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Supplementary information
Here we focus on three key results of this paper, expanding on their robustness to
possible artefacts of sampling.
1. At least 50% of the global population of Euphausia superba are within the
SW Atlantic sector
Fig. 1b covers most of the krill habitat in the Southern Ocean, and is compiled from all
suitable data (see Methods) from all net types. Within the area of the grid cells, 68% of
total krill are situated from 10o-70oW, based on mean density within each cell and its
area. This analysis is based on similar times of year and depths of sampling
(Supplementary Table 3), so these could not lead to a spurious concentration of krill in
the SW Atlantic. However, we have re-analysed the data based on comparable net types
and sampling eras. Obliquely towed 1 m ring nets (1926-1939) show that 58% of the
krill within the sampled grid squares were between 10o and 70oW (Supplementary Fig.
4a. This figure is 71% based on Isaac Kidd- and 8m2 Rectangular Midwater Trawls of
the post 1976 era (Supplementary Fig. 4b). Thus the krill hotspot in the SW Atlantic
existed both in the Discovery and the modern eras and is not a sampling artefact.
2. Krill and salp densities have shown significant temporal changes
The main text covers potential artefacts of sampling, including spatial effects at the
scale of our grid. Temporal trends can be affected by changing sampling emphasis at
finer scales, for example in relation to krill-rich shelf breaks17. There was indeed a
change last century (Supplementary Fig. 5), but it was towards sampling such areas.
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Thus the observed decline in krill is certainly not a result of localised change in
sampling. Likewise, the increasing focus on productive shelf regions, compared to the
wider, more oceanic coverage of the Discovery era, has not caused the observed
increase in salps south of the SB last century, as salps are an oceanic species, reduced
over shelf areas and in zones of the highest chl a concentration2,3,4,12.
Changes in sampling gear or method could lead to erroneous temporal trends.
However, systematic trends in net size, time of year or depth of sampling were small
(Supplementary Table 3), with two exceptions. First, small krill nets tended to be used
earlier in the summer in the latter part of the post 1976 era and second, Discovery nets
were smaller than those in the post 1976 era. Both trends, however, tend to counteract
the changes observed, rather than lead to them. First, krill abundance tends to decline
through the season from mortality7, and second, the Discovery ring nets are probably
much less efficient in catching krill than the RMT 8s and Isaac-Kidd Midwater trawls
characteristic of the modern era5.
3. Krill density across the S.W. Atlantic sector is related to ice duration.
The positive relationship between sea ice extent and krill density requires
independent verification from separate data sets. In Supplementary Table 4 we thus
used combinations of sea-ice indices and krill data sets to examine the consistency of
the positive relationships observed in Fig. 3. For the satellite data set we recorded the
mean northern latitude of the 15% sea ice concentration in September along two
transects. The first was across the western Scotia Sea, on a transect line used for a
similar exercise10, the second being northwards through the South Orkneys for
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comparison with the long-term records of fast ice duration27 used in Fig. 3. The
September ice latitudes from these two transects were closely related (R2 = 0.82, P <
0.001, n = 24 years) and that from the South Orkneys transect was also related
positively to fast ice duration there (R2 = 0.25, P = 0.018, n = 22 years).
Supplementary Table 4 shows that the general positive relationship between winter
ice cover and krill abundance persists for krill data just from scientific trawls (IsaacKidd and RMT8s) and from just the eastern or the western parts of the SW Atlantic
(albeit not significantly, possibly related to the smaller sample sizes). This confirms that
these krill-ice relationships are not, for example, chance correlations due to the pattern
of inter-annual sampling of krill with different gears. One possibility is that the
relationships represent gross spatial changes of krill distribution, e.g. to the south after
light ice years. However, we have not focussed on this explanation for 3 reasons. First,
in contrast to the ample evidence for effects of sea ice on krill recruitment7-11, no studies
have found that the large scale distribution of krill in summer relates to ice edge extent
the previous winter. Secondly, as stressed in the main text, positive temporal
relationships between krill density and sea-ice cover (Fig. 3b) contrast to the absence of
positive spatial relationships between krill density distribution and the SIZ (Fig. 1a,b).
There is thus no theoretical support for such an explanation. Thirdly, our integration
area for calculating mean krill density spans 50o in longitude and 20o in latitude. This is
a large area compared to the latitudinal range of winter ice variability (Fig. 3b) so any
redistribution of populations would have to be on a very large scale.
