ICES Journal of Marine Science, 62: 1597e1602 (2005)
doi:10.1016/j.icesjms.2005.06.003
Analyses of Bering Sea bottom-trawl surveys in Norton Sound:
absence of regime shift effect on epifauna and demersal fish
Toshihide Hamazaki, Lowell Fair, Leslie Watson,
and Elisabeth Brennan
Hamazaki, T., Fair, L., Watson, L., and Brennan, E. 2005. Analyses of Bering Sea bottomtrawl surveys in Norton Sound: absence of regime shift effect on epifauna and demersal fish. e
ICES Journal of Marine Science, 62: 1597e1602.
This study retrospectively examined evidence of ocean climate regime shift effects on
epifauna and demersal fish of Norton Sound, Alaska, northeast Bering Sea, based on
triennial bottom-trawl surveys from 1976 to 2002. Throughout the period, benthic fauna
was dominated by sea stars (48e78%), followed by cods (5e19%), flatfish (5e15%),
sculpins (1.5e7%), and crabs (2e6%). From 1976 to 2002, the cpue index of total species
increased exponentially (4.5% yÿ1) by threefold with some declines in 1991 and 1999. The
increase was also observed in sea stars (5.1% yÿ1), flatfish (6.1% yÿ1), and crabs (2.5%
yÿ1). However, trends of cods and sculpins were mixed. Regression analysis showed the
cpue index of total species to be positively correlated with survey years and bottom-water
temperature. However, bottom-water temperature, when considered by itself, was not
significant. Results suggest that regime shifts caused biomass increases of Norton Sound
epifauna and demersal fish.
Ó 2005 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.
Keywords: Bering Sea, demersal fishes, epifauna, Norton Sound, regime shift.
Received 29 November 2004; accepted 14 June 2005.
T. Hamazaki and L. Fair: Alaska Department of Fish & Game, Commercial Fishery
Division, Anchorage, AK 99518, USA. L. Watson: Alaska Department of Fish & Game,
Commercial Fishery Division, Kodiak, AK 99615, USA. E. Brennan: Alaska Department of
Fish & Game, Commercial Fishery Division, Nome, AK 99762, USA. Correspondence to
T. Hamazaki: tel: C1 907 267 2158; fax: C1 907 267 2442; e-mail: hamachan_hamazaki@
fishgame.state.ak.us.
Introduction
It is widely recognized that the Bering Sea has been
experiencing abrupt ocean-wide climatic and oceanographic
shifts (regime shifts), occurring on a decadal scale in 1925,
1947, 1977, 1989, and possibly in 1998 (Graham, 1994;
Miller et al., 1994; Hare and Mantua, 2000; Benson and
Trites, 2002). These shifts are not limited to climatic and
oceanographic conditions. As biota of the Bering Sea is
largely influenced by physical and oceanographic conditions, many biological indices (e.g. fishery catch statistics)
also showed abrupt changes coincident with regime shifts.
From those associations, the 1977e1988 period is generally
considered a warm regime with high biological productivity, whereas the 1989epresent period is a cold
regime with low biological productivity (Hare and Mantua,
2000; Benson and Trites, 2002).
Mechanisms describing the effect of regime shifts on
biota are understood to involve ocean currents, and
1054-3139/$30.00
cascading (Niebauer et al., 1981, 1995; Gargett, 1997;
Sugimoto and Tadokoro, 1997; Francis et al., 1998;
McGowan et al., 1998; Anderson and Piatt, 1999; Hunt
et al., 2002). Changing climatic conditions alter oceanographic current patterns and the spatial/temporal distribution of sea ice, which alter spatial/temporal distribution of
water temperature and nutrient upwelling, which affect
spatial/temporal distribution of phytoplankton and watertemperature sensitive species, biological productivity,
species composition, and ecosystem structure. These
understandings, however, are based largely on studies in
the southeast Bering Sea and the Gulf of Alaska where
ocean currents have significant impacts on biota.
