Antarctic krill and ecosystem monitoring survey off the South Orkney Islands in 2024

The Antarctic krill (Euphausia superba) fishery started in the early 1970s and peaked in the late 1980s with catches up to 500,000 tons per year, by USSR and Japanese vessels operating in several areas of the Southern Ocean (Figure 1). The fishing for Antarctic krill is currently concentrated within the Southwest Atlantic sector of the Southern Ocean (Commission on the Conservation of Antarctic Marine Living Resources (CCAMLR) subareas 48.1, 48.2, 48.3 and 48.4 (see fig 4)). A fixed precautionary annual catch limit for this sector was set to 620,000 tons (termed the ‘trigger level’) by CCAMLR in 1991 (CCAMLR Conservation Measure 51–01).

From the total krill catch in the 2022/23 season, of 424 200 tons, 36% was taken in subarea 48.1, 44% in subarea 48.2, 20% in subarea 48.3 and 0 in subarea 48.4 (Figure 2).

The overall catch proportion during the 2022/23 fishing season taken by the Norwegian krill industry represented 67%, the Chinese vessels caught 17%, South Korea 8%, Chile 4%, and Ukraine 3% (Figure 3).

Two large-scaled acoustic trawl surveys have been executed in the Southwest Atlanticsector of the Southern Ocean (subarea 48.1-48.4), calculating a total biomass of krill in these fishing areas to be 62.6 Mt in 2019 (Krafft et al., 2021), which was similar to the synoptic survey in 2000 (60.3 Mt; CCAMLR, 2010). Regular monitoring on meso-scale of krill distribution and demographic composition in these fishing areas, have been carried out by the US Antarctic Marine Living Resources program (AMLR) since 1996 (Reiss et al., 2008, 2017), in the Bransfield Strait and Elephant Island area (subarea 48.1). During recent years also Chinese and Korean vessels survey these same AMLR transects (Yu et al., 2019; Krafft et al., 2021; Wang et al., 2021). Previously these surveys were made during the austral summertime but presently also during the austral winter. The British Antarctic Survey run an annual acoustic survey off the South Georgia Islands (“Western core-box” subarea 48.3, since 1997 (Fielding et al., 2014)) during the summer season.

A Norwegian fishing company offered to carry out an annual krill monitoring survey commencing in 2011, for as long as they have commercial activity in the Southern Ocean (Jensen et al. 2010). Through discussions in CCAMLR WG-EMM (Working Group on Ecosystem Monitoring and Management) in 2010, it was agreed that the survey could be carried out in the CCAMLR statistical Subarea 48.2 according to similar standards as the annual scientific surveys undertaken in 48.1 and 48.3 and that the surveys should be executed with scientists from the Institute of Marine Research, Norway. The three surveys could form an integrated monitoring effort extending across the Scotia Sea (Figure 4), linking three of the areas with highest concentrations of krill and highest fishing activity. The results will help evaluate and develop the management of the krill fishery (Hill et al., 2016).

The first annual survey was carried out in January/February 2011 using the FV ’Saga Sea’ (Aker Biomarine ASA). The results and study design from this survey was presented at the CCAMLR WG-EMM in 2011. The original survey design (suggested during WG-EMM 2010) consisted of six parallel north-south bound transects extending 100 nmi. During this first survey season it was recognized a need to extend the monitoring effort covering the waters over the shelf edge, north of the South Orkney archipelago, where most krill in this region traditionally aggregate. During the WG-EMM meeting in 2011 it was agreed to extend the survey transects 20 nmi northwards and to omit the westernmost transect line from the 2011 survey. Before the survey in 2014, it was also agreed to extend the transect lines further to the south to cover the northern part of the Marine Protected Area south of the South Orkney Islands, and the design has since remained unchanged.

Results and activities from these annual South Orkney surveys are reported to WG EMM (e.g. Krafft et al., 2018b, Skaret et al., 2019) and are published in the primary literature (e.g. Krafft et al 2018a, 2019, Skaret et al. 2023). This report presents the survey activities from the 2024 season off the South Orkney Islands including results from acoustic recordings, taxonomic sorting of trawl catches and krill predator sighting data collected during daylight hours along the transects. Given available funding and survey ship time, implementation of various projects in addition to the prioritized core activities is possible some survey years as presented in this report.

