Large mesopelagic fish biomass in the Southern Ocean resolved by acoustic properties.

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

doi:10.1098/rspb.2021.1781

Tracey Dornan, Martin J. Genner + Show 2 more

Open Access

https://doi.org/10.1098/rspb.2021.1781

Copy DOI

Abstract

The oceanic mesopelagic zone, 200–1000 m below sea level, holds abundant small fishes that play central roles in ecosystem function. Global mesopelagic fish biomass estimates are increasingly derived using active acoustics, where echosounder-generated signals are emitted, reflected by pelagic organisms and detected by transducers on vessels. Previous studies have interpreted a ubiquitous decline in acoustic reflectance towards the Antarctic continent as a reduction in mesopelagic fish biomass. Here, we use empirical data to estimate species-specific acoustic target strength for the dominant mesopelagic fish of the Scotia Sea in the Southern Ocean. We use these data, alongside estimates of fish relative abundance from net surveys, to interpret signals received in acoustic surveys and calculate mesopelagic biomass of the broader Southern Ocean. We estimate the Southern Ocean mesopelagic fish biomass to be approximately 274 million tonnes if Antarctic krill contribute to the acoustic signal, or 570 million tonnes if mesopelagic fish alone are responsible. These quantities are approximately 1.8 and 3.8 times greater than previous net-based biomass estimates. We also show a peak in fish biomass towards the seasonal ice-edge, corresponding to the preferred feeding grounds of penguins and seals, which may be at risk under future climate change scenarios. Our study provides new insights into the abundance and distributions of ecologically significant mesopelagic fish stocks across the Southern Ocean ecosystem.

Highlights

Mesopelagic fish inhabit the twilight zone of the world’s oceans, 200–1000 m below sea level
Where it is assumed that krill contribute to the acoustic signal at a relatively high abundance of 64 krill m−2 [43], the mean biomass of mesopelagic fish was reduced to 16.96 million tonnes (Mt) and 273.78 Mt in the Scotia Sea and Southern Ocean, respectively
Active acoustics indicate that net-based mesopelagic fish biomass estimates are likely to be underestimates [13]

Summary

Introduction

Mesopelagic fish inhabit the twilight zone of the world’s oceans, 200–1000 m below sea level These typically small (less than 20 cm) fish are globally the most abundant vertebrates, and their communities are often dominated by lanternfish (family Myctophidae) in terms of number and biomass [1]. Acoustic methods show declines in mesopelagic backscatter towards the poles, which has been interpreted as lower fish densities in the Southern Ocean relative to temperate latitudes [14,15]. We develop the first taxon-specific TS estimates for key Southern Ocean mesopelagic fish species, using empirical and literature-derived information on tissue density and swimbladder gas content. We assign proportions of this acoustic backscatter to individual species using mesopelagic fish assemblage data and unique TS values, enabling estimations of mesopelagic fish biomass for both the Scotia Sea and the wider Southern Ocean

Methods

Results

Discussion

Findings

63. Bost CA et al 1997 Foraging habitat and food