The Bering Sea consists of several regions with distinct
water masses (Takenouti and Ohtani, 1974) each with their
own unique ecological characteristics of physical oceanographic environment, biological community composition,
and ecosystem function (NRC, 1996). Since not all regions
are significantly influenced by ocean currents, mechanisms
Ó 2005 International Council for the Exploration of the Sea. Published by Elsevier Ltd. All rights reserved.
1598
T. Hamazaki et al.
Table 1. List of survey protocols and nets used for the Norton
Sound trawl surveys from 1976 to 2002.
Year
Date
Agency
9/2e9/5 NMFS
9/16e10/6
1979 7/26e8/5 NMFS
Gear type
83e112 Eastern
Otter Trawl
83e112 Eastern
Otter Trawl
1982 9/3e9/11 NMFS
83e112 Eastern
Otter Trawl
1985 9/16e10/1 NMFS
83e112 Eastern
Otter Trawl
1988 8/16e8/30 NMFS
83e112 Eastern
Otter Trawl
1991 8/22e8/30 NMFS
83e112 Eastern
Otter Trawl
1996 8/7e8/18 ADF&G 400 Eastern
Otter Trawl
1999 7/28e8/7 ADF&G 400 Eastern
Otter Trawl
2002 7/27e8/6 ADF&G 400 Eastern
Otter Trawl
1976
Sampling
protocol
24-h basis
24-h basis
24-h basis
Daylight hours
24-h basis
Daylight hours
(Lepidopsetta spp.) (Conners et al., 2002), Norton Sound is
dominated by invertebrates, especially the sea star, Asterias
amurensis, a generalist predator, which mainly inhabits the
depth range of 0e40 m along the coast across the Bering
and Chukchi Seas (Sloan, 1980; Jewett and Feder, 1981;
Fukuyama and Oliver, 1985). The southeast Bering Sea has
the highest primary and secondary productivity, whereas
Norton Sound has the lowest (NRC, 1996).
The above differences suggest that the effects of regime
shifts on benthic fauna in Norton Sound would compare
with the southeast Bering Sea. The objective of this study is
to retrospectively examine the presence and extent of
regime shift effects on Norton Sound epifauna and demersal
fish. As Norton Sound is an inshore system, we hypothesize
that any effect of a regime shift on the Norton Sound
benthic epifauna and demersal fish would be minimal.
Daylight hours
Daylight hours
Material and methods
Daylight hours
Norton Sound epifauna and demersal fish have been
monitored triennially since 1976 in response to the development of a red king crab (Paralithodes camtschaticus)
pot fishery initiated in 1977 (Brennan, 2003). The triennial
trawl survey has been conducted by the National Marine
Fisheries Service (NMFS) (1976e1991) and by the Alaska
Department of Fish & Game (ADF&G) (1996 to the present)
to monitor the distribution and abundance of red king crab
and demersal fish (e.g. flounders). NMFS and ADF&G
conducted trawl surveys with a similar format, except for
trawl gear used, sampling schedule, and total area trawled
(Table 1, Figure 1; Fair, 1998). The NMFS survey was
conducted over the entire Norton Sound, whereas the
ADF&G survey was limited to areas where the commercial
crab pot fishery operates (Figure 1). The trawl gear used by
NMFS (83e112 Eastern) is more efficient at catching
walleye pollock (Theragra chalcogramma) and Pacific cod
(Gadus macrocephalus), while gear used by ADF&G (400
Eastern) is more efficient at catching arrowtooth flounder
(Atheresthes stomias) and flathead sole (Hippoglossoides
elassodon) (Szalay and Brown, 2001). However, this
difference is considered negligible in examining total
abundance estimates (Conners et al., 2002).