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The presence of any significant relationship between krill and ice over such large
scales is surprising. Our indices of sea ice cover a tiny fraction of its total area in the
SW Atlantic and they are only proxy variables. Likewise the annual krill densities are
simple arithmetic means from various net types, fished at various times within the
November-April period and across a large and diverse area. The fact that we see any
link at all must reflect the following combination of circumstances.
a)
The sea ice indices are reasonable proxies for ice cover over larger areas;
areas important for the life cycle of krill in the SW Atlantic sector.
b)
Inter-annual fluctuations in krill density are harmonic across the whole region
(this is shown independently within the main text). If they were not harmonic,
relationships between krill and ice would not be seen across such a large area.
c)
Inter-annual fluctuations in measured krill density across the region (Fig. 2a)
bear some resemblance to reality, despite the problems of variable sample
size, sampling method and region of sampling.
d)
Total krill density in any particular year depends mainly on larval survival
from the previous winter. Sea ice may also be involved with increased
juvenile survival and adult spawning9,11 but the fact that the only clear
relationships were with just a half year lag are explained by larval survival
being key.
When sample sizes are decreased, for example by restricting analyses to eastern or
western portions of the SW Atlantic, the relationships weaken - probably due to the
inherent imprecision of krill sampling with nets5. Only with ~25 years of data and
thousands of net hauls (more than that currently available to Southern Ocean
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monitoring programs) do we start to see significant relationships, underscoring the
value of such data composites alongside monitoring.
Supplementary Table 3 Net sampling details for the circumpolar comparison of
abundance and the spatio-temporal model of density change (Table 1). Krill values are
in Roman, salps in italics.
Analysis
Data set
Number
of
stations
Sampling details: median values
Day of
sampling
from 1
Nov
Nominal
mouth area
of net (m2)
Top
sampling
depth
Bottom
sampling
depth
Circumpolar
abundance of krill
and salps from all
nets (Fig 1a, 1b)
S.W. Atlantic
(10o-70oW)
4828
3302
91
86
3.1
3.1
0
0
170
180
Rest of
Southern Ocean
2147
1910
97
97
0.5
0.5
0
0
185
200
Krill density in
SW Atlantic since
1976 from large
nets (Table 1)
Early period
(1976-1990)
3112
91
3.1
0
170
Later period
(1991-2003)
1825
91
3.1
0
170
Krill density in
SW Atlantic since
1976 from small
nets (Table 1)
Early period
(1976-1990)
496
135
0.6
0
200
Later period
(1991-2003)
267
69
0.6
0
200
Circumpolar
change in krill and
salp density since
1926 from all nets
(Table 1)
Early period
(1926-1951)
545
257
82
91
0.8
0.8
0
0
119
140
Later period
(1976-2003)
4227
1990
90
91
3.1
3.1
0
0
173
193
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Supplementary Table 4. Regressions (for the post 1976 period only ) of log10 (summer mean krill
density) against indices of winter sea ice, either latitudes south or duration. Krill density (no. m -2) was
calculated as the mean value of all hauls south of the PF within either the whole SW Atlantic sector or
within its eastern or western halves. Results are based either on all nets combined or just the large nets
(Isaac Kidd or Rectangular-midwater trawl). Bold values are significant (p < 0.05), italicised values
are plotted as blue circles in Fig. 3a although the whole post-1926 dataset is regressed there.
Index of winter sea ice cover
Index of krill density,
log10 (mean no. m-2)
during summer
Sept latitude along a
transect in western
Scotia Sea11*
Sept latitude along a Duration of fast ice
transect north
recorded at the South
through the South
Orkneys23 (Days)
Orkneys
o
o
2
30 -70 W, all net types
Slope=0.002 r2=0.051
Slope=0.21 r =0.21 Slope=0.13 r2=0.11
p=0.12, n=24
p=0.29, n=24
p=0.02, n=25
o
o
2
2
30 -70 W just large nets
Slope=0.0016 r2=0.038
Slope=0.18 r =0.19 Slope=0.13 r =0.13
p=0.09, n=23
p=0.4, n=23
p=0.03, n=24
o
o
2
2
50 -70 W all net types
Slope=0.13 r =0.11
Slope=0.086 r =0.05 Slope=0.0017 r2=0.052
p=0.1, n=23
p=0.33, n=22
p=0.3, n=22
o
o
2
2
30 -50 W all net types
Slope=0.20 r =0.10
Slope=0.15 r =0.055 Slope=-0.0006 r2=0.02
p=0.22, n=16
p=0.4, n=15
p=0.9, n=15
*The transect line is figured in Fraser et al. (1992) Polar Biology 11:525-531.
Supplementary Figure captions
Supplementary Fig. 4. Krill distribution based on a; 1 m ring nets from the Discovery
era (1926-1951) and b; RMT 8 and Isaac-Kidd Midwater trawls from the post 1976 era,
and b..
Supplementary Fig. 5. Fine scale sampling coverage in the SW Atlantic sector on a 1o
by 1o grid in relation to the 500 m isobath. It shows the wide, oceanic coverage during
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the Discovery era with some emphasis at South Georgia, the Scotia Sea-South GeorgiaAntarctic Peninsula focus during the 1970s and 1980s changing to a tighter focus on the
shelf/shelf breaks north of the South Shetlands and South Georgia in more recent years.