Swimbladder morphology masks Southern Ocean mesopelagic fish biomass

Abstract

Within the twilight of the oceanic mesopelagic realm, 200–1000 m below sea level, are potentially vast resources of fish. Collectively, these mesopelagic fishes are the most abundant vertebrates on Earth, and this global fish community plays a vital role in the function of oceanic ecosystems. The biomass of these fishes has recently been estimated using acoustic survey methods, which rely on echosounder-generated signals being reflected from gas-filled swimbladders and detected by transducers on vessels. Here, we use X-ray computed tomography scans to demonstrate that several of the most abundant species of mesopelagic fish in the Southern Ocean lack gas-filled swimbladders. We also show using catch data from survey trawls that the fish community switches from fish possessing gas-filled swimbladders to those lacking swimbladders as latitude increases towards the Antarctic continent. Thus, the acoustic surveys that repeatedly show a decrease in mesopelagic fish biomass towards polar environments systematically overlook a large proportion of fish species that dominate polar seas. Importantly, this includes lanternfish species that are key prey items for top predators in the region, including king penguins and elephant seals. This latitudinal community switch, from gas to non-gas dominance, has considerable implications for acoustic biomass estimation, ecosystem modelling and long-term monitoring of species at risk from climate change and potential exploitation.

Mesoscale Eddies Structure Mesopelagic Communities

“…It has been observed that these factors present a strong variability across species and life stages (Davison et al, 2015). In particular, a recent study conducted in the Southern Ocean highlighted how, going from sub-Antarctic to Antarctic waters, the proportion of mesopelagic fish with a gas-filled swim bladder -and therefore the biomass estimated from acoustics-decreased while the biomass measured by midwater trawling remained approximately constant (Dornan et al, 2019). It is therefore important to emphasize that the patterns in acoustic backscattering we identify in this study may result from differences in micronekton biomass, community composition, or fish physiology.…”

Spatial Distribution and Abundance of Mesopelagic Fish Biomass in the Mediterranean Sea

“…Some species, in fact, do not have one or some lose the gas component in the adulthood. In addition, other organisms at midwater depths, e.g., siphonophores, possess gas-filled organ, which might be subsumed to the biomass from fish, leading to an overall overestimation of the midwater fish biomass (Kaartvedt et al, 2012;Davison et al, 2015;Dornan et al, 2019). Last, the relationship between targetstrength (an acoustic parameter) and biomass can introduce a potential bias in the estimate of the biomass (Saunders et al, 2013).…”

Mesopelagic Scattering Layer Behaviors Across the Clarion-Clipperton Zone: Implications for Deep-Sea Mining

“…The Northern Central Pacific ecoregion appears to hold high mesopelagic fish diversity and a dominant abundance of fish in the genus Cyclothone (Clarke, 1973(Clarke, , 1974(Clarke, , 1976Maynard et al, 1975;Barnett, 1983). Despite high abundances of Cyclothone collected in trawls, several species within this genus have regressed or fat-invested swim bladders (Phleger, 1998;Davison, 2011;Dornan et al, 2019), rendering them weak acoustic scatterers. Thus, Cyclothone may not contribute significantly to the acoustic backscatter we observed and could be considered "acoustically cryptic" (Dornan et al, 2019).…”