In Norton Sound sampling stations were evenly spaced to
represent an approximately 18.5 ! 18.5 km (10 ! 10
nautical miles) square grid (Figure 1). At each station,
a trawl was towed once for approximately 2.6 km
(1.5 miles; NMFS), or 1.9 km (1.0 miles; ADF&G). When
the first tow was unsuccessful, a second tow was
completed. For each tow, all red and blue king crabs
(Paralithodes camtschaticus, P. platypus, respectively),
Pacific halibut (Hippoglossus stenolepis), and Pacific cod
(Gadus macrocephalus) were separated, counted, and
weighed. For the remainder of each catch, two or three
subsample buckets were taken, sorted to the lowest possible
taxon, weighed, and counted. Total catch weight of each
of the regime shifts may differ among regions, or the regime
shift effects from the same mechanism could differ among
regions. Specifically, if the effects of regime shifts are only
ocean-current-driven, then regime shifts have little effect on
biota of Bering Sea regions where ocean currents are not
prevalent. Thus far, the effects of regime shifts have not
been examined in the other Bering Sea regions, primarily
because long time-series data are absent (NRC, 1996).
Norton Sound of the north Bering Sea is one of a few places
where long time-series data are available. Ecological
characteristics of Norton Sound differ from those of the
southeast Bering Sea. Whereas the southeast region is
characterized as an oceanic ecosystem, surrounded by strong
oceanic currents, such as Commander Currents, Bering Slope
Currents, and the Alaskan Stream, Norton Sound is
characterized as an inshore ecosystem, consisting of the
inshore Norton Sound Water Mass, a diversion of the Alaska
Coastal Water Current (Takenouti and Ohtani, 1974; Nelson
et al., 1981). In the southeast, the currents influence location
and duration of sea ice that serves as a major source of
freshwater, nutrients, and water mixing-major determinants
of biological productivity. In contrast, Norton Sound receives
most of its freshwater and nutrients from the Yukon and
Kuskokwim Rivers and other small rivers across the Seward
Peninsula; most water mixing is driven by tides and winds
(Goering and Iverson, 1981; Coachman, 1986).
The benthic fauna of Norton Sound also differs from that
of the southeast Bering Sea. Whereas the southeast is
dominated by demersal fish, such as walleye pollock
(Theragra chalcogramma), Pacific cod (Gadus macrocephalus), yellowfin sole (Limanda aspera), and rock sole
Bering Sea bottom-trawl surveys: regime shift effect on epifauna and demersal fish
168°
166°
167°
165°
163°
164°
1599
161°
162°
65°
65°
64°
64°
63°
63°
161°
166°
167°
70
0
165°
70
164°
163°
140
162°
Km
Figure 1. Locations of the trawl survey stations in Norton Sound. Stations in the main squares were continuously surveyed from 1976 to
2002. Stations in the peripheral squares were surveyed when additional time was available. Stations outside the squares were surveyed by
NMFS (1976e1991) but discontinued when ADF&G took over the survey in 1996.
taxon was estimated by multiplying its subsample percentage weight with total catch weight (excluding red and blue
Table 2. List of major species in each dominant taxon group.
king crabs, Pacific halibut, and Pacific cod). Bottom-water
temperature was also collected at each site. Catch weights
Number
of each haul were converted into catch per unit effort
Taxon of identified
(cpue): weight per area swept (kg kmÿ2) calculated as
group
species
Major species
effective width of the net times distance towed. The cpue
index of each taxon was calculated as the geometric mean
Sea stars
16
Asterias amurensis, Lethaserias nanimensis,
of non-zero cpue multiplied by the percentage of non-zero
Evasterias spp.
hauls for each taxon (Pennington, 1983; Conners et al.,
Crabs
10
Telmessus cheiragonus, Paralithodes
2002). In this retrospective analysis, only stations within
camtschaticus, Paralithodes platypus
Snails
49
Neptunea heros, Volutopsius harpa, Tritonia
ADF&G surveyed areas were used for calculation of the
diomedea
cpue index.
