https://doi.org/10.20575/00000054
Polar Data Journal, Vol. 8, 34–43, June 2024
Ⓒ 2024 National Institute of Polar Research. This work is distributed
under the Creative Commons Attribution 4.0 International license.

Zooplankton monitoring using a twin NORPAC net during the 63rd
Japanese Antarctic Research Expedition in austral summer 2021–2022
Kunio T. TAKAHASHI 1, 2, 3

1

National Institute of Polar Research, Research Organization of Information and Systems,
10-3, Midori-cho, Tachikawa, Tokyo 190-8518.

2 Polar Environment Data Science Center, Research Organization of Information and Systems,

10-3, Midori-cho, Tachikawa, Tokyo 190-8518.
3 Polar Science Program, Graduate Institute for Advanced Studies, SOKENDAI,

10-3, Midori-cho, Tachikawa, Tokyo 190-8518.
E-mail: takahashi.kunio@nipr.ac.jp(Received April 4, 2024; Accepted May 15, 2024)

Abstract: As a part of the monitoring programs of the Japanese Antarctic Research Expedition(JARE), zooplankton sampling using a NORPAC (NORth PACific) standard net has been routinely
carried out since JARE-14 (1972/73 season) to estimate the long-term trends of abundance, species
composition and their temporal/spatial variation in the uppermost 150 m of the Indian Ocean sectorof the Southern Ocean. The icebreaker Shirase has a fixed schedule and route, traveling down
longitude 110ºE each December. Data collected on fixed schedules seems to be ideal for long-termmonitoring work, and can be used as time series and/or seasonal analysis. This report provides thelatest zooplankton data collected during the Shirase cruise in the 2021/22 seasons, and is the fifth
report for the zooplankton monitoring on board Shirase since the first report of the 2016/17 season
in this journal.

1. Background & Summary
The need to conduct monitoring of zooplankton in the Southern Ocean has long beenrecognized. Since zooplankton have short life spans and faster population turnover than vertebrate
predators, zooplankton communities tend to change rapidly reflecting ambient environmental
conditions1. They are closer to the base of the marine food web, and they form extremely important
links between phytoplankton and higher predators. Therefore, zooplankton community structure is a
useful indicator of environmental variability in the Southern Ocean1,2. Moreover, long-termmonitoring of zooplankton communities provide a valuable early warning of any changes in the34

Polar Data Journal, Vol. 8, 34–43, June 2024

Southern Ocean and Antarctic ecosystem2,3,4. Numerous intensive surveys have been conductedaround Antarctica and its surrounding waters to define zooplankton composition, abundance, andcommunity structures. However, longer-term monitoring is difficult in the remote Southern Ocean,because research opportunities are limited in both time and space.
The Japanese Antarctic Research Expedition (JARE) has been carrying out routine zooplankton
monitoring in the Indian Ocean sector of the Southern Ocean every austral summer (December–March) since 1972 (JARE-14). This monitoring program is conducted from an icebreaker, which
travels along much the same cruise track at approximately the same time each year. This routine
sampling is ideal for long-term monitoring of this region. Although several kinds of plankton nets
have been deployed from the icebreakers, vertical hauls using a NORPAC (NORth PACific)
standard net have been routinely and frequently carried out to estimate the long-term trends ofabundance and species composition of surface zooplankton and its spatiotemporal variability in theupper layers of the Indian Ocean sector of the Southern Ocean. Sampling was conducted from theicebreaker Fuji during JARE-14 until JARE-24 (1972–1983)5,6, from the icebreaker Shirase during
JARE-25 until JARE-49 (1983–2008)7,8,9, from RSV Aurora Australis during JARE-50 (2009)10,
and from the new icebreaker Shirase during JARE-51 to JARE-61 (2009–2020)10,11,12,13,14,15,16,17.
The JARE NORPAC monitoring is the longest ongoing zooplankton study within the Antarctic
regions; it has been carried out for the last 50 years. In addition, zooplankton monitoring has also
been carried out by the T/V Umitaka-maru of Tokyo University of Marine Science and Technology,
since the 2013/2014 season (JARE-55) as a part of the JARE monitoring programs11,12,13,14,15,16.
Due to the influence of COVID-19, monitoring program of JARE-62 (2020–2021) was
cancelled, and the sampling plan on JARE-63 (2021–2022) was reduced. This report, which is thefifth report for the zooplankton monitoring on board Shirase in this journal, presents the dataobtained from the NORPAC standard net at two sites during JARE-63 (December 2021).

