Stock-specific distributions of Asian and North American salmon in the open ocean, interannual changes, and oceanographic conditions

Abstract

Knowledge of migration routes, migration timing, and resident areas for populations of Pacific salmon in the open ocean is vital to understanding their status and role in North Pacific marine ecosystems. In this paper we review information from the literature, as well as some previously unpublished data, on stock-specific distribution and migration patterns of salmon in the open ocean, interannual variation in these patterns, and associated ocean conditions, and we consider what this information can tell us about ocean conditions on small-to mid-size scales. We conclude that climate-driven changes in open-ocean feeding areas and along the migratory routes of Asian and North American salmon can result in predictable interannual changes in stock-specific distribution, migration patterns, and other biological characteristics. Global climate change is currently causing more frequent and un-predictable environmental changes in the open ocean habitats through which salmon migrate. Data on changes in the distribution and migration of indicator stocks of adult salmon returning from the open ocean might provide an "advance warning" of interannual changes in North Pacific marine ecosystems.

Full text

North Pacic Anadromous Fish Commission
Bulletin No. 4: 159–177, 2007
Abstract: Knowledge of migration routes, migration timing, and resident areas for populations of Pacic salmon in
the open ocean is vital to understanding their status and role in North Pacic marine ecosystems. In this paper we
review information from the literature, as well as some previously unpublished data, on stock-specic distribution
and migration patterns of salmon in the open ocean, interannual variation in these patterns, and associated ocean
conditions, and we consider what this information can tell us about ocean conditions on small- to mid-size scales.
We conclude that climate-driven changes in open-ocean feeding areas and along the migratory routes of Asian
and North American salmon can result in predictable interannual changes in stock-specic distribution, migration
patterns, and other biological characteristics. Global climate change is currently causing more frequent and un-
predictable environmental changes in the open ocean habitats through which salmon migrate. Data on changes
in the distribution and migration of indicator stocks of adult salmon returning from the open ocean might provide
an “advance warning” of interannual changes in North Pacic marine ecosystems.
All correspondence should be addressed to K. Myers.
e-mail: kwmyers@u.washington.edu
Stock-Specic Distributions of Asian and North American Salmon in the Open
Ocean, Interannual Changes, and Oceanographic Conditions
Katherine W. Myers1, Natalia V. Klovach2, Oleg F. Gritsenko2,
Shigehiko Urawa3, and Thomas C. Royer4
1School of Aquatic and Fishery Sciences, University of Washington,
Box 355020, Seattle, WA 98195-5020, USA
2Russian Federal Research Institute of Fisheries & Oceanography (VNIRO),
17 V. Krasnoselskaya, Moscow 107140, Russia
3National Salmon Resources Center, Fisheries Research Agency,
2-2 Nakanoshima, Toyohira-ku, Sapporo 062-0922, Japan
4Center for Coastal Physical Oceanography, Department of Ocean, Earth and Atmospheric Sciences,
Old Dominion University, Norfolk, VA 23529, USA
Keywords: salmon, ocean, distribution, stocks, interannual variation, oceanographic conditions
INTROdUCTION
 Formore than fty years, research studies coordinated
by the International North Pacic Fisheries Commission
(INPFC 1955–1992) and the North Pacic Anadromous
Fish Commission (NPAFC 1993-present) have focused on
determining distribution and migration patterns of Pacic
salmon (Oncorhynchus spp.) in the open ocean (e.g., see
datasynthesesandreviewsbyGodfreyetal.1975;Frenchet
al.1976;Neaveetal.1976;Majoretal.1978;Takagietal.
1981;Burgner1991;Healey1991;Heard1991;Salo1991;
Sandercock1991).Thislargebodyofworkhasledtosome
generalhypothesesaboutoceanicdistributionandmigration
ofsalmonpopulations,andinparticularabout:(1)migration
routes,migrationtiming,andresidentareasofPacicsalmon
as population- or stock-specic traits, and (2) variation in
oceanconditions(e.g.,temperature,salinity,oceancurrents)
thatcan inuence stock-specic distributionand migration
patterns.Scientistshavelongrecognized,however,thatre-
Myers, K.W., N.V. Klovach, O.F. Gritsenko, S. Urawa, and T.C. Royer. 2007. Stock-specic distributions of Asian
and North American salmon in the open ocean, interannual changes, and oceanographic conditions. N. Pac.
Anadr. Fish Comm. Bull 4: 159–177.
© 2007 The North Pacic Anadromous Fish Commission
lations among salmon distribution, migration patterns, and
environmental conditions in the open ocean are obscured
whenstocksfrom different continents, geographic regions,
andsub-regions intermix (e.g.,Manzer et al.1965; Takagi
etal.1981). Untilrecently,signicantprogressinresearch
onthisissuehasbeenlimitedbythelackofcomprehensive
baselinedata on salmon populations throughout the Pacic
Rimandofaccuratemethodsforidentifyingsalmonstocks
migratingthroughtheopenocean(e.g.,NPAFC2004).
 Inarecentreviewand synthesisofinformation onsal-
monbehaviorandecology,Quinn(2005)concludedthat“we
stillhavelittledirectinformationonthemovementpatterns
and orientation mechanisms used by salmon on the open
ocean.”Whileitisbeyondthescopeofourpapertoresolve
these major questions, we hope to draw attention to these
issuesasafocusforfutureresearchonthestatusandroleof
PacicsalmoninNorthPacicmarineecosystems.