Corals
20
Styela rustica, Halocynthia spp., Boltenia spp.
To examine influences of regime shifts, a multiple logFlatfish
9
Hippoglossus stenolepis, Platichthys stellatus, linear regression model was constructed, in which the logLimanda aspera
transformed cpue index was regressed with survey year
Cods
5
Eleginus gracilis, Theragra chalcogramma,
and bottom temperature: log(cpue) Z b0 C b1(year ÿ 1976) C
Gadus macrocephalus
b
2(bottom temperature). This model has a biologically
Sculpins
28
Myoxocephalus jaok, Myoxocephalus
reasonable, exponential growth trajectory and multiplicative
verrucosus, Myoxocephalus scorpioides
lognormal error structure.
1600
T. Hamazaki et al.
Table 3. Cpue index (kg kmÿ2) of the Norton Sound benthic fauna taxon groups and mean bottom-water temperature at the trawling site.
Survey year
Sea stars
Crabs
Snails
Corals
Other invertebrates
Flatfish
Cods
Sculpins
Other fish
1976
1979
1982
1985
1988
1991
1996
1999
2002
1471.2
100.9
15.6
e
75.9
72.4
234.0
27.4
20.1
1709.8
66.6
17.7
27.5
40.2
180.7
940.8
45.4
47.1
2070.4
125.2
76.4
45.7
64.2
331.2
274.9
209.1
21.1
2657.7
126.3
66.9
146.3
290.8
369.7
674.2
223.5
72.0
2880.0
113.5
115.7
160.6
338.9
406.7
997.6
209.3
48.8
1924.9
138.3
39.6
52.9
216.6
240.5
969.7
316.5
43.3
4385.9
138.6
192.8
183.8
321.0
702.6
832.9
150.3
79.7
3936.4
199.1
18.8
53.8
77.8
510.8
218.4
117.0
36.0
6772.9
144.9
160.7
158.6
337.1
483.6
395.5
131.1
63.4
7.60
Bottom-water temperature
9.39
7.73
Results
From 1976 to 2002, 322 taxonomic groups were captured
and identified (224 species identified); these were combined
into 48 family/genus groups and further into 9 groups. Five
major groups represented the benthic fauna: sea stars
(49e78%), cods (4e31%), flatfish (4e10%), sculpins
(2e8%), and crabs (2e8%) (Tables 2, 3, Figure 2). The
percentage of sea stars tended to be lower during the warm
regime years (1976e1988) than during cold regime years
(1989e2002) (mean: 58.0 vs. 67.8%).
The cpue index of total species generally trended
upwards, trending down in 1991 and 1999 (Table 3). The
trend differed for each group. An increasing trend was
observed for sea stars, crabs, and flatfish, but trends of cods
and sculpins were mixed. Regression analyses showed that
the cpue index for total species, sea stars, crabs, and flatfish
was significantly positively correlated (p ! 0.05) with
survey years, with mean annual increase rates 4.5%,
5.1%, 2.6%, and 6.1%, respectively (Table 4). Annual
7.38
9.24
7.09
8.82
7.88
mean bottom temperature showed a mixed trend: a significant positive correlation with total species and a negative
correlation with crabs (p ! 0.05) (Tables 3, 4). However,
models considering temperature alone did not show
significant correlations (p O 0.05).
At family/genus level, 30 out of 48 groupings showed
increasing trends over the years (Spearman correlation
coefficient: rs O 0.1); however, only five groups had
statistically significant trends (p ! 0.05). This lack of
statistical significance was due to sparse and highly variable
data.
Discussion
In order to appropriately assess the effects of regime shifts on
biota, data should be collected in a continual and consistent
manner, should reflect characteristics of the biota, and
confounding effects of the fishery should be minimized. From
these perspectives, we acknowledge that Norton Sound
100%
90%
80%
CPUE Index
5.58
Other fish
70%
Sculpins
60%
Flatfish
Cods
50%
Other invertebrates
40%
Snails
Corals
Crabs
30%
Sea stars
20%
10%
0%
1976 1979 1982 1985 1988 1991
1996 1999 2002
Year
Figure 2. Percentage of Norton Sound benthic fauna taxon groups in the cpue index.
