Download A review of the Sea of Okhotsk ecosystem response to the climate

ICES Journal of
Marine Science
ICES Journal of Marine Science (2012), 69(7), 1123–1133. doi:10.1093/icesjms/fss107
A review of the Sea of Okhotsk ecosystem response to the climate
with special emphasis on fish populations
Sen Tok Kim
Sakhalin Scientific Research Institute of Fisheries and Oceanography (SakhNIRO), 198 Komsomolskaya Street, Yuzhno-Sakhalinsk, Russia;
tel: +89147628718; fax: +8(4242)456783; e-mail: [email protected]
Kim, S. T. 2012. A review of the Sea of Okhotsk ecosystem response to the climate with special emphasis on fish populations. – ICES Journal of
Marine Science, 69: 1123– 1133.
Received 20 September 2011; accepted 23 April 2012; advance access publication 24 June 2012.
This article provides a brief review of climatic, oceanographic, and biological changes in the Sea of Okhotsk in recent decades. The Sea
of Okhotsk is distinguished by its high biological productivity and its significant impact on the Pacific Ocean through water exchanges.
Long-term temperature data have shown periodic cooling and warming of the Sea that in turn have resulted in changes to its biological communities. In the 1980s, a generally warm period, the Sea of Okhotsk had abundant fish, primarily large stocks of gadoids,
especially walleye pollock. The second half of the 1990s was a transitional period when the marine ecosystem was being restructured.
In particular, by the mid-1990s, the total biomass of fish in the Sea of Okhotsk had decreased significantly. In the early 2000s, the
situation reached a critical level, but by the end of that decade, there was a renewed warming and an increase in the abundance
of walleye pollock.
Keywords: climate, currents, demersal fish, herring, regime shift, sea ice, walleye Pollock.
Introduction
The study of marine ecosystems is a complex and multifaceted task
accomplished only by a synthesis of the knowledge of the nature of
atmospheric, oceanographic, and biological processes. At the same
time, such observations in different regions can be useful in improving the understanding of natural processes as a means of
better managing marine resources in future (McGowan et al.,
1998; Astthorsson et al., 2007; Loeng and Drinkwater, 2007;
Shuntov et al., 2007). This paper provides a brief review of the
Sea of Okhotsk ecosystem and is meant to add to those of other
Subarctic seas published in Hunt et al. (2007).
Although early records of Far Eastern seas exist from the 18th
century, the Sea of Okhotsk still remains an area that has been insufficiently investigated. Integrated investigations combining
knowledge on the physical environment and marine biological
communities began in the second half of the 20th century, but
the results have been discussed relatively recently. One example
is found in the PICES review “Marine Ecosystems of the North
Pacific Ocean 2003–2008” (McKinnell and Dagg, 2010), which
included a discussion on several aspects of the Sea of Okhotsk
ecosystem (Radchenko et al., 2010). Although the warming trend
in the Sea of Okhotsk, pointed out in the PICES report, has
been confirmed by the results of the present work, there are some
# 2012
differences for certain other environmental parameters. Moreover,
after the report’s focus period of 2003–2008, there was cooling in
2009 and 2010, underscoring the need for continued monitoring
of periodic environmental changes to predict more accurately
likely upcoming events in biological communities.
It is known that the warming has resulted in melting of sea ice
in the Arctic, which decreased by 40% between the early 1980s
and 2007, when the minimum was recorded and since when it
has remained low (Kauker et al., 2009). Exhaustive study of
Subarctic regions in recent decades indicates similar tendencies
of climate warming over those regions, including the Sea of
Okhotsk (McKinnell and Dagg, 2010; Radchenko et al., 2010).
There are 12 distinguishable boundary currents in the Sea of
Okhotsk, the most important being the West Kamchatka, East
Sakhalin, and Soya Currents (Chernyavsky et al., 1993; Figure 1).
The warm West Kamchatka Current, which dominates the northeastern part of the Sea, is a continuation of the East Kamchatka
Current that in turn is part of the Western Subarctic Gyre. In
the northwestern basin of the Sea of Okhotsk, the waters of the
Amur River contribute to the cold East Sakhalin Current, and in
the southern part of the Sea, the warm Soya Current dominates.