2. Sampling Location
From JARE-14 to JARE-28 (1986/1987), NORPAC standard net sampling sites were mainly
within the western part of the Indian Ocean Sector. Thereafter, sampling stations were shifted to theeast where the cruise tracks of the icebreaker Shirase remained along the same cruise track eachseason, beginning with JARE-29 (1987/1988). Regular sampling has been conducted from theicebreaker Shirase at five stations along longitude 110°E, ranging from 40°S to 60°S in December.
Typically, routine sampling stations are located at intervals of 5 degrees of latitude. In JARE-63,
sampling station L01 (40°S, 110°E), L02 (45°S, 110°E), and L03 (50°S, 110°E) were omitted
because of rough seas. The location of sampling stations is shown in Figure 1.

35

K. T. Takahashi

3. Methods
The NORPAC standard net was established as a standard method for collecting zooplankton ininternational cooperative surveys at an international meeting held in Honolulu in February 195618. A
twin NORPAC standard net made of nylon bolting cloth (NGG 54, mesh size 315 µm; NXX 13,
mesh size 100 µm) was used at all sampling stations (Fig. 2). The net was hauled vertically at a
speed of about 1 m/s from a depth of approximately 150 m. The maximum depth reached wasestimated from the wire angle and length of the wire paid out. All samples obtained wereimmediately preserved on board in seawater containing 5%–10% buffered formalin. The volume ofwater filtered by each net haul was estimated using a flow-meter (Rigo Co., Ltd., Saitama, Japan)
mounted at the center of the mouth ring of the net. Sampling was conducted during the daytime attwo stations, reducing the contribution of diel vertically migrating zooplankton to the biomass.

4. Data Records
Zooplankton monitoring datasets are presented in three data sheets, species/taxa list, abundance
data, and wet weight data. The fields in the dataset are:
JARE number – the JARE number of this sampling season
Ship name – the name of the ship on which the sampling was conducted
Station number – the name of the station on which the sampling was conducted
Latitude – the decimal latitude of the sampling station (negative value for South)
Longitude – the decimal longitude of the sampling station (positive value for East)
Sampling season – two-year Antarctic season based around the austral summer, e.g., “20212022” runs from December 2021 to March 2022
Sampling year – the sampling date year
Sampling month – the sampling date monthSampling day – the sampling date day
Sampling time – the sampling date time (UTC)Sampling depth – the depth of sampling tow
Mesh size – the mesh size of plankton net
Estimated volume of water filtered – the estimated volume of water filtered using a flowmeter
Abundance – the abundance of each species/taxa
Total abundance – total abundance of all zooplankton in a sample
Number of species/taxa – the number of species/taxa in a sample
Wet weight – the wet weight of each category
Total wet weight - total wet weight of all zooplankton in a sample

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Polar Data Journal, Vol. 8, 34–43, June 2024

5. Technical Validation
5.1. Zooplankton identification
Zooplankton were identified to the lowest practical taxonomic level, generally to species orgenus, using a stereo-microscope (SMZ1500, Nikon, Japan). Copepodite stages of copepod species,
calyptopis stages and furcilia stages of euphausiid species were subdivided from the adults. The
nauplius stages of Rhincalanus gigas (Copepoda: Calanoida) were distinguished from other calanoid
nauplii by their large size and morphological characteristics. Zooplankton abundance was converted
to individuals per cubic meter of water filtered.
The species list for this dataset was checked using the Taxon Match of the World Register ofMarine Species (WoRMS: http://www.marinespecies.org/index.php) name validation tool. WoRMS
is an open-access inventory of all marine species, being >90% complete19. The tool performs across-check of the spelling and taxonomic status of species against the WoRMS database; it returns
standard taxonomic information with valid names.

