There’s plenty of fish in the sea… but how many, exactly? How do you go about counting the number of fish in the sea, especially in remote areas like the “twilight” zone (200-1000 meters below sea level) of the Southern Ocean, close to Antarctica? Obviously, going at it one by one is not an efficient approach. So instead, researchers try something else: echo sounding.

An echosounder sends out an acoustic pulse and when the pulse encounters something with different physical properties than the space around it, it is reflected back to the receiver. The fishes’ swim bladders, the gas-filled organs they use for buoyancy, are particularly sensitive to these pulses and offer very good reflectors.

But there’s a catch: a lot of times, in colder waters, fish lose the highly-reflective gas in their swim bladders. This means they produce a smaller acoustic signal, and their numbers could have been underestimated — this is what the authors of a new study are claiming.

Judging by this difference, the researchers say, the biomass (total weight of the fish) in the areas close to the Antarctic could be 1.8-3.6 greater than previously expected. Lead author Dr. Tracey Dornan, a fisheries acoustician at British Antarctic Survey says:

“This research is hugely important as people have interpreted declines in acoustic survey signals towards Antarctica as a decrease in fish biomass—but this isn’t the case. We’ve shown that there is actually likely to be a peak in fish biomass towards the seasonal ice edge, but the typically larger fish that live here are relatively poor at scattering the acoustic signal back because they lack a gas-filled swimbladder, so their biomass has been underestimated.”

The research is particularly important for researchers studying fish in the Antarctic, especially those trying to assess their numbers. If we’re undercounting the total number of fish, it’s hard to have an accurate idea of how their numbers are changing. Fish play a vital role in the oceanic carbon cycle, and they’re also a key component of their ecosystems. These typically small fish (less than 20 cm or 8 inches) migrate vertically, feeding on plankton closer to the surface during the day, and returning to the depths during the night. So understanding variations in their population is essential not just for their own sake, but for the sake of the entire Antarctic ecosystem and its evolution.

Researchers caution that climate change (and warming waters) could produce more changes in how acoustic data reflects off fish, Co-author and ecologist Sophie Fielding, who supervised the project, concludes:

“This research highlights the importance of accounting for the unique scattering properties of individual species. This is particularly important in the face of climate change, as studies have predicted that acoustic signal is likely to increase. However, this could indicate a decrease in fish biomass as smaller, highly-reflective species move further south.”

Journal Reference: Large mesopelagic fish biomass in the Southern Ocean resolved by acoustic properties by Dornan, T., Fielding, S., Saunders, R.A., and Genner, M.J. is published in Proceedings of the Royal Society B. DOI: 10.1098/rspb.2021.1781