Bahlburg, D., Hüppe, L., Böhrer, T., Thorpe, S. E., Murphy, E. J., Berger, U., & Meyer, B. (2023). Plasticity and seasonality of the vertical migration behaviour of Antarctic krill using acoustic data from fishing vessels. Royal Society Open Science, 10(9), 230520. https://doi.org/10.1098/rsos.230520
Krill are one of the most abundant animals on Earth, with a biomass of more than 370 million tons. These shrimp-like crustaceans are famously whale food, but they are an essential food source for many sea creatures, including whales, seals, penguins, and squids. Like many other zooplankton, krill undergo diel vertical migration, or a daily vertical movement pattern through the water column. This allows animals to
surface at night to feed on algae, and descend to deeper waters during the day to hide from predators.
However, the movement patterns of krill are mysterious. They may perform large vertical migrations sometimes, but in other situations they don’t migrate at all. Scientists don’t yet understand the environmental factors that contribute to krill movement patterns because of the extreme conditions near their Antarctic habitat. In particular, there are very few studies in the winter, when harsh conditions make scientific studies extremely difficult.
A krill cruise of opportunity
Fishing corporations are moving into Antarctic waters, and some of these fishers are harvesting krill for fish food and nutritional supplements. These fishing fleets provide opportunities for scientific research. Scientists recently partnered with a commercial fishing operation to study krill movement patterns over an eight-month cruise.
Oceanographers can use acoustic methods like SONAR to detect animals in the water. These devices send out sound waves, which bounce back at different frequencies depending on the distance of the object. In this study, the researchers used acoustic instruments on the fishing vessel to monitor krill swarms.
Swimming patterns and seasonal cycles
During the study, researchers observed that some krill swarms would spend most of their time close to the surface, without performing vertical migrations. At other time, however, krill migrated over vertical distances of more than 150 meters. Krill behavior also differed across seasons. In the winter, krill swarms shifted to deeper waters, and performed diel vertical migrations to deeper depths.
The next step was to connect these patterns with environmental factors such as feeding conditions, with a focus on concentrations of krill’s food, algae. Sometimes krill would spend more time at the surface when food concentrations were low, perhaps because they were forced to spend more time foraging. Overall, however, there was a lot of variability in migratory behavior even during similar environmental conditions and locations. This variability could not be easily explained. It could be that differences between individuals—based on hunger or body size—are affecting behavior.
Managing our future fisheries
Krill living near Antarctica are being harvested more and more each year, and this trend will only continue as warming makes the Southern Ocean more accessible. Managing healthy fisheries requires basic knowledge of the population distributions of animals—where are they, and how many are there? We are probably exploiting Antarctic fisheries faster than we can answer these questions. As this study highlights, there is a lot we don’t know and the drivers of krill migration remain a mystery.
Facilitating the help and expertise of the people whose livelihood depends on a sustainable fishery is a great example of how researchers can collaborate with industries to tackle these important questions and effectively manage marine ecosystems.
Cover image source: Wikimedia Commons
I am a PhD student at MIT and the Woods Hole Oceanographic Institution, where I study the evolution and physiology of marine invertebrates. I usually work with zooplankton and sea anemones, and I am especially interested in circadian rhythms of these animals. Outside work, I love to play trumpet, listen to music, and watch hockey.