Summer Distribution and Demography of Antarctic Krill Euphausia Superba Dana, 1852 (Euphausiacea) at the South Orkney Islands, 2011–2015
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Downloaded from orbit.dtu.dk on: Oct 23, 2019 Summer distribution and demography of Antarctic krill Euphausia superba Dana, 1852 (Euphausiacea) at the South Orkney Islands, 2011–2015 Krafft, Bjørn A.; Krag, Ludvig Ahm; Knutsen, Tor; Skaret, Georg; Jensen, Knut H. M.; Krakstad, Jens O.; Larsen, Stuart H.; Melle, Webjørn; Iversen, Svein A.; Godø, Olav R. Published in: Journal of Crustacean Biology Link to article, DOI: 10.1093/jcbiol/ruy061 Publication date: 2018 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Krafft, B. A., Krag, L. A., Knutsen, T., Skaret, G., Jensen, K. H. M., Krakstad, J. O., ... Godø, O. R. (2018). Summer distribution and demography of Antarctic krill Euphausia superba Dana, 1852 (Euphausiacea) at the South Orkney Islands, 2011–2015. Journal of Crustacean Biology, 38(6), 682-688. https://doi.org/10.1093/jcbiol/ruy061 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Journal of Crustacean Biology Advance Access published 11 September 2018 Journal of Crustacean Biology The Crustacean Society Journal of Crustacean Biology 38(6), 682–688, 2018. doi:10.1093/jcbiol/ruy061 Summer distribution and demography of Antarctic krill Euphausia superba Dana, 1850 (Euphausiacea) at the South Orkney Islands, 2011–2015 Downloaded from https://academic.oup.com/jcb/article-abstract/38/6/682/5095293 by guest on 08 July 2019 Bjørn A. Krafft1, Ludvig A. Krag2, Tor Knutsen1, Georg Skaret1, Knut H.M. Jensen3, Jens O. Krakstad1, Stuart H. Larsen1, Webjørn Melle1, Svein A. Iversen1 and Olav R. Godø1 1Institute of Marine Research, Nordnesgaten 50, 5005 Bergen, Norway; 2Technical University of Denmark, National Institute of Aquatic Resources, Niels Juelsvei 30, DK-9850 Hirtshals, Denmark; and 3University of Bergen, Musèplassen 1, 5007 Bergen, Norway Correspondence: B.A. Krafft; email: [email protected] (Received 4 April 2018; accepted 21 June 2018) ABSTRACT We carried out a survey of Antarctic krill (Euphausia superba Dana, 1850) from 2011 to 2015 to establish a long-term, time-series dataset of distribution, abundance, and demography for this species in the South Orkney Islands sector of the Southern Ocean. This species is abundant in this region and is subjected to high-intensity fishing, but previous assessments of density and population dynamics are few and outdated. Our data for Antarctic krill was collected from trawl stations along survey line transects covering the South Orkney plateau and shelf region during the summers of five consecutive years. We used concurrent data on hydrography, ba- thymetry, and proxies for algal biomass to describe potential spatial patterns of demography and abundance of E. superba. Comparative analysis of the demographic composition showed that 2012 differed from the other years by having a higher proportion of juveniles; otherwise a consistent pattern was found among years and within the study area. The highest biomass during the study period occurred along the northern shelf edge of the South Orkney Islands. Results of the linear mixed-effect model used to evaluate a diverse range of variables revealed that the only predictors for this hotspot were the short distance from land and great bottom depth. No clear differences in demographic composition for the study area were detected, which indicates that the area is highly dynamic and dominated by flux and advection of krill, both to, from, and within the area. Despite this finding, the results demonstrate that the shelf break on the northwest South Orkney Islands is predictable over time as a krill concentration and retention hotspot during the summer season. Key Words: fisheries, maturity stage, Southern Ocean, time series, zooplankton THIRD INTERNATIONAL SYMPOSIUM ON KRILL INTRODUCTION (off South Georgia Islands). It is difficult to describe and quantify how the population dynamics of the Antarctic krill change tem- The western South Atlantic sector of the Southern Ocean con- porally and regionally, especially at somewhat larger spatial scales, tains the highest concentrations of Antarctic krill (Euphausia superba because actual monitoring of the stock has been very limited in Dana, 1850, hereafter krill) (Atkinson et al., 2009). The krill fishery time and space. Nonetheless, the few existing small spatially is regulated by the Commission for the Conservation of Antarctic