2.1 - Survey design, area, and vessel

The Cargo vessel “Antarctic Provider” (Aker Biomarine AS) departed Montevideo, Uruguay, on the 22. January. On the 22. February the survey ended by reaching port in Montevideo. The survey design covers 60 360 km2 of the waters around the South Orkney Islands and includes five parallel transects extending from the northernmost waypoints at 59.67°S and southernmost waypoint at 62.00°S. Longitudes for transects 1 through 5 are at 44°W, 45°W, 45.75°W, 46.5°W and 47.5° W, respectively.

In addition, a small Shelf edge stratum at 2368 km2 was covered with a denser transect grid. The Shelf edge stratum covers the locations with the highest historical krill catches. The acoustic sampling effort is higher than in the main strata, but otherwise, the design is the same (Figure 5).

Acoustic data collection

2.1.1 - Vessel mounted echosounders

During the survey ‘Antarctic Provider’ was equipped with Simrad EK80, 18 kHz and EK60 echo sounders operating at 38, 70, 120 and 200 kHz.

The echosounders were calibrated while at anchor off Signy Island using the standard sphere calibration method (Foote et al., 1987). The echo sounder was operating with a ping interval at or close to 1 per second. Nominal vessel speed during surveying is 10 knots and could be kept during most of the survey. Acoustic data were collected down to 600 m on all five frequencies. Other transceiver settings are specified in Table 1.

Acoustic moorings

2.1.1.1 - Mooring recovery from the survey deployments in 2023

Mooring 1: Aural hydrophone, ASL and CTD at S 60° 21.917 and W 046° 33.365

Mooring 2: two NorTek Signature 55 combined echo sounder and Acoustic Doppler Current Profiler (ADCP), with one pointing upwards and the other downwards, and two CTDs were deployed at S 60° 17.56576 and W 045° 46.21143 in 2023. The release unit did not function. Planning to resque this rig in 2025 by fishing after it using a rope.

Mooring 3: Nortec Signature 100 and a CTD at S 60° 21.90475 and W 046° 45.16915.

Mooring 4: Signature 100 and a CTD at S 60° 19.11244 and W 046° 44.90525.

2.1.1.2 - Mooring re-deployment

Mooring 1: Nortek Signature 100, CTD at S 60°21.900 and W 046°45.320W (Appendix 1)

Mooring 2: Nortek Signature 100, 2 CTDs at S 60°19.110 and W46°44.887 (Appendix 2)

2.2 - Acoustic data analyses

Discrimination of targets

In this work, we applied the standard swarm-based technique for krill discrimination on 120 kHz data using an Echoview work template, which includes noise removal filters, a swarm detection filter and an integration module (CCAMLR, 2017). Swarm detection was implemented using the Large Scale Survey System (LSSS) computer program (Korneliussen et al., 2016).

Target strength prediction

The NASC were converted to biomass density (g m-2) using the SDWBApackage2010 (Conti and Demer, 2006; SG-ASAM 2010; Calise and Skaret, 2011) according to the CCAMLR protocol. The model was parameterized according to Table 1, or if nothing else specified according to Calise and Skaret (2011).

The predicted target strengths were used to calculate weighted conversion factors (CF) from NASC-values to biomass density

where f is the frequency of a specific length group (i) and W(TL) is weight at total length, which was calculated following Hewitt et al. (2004):

σ(TL) is the backscattering cross-section at a specific total length and was calculated using the full SDWBA model.

2.2.1 - Estimation of biomass

Based on the average biomass density for each nautical mile, a weighted biomass density for each transect line could be calculated and the sampling variance from the averages of each transect line according to Jolly and Hampton (1990). 

2.3 - Biological sampling

The survey design included trawl stations spaced ~20–25 nautical miles apart along the set parallel north-south oriented transect lines (Figure 5). The standard survey trawl used was 42 m long, with a 36 m2 mouth opening, constructed of 7 mm (stretched) diamond shaped meshes from mouth to rear, or a 3 mm light opening (Figure 6). The trawl was towed using a 6 m wide steel beam with 200 kg weights at each lower wing tip and 1500 kg attached to the beam to ensure fast deployment to depth and the best possible geometric stability of the trawl during sampling.