The latter is a continuation of the Tsushima Current, which
itself is a branch of the Kuroshio Current. These three main
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S. T. Kim
Figure 1. The Sea of Okhotsk cyclonic Gyre in the warm period (July –September), after Chernyavsky et al. (1993). Currents: 1, West
Kamchatka; 2, Northern; 3, Median; 4, Penzhinskoye; 5, Yamskoye; 6, North Okhotsk; 7, North Okhotsk Counter; 8, Amur; 9, East Sakhalin; 10,
East Sakhalin Counter; 11, North Eastern; 12, Soya.
currents vary seasonally, with flow increasing in summer and decreasing in winter (warm currents), or vice versa (cold East
Sakhalin Current). Overall, the Sea of Okhotsk has a generally cyclical circulation caused by the prevalent cyclonic atmospheric circulation over the region. This circulation pattern plays a decisive
role in the spatial distribution of biological resources within the
Sea.
Unquestionably, the Sea of Okhotsk plays a hugely important
role in the northwestern Pacific. For example, cold dense
Okhotsk waters, formed through brine rejection during sea-ice
formation in the Amur River region, influence the characteristics
of the intermediate layer of the Pacific Subarctic Gyre (Okuda
et al., 1991; Yasuda, 2004). This dense water sinks through convection to the deep shelf of the Sea of Okhotsk and is then transported
by the East Sakhalin Current, which flows along the eastern
Sakhalin coast to Bussol Strait before entering the Oyashio
region (Nakatsuka et al., 2002). This water causes a cooling of
the Northwest Pacific as well as an increase in nutrient concentration, including iron. These nutrients eventually reach the surface
and contribute to the creation of large-scale productive zones in
the northwestern Pacific for the abundant epipelagic subtropical
fish (Pacific saury Cololabis saira, Japanese sardine Sardinops melanostictus, Japanese anchovy Engraulis japonicus) and squid there
(Belyaev, 2003; Filatov et al., 2011).
It is well known that climate variability influences marine environmental processes, but it is often difficult to determine the
cause-and-effect relationships. The long-term dynamics of biological communities in the Sea of Okhotsk at various levels from
phytoplankton to fish have not been investigated adequately, and
more work is needed to find and compare the scattered information on the dynamics of the basic groups of marine organisms in
the region. Hence, the main purpose of this review is to provide
information on recent climatic, oceanographic, and biological
changes, as well as on the basic trends of fish resources in the
Sea of Okhotsk. In addition, near-future scenarios are considered
based on previously published work and the author’s own research
on fish dynamics in limited areas of the Sea.
Material and methods
This review is based largely on publications, mainly in the Russian
language, that have examined climate and ocean shifts in recent
decades that might have influenced biological communities in
the Sea of Okhotsk (e.g. Figurkin, 2006; Dulepova and
Merzlyakov, 2007; Kim, 2007; Ohshima et al., 2008; Kim and
Biryukov, 2009; Glebova et al., 2009; Radchenko et al., 2010;
Savin et al., 2011). Information is presented on the atmosphere,
circulation, ice processes, the temperature regimes of seawater,
primary and secondary plankton production, and fish resources
in the Sea. This summarized information is then used in an
attempt to provide a long-term perspective on shifts in marine
processes in the region.
Sea of Okhotsk ecosystem response to climate, with emphasis on fish populations
The methods used include standard statistical analyses of timeseries for environmental parameters, fishery statistics, and observations from trawl surveys (Figurkin et al., 2008; Glebova et al.,
2009; Zhigalov and Luchin, 2010; Savin et al., 2011). The GIS technology was used to estimate indices and to visualize results.
Satellite remote sensing data were analysed for chlorophyll distributions, ice-cover thickness, and the thermal regimes of local and
vast areas of the Pacific Ocean (Mantua et al., 1997; Ohshima et al.,
2006; Kasai et al., 2010). Ship and coastal observations provided
information on temperature, salinity, oxygen, and nutrient concentrations in seawater as well as on biological diversity and
where possible abundance of fauna in trawl surveys
(Chernyavsky et al., 1993; Shuntov and Bocharov, 2003). Often,
source data were compared by correlation analysis (Figurkin,
2006; Ogi and Tachibana, 2006; Ohshima et al., 2006; Glebova
et al., 2009). A correlation coefficient (r) of .0.60 was generally
considered to indicate a strong relationship between variables.
