Within the twilight zone of the ocean, 200-1000 m below sea level, there are potentially vast resources of fish. Echosounders are commonly used to estimate fish abundance, by transmitting pulses of sound of known frequency into the water column and measuring the amount of signal (acoustic backscatter) reflected back. Fish with gas filled swimbladders return strong signals, particularly compared to those without.

Using a combination of computed tomography (CT) scans, x ray and lab dissection scientists at the British Antarctic Survey and the University of Bristol examined the swimbladders of key mesopelagic fish in the South Atlantic (see e.g. Figure above).

They found that Southern Ocean fish closer to the Antarctic continent are more likely to lack gas in their swimbladders, and so are more likely to be overlooked by acoustic surveys. Thus, acoustic surveys that show a decline in acoustic backscatter and therefore indicate a decrease in fish biomass towards the poles may overlook a large proportion of fish species that dominate polar seas.

(a) Mean abundance of fish (individuals per 1000 m3) in RMT25 net samples by latitude, (b) Biomass of fish (grams per 1000 m3) in RMT25 net samples by latitude, (c) Relative proportions of fish by swimbladder contents in net samples at latitude. From Swimbladder morphology masks Southern Ocean mesopelagic fish biomass, Volume: 286, Issue: 1903, DOI: (10.1098/rspb.2019.0353)

(a) Mean abundance of fish (individuals per 1000 m3) in RMT25 net samples by latitude, (b) Biomass of fish (grams per 1000 m3) in RMT25 net samples by latitude, (c) Relative proportions
of fish by swimbladder contents in net samples at latitude. From Swimbladder morphology masks Southern Ocean mesopelagic fish biomass, Volume: 286, Issue: 1903, DOI: (10.1098/rspb.2019.0353)

Lanternfish play a vital role in Southern Ocean ecosystems and are key prey items for top predators in the region, including king penguins and elephant seals. Understanding the capacity for acoustics to detect and monitor lanternfish in the Southern Ocean is vital for interpreting acoustic data, and monitoring changes in the mesopelagic community.

Reference