 Ourspecicobjectivesinthispaperareasfollows: (1)
provide a brief overview of information on stock-specic
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Myers et al.
distribution and migration patterns of salmon on different
spatial and temporal scales in the open ocean, (2) provide
examplesofinterannualvariationinsalmondistributionand
migrationin relation toocean conditions, (3)review some
recentchangesinoceanconditions(physical)thatmayaffect
stock-specic salmon distribution and migration patterns,
and(4)considerwhether annual variation in stock-specic
distribution and migration patterns can tell us something
aboutchangesinoceanconditions.
MATERIALS ANd METHOdS
 This paper synthesizes previously published scientic
literatureandprocessedresearchreports,and analyzesboth
publishedandunpublisheddata.Overtheyears,manydiffer-
entmaterialsandmethodshavebeenusedtosampleandana-
lyze salmon andsurrounding oceanic conditions.  Samples
werenotcollectedconsistentlybylocation,time,orintensity.
Capturemethodsincludedpurseseines, longlines,gillnets,
andsurfacetrawls(e.g.,Hartt 1975;Karpenkoet al. 2005).
Stockidentication techniques haveincludedhighseastag-
ging,serology,morphometry,scales(measuringage,circuli
patterns,or both), natural parasitetags,geneticanalysis(al-
lozymeandDNA),otolithmarks,andcoded-wiretags (e.g.,
Hartt1962;Myersetal.2004).Oceanographicobservations
weremadeindependently(e.g.,Favoriteet al.1976),direct-
lyaboardcharteredshing vessels at shing stations,(e.g.,
Eisneretal.2005),orjointlyinsheries-oceanographicsur-
veysaboardresearchvessels(e.g.,KhenandBasyuk2005).
 Becauseoflimitedspace,wedonotattemptacompre-
hensivereview,andinsteadwefocusonresearchpertaining
toafewmajorpremises.Thesepremisesinclude:(1)Pacic
salmon in the open ocean have stock-specic distribution
andmigration patterns, (2) interannualvariation in salmon
distributionintheopenoceanduringthespring-summersea-
sondependslargelyonoceanconditionsduring thepreced-
ingwinter,(3)circulationandclimatesystemsinthe North
Pacic Ocean and Bering Sea are interconnected, and (4)
oceanographicconditionsintheseregionshavebeenchang-
ingsignicantlyoverthepastseveraldecades,evenpriorto
theclimaticregimeshiftof1977.
 We use the term “juvenile” to denote salmon in their
rstocean year, and “immature” or“maturing” to indicate
older sh.  By our denition, the “open ocean” refers pri-
marilytodeep-wateroceanicregions beyondneriticwaters
(<200mdeep) overthecontinental shelf.Themajorityof
oursampleswerematuringpinksalmon(O. gorbuscha)and
immature and maturing chum (O. keta) and sockeye (O.
nerka)salmon,whicharethemostabundantmaturitygroups
and, more generally,Pacic salmon species inhabiting the
openocean.  Thedata from high-seas, salmon-tagging ex-
perimentsusedinafewexamplesarefromasharedNPAFC
databasethatiscurrentlyupdatedandarchivedbytheHigh
SeasSalmonResearchProgram,SchoolofAquaticandFish-
erySciences,UniversityofWashington,Seattle.
RESULTS ANd dISCUSSION
Stock-specic Distribution and Migration Patterns of
Salmon in the Open Ocean
 AmajorpremiseofthisreviewisthatPacicsalmonin
theopen ocean havestock-specic distribution andmigra-
tionpatterns.Thisis not a new idea.Moiseev(1956)was
oneoftherstscientiststopublishevidencethatthemarine
habitatsofindividualstocksofsalmonarelocatedinspecic
areasoftheopenocean.Recentgeneticworkinfreshwater
habitats has revealed a strongly hierarchical structuring of
geneticvariationthatdescendsbygeographyfromthelarg-
est scale (i.e., ancestral) geographic lineages, to regional
geographic subdivisions, to individual subbasins, and to
life-historysubdivisionswithinthesesubbasins(Utteretal.
1989;seereviewbyWilliamsetal.2006).Wehypothesized
that the distribution patterns of salmon populations in the
openoceanwouldalsohaveahierarchicalgeographicstruc-
ture,i.e.,stocksthataregeneticallysimilarorgeographically
adjacenttoeachotherinfreshwaterhabitats,or both, have
oceandistributionandmigrationpatternsthataremoresimi-
lartoeachotherthanthoseofpopulationsthataregenetically
orgeographicallydistant.Individualpopulationsorlife-his-
toryvariantswithinpopulationsusually occupyonlyapor-
tionoftheentireoceanicrangeoccupiedbylargergroupsof
populations,e.g.,regionalstockcomplexes.
 Onthelargestspatial scale,Pacicsalmonspecies mi-
gratinginopenwatersoftheNorthPacicOceanaredistrib-
utedprimarilyintheregionnorthofthesub-arcticboundary
(Fig.1).Pearcy(1992)speculatedthattheevolutionary“di-
vergenceofNorthPacicsalmonidsandtheiremergenceas
successfulandabundantshesisrelatedtotheformationof
thecoldSubarcticWaterMassintheNorthPacic.”Across
this immense marine region, the known ranges of salmon
encompassmostmajor oceanic currents anddomains(Fig.
1).Marinehabitatconditions(e.g.,seatemperaturesandsa-
linities) within acceptable limits for salmon, however, can
sometimes extend south of the sub-arctic boundary,which
expandsthesalmon’sknownopenoceanrangeintosubtropi-
calwaters(Azumayaetal.2007).
 Earlymodelsofopen-oceanmigrationpatternsfromIN-
PFC-coordinated research described salmon movements at
seaas counterclockwisecircles,generally “downstream”in
cyclonicgyres andthroughassociatedcurrents inthewest-
ernNorthPacic,GulfofAlaska,andBeringSea(Royceet
al.1968). Althoughthis outdated modelis still frequently
citedintherecent scienticliterature,the prevailingtheory
amongexpertsisthatsalmonintheopenoceanmoveacross
broadfronts–-tothesouthandeastinwinterandspringand
tothenorthandwestinsummerandfall(e.g.,Frenchetal.
1976;Burgner1991;Shuntovetal.1993).Thesebroadsea-
sonalshiftsindistributionlikelyreectbothgeneticadapta-
tionsand behavioral responses toenvironmentalcues(e.g.,
preyavailabilityandwatertemperature)thataremediatedby