The analysis of synoptic atmospheric pressure data is based on
maps produced by the Hydrometeorological Centre of Russia
(1974– 1989) and the Japan Meteorological Agency (JMA;
1990–2007; Glebova et al., 2009). Variables such as the latitude
and longitude of the centres of pressure systems, their amplitude,
and the mean pressures at the sea surface and at a height of
500 hPa were investigated for the area 30– 708N 808E–1608W
between 1948 and 2005 (Shatilina and Anzhina, 2008). Trends in
sea surface temperature (SST) in the Sea of Okhotsk for the
period 1950– 2006 were examined using monthly mean data
(Khen et al., 2008). The thermal regime at the sea surface was
determined from monthly data provided by the JMA for the
period 1974–2007 (Glebova et al., 2009) and temperature data
collected during 36 surveys conducted by TINRO during the
years 1982–2004 (Figurkin, 2006). Sea-ice data for the Sea of
Okhotsk were derived from regular observations of FERHRI (the
Far Eastern Regional Hydrometeorological Research Institute)
for 1974– 1991 and for 1992–2005 from maps generated by
NOAA (Glebova et al., 2009). Additionally, daily sea-ice data
were obtained from the Scanning Multi Microwave Radiometer
(SMMR) for the period 1978– 1987 and the Special Sensor
Microwave Imager (SSM/I) for the years 1987–2001 (Ohshima
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et al., 2006). Amur River discharge data have been recorded at
Bogorodskoye (1971 –2004; Ohshima et al., 2006) and at
Khabarovsk (1986 –2005) by FERHRI (Novorotsky, 2007).
The PDO (Pacific Decadal Oscillation) index was taken from
http://jisao.washington.edu/pdo (Mantua et al., 1997).
Time-series data of PO4, NO3, and chlorophyll a (Chl a)
between 3 and 348N along the 1378E line of longitude during
the period 1971–2000 were available from the JMA (http://
www.jodc.go.jp/service.htm) and used for studying nutrient and
phytoplankton dynamics in the northwestern Pacific (Watanabe
et al., 2005). Zooplankton dynamics were evaluated from data
collected during plankton surveys made by TINRO in the years
1984–2005 (Dulepova and Merzlyakov, 2007). Trawl and plankton
surveys were carried out within the standard biostatistical areas of
the Sea of Okhotsk (Shuntov et al., 1986; Shuntov and Bocharov,
2003). Bottom-trawl surveys included 230– 250 stations off eastern
Sakhalin Island, 160–180 stations off the southern Kuril Islands,
and nearly 180 stations off the West Kamchatka Peninsula (Kim,
2007; Kim and Biryukov, 2009; Savin et al., 2011). The large-scale
pelagic surveys in the Sea of Okhotsk conducted for assessing
meso- and epipelagic fish resources are described by Melnikov
(2006); qualitative and quantitative compositions of catches
during 30-min trawls were used to assess fish biomass. Feasible
coefficients of catchability were used to estimate fish and plankton
biomasses, fitting them to reliable levels of resources (Boretz, 1985;
Volkov, 1986).
Results and discussion
Physical environment
The thermal regime of the Sea of Okhotsk is determined by three
factors: (i) its northern location; (ii) the effect of atmospheric processes including warming or cooling on surface waters, formation
or melting of sea ice, and convection in the water column; (iii) the
effect of currents, particularly the advection of warm water from
the Pacific Ocean.
In the past 50 years, ice cover in the Sea of Okhotsk has
decreased by nearly 10% (Figurkin, 2006; Ohshima et al., 2008;
Figure 2), and from 2002 to 2006, the area of cover continued to
Figure 2. Ice cover averaged over February –March of 1960 –2004, January – April of 1985– 2010, and bottom-water temperature averaged for
depths of 150 – 200 m along the West Kamchatka coast in July of 1965 –2004 (after Figurkin, 2006, 2011).
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S. T. Kim
Figure 3. Water transport by the West Kamchatka Current (northern branch) within the 0– 200 m layer in April of 1983 – 2006 (after Figurkin
et al., 2008).
decrease gradually. Then, after the 2007/2008 winter, ice cover
increased again, but through to 2010, the level of coverage
remained less than its long-term mean (Ishizaki, 2009; Figurkin,
2011). The period of maximum ice cover has also recently
shifted from March to February (Radchenko et al., 2010).
Interannual variability in the Sea of Okhotsk ice extent is highly
correlated with surface temperature in autumn and winter
(Ohshima et al., 2006). Air temperature has increased steadily
over the past 50 years, possibly under the influence of global
warming.
