水研センター研報，第27号，43−94，平成21年
Bull. Fish. Res. Agen. No. 27, 43‑94, 2009

博士号論文

The effect of sea ice on the oceanographic structure and phytoplankton
biomass in the Nemuro Strait, a nursery area of salmon ＊1
Ikutaro SHIMIZU＊2

Abstract：Coastal waters of the Okhotsk Sea and the Nemuro Strait of east Hokkaido areimportant nursery grounds for salmon fry released from hatcheries to the rivers of these
districts. As an annual event in these coastal regions, sea ice drifting from the north of
the Okhotsk Sea covers the area in winter and phytoplankton blooms occur following theretreat of sea ice in the spring. Spring phytoplankton blooms are thought to be important
precedents to zooplankton production which supports the growth of salmon fry during theirfirst oceanic stage. With establishment of a better management scheme of salmon resources
as the ultimate goal, the objective of this study is to fill the gaps in our knowledge aboutmechanisms through which sea ice affects hydrography and phytoplankton production in
the Nemuro Strait.
Through analyzing water temperature, salinity, nutrients（nitrate, phosphate and silicate）
and chlorophyll collected during January and July in 1998‑2002, the water under the ice
cover was revealed as being nutrient‑rich. As judged by nutrient ratios（N：P=11 2 and
Si：P=23）, the origin of the water under sea ice was considered to be Okhotsk Intermediate Cold Water which entered the Nemuro Strait before sea ice encroached. Nutrient‑poor
Soya Warm Water was found in the Nemuro Strait during summer only.
Intensive study in 1998 revealed that massive phytoplankton blooms occurred after sea
ice retreat in late March. Size fractionated chlorophyll data showed the majority of phytoplankton was composed of >10 μm size, but small size fractions（2‑10 μm and <2 μm）
increased as the season progressed. Zooplankton abundance peaked following the spring
phytoplankton bloom in 2001, and the most dominant component was Copepoda, followed by
Sagittoida or jellyfishes.
Massive phytoplankton blooms occurred after sea ice retreat in most years with the exception of 1999 when sea ice retreat was delayed to late April. The major mechanism of theonset of the phytoplankton blooms is the water column stratification due to ice melt combined with the development of a deep mixing layer within the critical depth. A moderate
phytoplankton bloom in 1999 was attributed to a shallow mixing depth which limited nutrient availability for phytoplankton.
The present results regarding the effect of the timing of sea ice retreat on the magnitudeof phytoplankton blooms are opposite to those predicted from Hunts hypothesis for
the Bering Sea. This difference in the consequences of sea ice-phytoplankton bloom relationships between the Nemuro Strait and the Bering Sea may be reconciled if one takes into account of dissimilar daily solar radiation at equivalent seasons which would have a dissimilareffect on sea ice melting. The Bering Sea is located at higher latitudes than the Nemuro
Strait, hence daily solar radiation in the former are much less than in the latter, and in theformer early retreat of sea ice is not associated with meltwater leading to water column
stratification.
2008年12月15日受理（Received on December 15, 2008）＊1
北海道大学審査学位論文（Ph.D. Thesis, Hokkaido University）（掲載に際し投稿規定に沿って一部修正した）＊2
中央水産研究所水産経済部 〒236‑8648 神奈川県横浜市金沢区福浦2‑12‑4（Fisheries Economy Division, National Research Institute of Fisheries Science, 2-12-4, Fukuura, Kanazawa, Yokohama 236-8648, Japan）

Ikutaro SHIMIZU

Analysis of historical data showed that accumulated ice concentration has both positive
and negative effects on fisheries resources. The return rates（percentage of mature salmon
returning relative to the total number of fry released）of chum and pink salmon were negatively correlated with accumulated ice concentrations. Return rates of chum salmon werehigh in the years when their fry were released well after the sea ice retreat. Provision offood for fry before release improved the return rate of mature chum salmon, but the rate
has been variable year to year due to reasons which are unknown at present.
To improve the return rate of mature salmon, future study should be directed to ward
evaluating the change of food environments for salmon juveniles（zooplankton）in relation
to the force of sea ice, especially during periods of sea ice retreat. Given that migration of
salmon juveniles from the coastal waters into the Okhotsk Sea represents the first stage of
their oceanic life, zooplankton production cycles in the Okhotsk Sea should be evaluated inorder to diagnose feeding conditions of salmon juveniles, which affect their subsequent survival.
Key Words：sea ice, phytoplankton, salmon, Nemuro Strait, Okhotsk Sea

Contentsdepth
4 2 6 Nutrients
4 2 7 Chlorophyll

Abstract

4 3 Discussion

Chapter 1. Introduction
Chapter 2. Oceanographic conditions in the Nemuro
Strait during the ice season
2 1 Materials and methods

5 1 Sea ice and fishery production along the
Okhotsk coast of Hokkaido
5 2 The return of salmon to the Okhotsk Sea coast

2 2 Resultsof Japan

2 2 1 Water mass
2 2 2 Phytoplankton blooms and zooplankton

5 3 Sea ice and the return of mature salmon
5 4 Sea ice and artificial salmon enhancement

2 3 Discussion
Chapter 3. Chlorophyll

Chapter 5. Historical data analysis

and nutrient concentrationsin the Nemuro Strait after sea ice retreat
3 1 Materials and methods

Chapter 6. General Discussion
Acknowledgments
References

3 2 Results
Chapter 1. Introduction

3 2 1 Water temperature, salinity and nutrients
3 2 2 Phytoplankton biomass and its size
composition
3 3 Discussion
Chapter 4. Between-year variations in chlorophyll

Sea ice is distributed in high latitude seas,
typically Antarctic and Arctic waters, covering
an area of up to 7 % of the surface of the earth inand nutrients in the Nemuro Strait after

winter（Comiso

1990）. Our knowledge about

sea ice retreat

the impacts of sea ice on biological process in the

4 1 Materials and methods

pelagic ecosystem has increased rapidly in both the

4 2 Results

Arctic（Wassman2006）and Antarctic Oceans

4 2 1 Dynamics of sea ice（Smith and Nelson, 1985; Garrison

1987;

4 2 2 Light conditions

Sakshaug and Skjoidal, 1989; Comiso

1990;

4 2 3 Water temperature

Lancelot4 2 4 Salinity and density
4 2 5 Mixed layer, euphotic zone and critical

1991; Riebesell

1991; Bianchi

1992; Schloss and Estrada, 1994; McMinn, 1996;
Dunbar, 1998）.

Effect of sea ice on the oceanographic structure in the Nemuro Strait
In the northern North Pacific Ocean, sea ice

was paid to the incidence of phytoplankton blooms

occurs during part of the year （winter） in theduring the ice retreat season（Chapter 3）, and theBering and Okhotsk Seas. Intensive studies havemechanism determining the year‑to‑year variations

been made on the effect of sea ice on the productionin the magnitude of phytoplankton blooms wascycle in the Bering Sea（Alexander, 1980; Alexander

explored（Chapter 4）. To link these data to salmon

and Niebauer, 1981; Schandelmeier and Alexander,

resources, historical data on sea ice coverage,

1981; Niebauer and Alexander, 1985; Niebauer

hatchery production, and fishery yields of salmon

1990; Niebauer, 1998; Huntand other commercial species were summarized, 2002）. While comparable studies in theand analyzed（Chapter 5）. As a synthesis, future

Okhotsk Sea are still lacking, results from other ice

study needed for the establishment of a better

covered pelagic ecosystems show that ice meltwater

management scheme of salmon resources throughenhances the density stratification during ice retreat

adjusting the activities of hatcheries is proposedin the spring and facilitates phytoplankton blooms（Chapter 6）.

Jr.

1995; Stabeno（Alexander and Niebauer, 1981）. Biological processes
other than phytoplankton in sea ice covered areas
were reviewed by Diekmann and Hellmer（2003）.

Chapter 2. Oceanographic Conditions in the
Nemuro Strait during the Ice Season

As an area which is covered by ice in winter in
the North Pacific, the Nemuro Strait located in the

Sea ice begins to form in coastal waters of thesouthern Okhotsk Sea is of special interest in thisnorthern Okhotsk Sea off Magadan province instudy because it is an area in which hatchery-raised

Russia early in the winter. The southward advancefry are released to rivers flowing into the strait inof sea ice is affected by a seasonal wind in winterorder to improve management of salmon resources.

and by the East Sakhalin Current of which volume

Chum salmon（

transport reaches a maximum in December‑January）fry are releasedin the spring, spend their early life in the coastal（Itoh and Ohshima, 2000; Ohshima

2002; Itowaters of the Nemuro Strait, and then migrate into2003）. Sea ice advances southward and reaches

the Okhotsk Sea to spend their first summer. Chumthe coast of the Okhotsk region of east Hokkaido.

salmon move to the western subarctic gyre for their

The coastal waters of the Okhotsk region and thefirst winter, then they migrate between summer

Nemuro Strait of east Hokkaido are covered with sea

feeding grounds in the Bering Sea and wintering

ice between late January and middle or late Marchgrounds in the Alaskan Gyre for a period of up toof every year. As a hydrographical feature of thefour years, and finally return to their mother rivers

southwestern Okhotsk Sea and the Nemuro Strait,

to spawn（Ueno and Ishida, 1996; Urawa, 2000）.

these regions are under the influence of the Soya

A historical record of salmon hatcheries in thisWarm Current which flows along the Okhotsk Sea

district which goes back to the 1950s shows a strong

coast of Hokkaido（Aota, 1979; Aota and Takizawa,

relationship between the seasonal occurrence of1994）. Thus, the hydrography of the Nemuro Strait

sea ice and the return of mature salmon to motheris complicated by the effect of not only sea ice but

rivers as discussed in this study（see Chapter 5）.

also by the warm current.

