Glider-Based Estimates of Meso-Zooplankton Biomass Density: A Fisheries Case Study on Antarctic Krill (Euphausia superba) Around the Northern Antarctic Peninsula

Abstract

We compare estimates of krill density derived from gliders to those from contemporaneous and previous ship-based surveys. Our comparisons cover several temporal and spatial scales within two strata around the northern Antarctic Peninsula (off Cape Shirreff on the north side of Livingston Island and in the Bransfield Strait). Our objective is to explore the feasibility of using gliders to supplement or replace vessel-based surveys of fishery resources. We deployed two long-duration Slocum G3 gliders manufactured by Teledyne Webb Research (TWR), each equipped with a suite of oceanographic sensors and a three-frequency (38, 67.5, and 125 kHz, each single-beam) Acoustic Zooplankton Fish Profiler. We used the acoustic data collected by these gliders to estimate biomass densities (g⋅m–2) of Antarctic krill (Euphausia superba). The two gliders were, respectively, deployed for 82 and 88 days from mid-December 2018 through mid-March 2019. Off Cape Shirreff, glider-based densities estimated from two repeat small-scale surveys during mid-December and January were 110.6 and 55.7 g⋅m–2, respectively. In Bransfield Strait, the glider-based estimate of biomass density was 106.7 g⋅m–2 during December–January. Contemporaneous ship-based estimates of biomass density, from a multi-ship broad-scale krill survey (Macaulay et al., 2019) restricted to the areas sampled by the gliders, were 84.6 g⋅m–2 off Cape Shirreff and 79.7 g⋅m–2 in Bransfield Strait during January. We compared two alternative krill-delineation algorithms (dB differencing and SHAPES); differences between biomass densities estimated by applying these algorithms were small and ranged between 4 and 7%. Alternative methods of sampling krill length-frequency distributions (LFDs) (nets or predator diets), which are required to convert acoustic energy to biomass density, also influenced the glider-based results. In Bransfield Strait, net-based estimates of biomass density were 6% less than those based on predator diets. Off Cape Shirreff the biomass density of krill estimated from a net-based LFD was 20% greater than that based on predator diets. Development of a variance estimator for glider-based biomass surveys is ongoing, but our results demonstrate that fisheries surveys using acoustically-equipped gliders are feasible, can provide density estimates to inform management, and may be conducted at lower cost than ship surveys in some cases.

The Oceans’ Biological Carbon Pumps: Framework for a Research Observational Community Approach

“…However, these organisms may actively transport significant amounts of carbon from the EZ deep into the TZ, and thus contribute to the diel vertical migration pump (Pinti et al, 2021). Studies based on acoustic sensors carried by gliders (Reiss et al, 2021) or USV (saildrones) (De Robertis et al, 2021) have showcased the potential of this approach to monitor macrozooplankton or fish. Such technological advances may become tools of choice for characterizing diel vertical migrations of large organisms, but their role is not explicitly considered in the remainder of this paper or included in Figure 1 because the study advocating it (Pinti et al, 2021) has not been published in peer-reviewed literature as yet.…”

Uncrewed surface vehicle (USV) survey of walleye pollock, Gadus chalcogrammus, in response to the cancellation of ship-based surveys

“…In some circumstances (e.g. low-diversity high latitude ecosystems) acoustic backscatter can be attributed to species or important taxonomic groups without additional biological sampling (Geoffroy et al, 2011;Ressler et al, 2015;Reiss et al, 2021). This is the case in the EBS survey area, where pollock account for almost all the backscatter from semi-demersal fishes (De Robertis et al, 2010), and an acoustic-only index is used in the stock assessment (Honkalehto et al, 2011;Ianelli et al, 2020).…”

“…Sampling methods that can be adapted to USVs such as small cameras (Fernandes et al, 2016), and environmental DNA samplers (Yamahara et al, 2019;Berger et al, 2020) may prove valuable. It may be useful to incorporate information from predator diets (Reiss et al, 2021) or fishery-dependent biological sampling (McClatchie et al, 2000;Chu et al, 2019), particularly if corrected for size and species selectivity (Millar and Fryer, 1999). Hybrid approaches in which ships and USVs work in tandem, with the ships trawling and the USVs increasing the density of acoustic measurements may prove effective.…”

“…This work is written by (a) US Government employee(s) and is in the public domain in the US. Reiss et al, 2021). USVs using wind and wave power for propulsion are promising for large-scale fisheries surveys (Greene et al, 2014;Swart et al, 2016) due to their long endurance.…”

“…As a result, targeted trawl samples are generally used to identify species and size/age composition of acoustic scatterers during ship-based surveys (McClatchie et al, 2000, Simmonds andMacLennan, 2005). However, in some instances, diversity may be sufficiently low that a single species or group dominates acoustic backscatter and species identification may be less of a concern (Geoffroy et al, 2011;Ressler et al, 2015;Reiss et al, 2021). In the case of the EBS, age 1 + pollock dominate acoustic scattering in deeper parts of the water column to the point that this backscatter can be attributed entirely to pollock (De Robertis et al, 2010;Honkalehto et al, 2011).…”