Sea-ice extent is a good indicator of the thermal regime in nearbottom waters of the northern Sea of Okhotsk (Figurkin, 2006),
because the cold water sinks as a result of brine rejection during
ice formation. In the 1980s and 1990s, as the extent of sea ice
decreased, thermal conditions in the bottom layer of the sea
were relatively warm. The cold period of 1998– 2001 with its
heavy ice cover was followed by significant warming as the ice
cover generally decreased during the past decade (Figurkin et al.,
2008). As in the southern part of the Sea of Okhotsk, 2001 was
the coldest and 2003 the warmest year in the past decade, although
temperatures were warmer than normal from 2004 to 2010
(Figurkin, 2011).
The overall thermal regime in the northern Sea of Okhotsk
is determined to a large extent by the inflow of the Western
Kamchatka Current (WKC; Figure 3). In recent decades, the advection of warm ocean waters by the WKC varied considerably
(Figurkin et al., 2008), with the start of the 1980s and at the end
of the 1990s characterized by increasing flow. In contrast, during
the late 1980s and early 1990s and 2000s, that advection decreased.
In general, after fluctuating significantly in the late 1990s, WKC
transport became more stable, near its mean annual value. An association between the intensity of the WKC and changes in mean
temperature in the 150 –200-m layer is evident despite range
differences (Figurkin et al., 2008; Zhigalov and Luchin, 2010).
Moreover, in years of less advection, the extent of ice cover
increases.
In the northwestern Sea of Okhotsk, Amur River discharge is
another factor influencing the temperature regime. Every year,
the river supplies the Sea of Okhotsk with 362.5 km3 of water
(Zhabin and Dubina, 2008). In late autumn, river discharge
causes strong stratification and low salinities that promote freezing (Ohshima et al., 2001), then in spring, the discharge
increases and its relatively warm waters extend far south.
During the past century, four cycles of 21 – 31 years duration
each exist in the Amur River run-off time-series (Figure 4;
Novorotsky, 2007). The period from 1983 to 2005 consisted of
low river flow, after which discharge increased, corresponding
to a warm period. There is also decadal variability in the
Amur discharge record (Figure 4). With minimum flow in
2002 and assuming that the decadal cycle continues, this suggests that river discharge may decrease sometime after 2010/
2011. Opposite trends in long- and short-term fluctuations in
Amur River run-off could support the notion of modest
cooling in the immediate future.
In the southern Sea of Okhotsk, the summer temperature
regime is defined by the intensity of the warm Soya Current
(Figure 5). However, during cold periods, the strength of the
cold East Sakhalin Current becomes the main factor determining
the temperature regime of the southern Sea (Figurkin et al.,
2008). The summer water temperature regime around the southern Kuril Islands within the zone of influence of the Soya
Current is characterized by long-term temperature variability
that is out of phase with the extent of ice cover in the north
(Shatilina, 1996, 1998; Zhigalov and Luchin, 2010). Hence, the intensity of the Soya Current is greatest in years when winter ice
cover is larger. Warming of the northern Sea of Okhotsk generally
Sea of Okhotsk ecosystem response to climate, with emphasis on fish populations
1127
Figure 4. (a) Annual mean discharge of the Amur River and sea-ice area over the Sea of Okhotsk the thick lines are filtered with a 3-year
moving average, and the sea-ice series is shifted to the left by 1 year (after Ogi and Tachibana, 2006). (b) Long-term discharge from the Amur
River, with the thick line filtered with a 10-year moving average (after Novorotsky, 2007).
Figure 5. Water temperature anomalies at around the south Kuril Islands (a) in June – October of 1961 – 1990 (after Shatilina, 1996), and (b) in
March of 1982 –2007 (after Zhigalov and Luchin, 2010).
corresponds to cooling in the south, particularly in the southern
Kuril area. The Kuroshio and Soya Currents have similar variability, which is not surprising because the latter is a branch of the
former (McKinnell and Dagg, 2010).
The temperature regime of the waters of Bussol Strait and off
Iturup Island’s oceanic coast is influenced by the cool Oyashio
Current, and its variability is similar to the long-term trend in
sea ice in the Sea of Okhotsk.
1128
Average SST throughout the Sea of Okhotsk increased until the
mid-1990s, but from then to 2001, there was a decline in temperature, particularly notable in the north. Warming then took place
after 2003 (Glebova et al., 2009), but by 2006, it was cooling
again, and ice cover increased (Ustinova et al., 2004; Khen et al.,
2008). Similar decreasing trends in SST were documented for
the northwestern Pacific, suggesting common atmospheric
forcing throughout the region. For the Sea of Okhotsk, the PDO
index has been used widely to examine the dynamics of the
region (Khen et al., 2008). Analysis of century-long time-series
of the main climatic indices for the North Pacific, including
the PDO, reveals close correlation between them and an
60-year cycle. The new maximum was in the 1990s, and it
appears to be the start of a new phase of a decreasing PDO index
(http://jisao.washington.edu/pdo).