The objective of this study is to fill gaps in

In this chapter, the properties of water masses

our knowledge about the effects of sea ice on theprevailing in the Nemuro Strait are discussed withoceanographic conditions and phytoplankton growth

special reference to the winter ice season.

through the collection and analysis of field datain the Nemuro Strait. As the first attempt in the

2.1 Materials and methods

Nemuro Strait, field surveys were carried out during

Oceanographic observations were conducted at

ice-covered periods using icebreaking vessels and insix stations on a transect（0 5, 1, 2, 4, 6 and 8 km

so doing seasonal sequences of water mass, nutrientfrom coast） off Rausu in the northern Nemuro

regime, chlorophylland zooplankton abundance

Strait in January（before sea ice reaches the coasts）

data were collected（Chapter 2）. Special attention

through July（no ice）in 1998-2002（Fig. 2‑1）. An

Ikutaro SHIMIZU

Fig. 2‑1. Nemuro Strait（arrowed）as a marginal seaof the southwestern Okhotsk Sea（ ）, and MODIS
satellite image of sea ice extension to the Okhotsk
Sea and the Nemuro Strait in winter（）. TheEast Sakhalin Current and Soya Warm Current aresuperimposed in the top panel.

extensive sampling program was conducted in 2002

with a Memory STD（Alec Electronic Co., Ltd.）.

（9 times including 20 March; 9, 16 and 23 April; 8Transparency was measured with a Secchi disk.

and 22 May; 5 and 26 June; and 17 July）during theWater samples were routinely collected from 9

period in which there occurred an ice edge bloom

depths in the top 200 m（0, 10, 20, 30, 50, 75, 100,and an open water bloom of phytoplankton. Under

150 and 200 m）with a Rigo‑B sampler（Rigosha

sea ice observations were made on board the Japan

Co., Ltd.）. At the stations shallower than 200 m,

Coast Guard icebreakeron 4 April in 1999.

the number of sampling depths was reduced

Water temperature and salinity were measured

accordingly. Sampled water was filtered throughEffect of sea ice on the oceanographic structure in the Nemuro Strait
Whatman GF/F filters. Nutrient（nitrate, phosphateand Brandt, 1997）and assumed to be an index ofand silicate）concentrations were analyzed by using

zooplankton abundance in the water column ina Hitachi 100‑60 spectrophotometer（Hitachi Co.,

this study. The entire observation period rangedLtd.）and Flow Injection Analyzer 5020（Tecater

between late January and late July in 2001.

Co., Ltd.）. Chlorophyll

was extracted fromfilters with methanol solutions（Otsuki, 1987）

2.2 Results

and quantified with Turner AU‑10 fluorometer（TurnerDesign Co., Ltd.）.

2.2.1 Water mass

Continuous monitoring of water temperature,Continuous temperature and salinity records atsalinity, dissolved oxygen concentration, current

15 m depth at the station off Shibetsu showed that

direction and speed in real time was accomplishedalthough salinity was >33 psu from mid February

through use of the Coastal Area information

to the end of March, it decreased rapidly to 32 psu

Network System（CANS）, which was located 5 km

in early April and this low salinity was maintained

off Shibetsu in the southern Nemuro Strait at a

until the end of April（Fig. 2‑3）. It is noted that thedepth of 15 m（Fig. 2‑2）. A sediment trap bottle

station was ice-covered from early February to late（diameter; 15 cm, length; 40 cm, filled with 8 %March. In the northern Nemuro Strait, temperatureformalin‑40 psu salted water to prevent advection

was about ‑2℃ （close to the freezing point ofof the fixative from the bottle）was mounted on theseawater）and salinity was around 33 psu during

CANS to collect zooplankton, which was recovered

the ice-covered season（Figs. 2‑4 and 2‑5）. Salinity

seven times at two week intervals. Zooplankton

decreased suddenly to 32 psu and temperaturethus collected are so called swimmers （cf. Seiler

increased to 0℃ in early April when the sea ice

Fig. 2‑2. Sampling stations off Rausu in the northern Nemuro Strait and thelocation of CANS buoy off Shibetsu in the southern Nemuro Strait.

Ikutaro SHIMIZU

Fig. 2‑3. Continuous records of temperature and salinity measured with a CTD fixed at 15 m
depth at CANS buoy station located 5 km off Shibetsu during February‑July 2001.

Fig. 2‑4. Vertical profile of water temperature at the station 2（1 km off Rausu）in
the Nemuro Strait during March‑July 2002.

Effect of sea ice on the oceanographic structure in the Nemuro Strait

Fig. 2‑5. Vertical profile of salinity at the station 2（1 km off Rausu）in the Nemuro
Strait during March-July 2002.

Fig. 2‑6. Schematic presentation of seasonal sequences of water masses in the Nemuro
Strait and associated changes in nutrient regimes and chlorophyll in 1998 and 1999.
See text for details.

retreated from Rausu. Temperatures continued

l‑1, and during phytoplankton blooms concentrations

to rise in early May. Phytoplankton blooms were

were 2 3 μmol N, 0 5 μmol P, 9 1 μmol Si and 7 1 μobserved late April when salinity decreased. Waterg chll‑1（Fig. 2‑6）.

temperatures of less than 0℃ were seen until early
April, after which they increased gradually and2.2.2 Phytoplankton blooms and zooplankton
Phytoplankton blooms occurred at the ice edge,

reaching 1‑7℃ in mid June.
were measured

and then in open water at the Station 2 off Rausu

in 1998 and 1999. Mean nitrate‑N, phosphorus-P,

in 2002（Fig. 2‑7）. The ice edge bloom was alreadysilicate‑Si and chlorophyll

underway when the present sampling program was

The nutrients and chlorophyll

concentrations in

the top 10 m across the six stations were 6 8 μ

initiated on 20 March 2002, and the chlorophyll

mol N, 0 9 μmol P, 18 3 μmol Si and 0 2 μg chl

maximum（8‑9 μg chl−1

l , respectively, before the sea ice area was reached.

depth deepened to 20‑25 m depth on 9 April, andUnder the sea ice, respective figures were 11 2 μthen collapsed on 16 April. On 23 April, moderate

mol N, 1 0 μmol P, 23 0 μmol Si and 0 4 μg chl

concentrations of chlorophyll （3‑4 μg chll‑1）observed at 5‑10 m

l ‑1）

Ikutaro SHIMIZU

Fig. 2‑7. Vertical profile of chlorophyll
Nemuro Strait during March‑July 2002.

at the station 2（1 km off Rausu）in theFig. 2‑8. Zooplankton abundance and composition at CANS buoy station located
5 km off Shibetsu in the Nemuro Strait during January‑July 2001.

Effect of sea ice on the oceanographic structure in the Nemuro Strait
were detected in the top 10 m（open sea bloom）,

objects（Aota, 1984; Aota and Matsuyama, 1987）.

but this was not observed in the next sampling on 8

The present results of a rapid increase in water

May（Fig. 2‑7）.

temperature and salinity in mid June off Shibetsu

Zooplankton samples collected in the sediment
trap located at a depth of 15 m were composed of（Fig. 2‑3）are indicative of inflow of the Soya Warm
Water into the Nemuro Strait.

Copepoda, Ampipoda, other Crustacea, Sagittoidea,

The ratios of nutrients（N:P and Si:P）may bejellyfish and others （Fig. 2‑8）. Because of theused as a marker of water masses occurring in thenature of the collection method（sediment trap）, theNemuro Strait（Table 2‑1）. From this perspective,

results cannot be regarded as quantitative, but may

the Soya Warm Water and the Surface Low Salinity

still reflect their abundance in the water column.

Water are characterized by low N:P ratio and low

Numbers of zooplankton were low from January to

Si:P ratio, respectively. According to Aota（1979）

March, increasing gradually during the ice season,and Aota and Takizawa（1994）, the Soya Warmand forming a peak in early May. After the peak,

Water is usually replaced with the Surface Low

zooplankton numbers decreased toward the end

Salinity Water in late autumn before sea ice reachesof the sampling program of this year （15 July

the Okhotsk coast of Hokkaido in the following year.

2001）. Throughout the samples, the most dominant

The Soya Warm Current is nutrient poor（0 4 μ

zooplankton taxon was Copepoda, followed bymol N, 0 2 μmol P and 3 1 μmol Si; Maita and Toya,

Sagittoidea or jellyfish（Fig. 2‑8）.

1986）. Itoh and Ohshima（2000）suggested thatthe sea level at the Hokkaido coast of the Okhotsk

2.3 Discussion

Sea reaches its maximum in December due to the

A rapid increase in water temperature from 0

intrusion of cold East Sakhalin Current Water.

to 8℃ was seen in mid June 2001, accompanied by

From these seasonal sequences, it is clear thatmoderate increase in salinity from 33 6 to 34 psu

the nutrient-rich Intermediate Cold Water of the（Fig. 2‑3）. The water mass of salinity >33 6 psu

Okhotsk Sea（12 9 μmol N, 1 4 μmol P and 23 5 μobserved in the Okhotsk coast of Hokkaido has been

mol Si; Maita and Toya, 1986）reaches the Okhotsk

identified as the Soya Warm Water（Aota, 1979）.

coast of Hokkaido before the sea ice does, and

The main factor affecting the transport volume ofremains under the sea ice during the winter season

the Soya Warm Water is the difference in water（Fig. 2‑6）.

levels among the northern Japan Sea, Soya Strait

The open water bloom of phytoplankton（Fig.

and Shiretoko Peninsula（Aota, 1984）. The current2‑7）was observed in the top 10 m during mid tofrom the Japan Sea coast of Hokkaido to the Nemuro

late April 2002（Fig. 2‑7）when salinity decreased

Strait has been evaluated by tracking floating

suddenly to <32 psu （Fig. 2‑5）. According toTable 2‑1. Mean ratios of nutrients（N/P and Si/P）and water masses occurred in the
Nemuro Strait.