5.2. Wet weight measurement
Processing of samples was carried out according to the four-step procedure outlined below andshown in Fig. 3.
Step 1: the large-sized zooplankton (more than 10 mm in size) were sorted for the wholesample in the laboratory. Zooplankton were classified into five categories, and counting number ofindividuals and measured the wet weight each category using an electronic balance (Sartorius
Quintix124-1SJP, readability 0.1 mg).
Step 2: after removing the larger-sized zooplankton in the step 1, all other species (<10 mm insize) were counted from 1/2 to 1/32 aliquots of the whole sample, and identified to the lowest
taxonomic level, generally species or genus, level, using a stereo-microscope. We used a Motoda
box-type plankton sample splitter to subdivide a sample. While sorting and counting this size
fraction, the wet weight of zooplankton typically reached more than 10 mg per aliquot.
Step 3: given that the wet weight of zooplankton of 1 mm size or less were hard to sort, thisfraction was offered to count the individual numbers then the wet weight was estimated by using
conversion factors listed in Table 1. The lengths of body parts for calculation were measured using astereo-microscope (SMZ1500, Nikon, Japan).
Step 4: the weights obtained in steps 1–3 were summed to give a total wet weight per eachsample.
Zooplankton abundance per haul was converted to mg m-3. For a detailed description of
zooplankton processing for wet-weight measurements, see Ukai et al. (2014)28.

37

K. T. Takahashi

6. Figures

Fig. 1.

Locations of the icebreaker Shirase sampling stations during JARE-63 in 2021/2022. Stations
L01, L02, and L03 were omitted because of rough sea conditions.

Fig. 2.

The twin NORPAC standard net. Flow-meter is attached at each mouth ring of net.

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Polar Data Journal, Vol. 8, 34–43, June 2024

Fig. 3.

Four-step procedure used to measure and to estimate the wet weight of NORPAC net samples.

39[3] logDW(μg)=-5.10+2.46logSL(μm)

WW(μg)=((3.14*BW(μm)^2*L(μm))/12)*10^-6 [2][3] logDW(μg)=-2.70+1.47logSL(μm)[3] logDW(μg)=-5.68+2.10logL(μm)

WW(μg)=DW(μg)/0.256

WW(μg)=DW(μg)/0.256
WW(μg)=DW(μg)/0.097

Thecosomata, Gastropoda larvae

Cavolinidae（Cone）

Bivalve40[3] logDW(μg)=-6.54+2.65logL(μm)

WW(μg)=DW(μg)/0.182

Nauplius[1]

WW(μg)=DW(μg)/0.050
WW(μg)=((3.14*BW(μm)^2*L(μm))/6)*10^-6

Appendiculata

Conversion factors of carbon weight[5] logCW(μg)=-7.58+2.83logTL(μm) [7]

[5]

[5]

[5] CW(ng)=1.51*10^-5*L(μm)^2.94 [6]

[10]

[5]

[5]

[5] CW(ng)=1.51*10^-5*L(μm)^2.94 [6]

[5]

[5]

[5]

[5]

[5]

[5]

[5]

[5]

[5]

[5]

[5]

[5]

[9]

Japan (1986). [6]: Uye et al. (1996). [7]: Sato et al. (2001). [8]: Ikeda (1990). [9]: Ikeda and Imamura (1996). [10]: Iguchi et al. (1999).

[1]: Wet weight was calculated from the volume
gravity = 1). [2]: Wet24weight was calculated 25
from the volume of the cone (Specific26gravity = 1). [3]: Beers (1966).27[4]: Ikeda (1970). [5]: The Oceanographic Society of
22 of the ellipsoid body (Specific23[1]: Wet weight was calculated from the volume of the ellipsoid body (Specific gravity = 1). [2]: Wet weight was calculated form the volume of the cone (Specificgravity = 1).21[3]: Beers (1966)2220. [4]: Ikeda (1970)21. [5]: The
20
Oceanographic SocietyofJapan(1986)
.
[6]:
Uye
et
al.
(1996)
.
[7]:
Sato
et
al.
(2001).
[8]:
Ikeda(1990)
.
[9]:
Ikeda
and
Imamura(1996).
[10]:
Iguchietal.
(1999)
.
23
24
25
26
27

including eggs

[4] DW(μg)=CW(μg)/0.442WW(μg)=DW(μg)/0.050

Doliolida, Salpida

Others Larvae[3] logDW(μg)=-0.553+2.79logL(mm)[4] logDW(μg)=-6.94+2.54logL(μm)

