DOI: 10.5061/dryad.h44j0zpgs
title: Data from: Spatial structuring and life history connectivity of Antarctic silverfish along the southern continental shelf of the Weddell Sea

description: A multidisciplinary approach was employed to examine a physical-biological
 population hypothesis for a critical forage species, the Antarctic
 silverfish (Pleuragramma antarctica). A previous study had shown strong
 gene flow along the westward Antarctic Slope Current, in addition to
 spatially recurring length modes that provided evidence for episodic
 connectivity. In this paper, otolith nucleus chemistry from a subset of
 fish collected in the southern Weddell Sea as part of a hydrographic
 survey of the Filchner Trough system was used to test between connectivity
 scenarios. Nucleus chemistry, which reflects environmental exposure during
 early life, showed significant spatial structuring despite homogeneity in
 microsatellite allele frequencies. Mg×Ca-1 and Sr×Ca-1 differentiated
 length modes, and Mg×Ca-1 showed significant contrasts between Atka Bay,
 Halley Bay, and Filchner Trough. Physical-biological mechanisms may help
 reconcile structuring shown by otolith chemistry, length, and abundance
 data with prior evidence of gene flow. Such mechanisms include
 self-recruitment shaped by circulation associated with the Filchner
 Trough, fluctuations in mixing between immigrant and locally-recruited
 fish, and feeding opportunities between inflowing Modified Warm Deep Water
 and outflowing Ice Shelf Water. The results illustrate how comparisons
 between multi-disciplinary techniques based on integrated sampling designs
 that incorporate hydrography can enhance understanding of population
 structure and connectivity around the Southern Ocean.
description: <b>Field sampling</b> Antarctic
 silverfish were collected in the southern Weddell Sea from January –
 February 2014 as part of the research cruise PS82 (ANT-XXIX/9) by the RV
 <i>Polarstern</i>, investigating the Filchner Outflow System.
 For this, a Seabird 911+ CTD (Conductivity-Temperature-Depth), measuring
 salinity, temperature, and pressure, attached to a carousel with 24 water
 bottles, was deployed to investigate the physical environment in the
 region around the Filchner Trough, including the course of the AACC and
 MWDW flowing towards the Filchner Ice Shelf (Knust & Schröder,
 2014). Conductivity and temperature sensors were calibrated before and
 after the cruise by Seabird Electronics; in addition, conductivity was
 corrected after calibration using salinity measurements from water samples
 measured by two Optimare Precision Salinometers. A total of 142 CTD
 profiles were obtained for oceanographic transects across and along the
 continental slope and shelf-break, and across the eastern shelf adjoining
 the trough. The distribution of bottom water masses over the slope and
 continental shelf were reported by Schröder et al. (2014). The CTD section
 at 76˚S off Coats Land was described by Ryan et al. (2017), including
 seasonal evolution based on a 2-year time series retrieved from three
 moorings. Integrated with the physical measurements,
 silverfish were sampled from stations around the Filchner Trough including
 the adjoining eastern shelf. All sampling was undertaken during the day
 when fish were near the bottom during their diurnal migration, using a
 commercial benthic trawl with a cod-end mesh line of 20 mm. Standard
 length (SL) was measured for all individuals, and biomass, abundance, and
 size distribution determined for each haul. Catches were randomly
 sub-sampled for tissue and otoliths for subsequent analyses. For
 comparison, samples were also collected in Atka Bay and off Camp Norway,
 with corresponding CTD data. <b>Laboratory
 procedures</b> Biomass and abundance indices of
 Antarctic silverfish were reported by Wetjen et al. (2014a) and (Wetjen et
 al. 2014b) and genetic analyses were undertaken and reported by Caccavo et
 al. (2018). Length distributions showed two recurrent modes, of immature
 fish at approximately 10 cm and mature fish at approximately 15 cm SL.