Ikutaro SHIMIZU
Alexander and Niebauer （1981）, melting ice at

ice retreats. Ice algae may act as a seed population

the ice edge facilitates phytoplankton blooms infor a spring phytoplankton bloom after ice melt

the Bering Sea. This is because lowered salinities（Taguchi, 1993）, but the relationship between sea ice

caused by melting ice increase stability of the waterand phytoplankton growth in the Okhotsk Sea hascolumn. Niebauer（1990）suggested also thatnot yet been well established. Kudo and Matsunaga

stratification in the ice edge blooms resulted from（1999）have reported that the development of asalinity gradients, while in open water, stratification

spring bloom in Funka Bay located in southwestern

resulted from thermal gradients. Veth

Hokkaido is highly dependent on the timing of the（1992）

considered that the meltwater lens produced during

intrusion of the Oyashio water into the bay.

the spring ice retreat in the marginal ice zone of the

In this chapter, possible factors controlling thenorthwestern Weddell Sea was the main contributor

incidence of spring phytoplankton blooms in coastal

to the stability of the water column when surface

waters in the northern Nemuro Strait are discussed

temperatures were low. Such sea ice-phytoplankton

based on chlorophyll

bloom interactions have also been reported by otherperiod mid March‑July in 1998.

researchers（Stabeno

data collected during the

1998; Smith and Nelson,

1986; Sakshaug and Skjoldal, 1989）. All these results

3.1 Material and methods

strongly suggest that the mechanism determining

Starting before sea ice retreat on 21‑23 March

the open water blooms of phytoplankton observedin 1998（Sea Ice Information Center, First Regionalin the Nemuro Strait（Fig. 2‑7）is similar to those

Maritime Safety Headquarters）, samplings werebeing reported in the Bering and Weddell Seas.

carried out at 6 stations on a transect on 17 March,

All of these results of water mass and associatednutrient regimes and chlorophyll

14 April, 12 May, 9 June and 8 July in 1998. The

concentrations

six stations are 0 5（Stn.1）, 1（Stn.2）, 2（Stn.3）, 4

in the Nemuro Strait are summarized in Fig. 2‑6.

（Stn.4）, 6（Stn.5）and 8 km（Stn.6）distant from

While the abundance and composition of zooplankton

Matsunori located 4 km southwest of the town of

was investigated only in 2001（Fig. 2‑8）
, its seasonal

Rausu on the Shiretoko peninsula（all stations fallvariation pattern characterized by an abundancein an area delineated by 43 56 2N to 43 57 7N andor biomass peak in spring is a commonly reported（Fig. 3‑1）.
145 14 8E to 145 12 5E）

feature from the Oyashio region（Seki and Shimizu,
1997; Saito

2002; Ikeda, in press）. Ato

Vertical profiles of temperature and salinity wereobtained with an Alec Memory STD AST-200.

its peak, zooplankton abundance or biomass isTransparency was determined with a Secchi disclargely composed of copepods whose spawning or（SDR）at the stations where observations weredevelopment is accelerated by the phytoplankton

carried out during the daytime, and the extinction

blooms.

coefficient k（m‑1）was computed from an equation
where k = 1 7/SDR（Poole and Atkins, 1929）. The

Chapter 3. Chlorophyll a and Nutrient

depth of the euphotic layer（EL）was defined as theConcentrations in the Nemuro Strait after

depth where 1 % of the surface light penetrated（ELSea Ice Retreat= ‑ln（0 01）/k）.
Seawater samples for measuring chlorophyll

The northern Nemuro Strait is covered withand nutrient concentrations were collected fromsea ice from January to March or April. Sea ice

the surface, and at 5, 10, 20, 30, 50, 75, 100, 150 andhas been thought to be a cause of high primary

200 m depths using a Rigo‑B water sampler. For

production in spring season in this region（Aota,

chlorophyll , 200 ml to 250 ml aliquots of seawater

1986; Kudo, 1996）. Spring phytoplankton blooms

were filtered through a 47 mm Whatman GF/F

have been confirmed through shipboard chlorophyll

glass fiber filter. Chlorophyllmeasurements in coastal waters of the Okhotsk
1989;

fractionation, additional seawater samples（50 ml）

1989）, especially in April when sea

were filtered through 25 mm Nuclepore filters of

Sea （Watanabe, 1989; Nishiyama,
Shimizu

was size-fractionatedat all stations during April and July. For the sizeEffect of sea ice on the oceanographic structure in the Nemuro Strait

Fig. 3‑1. Sampling stations off Rausu in the northern
Nemuro Strait during March‑July 1998.

10 μm, 2 μm and 0 2 μm pore size and the vacuum

psu on 12 May, from 32 02 to 33 12 psu on 9 June,pressure applied was less than 100 mm Hg（Joint

and from 31 69 to 33 56 psu on 8 July（Fig. 3‑3）.

and Pomroy, 1983）. Chlorophyll

concentrations

Temperature and salinity were vertically uniform

were measured with the same method described inon 17 March. Low salinity water（<32 0 psu）was

Chapter 2 1

observed in the upper 30 m with a weak salinity
gradient below 40 m depth on 14 April. A weak

3.2 Results

temperature gradient was observed in the upper
30 m water column and saline water（>33 0 psu）

3.2.1 Water temperature, salinity and nutrients

occurred below about 80 m depth during the period

Temporal variations of water temperature andof 12 May ‑ 9 June. A steep temperature gradientsalinity were generally well synchronized at thewas seen in the upper 10 m and saline water（>33 4

six stations during the observation periods. Acrosspsu）appeared below about 40 m depth at Stns. 1 tostations and depths, water temperature varied from5, and below about 70 m depth at Stn.6 on 8 July.

‑1 66 to ‑0 21℃ on 17 March, from ‑1 26 to 0 16℃

The Okhotsk Intermediate Cold Water

on 14 April, from 0 07 to 4 66℃ on 12 May, from 0 06

（temperature; ‑1 5 to 1 5℃, salinity; <33 0 psu）

to 9 32℃ on 9 June, and from 4 50 to 14 08℃ on 8

defined by Maita（1986）was observed below 80 m

July（Fig. 3‑2）
. Across stations and depths, salinity

depth in March, in the upper 100 m depth in April

varied from 32 07 to 32 62 psu on 17 March, fromand between 30 and 80 m depth in May.

31 90 to 32 86 psu on 14 April, from 32 05 to 33 16

temperature and salinity between 40 and 80 m

The

Ikutaro SHIMIZU

Fig. 3‑2. Vertical profiles of temperature（℃）at Stns. 1‑6 off Rausu in the Nemuro Strait
during March‑July 1998.

Fig. 3‑3. Vertical profiles of salinity（psu）at Stns. 1‑6 off Rausu in the Nemuro Strait
during March‑July 1998.

Effect of sea ice on the oceanographic structure in the Nemuro Strait
depth in July were 6 5‑8 3℃ and 33 4‑33 6 psu,

In the top 30 m, a rapid decline in nutrients wasrespectively, which was considered to be indicative

observed during March and April；e.g. nitrate fromof its origin in the Soya Warm Water（temperature;>7 μmol（all stations）to <3 μmol（Stns. 2‑5）（Fig.

7 3‑13℃ and salinity; 33 6‑34 0 psu, cf. Maita3‑4）, phosphate from 1 3 μmol to <0 7 μmol（Stn.4

1986）. The depth of the surface mixed layer was

to Stn. 6）or to <1 0 μmol（Stns. 1‑3）
（Fig. 3‑5）,

defined as the depth where the vertical gradientand silicate from >8 μmol（Stns. 4‑6）to <8 μmolof sigma-t reached its maximum. The depth of the（Stns. 4‑6）
（Fig. 3‑6）. Silicate concentrations insurface mixed layer at six stations was 79‑93 m inthe upper 20 m depth at Stns. 1‑3 were very highMarch, 24‑32 m in April, 26‑31 m in May, 7‑16 m（11‑17 μmol）but decreased to 8‑13 μmol in April.

in June and 7‑10 m in July, thus showing a trend of

Nitrate was >25 μmol throughout the water column

decreasing depth as the season progressed. Secchi

on June 9 when the water temperature was uniform

disk readings at six stations were 13‑21 m in March,

below about 40 m depth.

6‑10 m in April, 11‑13 m in May, 7 7‑10 8 m in

The mean concentrations of nitrate, phosphate

June, and 7‑9 m in July. Hence, the depths of theand silicate in the euphotic layer during each montheuphotic layer were calculated to be 35 2‑56 9 m inwere as follows; 8 82 ± 0 84 μmol, 1 28 ± 0 08 μmol

March, 16 3‑27 1 m in April, 29 8‑35 2 m in May,and 11 19 ± 2 93 μmol, respectively, in March; 2 42

20 9‑29 3 m in June, and 19 0‑24 4 m in July. The±1 09 μmol, 0 53±0 11 μmol and 9 05±3 50 μmol,

depth of the surface mixed layer greatly exceeded

respectively, in April；5 38±2 40 μmol, 0 63±0 23 μthat of the euphotic layer in March and was slightly

mol and 12 92±3 36 μmol, respectively, in May; 5 14

deeper in April, but in May through July the±4 49 μmol, 0 30±0 31 μmol and 4 80±5 13 μmol,

euphotic layer was deeper than the surface mixed

respectively, in June; and 0 66 ± 0 41 μmol, 0 10 ±layer.