WW(μg)=DW(μg)/0.068-

Chaetognatha

Chordata[3] DW(μg)=CW(μg)/0.407[3] logDW(μg)=-13.77+4.99logSL(μm)

WW(μg)=DW(μg)/0.182

CyprisCirripedia[3] DW(mg)=9.954*10^-4*L(mm)^3.156[3] DW(μg)=CW(μg)/0.457

WW(μg)=DW(μg)/0.135

Other nauplii

Copepoda

WW(μg)=DW(μg)/0.159[3] logDW(μg)=-9.59+3.41logL(μm)

WW(μg)=DW(μg)/0.135

Nauplius

Copepoda：Eucalanoidae

WW(μg)=DW(μg)/0.159[3] logDW(μg)=-9.07+3.26logL(μm)

WW(μg)=DW(μg)/0.135

Adult, Copepodite

Copepoda：Others

Nauplius[3] logDW(μg)=-5.59+2.25logL(μm)

WW(μg)=DW(μg)/0.135

Adult, Copepodite

Copepoda：Oncaea

Calyptopis, Furcilia, Adult[3] logDW(μg)=-6.45+2.43logL(μm)

WW(μg)=DW(μg)/0.135

Adult, Copepodite

Copepoda：Corycaeus

Euphausiacea[3] logDW(μg)=-7.59+2.88logL(μm)

WW(μg)=DW(μg)/0.135

Adult, Copepodite

Copepoda：Microsetella[8][3] logDW(μg)=-6.05+2.10logL(μm)

WW(μg)=DW(μg)/0.135

Adult, Copepodite

Copepoda：Cyclopoida

logWW(mg)=-1.517+2.832logL(mm)[3] logDW(μg)=-9.59+3.41logL(μm)

WW(μg)=DW(μg)/0.135

Adult, Copepodite

Copepoda：Calanoida

Amphipoda[3] logDW(μg)=-13.77+4.99logSL(μm)

WW(μg)=DW(μg)/0.182

Ostracoda

Arthropoda

LarvaePolychaeta

Larvae[3] logDW(μg)=-7.67+2.75logBD(μm)

WW(μg)=DW(μg)/0.043

BH/BD<1

Conversion factors of dry weight[3] logDW(mg)=-2.333+1.268logBH(mm)+1.125(logBH(mm))^2

Conversion factors of wet weightWW(μg)=DW(μg)/0.040

Developmental stage

BH/BD≧1

Species/taxa and/or Form

Conversion factors between Carbon weight (CW), Dry weight (DW), Wet Weight (WW), Body length (L), Bell height (BH),
Bell diameter (BD), Shell length (SL), Trunk length (TL) and Body width (BW) used in this study.

Annelida

Mollusca

Cnidaria

Category

Table 1.

K. T. Takahashi

7. TablePolar Data Journal, Vol. 8, 34–43, June 2024

Author contributions
K. T. Takahashi performed the processing of samples and writing of the manuscript anddirected the JARE-63 monitoring program.

Acknowledgments
The authors thank Mr. Hironori Sato, Marine Works Japan Ltd., for collecting samples onboard the icebreaker Shirase. We express our heartfelt appreciation to all members of JARE-63 fortheir support. We also thank the officers and crew of the icebreaker Shirase.

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Data Citation
Takahashi, K. T. Zooplankton monitoring using a twin NORPAC net during the 63rd Japanese Antarctic
Research Expedition in austral summer 2021-2022. 1.00, Japan, Arctic Data archive System (ADS), 2024.
https://doi.org/10.17592/001.2024040411.

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