scaled krill monitoring programmes provide valuable biomass Marine Living Resources (CCAMLR). During the last two decades, and demography data (indices) that answer important questions this fishery has been focused mainly in subareas 48.1 (Bransfield about change in the krill stock. Monitoring of krill during the last Strait and Elephant Island), 48.2 (South Orkney Islands), and 48.3 two decades has been regularly performed by the U.S. Antarctic © The Author(s) 2018. Published by Oxford University Press on behalf of The Crustacean Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact [email protected] SUMMER DISTRIBUTION OF ANTARCTIC KRILL AT THE SOUTH ORKNEY ISLANDS Marine Living Resources Program in subarea 48.1 (Kinzey et al., Islands. The research platforms employed were two commercial 2015) and the British Antarctic Survey in subarea 48.3 (Fielding Norwegian ramp trawlers: FV Saga Sea (Aker Biomarine AS, Oslo, et al., 2014). Knowledge about the distribution and biology of the Norway) in 2011, 2013, and 2014, and FV Juvel (Rimfrost AS, species in the South Orkney Islands, which has a high abundance Fosnavåg, Norway) in 2012 and 2015. The study area included of krill (Atkinson et al., 2008) and is subjected to particularly high the waters between 59°40’S and 62°00’S and from 44°00’W to fishing activity (Nicol et al., 2012), is fragmented and outdated. 48°30’W. The survey design included trawl stations spaced 37.0– Most fishing in subarea 48.2 occurs in a concentrated area 46.30 km (~20–25 nautical miles) apart along predetermined par- over the shelf break northwest of the South Orkney Islands. The allel north-south oriented transect lines (Fig. 1). Some parts of the area includes two north-south oriented canyons, the Monroe and study area could not always be covered due to drifting pack ice, Coronation Troughs (Dickens et al., 2014), which are described as which prevented ship operations, including trawling. This was par- hotspots for krill (Nicol et al., 2012; Krafft et al., 2015). The area ticularly the case during the 2013 and 2015 surveys. is likely important for retention of krill that are advected along The standard survey trawl used was 42 m long, with a 36 m2 the shelf and slope region from areas further west and southwest mouth opening, constructed of 7 mm (stretched) diamond-shaped or via deeper currents from the Weddell Sea region flowing east mesh from mouth to rear (Fig. 2). The trawl was towed using a and turning north in a counter clockwise direction around the 6 m wide steel beam with 200 kg weights at each lower wing tip South Orkney plateau (Gordon et al., 2001). The South Orkney and 1,500 kg attached to the beam to ensure fast deployment to Downloaded from https://academic.oup.com/jcb/article-abstract/38/6/682/5095293 by guest on 08 July 2019 plateau rises from abyssal depths to an average depth of approxi- depth and the best possible geometric stability of the trawl dur- mately 300 m, with shallower parts closer to the islands. The shelf ing sampling. A different trawl with a codend mesh of 11 mm was is defined as the area that is shallower than 1,000 m deep (Clarke employed during the 2013 survey. Our standard trawl was lost at the & Johnston, 2003). Montevideo, Uruguay warehouse prior to departure, so we made The current fisheries management protocol for the Antarctic a new trawl with the material we had available. A depth sensor region considers large-scale distribution and abundance infor- (Marport™, Reykjavik, Iceland) attached to the headline transferred mation. Overall, 87% of the stock is found over deep oceanic data to the wheelhouse to monitor trawl operations. At each station water (Atkinson et al., 2004). Smaller scale shelf and/or shelf the trawl was lowered vertically from surface to ~200 m depth (or break krill hotspots exist, and the current fishery mainly operates ~20 m above bottom if the water was < 200 m) and then hauled in in these areas. These hotspots represent 13% of the total stock, at 3.7 km hr–2 (~2.0 knots) including vessel and wire speed. and they are significant because they support key ecosystem func- When landed on the trawl deck, the codend was opened and tions, provide resilience for the stock, and are important areas the catch was removed. The towing rig was then hung from a for krill-dependent predators (Santora et al., 2011). Hotspots may crane and flushed on deck to wash out any biological remains offer favourable food availability for krill as well as shelter from stuck in the net.