 However, the length modes were not uniformly present at every sampling
 area, precluding a cross-wise experimental design with area and length
 group as factors. Instead, five stations were selected along the shelf in
 relation to the position of the AACC, the ASC, and inflows across the
 shelf-break. These were in Atka Bay; across the eastern shelf adjoining
 Filchner Trough corresponding to three CTD sections across Halley Bay, at
 76˚S off Coats Land; at 77˚S just east of the trough flank; and near the
 shelf-break corresponding to a section across the western shelf downstream
 of the trough mouth. All fish in a given sampling area were pooled where
 only one mode was present; where both were present, fish were pooled into
 groups of large and small individuals. A cut-off of 13 cm was used to
 separate the length modes of 10 cm and 15 cm, corresponding approximately
 to the length at which sexual maturation begins in males and females (La
 Mesa & Eastman 2012). This resulted in eight groups from each of
 which 25 fish were randomly sub-sampled to form experimental treatments.
 In Halley Bay, both large and small groups at Station 126 were
 supplemented by randomly selected fish from nearby at Station 129.
 Similarly at 77˚S, six fish from Station 84 supplemented those sampled
 from Station 78, resulting in eight treatments: Atka Bay (AB), Halley Bay
 small mode (HB-S) and large mode (HB-L), Coats Land small (CL-S) and large
 (CL-L) mode, East Filchner Trough (EF), West Filchner Trough small (WF-S)
 and large (WF-L) mode. One randomly selected otolith
 from each fish was used for elemental analysis. Otoliths were prepared
 using a standard protocol developed at the Center for Quantitative
 Fisheries Ecology at Old Dominion University. They were initially cleaned
 using glass probes and rinsed in Milli-Q water. Any remaining surface
 contamination was removed by 5 minutes incubation in 20% Ultra-Pure
 hydrogen peroxide, followed by rinsing in Milli-Q water. Otoliths were
 then mounted onto slides using crystal bond, and ground from the anterior
 side using a Crystal Master 8 Machine with 3-micron 3M™ polishing film to
 reveal a transverse surface just above the primordium. In a clean room,
 mounted otoliths were given a final polish by hand to reveal the
 primordium using 3-micron 3M™ polishing film, and once again rinsed with
 Milli-Q water and allowed to dry. Otoliths were removed from the polishing
 slide and mounted on petrographic slides in a randomized block design,
 with slide as the blocking factor. In this way, all eight treatments were
 represented on each petrographic slide, and the slides were then rinsed
 and sonicated individually for 5 minutes in Milli-Q water, before
 placement under a laminar-flow hood to dry. Minor and
 trace elements were measured using a Finnegan Mat Element 2
 double-focusing sector-field Inductively Coupled Plasma Mass Spectrometer
 (ICP-MS) located in the Plasma Mass Spectrometry Facility at Woods Hole
 Oceanographic Institution (WHOI). Samples were introduced into the ICP in
 an automated sequence (Chen et al. 2000) in which otolith material ablated
 with a 193 nm laser ablation system was combined with
 HNO<sub>3</sub> aerosol introduced by a microflow nebulizer.
 The subsequent mixture was then carried to the ICP torch. For quality
 control, we used a calcium carbonate reference produced by the United
 States Geological Survey (Microanalytics Carbonate Standard, MACS-3), for
 which elemental fractionation, mass-load, and matrix effects have been
 shown to be small for lithophile elements, especially when used with 193
 nm laser ablation (Jochum et al. 2012). In the same way as the samples,
 MACS-3 material was ablated and introduced into the spray chamber as an
 aerosol; HNO<sub>3</sub> aerosol alone was also introduced
 into the spray chamber by the nebulizer as blanks. The randomized block
 design controlled for operational variability in the instrument. Blank and
 reference readings of count rate (count s<sup>-1</sup>) were
 taken before and after each block, and every four otoliths within blocks.
 Readings consisted of 60 scans with 200 samples/peak and a 5% window.
