DOI: 10.5061/dryad.9zw3r22jv
title: Shifts in the composition and distribution of Pacific Arctic larval fish assemblages in response to rapid ecosystem change

description: The Pacific Arctic marine ecosystem has undergone rapid changes in recent
 years due to ocean warming, sea ice loss, and increased northward
 transport of Pacific‐origin waters into the Arctic. These climate‐mediated
 changes have been linked to range shifts of juvenile and adult subarctic
 (boreal) and Arctic fish populations, though it is unclear whether
 distributional changes are also occurring during the early life stages. We
 analyzed larval fish abundance and distribution data sampled in late
 summer from 2010 to 2019 in two interconnected Pacific Arctic ecosystems:
 the northern Bering Sea and Chukchi Sea, to determine whether recent
 warming and loss of sea ice have restricted habitat for Arctic species and
 altered larval fish assemblage composition from Arctic‐ to
 boreal‐associated taxa. Multivariate analyses revealed the presence of
 three distinct multi‐species assemblages across all years: (1) a boreal
 assemblage dominated by yellowfin sole (Limanda aspera), capelin (Mallotus
 catervarius), and walleye pollock (Gadus chalcogrammus); (2) an Arctic
 assemblage composed of Arctic cod (Boreogadus saida) and other common
 Arctic species; and (3) a mixed assemblage composed of the dominant
 species from the other two assemblages. We found that the wind‐ and
 current‐driven northward advection of warmer, subarctic waters and the
 unprecedented low‐ice conditions observed in the northern Bering and
 Chukchi seas beginning in 2017 and persisting into 2018 and 2019 have
 precipitated community‐wide shifts, with the boreal larval fish assemblage
 expanding northward and offshore and the Arctic assemblage retreating
 poleward. We conclude that Arctic warming is most significantly driving
 changes in abundance at the leading and trailing edges of the Chukchi Sea
 larval fish community as boreal species increase in abundance and Arctic
 species decline. Our analyses document how quickly larval fish assemblages
 respond to environmental change and reveal that the impacts of Arctic
 borealization on fish community composition spans multiple life stages
 over large spatial scales.
description: <em>Data Collection and Methods</em>
 Field collections of environmental and ichthyoplankton data were
 conducted by a variety of research programs across the eastern (U.S.)
 region of the northern Bering-Chukchi seas in August and September of
 2010-2019 and included coordinated efforts by the Distributed Biological
 Observatory, North Pacific Research Board’s Arctic Integrated Ecosystem
 Research Program (Baker et al. 2020), and the National Oceanic and
 Atmospheric Administration’s Alaska Fisheries Science Center. Hydrographic
 data were collected using a lowered conductivity-temperature-depth (CTD)
 profiler (SeaBird Electronics 911 plus) and in-line CTD system (SeaBird
 Electronics FastCAT SBE 49) attached to the ichthyoplankton nets to
 provide <em>in situ</em> temperature and salinity measurements
 from surface to bottom. If hydrographic data were not collected at a
 station, data from the nearest station and closest date in time were used.