0 07 μmol and 4 27±1 63 μmol, respectively, in July.

Fig. 3‑4. Vertical profiles of nitrate concentration（μmol）at Stns. 1‑6 off Rausu in the
Nemuro Strait during March‑July 1998.

Ikutaro SHIMIZU

Fig. 3‑5. Vertical profiles of phosphate concentration（μmol）at Stns. 1‑6 off Rausu in
the Nemuro Strait during March‑July 1998.

Fig. 3‑6. Vertical profiles of silicate concentration（μmol）at Stns. 1‑6 off Rausu in the
Nemuro Strait during March‑July 1998.

Effect of sea ice on the oceanographic structure in the Nemuro Strait
3.2.2 Phytoplankton biomass and its size

. In June, the medium and small size fractions

composition
Chlorophyll

were 24‑28 % and 15‑19 %, respectively, of the total

concentration began to increasechlorophyll. The large size fraction increasedin the surface layer in March at Stn.6 close toagain from June to July and composed of 59‑79 % of

Kunashiri Island（Fig. 3‑7）
. Chlorophyll

the total chlorophyll .

increased

rapidly at 10‑20 m depth intervals at Stns. 2‑6in April（maximum; <12 μg l ‑1）, then decreased
suddenly in May.

Chlorophyll

3.3 Discussion

returned to

The depth of the Nemuro Strait becomes rapidly

previous levels at 5‑10 m depths at Stns. 3‑6 in Juneshallow in either directions towards Shiretokowith some higher concentrations extending to >30 m

peninsula or Kunashiri Island（Fig. 3‑1）. Given thisdepth. The mean chlorophyll

concentrations in

topographical feature, wind-driven upwelling may

the euphotic layer over the six stations were 1 45±

be a mechanism to transport nutrients to surface‑1‑1

0 85 μg l in March, 6 97±2 99 μg l in April, 0 66

layers and upwelling is considered to occur under±0 25 μg l‑1 in May, 1 48±0 85 μg l‑1 in June, andthe influence of the southwest wind in the Nemuro‑1

1 26±0 39 μg l in July.
The size compositions of chlorophyll

Strait. According to Otani（1986）, such upwellingin

is induced by continuous winds of speeds exceeding

the euphotic layer were characterized by the

5 m s‑1. The number of days in which the southwest

predominance of the large fraction（>10 μm）whichwind velocity exceeded 5 m s‑1（daily mean of thecomposed of 94 to 95 % of the total chlorophyll .

previous ten days）during the present field survey（Fig. 3‑8）. In May, the proportion of the large

was zero in March, two days（7 3 ms‑1）in April, five

fraction decreased, and the medium （2‑10 μm）

days（7 3 ms‑1）in May, zero in June and one day（5 3and the small size fractions（<2 μm）increased toms‑1）in July（the data from Nemuro Meteological

25 and 33 %, respectively, of the total chlorophyll

Station）. These results, suggesting possible

Fig. 3‑7. Vertical profiles of chlorophyll concentration（μg l‑1）at Stns. 1‑6 off Rausu in
the Nemuro Strait during March‑July 1998.

Ikutaro SHIMIZU

Fig. 3‑8. Size fractionated chlorophyll in the euphotic layer atStns. 1‑6 off Rausu in the Nemuro Strait during April‑July 1998.

upwelling in May, are consistent with those of highsalinity increased with depth in the euphotic layer,

nutrients observed at 50‑200 m depths in May（Figs.

this suggests that nutrients were supplied from the

3‑4 through 3‑6）.

deep layer to the euphotic layer between May andTemperature was homogenous below 30 m

June.

depth and a weak salinity gradient was observed

Relationships between nitrate（N）, phosphate（P）

below 50 m depth in June（Fig. 3‑3）. Based on

and silicate（Si）in the euphotic layer were analyzed

temperature and salinity data, the density（sigma‑t）

by plotting the data（Fig. 3‑10）and calculatingof seawater was calculated（cf. Sudo, 1974）. Sigma-t

linear regression equations and correlation

below 50 m depth was 25 84‑26 70 in March,coefficients. Since the data for March did not show

25 96‑26 62 in April, 26 20‑26 72 in May, 26 41‑26 70

any significant relationships, means ± SD werein June, and 26 05‑26 60 in July（data not shown）.

computed as 8 82 ± 0 84 μmol for nitrate（N）, 1 28Judging from the narrowest range of the sigma‑t± 0 08 μmol for phosphate（P）, and 11 19 ± 2 93 μin June, a mixed layer might be formed below 50 m

mol for silicate（Si）（all n=32）. For months otherdepth bringing nutrients from the deeper layer tothan March, the following regression equations were

the intermediate layer in that month. Relationships

obtained;

between salinity and nitrate, phosphate and silicate

P=0 087N+0 315 （ =0 83, <0 001）,

concentrations within the euphotic layer are shown

P=0 019Si+0 354 （ =0 58, <0 05）in April（n=23）

Fig. 3‑9. Except for March and April when salinities

P=0 085N+0 166 （ =0 87, <0 001）,

were vertically uniform, nutrient concentrations

P=0 055Si‑0 090 （ =0 79, <0 001）in May（n=29）

increased with salinity during May and July. Since

P=0 064N‑0 033 （ =0 94, <0 001）,

Effect of sea ice on the oceanographic structure in the Nemuro Strait

Fig. 3‑9. Relationships between salinity andconcentrations of nitrate（a）, silicate（b）, andphosphate（c）in the euphotic layer off Rausu
in the Nemuro Strait during March-July 1998.

Ikutaro SHIMIZU

Fig. 3‑10. Relationships between nitrateand silicate （a）, nitrate and phosphate（b）, and silicate and phosphate（c）in theeuphotic layer off Rausu in the Nemuro
Strait during March‑July 1998.

Effect of sea ice on the oceanographic structure in the Nemuro Strait
P=0 053Si+0 042 （ =0 86, <0 001）in June（n=24）

Dcr =0 5I 0 /（I c × k）,

P=0 026N‑0 012 （ =0 59, <0 05）,,
where I 0 is the surface light intensity（Ein m‑2 h‑1）

P=0 136Si+0 008 （ =0 79, <0 001）in July（n=23）

k is the extinction coefficient（m‑1）and I c is the

The slopes of the regression lines denote the uptake

compensation light intensity（Ein m‑2 h ‑1）. The

ratio of nutrients by phytoplankton（Shiomoto andcalculation of k was based on k=1 7SDR（Pool andHashimoto, 1999）. Therefore, the ratios（Si:N:P）in

Atkins, 1929）. For I c , 0 023 Ein m‑2 h‑1 converted

the euphotic layer were 9:7:1 in March, 52:11:1 infrom 0 002 ly min ‑1 （Parsons

April, 18:12:1 in May, 19:16:1 in June and 39:7:1 in

substituted. Data on total daily sunlight intensity

July. The nutrient Si:N:P ratios below the euphotic

and day length were those collected by the Nemuro

layer were 10:7:1 in March, 14:7:1 in April, 21:11:1 in

Meteological Station. Energy-heat conversion factors

May, 18:16:1 in June and 31:10:1 in July. Comparing

were: 1 joule=107 erg, 1 g cal=4 185×107 erg, and 1

the uptake ratios by phytoplankton in the euphotic

einstein =52×103 g cal.

1984） waslayer, it was suggested that the phytoplankton

The daily values averaged over ten days: I 0（Eincommunity in April took Si at a higher rate thanm‑2 h‑1）were 0 233 in March, 0 270 in April, 0 334 in

the rate supplied below the euphotic layer. The

May, 0 333 in June and 0 237 in July. I 0 increased

Si and N relationship in April was expressed asfrom March to May. Calculated D cr values were

Si=2 43N+3 19（ =0 76, <0 001, n=23）, indicating

48‑56 m in March, 27‑32 m in April, 54‑63 m inthat Si was removed 2 4 times faster than N. The

May, 38‑44 m in June and 72‑85 m in July. During

positive intercept（3 19）of the regression equation

the present study, the depths of the surface mixedmeans that silicate is left when nitrate is usedlayer were 24‑32 m in April, 26‑28 m in May, 7‑10

up. In other words, phytoplankton growth in them in June and 8 m in July. Thus, comparing Dcr

Nemuro Strait is possibly limited by N. At the samewith the surface mixed layer（27‑32 m vs 24‑32 m）,

time, the present results suggest that the Okhotsk

the Sverdrup model predicts the occurrence of theIntermediate Cold Water containing high levels of Si

phytoplankton bloom to be in April in the northern

is of special importance for phytoplankton blooms in

Nemuro Strait was observed in this study, in other

the Nemuro Strait.

wards there is good agreement between model

The chlorophyllof the size fraction >10 μm

reached a maximum in mid April and contributed
94‑95 % of the total chlorophyll

biomass（Fig.

predictions and field observations.
Although moderate concentrations ofphytoplankton（1 65‑2 12 μg chll ‑1）and high3‑8）. According to a seasonal study of the size

concentration of nutrients were observed in mid

composition of chlorophyll

in Monbetsu harbor

March（Figs. 3‑4 through 3‑7）, the peak of theon the Okhotsk coast of Hokkaido, the proportion

phytoplankton bloom occurred in April ratherof the large size fraction （>10 μm） peaked inthan March.

