Swimbladder morphology masks Southern Ocean mesopelagic fish biomass.

Tracey Dornan,Ryan A Saunders,Sophie Fielding,Martin J Genner

doi:10.1098/rspb.2019.0353

Tracey Dornan, Ryan A Saunders + Show 2 more

Open Access

https://doi.org/10.1098/rspb.2019.0353

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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.

Highlights

Mesopelagic fish inhabit the twilight zone of the world’s oceans, 200–1000 m below sea level
The underlying principle of active acoustics is to transmit a pulse of sound of known frequency and duration into the water column from an echosounder; when the soundwave encounters something of a different acoustic impedance, such as gas in the swimbladder of a fish, it is reflected or scattered back to the transducer
Analyses focussed on 11 of the most dominant Scotia Sea fishes, which accounted for greater than 94% of all fish captured by abundance in RMT25 net data

Summary

Introduction

Mesopelagic fish inhabit the twilight zone of the world’s oceans, 200–1000 m below sea level. In the Southern Ocean, 35 species of myctophids are known to occur [12], where they form a key component of the Antarctic ecosystem, acting as both predators of zooplankton [13,14,15] and prey for higher predators, including seabirds and seals [16 –19] In this food web, which is typically dominated by krill (Euphausia superba), myctophids have elevated importance for higher-trophic-level species during the years when krill are scarce [20]. The utility of active acoustics for this assessment has been hampered by limited data on swimbladder morphology both within and among key myctophid species It has been unclear if the reported latitudinal decline in backscatter towards the Antarctic continent [22,23] is a consequence of a decrease in fish biomass, or instead a consequence of the coincidental change in mesopelagic fish community composition [23]. We consider this result from the perspective of acoustic biomass assessment, and discuss the potential underlying ecological and evolutionary drivers of the observed shift in myctophid community composition and morphology

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