At each station the trawl was lowered vertically from surface to ~200 m depth (or ~20 m above bottom if the water was < 200 m) and then hauled in at 1.5 - 2.0 knots (includes both vessel and wire speed). When landed on the trawl deck, the cod end was opened, and catch was removed. Thereby the towing rig was hung from a crane and flushed on deck to wash out biological remains stuck in the net. The macro zooplankton and micronekton were sorted, identified to species or to the nearest possible taxonomic group, and weighed. For E. superba, the length of individual krill was measured (± 1 mm) from the anterior margin of the eye to the tip of telson excluding the setae, according to the “Discovery method” used in Marr (1962). Sex and maturity stages of E. superba were determined on fresh material using the classification methods outlined by Makorov and Denys (1981). In brief, juveniles are classified due to their lack of visible sexual characteristics. Males were divided into three sub adult stages: MIIA1, MIIA2 and MIIA3 and two adult stages: MIIIA and MIIIB, females were divided into one sub adult stage: FIIA and five adult stages: FIIIA, FIIIB, FIIIC, FIIID and FIIIE (see Krafft et al. 2015 for further details).

2.4 - Hydrographical sampling

A Seabird CTD sensor was mounted in an open metal frame for protection and welded on the trawl beam to obtain profiles of temperature and salinity during the trawl hauls.

2.5 - Marine mammal observations

Marine mammal observations were carried out by a dedicated observer during daylight hours, whenever visibility was sufficient. The observer was located on the bridge and continuously scanned the starboard forward quarter (0 – 90°), binoculars were used for species identification. Angle were estimated using a drawn angle board. Distance estimation was aided by using the reticles in the binoculars that marked one milliradian per line. Together with the platform and observer height (20.3+1.7m), each reticles line from the horizon marked a known distance.

Observations were only recorded on straight sections of the vessel track with a nominal vessel speed of 10 knots. The vessel GPS track and logged effort were recorded with custom software on a windows tablet (DOI 10.5281/zenodo.10228669). For each observation the species, time, location, distance, angle, visibility and number of individuals were noted.

To describe migration patterns and foraging behaviour of humpback whales, satellite tags (SPLASH 10-373, Wildlife Computers, Redmond, Wash., U.S.A.) were deployed using an Aerial Remote Tag System (ARTS, www.restech.no)  (Figure 7).

2.5.1 - Pilot study using an in-trawl stereo camera for krill body length estimation

The krill body length frequency distribution data and its acoustic properties are used to convert acoustic NASC values into biomass. To provide the best overall representation of the length-frequency distribution of krill in the survey area and at the time of the survey, this can be challenging when using the standard equipment on traditional fishing vessels. Commercial nets display size selection characteristics that differ from small-meshed gear used in traditional scientific sampling, so that certain size groups can be under-represented. Changing trawling gear on board a fishing vessel is a complicated and time-consuming affair that one would rather avoid.

In this study, we developed a method for extracting body length for krill using state-of-the-art automatic image processing to analyze the video-feed from a custom built stereo camera.

The collected video data will be used for estimating the body length of the individual krill using semi-automatic image processing. Scientific methods for biomass estimation in surveys are not directly transferable to commercial fishing vessels as the methodology and data analysis will need to be automated. We therefore aimed to develop a semi-automatic tool to estimate the length-frequency distribution of krill entering a trawl during towing.

The camera setup for this study consisted of two GoPro cameras in a stereo configuration and underwater flashlights all fixed within a designed housing made of Polyoxymethylene (POM) material (fig. 8A). The camera system was then installed into the standard survey trawl to record entire hauls (fig. 8B).

For the analysis, the video footage was synchronized, undistorted, and rectified (Figure 9) to be processed using a custom-trained machine learning model that takes in the images and detects and segments krill appearing in the footage. The synchronized footage was then used to construct three-dimensional images of krill that is the basis for length estimations using calibration parameters calculated on the vessel prior to the experimental hauls (Figure 10.). The estimated length distributions will be validated against manually measured sub-samples of the krill from each haul to evaluate the accuracy of the stereo camera measurements.

3.1 - Acoustic survey estimates of krill distribution and biomass

The distribution of acoustic backscatter allocated to krill is shown Figure 11. The highest NASC-values allocated to krill were observed around the shelf edge and canyons. The krill length distributions are obtained by trawl samples (Figure 14). The biomass estimates are shown in Table 2.

a) Shelf edge stratum

b) Overall stratum

superba (red) and other targets (grey) from the 120 kHz recordings. Log distance is 1 nautical mile. The symbols are not linearly scaled due to space.                                        

3.2 - Biological sampling

A total of 25 trawl stations were successfully completed during the survey, with the largest catch weight at the northernmost stations and centered around and in the deeper part of the survey area (Figure 12). The total catch weight was dominated by salpa (58,1%) and euphausiids (33,7%) (Figure 13). Three species of euphausiids were identified: Euphasia superba, Thysanoessa macrura and Euphasia gregaria, where E. superba was the most numerous and present at 19 stations. However, most of the catch was concentrated at station E3, which accounted for 83,0% of total sampled E. superba. T. macrura was present at 22 of the stations and together with the amphipod Themisto gaudichaudi (found at 21 stations) was the most frequent taxa occurring in the trawl samples. The rest of the biomass consisted of Cnidarians (0.97%), Teleostei (0.88%), Calanoids (0,02%), and other taxa (0,75%), of which the most occurring species and taxa were Chaetognatha spp., pteropods, polychaets, mysids, Dendrobranchiata, Calycopsis borchgrevinki and Electrona spp. 