The PDO index is based on SST anomalies in the North Pacific
(Mantua et al., 1997). The upper layer temperature regime of the
upper water layers is considered to be one of the most important
characteristics of the environment that influences biological communities. The index shows periodicity with two main cycles of 15 –
25 and 50 –70 years. It tends to match the multiyear periodicity in
climate change for the northern hemisphere in the 20th century
(Schlesinger and Ramankutti, 1994; Byshev et al., 1997). In the
20th century, cool PDO regimes dominated during the years
1890–1924 and 1947–1976, and warm periods in the years
1925–1946 and 1977–2006. Temporal trends in the PDO point
to the likelihood that we might expect cooling over the next 20 –
30 years.
Similar future trends are predicted from other climate indices.
In recent years, it has been suggested that the climate over the Sea
of Okhotsk is related to the location of large-scale air pressure
systems, i.e. the Siberian High and Aleutian Low (Glebova et al.,
2009). There is a link between the location of the Aleutian Low
and the direction of prevailing winds over the Sea of Okhotsk
(Shatilina, 1998; Shatilina and Anzhina, 2008; Glebova et al.,
2009). When the Low is located near the Kamchatka Peninsula,
i.e. southwest of its normal location (518N 1808W), easterly
winds prevail in winter accompanied by general warming in the
Sea of Okhotsk. Ocean cooling persists under cold northerly
winds, which happens when the Aleutian Low is more to the
north, near the Komandorski Islands or in the eastern Bering Sea.
During the past three decades, the centre of these pressure
systems in winter has shifted to the southwest and weakened, coinciding with a general trend of fewer cold years and more warm
years (Glebova et al., 2009). During the same period, the
number of cyclones in the Sea of Okhotsk has decreased, but
their intensity has inversely increased (Glebova, 2010). However,
along with these long-term trends, the intensity of cyclones and
their dominant tracks passing over the Sea have exhibited
decadal variability (Glebova, 2005, 2010). Winds tended to be
strong from 1996 to 2001, and there was a positive correlation
between the cyclones and sea-ice cover, but from 2002 to 2009
the same two variables were negatively correlated. That is why
the total correlation coefficient between ice cover and the intensity
of cyclone activity for the entire period was not high (nearly
20.30). From 2002 to 2009, mainly warm winds prevailed over
the Sea of Okhotsk, causing a warming of the sea surface, and
sea-ice cover decreased at the same time that mean spring SST
increased. Based on atmospheric dynamics over the Sea of
Okhotsk, it has been suggested that a decade of cooling should
have started around 2006–2008 (Glebova, 2007; Khen et al.,
S. T. Kim
2008; Lyubushin and Klyashtorin, 2012). However, in view of
the predicted long-term global warming, this cooling is expected
to be “soft” or “unclear” (Glebova et al., 2009).
Hence, although for the current century, it is thought that there
will be overall warming of the Sea of Okhotsk, in the decade of the
2010s, some cooling might be expected (Ustinova et al., 2004;
Glebova, 2007; Khen et al., 2008).
Biology
Here, variability in phytoplankton, zooplankton, and the fish
resources of the Sea of Okhotsk are discussed in relation to the
temperature variability discussed above. Although there are
several hypotheses on the mechanisms linking biological events
with changes in the atmosphere or ocean climate, it can be difficult
to link the biological events with the observed environmental phenomena. Even more difficult is the forecasting of biological
responses to future climate, making such forecasts rather unreliable because of the complexity of cause-and-effect relationships
from solar activity to the dynamics of biological organisms, including fish.
Plankton
Full-scale, continuous studies of primary production in the Sea of
Okhotsk started only recently, although primary production is
linked to changes in hydrological features. The Sea of Okhotsk
and the western Pacific share chemical features. As discussed
above, water exchange between the Sea of Okhotsk and the
North Pacific enriches the latter with nutrients, notably iron,
and iron in seawater supports the phytoplankton assimilation of
such nutrients as nitrate, silicate, and phosphate (Martin and
Fitzwater, 1988; Tsuda et al., 2007; Tsumune et al., 2009); phytoplankton growth quickens dramatically with the addition of a
single mmol of iron. It has been suggested that if iron concentration in the North Pacific, or the Sea of Okhotsk, drops, there
would be decreased biological productivity, including of fish
(Martin et al., 1994; Wakatsuchi, 2006).