 Based on markers identified during previous studies for the Southern
 Ocean, otoliths were analyzed for <sup>48</sup>Ca,
 <sup>25</sup>Mg, <sup>55</sup>Mn,
 <sup>88</sup>Sr, and <sup>138</sup>Ba and reported
 as ratios to <sup>48</sup>Ca. Background counts were
 subtracted from otolith counts by interpolating between the readings taken
 before and after every four otoliths, and the corrected otolith counts
 were then converted to element:Ca (Me•Ca<sup>-1</sup>) ratios
 using the standards. To measure ratios indicative of conditions to which
 fish were exposed during early life, a grid raster type 150 µm x 200 µm
 was placed over the otolith nucleus corresponding to the first austral
 summer of growth, during which larvae are thought to remain in their natal
 trough (Brooks et al. 2018). The area was ablated using a 20 µm diameter
 laser beam at a frequency of 10 Hz and a power of 60%, traveling over the
 raster at 6 µm s<sup>-1</sup>. As expected from earlier
 studies along the Antarctic continental shelf,
 Mn•Ca<sup>-1</sup> values were extremely low, frequently
 showing zero values after standard conversion. Background levels of Ba
 increased over the course of the analyses; however, the randomized blocks
 design guarded against any resulting biases. Brooks CM,
 Ainley DG, Abrams PA, Dayton PK, Hofman RJ, Jacquet J, Siniff DB (2018)
 Antarctic fisheries: factor climate change into their management. Nature
 558:177-180 Caccavo JA,
 Papetti C, Wetjen M, Knust R, Ashford JR, Zane L (2018) Along-shelf
 connectivity and circumpolar gene flow in Antarctic silverfish
 (<i>Pleuragramma antarctica</i>). Scientific Reports
 8:17856 Chen Z, Canil D,
 Longerich HP (2000) Automated in situ trace element analysis of silicate
 materials by laser ablation inductively coupled plasma mass spectrometry.
 Fresenius' Journal of Analytical Chemistry 368:73-78
 Jochum KP,
 Scholz D, Stoll B, Weis U, Wilson SA, Yang Q, Schwalb A, Börner N, Jacob
 DE, Andreae MO (2012) Accurate trace element analysis of speleothems and
 biogenic calcium carbonates by LA-ICP-MS. Chemical Geology
 318-319:31-44 La Mesa M,
 Eastman JT (2012) Antarctic silverfish: Life strategies of a key species
 in the high-Antarctic ecosystem. Fish and Fisheries 13:241-266
 Ryan S,
 Hattermann T, Darelius E, Schröder M (2017) Seasonal cycle of hydrography
 on the eastern shelf of the Filchner Trough, Weddell Sea, Antarctica.
 Journal of Geophysical Research: Oceans 122:6437-6453 Schröder M,
 Wisotzki A, Ryan S, Huneke W, Semper S, Osterhus S, Schwegmann S,
 Castellani G (2014) Observations of the hydrographic conditions and water
 mass compositions at the Filchner Sill and in the Filchner Trough. In:
 Knust R, Schröder A (eds) The expedition PS82 of the Research Vessel
 Polarstern to the southern Weddell Sea in 2013/2014, Book 680. Reports on
 Polar and Marine Research, Bremerhaven Wetjen M, Wätjen
 K, Knust R (2014a) The role of the Antarctic silverfish
 <i>Pleuragramma antarcticum</i> in the Antarctic waters. In:
 Knust R, Schröder A (eds) The expedition PS82 of the Research Vessel
 Polarstern to the southern Weddell Sea in 2013/2014, Book 680. Reports on
 Polar and Marine Research, Bremerhaven Wetjen M, Wätjen
 K, Papetti C, Babbucci M, Riginella E, Koschnick N, Knust R, Sandersfeld T
 (2014b) Fish communities, distribution and production. In: Knust R,
 Schröder A (eds) The expedition PS82 of the Research Vessel Polarstern to
 the southern Weddell Sea in 2013/2014, Book 680. Reports on Polar and
 Marine Research, Bremerhaven
description: <b>1_LAICPMS_data.xlsx</b>
 Spreadsheet with the metadata associated with samples analyzed
 for trace element levels using LA-ICP-MS containing the following
 sheets: <b>Cover</b> sheet
 explains the column headings in the subsequent sheets.
 <b>Raw Data</b> sheet compiles the raw data
 produced from the LA-ICP-MS (which can be accessed in the .txt files also
 available in this data publication), including intensity data (AVG and STD
 in counts per second [cps], as well as % RSD, the average Mass Offset) for
 all elements analyzed. <b>Intensity
 Data</b> sheet includes just the average intensity data (Intensity
 AVG from the Raw Data sheet) for each element analyzed, as well as
 associated metadata. <b>All other
 .txt files</b> These are the text file outputs
 from the LA-ICP-MS, containing the raw data for each blank, standard, line
 (otolith edge), and raster (otolith nucleus) reading. These raw data are
 compiled in the Raw Data sheet of the 1_LAICPMS_data.xlsx file included in
 this data publication, however, for certain downstream analysis programs,
 these raw text files are needed.