. That’s often not as easy for schools of fish. And while on one hand krill don’t reflect sound as well as many fish species, which makes them harder to acoustically detect on their own, they’re easier to detect when they form these dense swarms.

But surveying krill in Antarctica—a cold, distant, and vast place—means that surveys are often much longer in duration, and with harsher working conditions, than in any other place.

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In addition to using echosounders to conduct the biomass survey, the acoustics team is also deploying a “krill swarm study system” that consists of free-floating cameras within a metal frame. Observing inside krill swarms will help scientists understand the orientation and behavior of krill, information that helps increase the accuracy of biomass estimates.

Q. How important have recent technological advances in monitoring and locating krill populations been in shaping precautionary catch limits?

Kawaguchi: Technological advancements allow us to understand more about the hidden side of krill biology, ecology, and the 3D distribution of krill. This additional information not only provides us with the currently unaccounted krill biomass in the system, but more importantly may provide us with some clues to explain what causes differences in krill population sizes from year to year.

Q. What novel research techniques are you using during this voyage, and what are you hoping to learn from these studies?

Macaulay: We’re measuring the speed of sound through krill, and we’re also measuring the density of krill themselves. These data are important for us to be confident in the krill biomass estimates from the survey, as sound speed and density affect how much sound reflects off a krill—and we use the sound to convert the acoustic survey data into biomass of krill. This is a difficult measurement to do accurately at sea—it involves weighing the krill to an accuracy of 1 mg and measuring the time it takes for sound to travel through krill to within a millionth of a second, all on a moving ship. We hope to develop the method and procedures so that it can become a routine procedure as a standard part of acoustic krill surveys.
RV Investigator crew
A colorful chart captures the output from an acoustic tool called an echosounder, which bounces sound off underwater objects such as fish—or, in this case, krill—to create a visual representation of a krill swarm’s density, structure, and vertical distribution in the water column.
Linda Gaskell Australian Antarctic Division/CSIRO

We also have the swarm study system, a combined echosounder and camera system that we’ll deploy into krill swarms. One of the purposes of this kit is to obtain acoustic and photographic measures of krill orientation inside swarms, which will help us produce more accurate estimates of how much sound reflects off krill at different orientations. That, again, will help improve our estimates of krill biomass from acoustic surveys.

Q. What can you tell us about the sizes of krill swarms you see in East Antarctica?

Kawaguchi: I’m not sure what I can tell about the sizes of the krill swarms in East Antarctica, but we’re about to undertake an exciting swarm study that will for the first time follow up on the only reported observation, by a Japanese krill fishing vessel, of a southward krill migration within this expedition’s survey area. We plan to track various krill swarms we encounter by using a multibeam echosounder and sonar to map and track swarm movements within the current fields to see if there is any systematic direction in the swarm movements and potential relation to their life history stages.