The average body length of E. superba was 43.29 ± 10.0 (SD) mm, and the range was 22-60 mm (Figure 14). The demographic composition of E. superba caught in the trawl was dominated by juvenile specimens (28.1%), as well as adult males (MIIIB: 26,9%) and adult females (FIIID: 11,4%). The adult individuals were mainly found in the northernmost stations (B1, C1, C2, D2) while the earlier stages appeared in the shelf and shelf break stations such as A3, A4, C3, C5 and C6.

Hydrography

The hydrographical profiles from the survey area based on the trawl mounted CTD are shown in Figure 15. A thermocline was observed mostly around 50-60 m.

3.3 - Marine predator observations

The observed species were: Fin whales (Balaenoptera physalus), Antarctic fur seals (Arctocephalus gazella) and Humpback whales (Megaptera novaeangliae) and Orcas (Orcinus orca). Table 4 lists the number of individuals observed per species. The map in Figure 16 displays the location of the effort periods and observed marine mammals. 

The satellite tracks from a female humpback whale with calf is illustrated in figure 17.

This work is financed by the project KRILL (IMR, Ministry of Trade, Industry and Fisheries), SUFIANT (Research Council of Norway and Aker Biomarine ASA), Antarctic Wildlife Research fund and the Ministry of Foreign Affairs. We extend our gratitude to the Aker Biomarine AS for providing ‘Antarctic Provider’ and its crew to disposal for this research survey free of charge. We are most grateful to the captain, officers, and crew on board for all the help provided during the cruise.

Calise, L. and Skaret, G. 2011. Sensitivity investigation of the SDWBA Antarctic krill target strength model to fatness, material contrasts and orientation. CCAMLR Science, Vol. 18 (2011): 97–122

CCAMLR (Convention on the Conservation of Antarctic Marine Living Resources). (2010) Report of the 29th meeting of the scientific committee, Hobart, TAS, Australia, 25-29th October, 2010. SC-CAMLR- XXIX. CCAMLR, Hobart, TAS, Australia.

Conti, S.G. and D.A. Demer. 2006. Improved parameterization of the SDWBA for estimating krill target strength. ICES Journal of Marine Science, 63 (5): 928-935.

Fielding, S., Watkins, J.L., Trathan, P.N., Enderlein, P., Waluda, C.M., Stowasser, G.,  Tarling, G.A. & Murphy, E.J. 2014. Interannual variability in Antarctic krill (Euphausia superba) density at South Georgia, Southern Ocean: 1997–2013. ICES Journal of Marine Science, 71: 2578–2588.

Foote, K.G., H.P. Vestnes, D.N. MacLennan and E.J. Simmonds. 1987. Calibration of acoustic instruments for fish density estimation: a practical guide. ICES Cooperative Research Report, 144, 69 pp.

Hewitt, R. P., Watkins, J., Naganobu, M., Sushin, V., Brierley, A. S., Demer, D., Kasatkina, S., Takao, Y., Goss, C., Malyshko, A., Brandon, M., Kawaguchi, S., Siegel, V., Trathan, P., Emery, J., Everson, I., and Miller, D. 2004. Biomass of Antarctic krill in the Scotia Sea in January/February 2000 and its use in revising an estimate of precautionary yield. Deep-Sea Research Part II-Topical Studies in Oceanography, 51: 1215-1236.

Hill SL, Atkinson A, Darby C, Fielding S, Krafft BA, Godø OR, Skaret G, Trathan P, Watkins J. 2016. Is current management of the Antarctic krill fishery in the Atlantic sector of the Southern Ocean precautionary? CCAMLR Science 23:31-51

Jensen N, Nicoll R, Iversen SA (2010) The importance of obtaining annual biomass information in CCAMLR Sub-area 48.2 to inform management of the krill fishery. WG-EMM-10/9, 5 pp

Jolly GM, Hampton I (1990) A stratified, random-transect design for acoustic surveys of fish stocks. Canadian Journal of Fisheries and Aquatic Sciences 47: 1282–1291.