 CTD-derived temperature and salinity measurements at each station were
 averaged over the entire water column to align with ichthyoplankton
 sampling protocols. <em>Environmental
 Dataset</em> Additionally, we collated regional
 climate data sets from the National Centers for Environmental Prediction
 (NCEP) reanalysis project. Monthly estimates of sea surface temperature
 (SST) and surface wind velocities (meridional [south-north] and zonal
 components [west-east], m s <sup>-1</sup>) were acquired for
 the northern Bering Sea (60–65.9°N, 155–170°W) and the Chukchi Sea
 (66–74°N, 155–170°W). The National Snow and Ice Data Center Sea Ice Index
 provided estimates of sea ice concentration (%, Cavalieri et al. 1996),
 area (km<sup>2</sup>), and extent
 (km<sup>2</sup>), using the Defense Meteorological Satellite
 Program series of passive microwave remote sensing instruments (Fetterer
 et al. 2002). The ice retreat index was estimated as the time period from
 January 1 to July 15 where ice coverage consistently reached values below
 15% in the Bering Sea based on the 7-day running mean of the daily sea ice
 data. Bering Strait advective transport (Sv) was acquired from a
 year-round subsurface Acoustic Doppler Current Profiler moored ~35 km
 north of the strait (Woodgate & Peralta-Ferriz 2021). All
 environmental data were averaged over the period of larval fish collection
 for each year of the study (August and September) to provide annual
 estimates of late summer ocean and climate conditions for each
 region. <em>Ichthyoplankton
 Dataset</em> Ichthyoplankton sampling was
 conducted in late summer of 2010-2019 using paired 60-cm diameter bongo
 nets towed obliquely from the surface to a maximum depth of 300 m deep or
 10 m off the bottom, whichever was shallower (only n = 11 of 1,114
 stations [&lt;1%] were &gt;300 m). In 2010-2016, a few stations
 were also sampled using a 1-m<sup>2</sup> Tucker trawl affixed
 to a sled frame that was lowered to the seafloor and towed obliquely to
 the surface with a messenger-based opening-closing net system (Sameoto
 & Jaroszynski 1976). The bongo nets and Tucker trawls were
 equipped with either 333- or 505-µm mesh before switching to only 505-µm
 mesh beginning in 2015. All sampling configurations were used for analysis
 since previous studies showed no significant differences in
 ichthyoplankton catch rates between these gear types and mesh sizes (Shima
 & Bailey 1994, Boeing & Duffy-Anderson 2008). Flowmeters
 (General Oceanics) attached to each net were used to calculate the volume
 filtered for each net tow, enabling larval abundance estimation (ind. 10
 m<sup>-2</sup>; Matarese et al. 2003). Samples were preserved
 in 5% formalin buffered with sodium borate and seawater and identified to
 the lowest taxonomic level at the Plankton Sorting and Identification
 Center in Szczecin, Poland. Taxonomic verifications took place at the
 National Oceanographic and Atmospheric Administration, Alaska Fisheries
 Science Center in Seattle, WA, USA.
description: # Shifts in the composition and distribution of Pacific Arctic larval fish
 assemblages in response to rapid ecosystem change Date of data collection:
 2010-2019 Geographic locations: Northern Bering Sea (&gt;60 N) and the
 Chukchi Sea --- ##
 NBS\_Chukchi\_Ichthyoplankton\_haul\_spp\_CTD\_2010\_2019.csv This
 spreadsheet is an ichthyoplankton species matrix with associated haul
 (ichthyoplankton sampling gear) and CTD (temperature, salinity)
 information of ichthyoplankton captured in the Northern Bering and Chukchi
 Seas between 2010-2019. The icthyoplankton taxa abundances were
 fourth-root transformed (ind. 10 m^-2). Ichthyoplankton sampling was
 conducted in late summer of 2010-2019 using paired 60-cm diameter bongo
 nets towed obliquely from the surface to a maximum depth of 300 m deep or
 10 m off the bottom, whichever was shallower (only n = 11 of 1,114
 stations [&lt;1%] were &gt;300 m). In 2010-2016, a few stations were also
 sampled using a 1-m^2 Tucker trawl affixed to a sled frame that was
 lowered to the seafloor and towed obliquely to the surface with a
 messenger-based opening-closing net system (Sameoto & Jaroszynski