Phytoplankton productivity is also affected by such factors as
sunlight, temperature, and the dynamics of the waters. Locally
strong productivity of phytoplankton in cold Subarctic seas is
related to the extent of upwelling of nutrient-enriched deep
waters. Nutrients, including iron, are carried by the Amur River
into the Sea of Okhotsk, and then, through brine formation and
subsequent sinking, they reach the cold intermediate layers of
the Sea and eventually mix vertically back into surface layers
(Selina et al., 2004; Andreev and Pavlova, 2010). The productivity
of the Sea of Okhotsk is also influenced by the duration of sea-ice
cover. Ice limits the penetration of sunlight and hence the duration
of photosynthetic activity, which takes place on average throughout the region for nearly 270 days a year, although in the southeast,
it can extend throughout the year (Chernyavsky et al., 1993).
Currently, only seasonal stages of phytoplankton development
are known reliably. In the coastal regions of the Sea of Okhotsk,
initial photosynthesis commences in March, coinciding with the
start of the ice-melt. Concentrations of Chl a are high near the
western Kamchatka Peninsula and in the largest bays of eastern
Sakhalin Island (Terpeniye and Aniva). Then, in April, the phytoplankton blooms actively, with peak blooming tending to be
in May when the sea-ice melt is practically finished (Saitoh
et al., 1996; Matsumoto et al., 2004, Kasai et al., 2010).
Photosynthetisis is widespread, but peaks in the estuaries of the
large Sakhalin rivers. Highest phytoplankton concentrations
Sea of Okhotsk ecosystem response to climate, with emphasis on fish populations
(.20 mg m23) tend to be in the Amur Estuary, where enriched by
nutrients, the river waters contribute to the high concentrations of
Chl a near northeastern Sakhalin (.10 mg m23) and Kashevarov
Bank (up to 5 mg m23). Generally, the phytoplankton bloom in
the Sea of Okhotsk is over by July, because with further
warming of sea surface waters and a stable shallow mixed layer,
nutrients become depleted and the intensity of photosynthesis
decreases. There is then a secondary bloom typically in October,
simultaneously with water cooling and increased wind-mixing,
and phytoplankton concentrations peak (.10 mg m23) again in
the Amur Estuary. By year end, though, phytoplankton production declines dramatically. The seasonal variability in Chl a is
related to the variability in the physical processes, including
sea-ice melting in spring, advection through the warm Soya
Current in summer and the intrusion of cold East Sakhalin
Current in autumn (Mustapha et al., 2011).
Interannual fluctuations in primary production in the Sea of
Okhotsk in relation to warm and cool periods are not yet clear.
From 2008 to 2010, spring and autumn phytoplankton blooms
were considerably weaker than in 2005 and 2006 (http://
teradata.sakhniro.ru). Average primary productivity in the sea
is 450 g S m22 year21) and total annual production reaches
720 × 106 g S (Chernyavsky et al., 1993).
In the western North Pacific, phytoplankton (diatom) abundance has been decreasing during the past three decades,
because of an increase in surface stratification caused by
warming (Watanabe et al., 2005; Ishida et al., 2009). The average
nutrient concentrations in the surface mixed layer have been decreasing concomitant with an increase in vertical stratification
(Watanabe et al., 2005). As the Sea of Okhotsk is a main source
of nutrients for the whole western Pacific (Wakatsuchi, 2006;
Takeda, 2011), the annual dynamics of chlorophyll concentrations
and the supply of nutrients are expected to be similar in both.
There is relatively little information on trends in total
zooplankton biomass in the Sea of Okhotsk (Dulepova and
Merzlyakov, 2007; McKinnell and Dagg, 2010). Zooplankton production is greater in the northern part of the Sea (where macroplankton makes up 76– 92% of total plankton biomass) than in
the south (56– 78%). Euphausiids and copepods dominate in
the north, and the biomass of both was high during the cold
period of 1999–2001, in the north and the south (Dulepova and
Merzlyakov, 2007). Zooplankton biomass was relatively low in
1997 and 1998 and again in 2003 and 2004. Since then, too,
1129
there was a trend of decreasing biomass until 2009 (McKinnell
and Dagg, 2010). A twofold change in average plankton biomass
the 7 years from 1998 to 2005 is evidence of considerable variability in the planktonic communities in the Sea of Okhotsk. Available
information over a longer period (1984–2006) cannot distinguish
a long-term trend (McKinnell and Dagg, 2010), and 25 years of
benthic data in the Sea of Okhotsk off eastern Sakhalin show
no significant changes, either quantitatively or qualitatively
(Nadtochy et al., 2004).