March when the harbor was covered with sea iceof phytoplankton was influenced by water（Hamasaki

temperatures and decreased with decreasing1998）. The major component

The compensation light intensityof the large and medium size fractions may betemperature （Takahashi

diatoms and flagellates, and the small size fraction istemperature was below 0℃ in the entire water

believed to be composed of cyanobacteria and othercolumn in March, hence the compensation light

picophytoplankton（Odate, 1994）. Unfortunately, no

intensity of phytoplankton in March is suspected

microscopic observation on phytoplankton species

to have been low. The critical depth（Dcr）was

was made in this study to confirm this.

1996）. The

calculated for March using Ic values of ice algae

According to the Sverdrup model （1953）,（6 48 × 10‑4‑3 6 × 10‑3 Ein m‑2 h‑1 converted fromphytoplankton net production occurs in the water

0 18‑1 0 μEin m‑2 s‑1 ）（cf. Kirk, 1994）. The Dcr

column when the critical depth is greater than thein March was 305‑1 692 m, which is much deeper

depth of the surface mixed layer. To test this model,

than the depth of the surface mixed layer, indicating

the critical depth（Dcr ; m）was estimated from thethat the net gain in photosynthetic products

simplified equation（Takahashi（phytoplankton biomass） was negative in mid., 1996）;

Ikutaro SHIMIZU
March despite the presence of rich nutrients.

above（Chapter 3 1）. Sampling dates were 4 and 21
April, 18 May, 1 and 22 June and 13 July in 1999.

Chapter 4. Between-Year Variations in ChlorophyllDays of bright sunshine and solar radiationa and Nutrients in the Nemuro Strait

were calculated based on the data in the Monthly

after Sea Ice Retreat

Bulletins of the Nemuro Meteorological Station,
Sapporo District Meteorological Observatory. The

Sea ice begins to be formed in the coastal waters

data of the first and the last dates when sea iceof the northern Okhotsk Sea off Magadan province,

were recognized along the Okhotsk Sea coast were

Russia in early winter. Sea ice advances southward

provided by the Abashiri Meteorological Station,by the influence of seasonal winds and the East

Sapporo District Meteorological Observatory. The

Sakhalin current（Itoh and Oshima, 2000; Oshima

dynamics of sea ice along the Okhotsk Sea coast

2001; Oshima

2002; Itoh2003）. On（the first and the last dates of sea ice recognized）

the coast of Nemuro Strait, sea ice arrives in latewere estimated from the images of a radar

January and retreats in mid to late March in most

network of Esashi-Monbetsu-Abashiri and theyears, but the effects of year‑to‑year variation inaccumulated daily sea ice concentrations（% day）

the dates of sea ice departure on the phytoplankton

along the Okhotsk Sea coast of Hokkaido reported

growth and the underlying mechanisms haveby the Sea Ice Laboratory of the Institute of Low

been little studied. The data available prior to theTemperature Science, Hokkaido University.

present study were those of Shiomoto（1998）

ice concentration is defined as percent coverage ofconcentrations in thesea ice in the sea area studied. According to Aota

who compared chlorophyllSeasouthern Okhotsk Sea during late fall in 1993 and（1993）, the force of sea ice along the Okhotsk

1996 before the arrival of sea ice, but not during sea

Sea coast of Hokkaido may be defined as theice retreat in the spring.

accumulated ice concentration. The information of

From the viewpoint of enhancement of salmon

sea ice distribution in the Okhotsk Sea was compiled

resources, the Okhotsk coastal waters of Hokkaido

from the home page of the Meteorological Bureau

and Nemuro Strait are the most important nursery（http://www data kishou go jp/kaiyou/db/seaice.

areas for juveniles.

Survival and growth ofhtml）. The information on the period between thejuvenile chum salmon depends on the abundance

first and last date when sea ice was recognized inand availability of food（Karpenko, 2003）. Coastal

the Okhotsk Sea coast of Hokkaido was obtained

environmental conditions including food organisms

from the home page of the Ice Information Center,

have been reported as major factors affecting theFirst Regional Coast Guard Headquarters（http://wsurvival success of juvenile salmon in the first

ww1.kaiho.mlit.go.jp/KAN1 /1center.html）.

oceanic period of life（Ryding and Skalski, 1999;
Willette

1999; Fukuwaka and Suzuki, 2002）.

4.2 Results

Clearly, the environmental conditions of the coastal
regions of the Okhotsk Sea and the Nemuro Strait

4.2.1 Dynamics of sea ice

for salmon could vary from one year to the next

The period of sea ice is given in days from the

because of interannual variations of the incidence ofdate when sea ice first reached the coast to the datephytoplankton blooms as discussed above.

when sea ice last left the coast. The period of sea

As typical cases for years with and without

ice in the Okhotsk Sea coast of Hokkaido was 61

massive phytoplankton blooms in the Nemuro Strait,days（from January 26 to March 27）in 1998, andthe results from 1998 mentioned in Chapter 3 2 are

113 days（from January 13 to April 25）in 1999.

compared with those from 1999 below.

From the inflection point of the accumulated ice
concentration curve, the area of sea ice area shifted

4.1 Materials and methods

to a decreasing phase on 8 March（Julian Day =Sampling stations, treatment of samples and67）in 1998 and April 15（Julian Day = 105）in 1999

analytical methods were identical to those described（Fig. 4‑1）. Therefore, the dates when sea ice beganEffect of sea ice on the oceanographic structure in the Nemuro Strait
to retreat in the Okhotsk Sea coast of Hokkaido inin 1998 and 1999. In early April, when sea ice was1998 and 1999 were early March and mid April,

retreating in 1998 but not retreating in 1999, therespectively.

daily solar radiation was much higher in 1998（18 48
MJ m ‑2）than in 1999（15.40 MJ m ‑2）（Fig. 4‑2）.

4.2.2 Light condition

Such district between-year differences were not

Taking into account daily variations, there wasobserved in the mean daily solar radiations during

no appreciable difference in the seasonal patterns

mid April and early May in 1998（17 79 MJ m‑2）andof daily solar radiations between January and July

1999（17 25 MJ m‑2）.

Fig. 4‑1. Accumulated daily ice concentration with progress of season in the
Okhotsk coastal area of Hokkaido in 1998 and 1999（sea ice radar observation of
Hokkaido University）. Arrows denote inflection points.

Fig. 4‑2. Seasonal changes in daily solar radiation at the Nemuro
Meteorological Station in 1998 and 1999. Arrows denote the dates
when solar radiation first exceeded 20 MJ m‑2 of each year.

Ikutaro SHIMIZU

Fig. 4‑3. Vertical profiles of temperatures at Stn. 1‑6 off Rausu in the Nemuro Strait during March‑July 1998.

4.2.3 Water temperature

200 m depth in July in both years.

While no data was available in March in 1999, theresults from April through July showed that the

4.2.4 Salinity and density

vertical profiles of water temperatures at six stations

Vertical profiles of salinities were characterized

in 1998（Fig. 4‑3）and 1999（Fig. 4‑4）were moreby the surface minimum（30‑32 psu）and a gradualor less similar when compared interannually byincrease with increasing depth from April throughmonth. No appreciable between-station differences

June in both 1998（Fig. 4‑5）and 1999（Fig. 4‑6）.

were detected. From April onward, a thermocline

The vertical profile in July was somewhat different

developed in the top 40 m.

In early June, thefrom those of the preceding months in that thethermocline was well established in the top 30‑40 m,

salinity maxima （33 5‑33 6 psu） was seen at

and water temperature was homogeneous below the40‑100 m depths and salinity decreased beyond

thermocline. In July, the water temperature of thethat depth. Between-station differences were notentire water column increased, forming a gradualappreciable in 1998, but lower values were seen in

declining pattern from the surface to 200 m depth.

offshore stations during April and May in 1999.

Very cold water（0℃ or less）seen at 50‑100 m

Vertical profiles of sigma-t during March‑July in

depths during April and June was observed only at

1998（Fig. 4‑7）and April-July in 1999（Fig. 4‑8）

Effect of sea ice on the oceanographic structure in the Nemuro Strait

Fig. 4‑4. Vertical profiles of temperatures at Stn. 1‑6 off Rausu in the Nemuro Strait during
April‑July 1999.

were 24‑25 in the surface layer and increased

method after sea ice retreated in 1998（Fig. 4‑7）and

gradually toward 200 m（sigma-t: 26‑27）. Vertical

1999（Fig. 4‑8）was 8‑30 m and 4‑13 m, respectively

profiles of sigma-t were similar to those of salinity,（Table 4‑1）. During the same period, the euphotic

with the exception of the profile in July in bothzone depth and critical depth ranged from 22 to 33

years. Specifically, the mid-depth maxima seen in

m and from 25 to 52 m, respectively, in 1998, andthe salinity profile in July（Figs. 4‑5 and 4‑6）wasfrom 28 to 42 m and from 33 to 50 m, respectively,

not seen in sigma-t profiles for that month.

in 1999（Table 4‑1）. Mixed layer depths became
shallower as the season progressed, but such a

4.2.5 Mixed layer, euphotic zone and critical depth
The mixed layer depth can be defined as thepattern was not detected in the euphotic zone depth
and critical depth.

depth at which the density reaches a maximum
when a vertical profile of density is constructed by

4.2.6 Nutrientsplotting at depth intervals of 1 m（Mann and Lazier,The first sampling dates in 1998 （17 March）

1996）. Mixed layer depth estimated using thisand in 1999（21 April）were before the onset of

Ikutaro SHIMIZU

Fig. 4‑5. Vertical profiles of salinities at Stn. 1‑6 off Rausu in the Nemuro Strait during
March‑July 1998.

Effect of sea ice on the oceanographic structure in the Nemuro Strait

Fig. 4‑6. Vertical profiles of salinities at Stn. 1‑6 off Rausu in the Nemuro Strait during April‑July1999.