Krafft BA, Macaulay GJ, Skaret G, Knutsen T, Bergstad OA, Lowther A, Huse G, Fielding S, Trathan P, Murphy E, Choi S-G, Chung S, Han I, Lee K, Zhao X, Wang X, Ying Y, Yu X, Demianenko K, Podhornyi V, Vishnyakova K, Pshenichnov L, Chuklin A, Shyshman H, Cox MJ, Reid K, Watters GM, Reiss CS, Hinke JT, Arata J, Godø OR, Hoem N. 2021. Standing stock of Antarctic krill (Euphausia superba Dana 1850 (Euphausiacea)) in the Southwest Atlantic sector of the Southern Ocean, 2019. 2021. Journal of Crustacean Biology. 41:1-17. https://doi.org/10.1093/jcbiol/ruab046

Krafft BA, Krag LA, Knutsen T, Skaret G, Jensen KHM, Krakstad JO, Larsen SH, Melle W, Iversen SA, Godø OR. 2018a. Summer distribution and demography of Antarctic krill (Euphausia superba) (Dana, 1852) (Euphausiacea) at the South Orkney Islands, 2011–2015. Journal of Crustacean Biology. P 1-7- Doi: 10.1093/jcbiol/ruy061

Krafft BA, Skaret G, Krag LA, Pedersen R. 2018b. Antarctic krill and ecosystem monitoring survey off the South Orkney Islands in 2018. WG-EMM CCAMLR 2018/04, 32 pp.

Makorov RR, Denys CJI (1981) Stages of sexual maturity of Euphausia superba. BIOMASS Handbook No. 11, pp 1-13

Marr J (1962) The natural history and geography of the Antarctic krill (Euphausia superba Dana). In: Discovery reports vol 32. National Institute of Oceanography, Cambridge University Press, Cambridge pp 33-464

Reiss C.S., A.M. Cossio, V. Loeb and D.A. Demer 2008. Variations in the biomass of Antarctic krill (Euphausia superba) around the South Shetland Islands, 1996-2006. ICES Journal of Marine Science. 65: 497-508.

Reiss C. S., A. Cossio, J. A. Santora, K. S. Dietrich, A. Murray, B. G. Mitchell, J. Walsh, E. L. Weiss, C. Gimpel, C. D. Jones, and G. M. Watters. 2017. Overwinter habitat selection by Antarctic krill under varying sea-ice conditions: Implications for top predators and fishery management. Marine Ecology - Progress Series 568:1-16.

SC-camlr. 2005. Report of the First Meeting of the Subgroup on Acoustic Survey and Analysis Method (SG-ASAM).  In: Report of the Twenty-fourth Meeting of the Scientific Committee (sc-camlr-xXIV/BG/3), Annex 6.  ccamlr, Hobart, Australia: 564–585.

SG-ASAM. 2010. Report of the fifth meeting of the subgroup on acoustic survey and analysis method. Cambridge, UK, 1 to 4 June 2010. Submitted for: Report of the Twenty-ninth Meeting of the Scientific Committee (sc-camlr-xXIX/6). ccamlr, Hobart, Australia: 23 pp.

Skaret G, Krafft BA, Macaulay G, Knutsen T, Bergstad OA. 2019. Results from the 2019 annual acoustic krill monitoring off the South Orkney Islands. CCAMLR WG-EMM-2019/69. 8 pp.

Skaret G, Macaulay GJ, Pedersen R, Wang X, Klevjer TA, Krag LA, Krafft BA.2023. Distribution and biomass estimation of Antarctic krill (Euphausia superba) off the South Orkney Islands during 2011-2020. ICES Journal of Marine Science, 2023, 0, 1–15 DOI: 10.1093/icesjms/fsad076

Wang X., X. Yu, X. Zhao, J. Zhang, G. Fan, Y. Ying and J. Zhu. 2021. Biomass estimates of Antarctic krill around the South Shetland Islands based on surveys conducted by a Chinese fishing vessel from 2013 to 2019. WG-ASAM-2021/13.

Yu X., X. Wang, X. Zhao, J. Zhang, G. Fan, Y. Ying and J. Zhu. 2019. Time series of Antarctic krill estimates around the South Shetland Islands from 2013 to 2019. SG-ASAM-2019/04 Rev. 1.

Appendix 1

Appendix 2