Fish and fisheries
Trawl surveying of the Sea of Okhotsk off West Kamchatka, East
Sakhalin, and south of the Kuril Islands is regular. The total
ichthyofauna in the Sea of Okhotsk contains at least 513 species
(Boretz, 2000), but just 20 –30 species are considered to be commercially important, some well studied because of their commercial importance.
The impact of the fisheries on the Sea of Okhotsk ecosystem
is very important. The annual total fish catch peaked at 2.6
million tonnes in the 1970s, dominated by walleye pollock
(Theragra chalcogramma; Figure 6), but subsequently have fluctuated mainly between 1.0 and 1.5 million tonnes. Significant
decreases in some local fish resources during the 20th
century has resulted from the intensive fishing pressure, e.g.
the Sakhalin – Hokkaido herring and yellowfin sole stocks of
Terpeniya Bay (Tarasyuk, 1994; Pushnikova, 1996). However,
natural factors are also part of the drivers of the changes in
fish population sizes.
For Far Eastern seas, the 1980s were characterized as a period of
high fish stock abundance. In the Sea of Okhotsk, maximum total
biomass was principally a result of there being large stocks of
gadoids, predominantly walleye pollock. In northern pelagic communities of the Sea of Okhotsk, 85 –99% of total fish biomass consists of that species, although mesopelagic fish are also fairly
abundant in the south (Balanov and Radchenko, 1995), where
walleye pollock constitutes 8 –30% of total fish biomass.
Northern smoothtongue (Leuroglossus schmidti) make up some
73% of total fish biomass in the south.
In the 1980s, total fish biomass reached 35 million tonnes, with
pelagic fish contributing 90%. Until the early 1990s, there was
a large biomass of sardine in the southern Sea of Okhotsk, up to
1.2 million tonnes; groundfish made up 3.5 million tonnes
(Zhigalin and Belyaev, 1999). These values were recently revised
Figure 6. Annual catches of walleye pollock in the Sea of Okhotsk (from official Russian statistics), with the line showing the catch for the
entire sea and the dashed line for just western Kamchatka waters.
1130
S. T. Kim
Figure 7. Composition and biomass of epipelagic fish communities in the Sea of Okhotsk (after Dulepova and Merzlyakov, 2007).
and are now estimated to have been at least 55 –60 million tonnes
(McKinnell and Dagg, 2010), of which demersal fish constituted
.11 million tonnes. Then, in the 1990s, there were changes to
the marine ecosystem of the Sea of Okhotsk, mainly affecting
pelagic fish. For example, by 1994, the biomass of the northern
stock of walleye pollock was estimated to have decreased by 5
million tonnes, whereas the Okhotsk –Ayan herring population
was thought to have increased by some 1.0 –1.5 million tonnes
(Shuntov and Dulepova, 1997). In the southern Sea of Okhotsk,
pollock and sardine biomass together declined by 5 million
tonnes, simultaneously with the decrease in mesopelagic fish.
Hence, the total fish biomass of the Sea of Okhotsk in the
mid-1990s reduced by not ,10 million tonnes.
In the 2000s, the total biomass of nekton in the northern Sea of
Okhotsk began to increase again, with both walleye pollock and
herring biomass rising (Figure 7; Dulepova and Merzlyakov,
2007). Walleye pollock were abundant from 2004 to 2006 but
thereafter declined. Even so, the total biomass of walleye pollock
in the northern Sea of Okhotsk was recently estimated to be
.10 million tonnes, and in 2010, the total pollock catch was .1
million tonnes. The southern Sea of Okhotsk stock of walleye
pollock is currently growing rapidly off the southern Kuril
Islands, recently reaching some 300 000– 400 000 t, whereas in
the late 1990s and early 2000s, it did not exceed some tens of thousands of tonnes. From 2003 to 2009, large biomasses of mainly
young walleye pollock have been recorded in the area
(Ovsyannikova et al., 2008). Hence, by the end of the first
decade of this century, the biomass of Sea of Okhotsk pollock
had increased substantially, but not to the level of the 1980s.
The northern Okhotsk walleye pollock stock seems since at least
2006, to be delivering less in the form of recruiting year classes,
a sign of decreasing resource, whereas the southern Okhotsk
stock still continues to increase.