Ikutaro SHIMIZU

Fig. 4‑7. Vertical profiles of sigma-t at Stn. 1‑6 off Rausu in the Nemuro Strait during
March‑July 1998.

Effect of sea ice on the oceanographic structure in the Nemuro Strait

Fig. 4‑8. Vertical profiles of sigma-t at Stn. 1‑6 off Rausu in the Nemuro Strait during April‑July1999.

Ikutaro SHIMIZU
Table 4‑1. Seasonal variations in critical depth, euphotic layer andmixed layer in 1998 and 1999 off Rausu in the Nemuro Strait. Eachvalue represent mean at 6 stations.

Table 4‑2. Seasonal variations in nutrients and chlorophyll concentrations in theeuphotic layer off Rausu in the Nemuro Strait in 1998 and 1999. Each value representsmean at 6 stations.

phytoplankton bloom.

Nitrate, phosphate andμmol, respectively, between 17 March and 14 April

silicate in the euphotic layer were all high in thesein 1998（Table 4‑2）, and from 6 11 to 2 35 μmol,

first sampling dates in both years（Figs. 4‑9 throughfrom 0 68 to 0 40 μmol and from 15 46 to 7 78 μ

4‑14, Table 4‑2）. As the season progressed, allmol, respectively, between 21 April and 17 May inthese nutrients were more or less consumed by a1999（Table 4‑2）. Nutrients in the euphotic layer

phytoplankton bloom in the euphotic layer. Nitrate,

reached at minimum levels in June and/or July inphosphate and silicate decreased from 8 82 to 2 42 μboth years. Below the euphotic layer, high nutrientsmol, from 1 28 to 0 53 μmol, and from 11 19 to 9 05

were maintained throughout the study periods, andEffect of sea ice on the oceanographic structure in the Nemuro Strait

Fig. 4‑9. Vertical profiles of nitrate concentrations at Stn. 1‑6 off Rausu in the Nemuro Strait
during March‑July 1998.

no appreciable differences were seen between 1998

no appreciable differences were seen in daily

and 1999.

solar radiation levels （Fig. 4‑2） and nutrient
concentrations in the euphotic zone（Table 4‑2）.

4.2.7 Chlorophyll a
While chlorophyll

Nevertheless, a massive plankton bloom wasconcentration in the mixed

observed in 1998 when sea ice retreated earlier

layer reached as high as 13 μg l‑1 in April after sea（late March）than in 1999（late April）. Assuming

ice retreated on 21‑23 March in 1998（Fig. 4‑15）,that there were no changes in water mass, thethe concentration barely exceeded 3 μg l ‑1 after

nutrient utilization by a phytoplankton bloom cansea ice retreated on 26‑28 April in 1999（Fig. 4‑16）.

be estimated by the difference between the nutrient

Average chlorophyllin the euphotic zone ranged

concentrations when sea ice began to retreat（before

from 0 66 to 6 97 μg l‑1 between March and July inblooming）and concentrations remaining after the‑1

1998, and from 0 65 to 1 77 μg l between April andJuly 1999（Table 4‑2）.

bloom. On this basis, possible yield of chlorophyllin the euphotic layer in 1998 and 1999 wasestimated from nutrient data（N and P, Table 4‑2）

4.3 Discussion
During the sea ice retreat in 1998 and 1999,

combined with the nutrient-chlorophyll

ratio ofdiatoms（Brzezinski, 1985）. The decrease in N or P

Ikutaro SHIMIZU

Fig. 4‑10. Vertical profiles of nitrate concentrations at Stn. 1‑6 off Rausu in the Nemuro Strait
during April‑July 1999.

Effect of sea ice on the oceanographic structure in the Nemuro Strait

Fig. 4‑11. Vertical profiles of phosphate concentrations at Stn. 1‑6 off Rausu in the Nemuro
Strait during March‑July 1998.

concentrations during the bloom season was fitted toChl.

a regression model;1（Redfied ratio, cf. Parsons

N or P = a × D + b,

= 30（Banse, 1977）and C：N：P = 106：16：
1984）. The total

amounts of N and P utilized by the phytoplankton

where D is Julian days since the beginning of sea

bloom during the study period（Julian Day = 104）in

ice retreat, and a and b are constants. Resultant

1998 were 4 90 μmol and 0 422 μmol, respectively,

regression equations are, N = ‑0 066D + 14 15（R22which equate 5 3‑7 0 μg chlorophyll

l ‑1.

The

= 0 895）and P = ‑0 011D + 2 065（R = 0 991）forsame calculation in 1999 showed that the total N

1998, and N = ‑0 079D + 16 05（R2 = 0 897）and P =and P utilized by phytoplankton during the study

2‑0 009D + 1 714（R = 0 985）for 1999. The amounts

period（Julian Day = 138）in 1999 was 2 78 μmolof N or P utilized by phytoplankton were calculated

and 0 072 μmol, respectively, which is equivalent tofrom these equations and converted to chlorophyll

1 19‑3 0 μg chlorophyllusing a set of conversion factors including C：

l‑1. Observed chlorophyllmaxima were 6 97 μg l‑1 in 1998 and 1 77 μg l‑1

Ikutaro SHIMIZU

Fig. 4‑12. Vertical profiles of phosphate concentrations at Stn. 1‑6 off Rausu in the Nemuro Strait
during April‑July 1999.

Effect of sea ice on the oceanographic structure in the Nemuro Strait

Fig. 4‑13. Vertical profiles of silicate concentrations at Stn. 1‑6 off Rausu in the Nemuro
Strait during March‑July 1998.

Ikutaro SHIMIZU

Fig. 4‑14. Vertical profiles of silicate concentrations at Stn. 1‑6 off Rausu in the Nemuro Strait
during April‑July 1999.

Effect of sea ice on the oceanographic structure in the Nemuro Strait

Fig. 4‑15. Vertical profiles of chlorophyll
Strait during March‑July 1998.

concentrations at Stn. 1‑6 off Rausu in the Nemuro

Ikutaro SHIMIZU

Fig. 4‑16. Vertical profiles of chlorophyll
Strait during April‑July 1999.

concentrations at Stn. 1‑6 off Rausu in the NemuroEffect of sea ice on the oceanographic structure in the Nemuro Strait

Fig. 4‑17. Possible mechanism for the consequences of early and late sea ice retreat
to the development phytoplankton bloom in the Bering Sea（Hunt s hypothesis）.

Fig. 4‑18. Possible mechanism for the consequences of early and late sea ice retreat
to the development phytoplankton bloom in the Nemuro Strait.

Ikutaro SHIMIZU
in 1999（Table 4‑2）, which fall within the rangeof slow sea ice melting during years of early sea ice

predicted from the decrease in nutrients.

retreat. The meltwater lens produced during sea ice

Comparing the depth of the mixed layer after

retreat in the marginal ice zone has been considered

sea ice retreat, the mixed layer depth just after theas the main contributor to the stability of the water

ice retreat in 1998 was deeper（14 April；30 m）

column（Veththan that（18 May；13 m）in 1999（Table 4‑1）. As

the onset of a phytoplankton bloom. In contrast tocompared with the depth of mixed layer, the depth

the Bering Sea, sea ice melts quickly even in yearsof euphotic zone at the same date was slightlyof early sea ice retreat because of higher daily solar

shallower（23 m）in 1998 but much deeper（42 m）

radiation, which facilitates stratification of the waterin 1999. The critical depth was nearly similar（29 m）

column and leads to the onset of a phytoplankton

in 1998 but much deeper （50 m） in 1999.

The

bloom. In years during which sea ice retreat ismagnitude of primary production is governed bydelayed, sea ice melts rapidly and the resultant low

the thickness of the mixed layer（nutrient pool）on

salinity water spreads at shallow depths（=shallow

the premise that the bottom of the mixed layer ismixed layer）leading to a less marked phytoplankton

not light limited（shallower than the critical depth）.

bloom（Fig. 4‑18）. Thus, the fundamental difference

While the daily solar radiation during sea ice retreat

in sea ice-phytoplankton growth relationships

periods in 1998 and 1999 was similar（Fig. 4‑2）, thebetween the Bering Sea and the Nemuro Strait lie

radiation in early March 1998 was less than that inin the differences in latitudes at the two locations

early April 1999, suggesting that the sea ice melted

which in turn govern daily solar radiation levels at

slowly in 1998 but quickly in 1999 as judged by

the sea surface.

1992）, a necessary precedent tovertical profiles of salinity and density in each year
（Figs. 4‑7 and 4‑8）.

Chapter 5.

