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.

Keywords: Southern Ocean; acoustics; biomass; ecosystem; mesopelagic fish; myctophid.

Figure 1.

Study location in the Scotia Sea, Atlantic sector of the Southern Ocean. RMT25 surface to 1000 m depth net sample locations (yellow diamond). Acoustic transects between the Falkland Islands and the South Orkney Islands (coloured lines): spring cruises, JR161 (October 2006), JR15002 (November 2015); summer cruises JR177 (January 2008), JR15004 (January and February 2016); autumn cruise, JR200 (March 2009). Mean frontal positions are represented in white. SAF, Sub-Antarctic Front; PF, Polar Front; SACCF, Southern Antarctic Circumpolar Current Front; SB, Southern ACC Boundary [–26]. Also shown are the 2° latitudinal bands used in analysis. Map generated in Quantum GIS v. 2.18 (www.qgis.org). (Online version in colour.)

Figure 2.

Relationship between the nautical area scattering coefficient (NASC, m² nmi⁻²), a proxy for biomass, and increasing latitude by cruise number. JR15004 had both north to south (NS) and south to north (SN) transits, all others are one way only. All data shown were collected in water greater than 1000 m depth. Linear regressions (black lines) are statistically significant (p < 0.001). (Online version in colour.)

Figure 3.

Single slice computed tomography scans of (a) Electrona antarctica showing loss of swimbladder gas and (b) Krefftichthys anderssoni showing gas presence (dark regions in tissue). Fish standard lengths shown in mm. (c) Polar plots of standardized proportions of species captured in 2° latitude bins, each colour segment proportionally corresponds to the abundance of individual species. (Online version in colour.)

Figure 4.

(a) Mean abundance of fish (individuals per 1000 m³) in RMT25 net samples by latitude. Bars indicate standard deviation between net samples and numbers in columns indicate numbers of individual net strata samples included. (b) Biomass of fish (grams per 1000 m³) in RMT25 net samples by latitude, box spans interquartile range (IQR), horizontal line is the median, whiskers include values up to 1.5 × IQR, outlying values plotted individually. (c) Relative proportions of fish by swimbladder contents in net samples at latitude. Numbers in columns are the individual number of total water column samples (each consisting of 4 depth strata) used in analysis. (Online version in colour.)