 1976). The bongo nets and Tucker trawls were equipped with either 333- or
 505-m mesh before switching to only 505-m mesh beginning in 2015. All
 sampling configurations were used for analysis since previous studies
 showed no significant differences in ichthyoplankton catch rates between
 these gear types and mesh sizes (Shima & Bailey 1994, Boeing &
 Duffy-Anderson 2008). Flowmeters (General Oceanics) attached to each net
 were used to calculate the volume filtered for each net tow, enabling
 larval abundance estimation (ind. 10 m-2; Matarese et al. 2003). Samples
 were preserved in 5% formalin buffered with sodium borate and seawater and
 identified to the lowest taxonomic level at the Plankton Sorting and
 Identification Center in Szczecin, Poland. Taxonomic verifications took
 place at the National Oceanographic and Atmospheric Administration, Alaska
 Fisheries Science Center in Seattle, WA, USA. ### Column headers
 explanation: Ichthyoplankton sampling gear haul information: HAUL_ID,
 DATE, CRUISE, LON, LAT, YEAR, BOTTOM_DEPTH (meters), STATION_NAME,
 HAUL_NAME, GEAR_NAME (60 cm diameter bongo net or sled oblique trawl),
 NET, MIN_GEAR_DEPTH (meters), MAX_GEAR_DEPTH (meters), HAUL_PERFORMANCE
 (Good or Quest), MESH (microns), VOLUME_FILTERED, PURPOSE, DOY (Day of
 Year) CTD sampling haul information: PROFILE_ID, TEMPERATURE1 (degrees
 Celsius), SALINITY1 Larval fish species abundances (fourth-root
 transformed ind. 10 m^-2) columns range from *Acantholumpenus mackayi* :
 *Triglops pingelii* taxa. --- ##
 NBS\_Chukchi\_Environmental\_Variables\_2010\_2019.csv Mean environmental
 conditions during late summer (August-September averaged) in the northern
 Bering-Chukchi Sea region (2010-2019). regional climate data sets from the
 National Centers for Environmental Prediction (NCEP) reanalysis project.
 Monthly estimates of sea surface temperature (SST) and surface wind
 velocities (meridional [south-north] and zonal components [west-east], m s
 -1) were acquired for the northern Bering Sea (6065.9N, 155170W) and
 Chukchi Sea (6674N, 155170W). The National Snow and Ice Data Center Sea
 Ice Index provided estimates of sea ice concentration (%, Cavalieri et al.
 1996), area (km^2), and extent (km^2), using the Defense Meteorological
 Satellite Program series of passive microwave remote sensing instruments
 (Fetterer et al. 2002). The ice retreat index was estimated as the time
 period from January 1 to July 15 where ice coverage consistently reached
 values below 15% in the Bering Sea based on the 7-day running mean of the
 daily sea ice data. Bering Strait advective transport (Sv) was acquired
 from a year-round subsurface Acoustic Doppler Current Profiler moored ~35
 km north of the strait (Woodgate & Peralta-Ferriz 2021). All
 environmental data were averaged over the period of larval fish collection
 for each year of the study (August and September) to provide annual
 estimates of late summer ocean and climate conditions for each region. ###
 Column headers explanation: Year (2010-2019) Chukchi_SST (Sea Surface
 Temperature, degrees Celsius, averaged over the Chukchi Sea during
 August-September) NBS_SST NBS_U_Wind (Northern Bering Sea Zonal Eastward
 Wind Velocity, m/s, +/- depending on wind direction; averaged over the
 Northern Bering Sea during August-September) NBS_V_Wind (Northern Bering
 Sea Meridional Northward Wind Velocity, m/s, +/- depending on wind
 direction; averaged over the Northern Bering Sea during August-September)
 Chukchi_U_Wind (Chukchi Sea Zonal Eastward Wind Velocity, m/s, +/-
 depending on wind direction; averaged over the Northern Bering Sea during
 August-September) Chukchi_V_Wind (Chukchi Zonal Northward Wind Velocity,
 m/s, +/- depending on wind direction; averaged over the Northern Bering
 Sea during August-September) Chukchi_SI_Area_km2 (Chukchi Sea Ice Area,
 km^2, averaged over August and September) Chukchi_SI_Extent_km2 (Chukchi
 Sea Ice Extent, km^2, averaged over August and September) BS_Transport
 (Bering Sea transport, Sverdrups, averaged over August and September)
 Mean_DOY (Mean Day of Year of ichthyoplankton sampling) IRI (Index of Ice
 Retreat for the Bering Sea, Mean Day of Year)