Historical Data Analysis

In the Bering Sea, delayed incidence ofphytoplankton blooms in open water is characteristic

According to Hokkaido fishery statistics 2002of years in which sea ice retreated early, while（Hokkaido, 2004）, the production of scallops, chum

phytoplankton blooms occurring immediately insalmon and seaweed was valued at 54 3 billion yen

open water is characteristics of a year in which sea（20 8 % of the total fishery production of Hokkaido）,

ice retreat was delayed（Hunt

2002: Hunt

34 2 billion yen （13 1 %） and 25 7 billion yen

and Drinkwater, 2005）（Fig. 4‑17）. According（9 9 %）,respectively. The production value of theseto the hypothesis in years of late sea ice retreat,

three fishery resources exceeded 40 % of the total

phytoplankton accumulates in cold meltwater,

value of the entire Hokkaido fisheries production

because there is sufficient light to support the bloom（260 5 billion yen）. Within Hokkaido, fisheryin a stabilized water column. In contrast, when sea

production of scallops, chum salmon and seaweed

ice retreats early in the spring, there is insufficientin the Ohkotsk Sea coast and the Nemuro Strait

light to support phytoplankton growth and strong

region in 2002 was valued at 35 0 billion yen（42 0 %seasonal winds prevent water column stratification.

of total fishery value of Hokkaido）, 17 4 billion yen

In contrast to this hypothesis, early retreat of sea（20 9 %）and 2 8 billion yen（3 3 %）, respectively,

ice in 1998 in the Nemuro Strait led to a massive

and all together accounted for more than 60 % of thephytoplankton bloom, and late retreat of sea ice

total fishery production（valued at 83 2 billion yen）

in 1999 in the Nemuto Strait led to a less marked

in these two regions. It is noted that the production

phytoplankton bloom.

of chum salmon alone in these two regions accounts

The Bering Sea（52‑62 N）is located at higher

more than 50 % of that of Hokkaido as a whole.

latitudes than the Nemuro Strait（43 7‑44 3 N）
. ThisSalmon juveniles are released from hatcheriesmeans that daily solar radiation in the former region

around Hokkaido and the northern part of Honshu

is less than that in the latter region（Sathyendranath

Island and migrate to and feed in the Ohkotsk Sea

and Platt, 1988）. Lower daily solar radiation in thefrom early summer to late autumn of their first year

Bering Sea suggests a deeper mixed layer because

before they migrate out of the Okhotsk Sea（UenoEffect of sea ice on the oceanographic structure in the Nemuro Strait, 1995; Urawa, 2000）. Compared with the Pacific5.1 Sea ice and fishery production along the
Okhotsk coast of Hokkaido

Ocean, the Okhotsk Sea is a preferable habitat forsalmon juveniles in that food zooplankton is very
abundant（Seki1995）, less saline-surface

water lessen osmotic stress（Shimizu

Sea ice exerts negative effects on local fisheries
in one sense in that it makes operation of fishing1995）,

boats difficult, and it damages seaweeds andand Hokkaido, Sakhalin, the Eurasian continent,

cultivation facilities. In another way though, theKamchatka and the Kuril Islands are naturalinevitable restriction of fishing boat operations may

barriers to prevent dispersion of the population

protect fishery resources from over exploitation.

（ H a n a w a , 1 9 9 6 ）. A s m e n t i o n e d a b o v e , t h eWalleye pollack （）,

environmental conditions of the Okhotsk Sea coast

flatfish （and the Nemuro Strait region are characterized by（sea ice cover in winter, but the relationship between

greenling （high salmon production and sea ice has never been

fishery resources in the coastal waters of theaddressed before.

Abashiri district in the southeastern Okhotsk Sea.

spp.）, giant Ezo scallop） and ArabesqueFig. 5‑1. Relationship between the accumulated ice concentrations and annual
fishery production of pollack, flatfish, scallop and greenling in the Okhotsk coast
during 1969‑1998.

） are major

Ikutaro SHIMIZU
The relationship between the accumulated ice

the fishery resource.

concentration off the Okhotsk Sea coast and thefishery production of walleye pollack, flatfish, giant
Ezo scallop and Arabesque greenling from 1969 to
1998 were analyzed（Fig. 5‑1）. Since most of the

5.2 Salmon returned to the Okhotsk Sea coast ofJapan
The kinds of salmon dealt with in this study

fisheries target of fish species including salmon

are largely chum salmon （which are 4> years old, year‑to‑year variations

but pink salmon（

were smoothed by calculating a moving average（forfor comparison. Data on the number of salmon

five years）before attempting correlations with sea

captured along coasts and in the rivers, and theice. A positive correlation was shown between theannual records of hatching and breeding of salmon

two parameters for flatfish and pollack（ <0 001）

in the hatcheries were compiled from the yearbooks

while a negative correlation was found for scallopof the Hokkaido Salmon Hatchery from 1950 to 1996,（ <0 001）and greenling（ <0 05）. The accumulated

and the reports of the National Salmon Resources

ice concentration off the Okhotsk Sea coast has

Center （NASREC） from 1997 to 2002.

been showing a decreasing trend for a long time

yearbooks and reports, detailed data from the Sharito date. Therefore, a positive correlation indicateshatchery of the Kitami branch of NASREC locateda decreasing trend of the fishery resource and ain the upper reaches of the Shari River draining into

negative correlation indicates an increasing trend of

the Abashiri coast region are of special interest（Fig.

Fig. 5‑2. Location of the Shari hatchery（arrowed）on the
Okhotsk Sea coastal region of Hokkaido.

）,）is also referenced

In theEffect of sea ice on the oceanographic structure in the Nemuro Strait
5‑2）. Fisheries production data and its commercial

even decreased from the 1980s to the 1990s although

values in Hokkaido were compiled from statisticalthis resulted in the improvement of the number of

yearbooks of the Hokkaido Department of Fisheries

returning mature chum salmon in this district in theand Forestry.

1990s（Fig. 5‑3）.

The number of chum salmon fry released from

The percentage of returning mature salmon fromhatcheries in Hokkaido increased rapidly from 300

the total number of released fry is the return rate ,million in the 1960s to one billion in the 1980s. A

which is on average 1 1 % in the 1950s, 1 6 % in the

total of about 200 million chum salmon fry have been

1960s, 2 5 % in the 1970s, 3 2 % in the 1980s andreleased from rivers in each of the five districts of

5 3 % in the 1990s in Hokkaido. Within Hokkaido,

Hokkaido; the Okhotsk Sea, Nemuro, the Japan Sea,

the return rate in the Nemuro district was less thanand east and west Erimo Peninsula. Accordingly,

the average in the 1950s and the 1960s but greater

the returning number of mature chum salmon has

than the average after the 1970s. In the Okhotsk

increased rapidly since the latter half of the 1970s.

Sea district, the figure was 3 1 % which is greater

During the 1980s, the sum of returning mature chumthan the average through the 1970s, decreased tosalmon increased remarkably except in the Okhotsk

2 1 % in the first half of the 1980s, recovered to

Sea and Nemuro districts. This reflects the fact

3 8 % in the latter half of the 1980s and afterwards

that the increase in the number of released salmon

increased as high as 8 5 % in the 1990s.

fry was less in these two districts as compared withresult, the sum of salmon fishery production by set

those in the other three districts. In the Okhotsk

net fisheries off Abashiri and Shari in the eastern

district, the number of released salmon fry in fact

Okhotsk Sea coast and off Rausu and Shibetsu in theFig. 5‑3. Long‑term changes of the return number in brood year of mature chumsalmon（）in the Okhotsk Sea, Nemuro and other coast districts（total of the Japan Sea, west coast of Erimo Peninsula and east coast of Erimo
Peninsula）around Hokkaido.

As aIkutaro SHIMIZU
Nemuro Strait region has exceeded 40 % of that ofextent of sea ice was assumed to be large when

Hokkaido as a whole since the 1990s.

the annual accumulated ice concentration is high（Aota and Ishikawa, 1993）. There was a positive

5.3 Sea-ice and return of mature salmon

and significant correlation（ <0 001）between the

Annual data of the accumulated sea ice

accumulated ice concentration off the Abashiri

concentration along the Okhotsk Sea coast fromcoast（by observation）and the summed values of1969 to 2004 used in this analysis are those fromthe accumulated ice concentration off the Okhotsk

the sea ice observation radar operated by the

Sea coast of Hokkaido （observed with radar at

Sea Ice Laboratory of Hokkaido University atEsashi, Mombetsu and Abashiri）. In other words,

Mombetsu, and those along the Abashiri coast

the annual variation in sea ice concentration off thefrom 1930 to 2004 are from observations by the

Abashiri coast reflected that of the Okhotsk Sea

Abashiri Meteorological Observatory. For detailed

coast of Hokkaido. Subsequently the relationshipaccounts of the area covered by the radar and the

between the extent of sea ice and the return rate

observations, see Aota

（1993）. The dates sea

of mature chum salmon to their home rivers on theice reached and retreated from the coasts during

Abashiri coast was analyzed using long‑term data.

the period 1950 to 2004 were from the Okhotsk Sea

The five‑year moving average of the accumulated

Ice Museum of Hokkaido homepage（http://giza-ryu

ice concentration off the Abashiri coast showed ahyo.com）.

clear trend of decline from 1930 to 2004（ <0 001）.

The relationship between sea ice distribution

Three peaks, each approximately 20 years apart,off the Abashiri coast and off the Okhotsk Sea

were observed（Fig. 5‑4）. The years of 1945 andcoast of Hokkaido was examined.

1985 correspond to the cold years broadly seen in

Because theextent of sea ice is indicated by a value integrated

the waters around Japan（Sakurai

over daily‑accumulated ice concentration, theWhile a trend of decrease is seen between the 1930sFig. 5‑4. Long‑term changes of the accumulated ice concentration（5‑year moving
averages）off Abashiri coast by watching observations during 1932‑1998.

2000）.

Effect of sea ice on the oceanographic structure in the Nemuro Strait

Fig. 5‑5. Relationship between the return rate of maturechum salmon（）and the accumulated
ice concentration in release year （5‑year moving
averages）off Abashiri（Okhotsk Sea）during 1951‑1995.

Fig. 5‑6. Relationship between the return rate of
mature pink salmon（）andthe accumulated ice concentration in release（5‑year
moving averages）off Abashiri（Okhotsk Sea）during
1968‑1996.

Ikutaro SHIMIZU
and the 1990s, large year‑to‑year variations in the

1960s, 1970s, 1980s and 1990s were 92, 95, 91 andextent of sea ice are also evident.