Bering Sea bottom-trawl surveys: regime shift effect on epifauna and demersal fish
Table 4. Multiple log-linear regression coefficient of log(cpue) Z
b0 C b1(year ÿ 1976) C b2(bottom temp) for selected taxon
groups.
Year
Bottom
temperature
((C)
r2
Average %
increase yÿ1
Sea stars
Crabs
Flatfish
Cods
Sculpins
0.023*
0.009*
0.028*
0.007
0.018
0.037
ÿ0.059*
0.576
0.168
ÿ0.002
0.86
0.85
0.67
0.45
0.21
5.1
2.6
6.1
e
e
Total
0.022*
0.057*
0.96
4.5
Taxon
group
n Z 9; *p ! 0.05.
triennial survey data are not ideal. The survey interval is too
sparse (9 survey years in 26 years) to examine detailed trends.
The effectiveness of trawl gear for assessing the true
abundance of invertebrates is also unclear. Catch of
invertebrates may be influenced by survey techniques (M.
Elizabeth Conners, pers. comm.). More significantly, this
study assumes that the 1976 survey reflects benthic faunal
characteristics of the period before the 1977 regime shift.
This assumption is unverifiable though reasonable. Norton
Sound did not have a major groundfish fishery until 1977, so
the 1976 benthic fauna is considered unexploited. Furthermore, since the crab pot fishery is small with minimum
bycatch (Brennan et al., 2003), the fluctuations are largely the
result of natural causes.
Despite the above deficiencies, a clear trend emerged.
The Norton Sound benthic biomass increased since 1976
and the benthic fauna remained dominated by sea stars
(Figure 2, Table 3). Interestingly, this trend was comparable to the southeast Bering Sea: north of Unimak Island,
Bristol Bay, and the Pribilof Islands (Conners et al., 2002).
While species composition and abundance differed between
areas, in all the three areas, cpue of total species, demersal
fish, and non-crab benthic invertebrates was stable from
1960 to the early 1970s, increased threefold to fivefold
from the late 1970s to the early 1990s, then declined
somewhat in the late 1990s. Furthermore, in the southeast
Chukchi Sea and Kotzebue Sound located just north of
Norton Sound, Feder et al. (2005) compared 1976 and 1998
trawling survey results. They found that the biomass of the
most dominant species/groups increased by twofold or
more in 1998, specifically sea stars (Asterias amurensis),
snow crabs (Chionoecetes opilio), tunicates, and sea
urchins (Strongylocentrotus droebachiensis), while little
change was observed in epifaunal invertebrate compositions. These surveys indicate that benthic fauna biomass
increased significantly without major species changes
across various Bering Sea regions since the 1977 shift.
However, it is unclear if those consistent responses were
caused by the same mechanisms. Ecosystem characteristics
1601
differ between the inshore Norton Sound ecosystem and the
ocean-current-dominated southeast Bering Sea ecosystem,
so ocean-current-driven regime shift effect mechanisms
(e.g. Francis et al., 1998; McGowan et al., 1998; Hunt
et al., 2002) would not be applicable in Norton Sound.
Further research to explore factors affecting productivity of
the Norton Sound benthic fauna is needed to enhance our
understanding of the structure and function of this
ecosystem and its relationship to regime shifts.
Acknowledgements
This work was the result of long-term survey efforts by
NMFS and ADF&G. We thank all the field crews who
participated in the surveys. We also thank Elizabeth
Conners and Bernard A. Megrey, NMFS Alaska Fisheries
Science Center, and Stephen Jewett and Howard Feder,
University of Alaska Fairbanks, for their critical review of
the manuscript.
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