The long-term dynamics of Pacific herring in the Sea of
Okhotsk can be deduced from its annual catch (Naumenko,
2007). From 1956 to 1975 (cold years), Okhotsk herring catches
peaked, but from 1976 to 1995 (warm years), the annual catch
of all Far Eastern populations of herring (mostly in the Sea of
Okhotsk) decreased threefold. Then, in the cold period of 1996–
2004, the annual catch of Okhotsk herring increased to some
290 000 t. According to the latest data, the biomass of Sea of
Okhotsk herring from 2004 to 2009 continued to increase, from
1 million to 1.9 million tonnes (Gorbatenko et al., 2010). Now,
of all the epipelagic fish in the northern Sea of Okhotsk, the
biomass of herring accounts for some 20 – 22% of the total, close
to its long-term annual average.
It is often difficult to determine long-term stock changes in demersal fish communities exposed to heavy commercial fishing.
However, the trawl surveys carried out in the early and late
1980s on the highly productive West Kamchatka shelf revealed
rapid growth of the resources from 0.8 to 1.4 million tonnes
(Dulepova and Boretz, 1994; Savin et al., 2011). Further, groundfish resources declined from the start of the new century, from 1.4
to 0.6 million tonnes, then expanded again in the mid-2000s, when
the biomass reached 1.5 million tonnes. The recent growth was
mainly of flatfish (60%), cottids (20%), and demersal gadoids
(15%). By the end of the recent decade (2009–2010), however,
the biomass of demersal fish was again decreasing (Savin et al.,
2011). In the southern Sea of Okhotsk, along the eastern
Sakhalin and southern Kuril coasts, the situation is similar to
that in the north. The dominant fish by biomass were gadoids, flatfish, and cottids. Overall, the biomass of demersal fish off the
southern Kuril Islands peaked in the late 1980s and early 1990s,
but then declined, followed recently by an increase again. The
same dynamic has been recorded for the eastern Sakhalin waters
(Kim, 2007).
Sea of Okhotsk ecosystem response to climate, with emphasis on fish populations
The various fish species in the Sea of Okhotsk have clear
dynamics, sometimes with opposing interannual variations in
biomass. In recent years, the abundance of the dominant
gadoids, flatfish, and atka mackerel off the south Kuril Islands
area has risen. Overall, there has been a steady rise in walleye
pollock and demersal fish resources there, but few notable recruitments of pollock and hence a decreasing total demersal fish
biomass in the northern Sea for the most recent 5-year period.
Concluding remarks
The influence of climatic –oceanographic variability on the dynamics of biological communities in the Sea of Okhotsk is
still poorly understood. In warm years, spring is earlier and replacement of coastal waters is more rapid. Early blooms of phytoplankton and zooplankton, and hence earlier fish spawning, and
greater survival of the early life stages, are likely links in the
same chain of events. However, this is only a hypothesis, because
research activity in the Sea of Okhotsk still needs to be expanded.
Attempts to allocate the main biotic or abiotic factors that affect
the survival of Sea of Okhotsk walleye pollock have been unsuccessful (Vasilkov and Glebova, 1984; Smirnov, 2005). Apparently
though, earlier ice melting in the Sea of Okhotsk favours
Okhotsk herring (Zavernin, 1972). Research has revealed that zooplankton biomass increases under a moderate but continuous
bloom of phytoplankton, which is the characteristic of colder
spring seasons (Nadtochy and Zuenko, 2000). However, almost
30 years of observations in the Sea of Okhotsk have revealed
that the total biomass of zooplankton has remained large,
despite fluctuations in the biomass of plankton-eating fish
(Dulepova, 2005).
The variability in some climatic and oceanic variables is obviously key in determining the dynamics of fish resources in the
Sea of Okhotsk. The recent decade (2000s) has yielded a reversal
of the downward trends in fish biomass, but the next decade is
expected to be cooler and may interrupt this process, especially
in the northern Sea. It is assumed that a transition from a cold
to a warm ocean regime followed by a sufficient duration of
warm conditions favours most of the fish stocks. Undoubtedly,
though, the information provided here is insufficient to predict
future changes in the fish community structure of the Sea of
Okhotsk. The natural mechanisms causing large-scale changes in
the dynamics of fish communities still remain poorly studied, although based on the current rate of data accumulation on the ecosystem of the Sea of Okhotsk, notable progress should be made
soon.
Acknowledgements
I thank Dr K. Drinkwater for his critical readings and many helpful
comments on an earlier draft.
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