78 days, respectively, along the Abashiri coast

The return rate of mature chum salmon on

by the first definition, and were 55, 59, 49 and 46

the Okhotsk Sea coast was negatively correlateddays, respectively, along the Abashiri coast bywith the annual accumulated ice concentration

the second definition. Under either definition, a

off the Abashiri coast（Fig. 5‑5）; the number oftrend of shortening of sea ice coverage in recent

returned chum salmon was less in years in which

years is evident in the waters around Abashiri.

the amount of sea ice was extensive. The same

Analyzing a historical data set（1930‑1992）, Aota

negative relationship was also confirmed for the（1993）noted a positive correlation between the

return rate of mature pink salmon（Fig. 5‑6）. In

duration of sea ice coverage and the accumulated

Fig. 5‑6, two anomalously high‑return cohorts were

ice concentration. This positive correlation was

those released in 1993 and 1995, which may reflect

confirmed in the present study by adding data sincehigher survival during their coastal lives although1993 to those used in Aotas（1993）.

the reasons for this potentially higher survival areunknown（perhaps high zooplankton abundance,large juvenile size, etc.）.

5.4 Sea ice and artificial salmon enhancement
The Shari River is the main river supporting the

The relationship between the duration of sea ice

salmon resources of the Abashiri coastal region andcover along the coasts and the release timing ofis one of the important salmon enhancement rivers

salmon fry was examined. The duration of sea ice

in the Okhotsk Sea coastal region（Fig. 5‑2）. The

coverage can be defined in two ways; one is the

Shari hatchery of the Kitami branch of the National

number of days between the first appearance of

Salmon Resources Center is located in the uppersea ice appeared and its complete disappearance;reaches of the Shari River and has a long history ofand the other is the number of days between thesalmon enhancement which goes back to 1950. The

date of sea ice reaching the coast and the dateentire history may be divided into three periodsof sea ice starting to retreat from the coast（Fig.

from the viewpoint of treatment of salmon fry before

5‑7）. The duration of sea ice coverage in therelease（Fig. 5‑8）. From 1950 to 1966, none of theFig. 5‑7. Long‑term changes of ice‑covered season off Abashiri. See text for details.

Effect of sea ice on the oceanographic structure in the Nemuro Strait
salmon fry were fed in the hatchery before release

release was the longest during Transitional period

to the river（Non-feed period）. From 1967 to 1986,

because the number of fry and the proportion ofpart of salmon fry were fed and the other part were

fed fry increased gradually. The breeding duration

not fed, before release to the river（Transitionalof female salmon inevitably became longer when

period）. The Transitional period may be subdivided

salmon fry were fed during the Feed period.

into First and Second by the difference in the

The return rate of mature chum salmon and thestarting dates of fry release to the stream（First:

body weight of released fry during each period

released late, Second: released early）. From 1987

are compared.

until the present, all salmon fry were fed in aweight of fry were 2 9 % and less than 0 5 g during

hatchery before being released to the river（FeedNon-feed period, 3 6 % and 1 2 g during First

period）.

Transitional period, 2 4 % and 0 5‑0 7 g during

The return rate and the body

In Fig. 5‑8, the release date in a given year was

Second Transitional period, and 7 0 % and 0 9‑1 4 g

shown by the number of days counted from January

during Feed period. The return rate of mature

first（Julian day）. For example, the release datechum salmon during Second Transitional period was

during the Non-feed period was between 89‑120

the lowest because a large number of the fry weredays, meaning the release of salmon fry started

needed to release over the longer period（including

on the 89th day and finished on 120th day. The

early in the season）due to the limited capacity of

release dates during the Transitional period were

hatchery facilities.

68‑145 days, and during the Feed period were107‑149 days. It is noted that the starting date

Chapter 6. General Discussion

and ending date of fry release were earlier duringNon-feed period than Feed-period. The duration of

While the present results clearly suggest that theFig. 5‑8. Historical accounts of raising procedures of chum salmon fry after
hatching to release in the Shari hatchery during 1950‑2000.

Ikutaro SHIMIZU
effects of sea ice on salmon of hatchery origin was

coastal regions in the course of their downstream

mediated by hydrography, phytoplankton blooms

migration.

and zooplankton（food for salmon）in the coastal

salmon are much faster than chum salmon andenvironments, the expression of these effects may

pink salmon return to home rivers for spawning invary depending on the physiological characteristics

two years（Heard, 1991）. Because of this shorterof different salmon species. There was a negative

generation time, the return rate of pink salmon iscorrelation between the accumulated sea ice

expected to be higher than that of chum salmon ifconcentration off the Abashiri coast and the return

they are not food limited during their oceanic lives.

rate of mature chum salmon in the Okhotsk Sea

Nevertheless, a large year-to-year variation in the

coast off Hokkaido（Fig. 5‑5）
. Compared with chumreturn rates of pink salmon may be indicative of

salmon, the correlation was weak in pink salmon

rather adverse food conditions during their oceanic（Fig. 5‑6）. Chum salmon are different from pink

lives.

The growth and maturation of pinksalmon in that the former spawn in spring water

The return rate of mature chum salmon decreased

and the latter in river water. Pink salmon can adapt

when chum salmon fry were released during the

to lower temperature environments than can chumperiod of sea ice（Fig. 5‑5 and 5‑6）. In addition,

salmon（Kobayashi, 1968）. This means that pink

the period of sea ice coverage was extended whensalmon is able to endure lower temperatures in thethe force of sea ice was strong（Aota

Fig. 6‑1. Relationship between the return rate in brood year of mature chumsalmon in the Okhotsk Sea coast and the releasing date of chum salmon fry fromthe Shari hatchery relative to the sea ice cleared date off Abashiri.

, 1993）.

Effect of sea ice on the oceanographic structure in the Nemuro Strait

Fig. 6‑2. Schematic diagram of two scenarios regarding the timing of sea ice
retreat which lead high or low production of phytoplankton, zooplankton andsurvival of salmon juveniles through food chain in the Nemuro Strait.

The relationship between the release date of salmon

temperature conditions in the Okhotsk coastal region

fry from the Shari hatchery relative to the dateand suffer a high mortality during their early oceanicof sea ice disappearance from the Abashiri coastlife（Shimizu, 1998）. While the provision of foodand the return rate of mature chum salmon infor the fry prior to the release could improve their

the brood year in the Okhotsk Sea coastal region

return rate as mature salmon（Kobayashi, 1987）,

was examined（Fig. 6‑1）. A positive correlation

the present results strongly indicate that the return（ <0 001）between these two parameters in Fig.

rates are also affected by coastal environments

6‑1 indicates that the return rate increased when（temperatures, food zooplankton etc.）encounteredchum salmon fry were released after the sea ice

downstream. Analyzing the relationship between

retreated from coastal regions. For this reason,

the return rate of chum salmon and sea surface

food（zooplankton）for salmon fry is considered totemperatures as determined by satellite images in

be abundant because of a preceding phytoplankton

the coastal waters around Hokkaido, Miyakoshi

bloom, especially in years when sea ice retreated（2007）noted that the return rate was low in years

early in the spring（Fig. 6‑2）.

of low temperature. It is suggested that the low

In examining records taken from 1958 to 1995, it

sea surface temperature is results from delayed sea

was noted that the return rate of mature salmon

ice retreat; if so, the suggestion by Miyakoshi

remained low in the Second Transitional period（2007）is consistent with the relationship between（1978‑1986, Fig. 6‑1）
. Despite the fact that the fry

the return rate and accumulated ice concentration ofwere fed prior to release, low return rates may bethis study（Fig. 5‑5）.

due to the strong force of sea ice（Fig. 5‑4）
, earlierIn this dataset the return rates of chum salmon

release of fry（Fig. 5‑8）
, and small body size of the

fry which were released after the sea ice retreated

fry released（as mentioned above）during thesefrom the coastal regions are still variable（Fig. 6‑1）,

years. Shimizu（1989）reported that watersuggesting that the causes of fry mortality observed

temperatures off Yubetsu, located in the north ofas high as >90% during their oceanic lives are still

Abashiri, were low during 1983 and 1985.

It islargely unknown. Yatsu and Kaeriyama（2003）

therefore considered that salmon fry released in theanalyzed causes affecting survival of Hokkaido

Second Transitional period might encounter low

chum salmon during their oceanic migrations andIkutaro SHIMIZU
concluded that while the body size at release wasat sea and during sample processing. I thank also

positively correlated the effects of the number of fry

Professor Masaaki Aota of the Sea Ice Laboratory

released, oceanic sea surface temperatures（SST）

of Hokkaido University（currently Director of theand climate（Aleutian Low Pressure）were all not

Okhotsk Sea Ice Museum of Hokkaido） for his

significant. It has also been suggested that thekind advice during the study. I also wish to thanksurvival salmon fry in coastal waters is affected by

Nichiei-Electronic Co. Limited for the deployment ofclimate（regime shift）（Kaeriyama, 2003）.

the CANS buoy station, the captain and the crews

As a next step to improve the return rate ofof the icebreakerof the First Regional Coast

mature salmon, study should be directed to theGuard Headquarters of Japan, andfeeding conditions of salmon fry during the sea

Rausu Fisheries Cooperative Organization for theice retreat season in the coastal waters of thekind cooperation during the implementation of field

Nemuro Strait（Shimizu and Aota, 2000; Shimizu

sampling. I am indebted to the Sea Ice Laboratory

2001; Shimizu

of2007）. Since zooplanktonof Mombetsu of Hokkaido University and Abashiri

are the major food component of salmon fry,Meteorological Station for making data available on

zooplankton production cycles in coastal waters andsea ice available for this study. Special thanks arein the Okhotsk Sea need to be studied as a basis todue to Dr. Shelley Clarke of the Imperial College

evaluate the mechanism of survival during their firstConservation Science for her polite correcting theoceanic year. Today, salmon hatchery programs

English manuscript.

aimed at enhancement of the resources have a
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