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Stock-specic ocean distribution of salmon
bioenergeticconstraints.
 Ageneralseasonalmodeloftheopenoceandistribution
of immature and maturing Pacic salmon indicates that in
winterandspringtheyareprimarilydistributedsouthofthe
CommanderIsland-AleutianIslandchainintheNorthPacif-
icOcean,andinsummerandfalltheyarewidelydistributed
throughouttheNorthPacicOceanandBeringSea(Fig.1).
However,thereare major exceptionstothisgeneral model.
For example, the Bering Sea is a major winter habitat for
Asianand NorthAmericanpopulationsof Chinook salmon
(O. tshawytscha)(RadchenkoandGlebov1998;Myersand
Rogers1988).
 Duringtheir rstyearintheocean, juvenileAsian and
NorthAmerican salmon interminglerarely. Although data
arelimited, most juvenile pink, chum,andsockeyesalmon
move in late fall or early winter from relatively shallow,
coastalwaterstosurface watersoverthe deepoceanbasins
(e.g.,HarttandDell1986;seerecentnationalreviewsofthe
earlymarineperiodinMyersetal.2000andNPAFC2003).
PossibleexceptionsareRussianandwesternAlaskanstocks
ofjuvenilesalmon,whichmayintermingleduringtheirrst
summerandfallinthenortheasternBeringSea(Farleyetal.
2005).
 Atthescaleofmajorgeographicallineagesorcontinent-
of-originandregionalstockcomplexes,openoceandistribu-
tionsof immatureandmaturingAsianand NorthAmerican
salmon are frequently depicted by composites of recovery
datafromINPFC/NPAFC-coordinatedhighseassalmontag-
gingexperiments(e.g.,Frenchetal.1975;Myersetal.1990,
1996;Klovachetal.2002;Beamishetal.2005).Thesedata
indicatethatAsian stocks areprimarily distributed west of
180°,whileNorthAmericanstocksareprimarilydistributed
eastof180°(Fig.2).Theapparentareasofmixingbetween
immatureandmaturingAsian and NorthAmericansalmon
intheopenoceanvarybyspecies,andarelargestforchum
salmon(174°E–140°W,44°N–61°N),smallerforpinksalm-
on(between175°E and 160°W,44°N–57°N),andsmallest
for sockeye salmon (165°E–175°W, 45°N–58°N) (Fig. 2).
Differencesbetween species in areas of mixing seem to be
positively correlated with their relative abundance in the
openocean.Forexample,chumsalmonaremoreabundant
inthe ocean thanpink salmon, inhabit the ocean for more
time (as many as ve winters compared to one winter for
pinksalmon),andhavealargerareaofmixing.
 Differencesinthe east-west extent ofdistributionalso
seemtobepositivelycorrelatedwiththerelativeabundance
ofAsianandNorthAmericansalmon. For example,Asian
pinkandchumsalmon aremoreabundantandhaveamore
extensiveeast-westrangethandoNorthAmericanpinkand
chumsalmon(Fig.2).Similarly,NorthAmerican sockeye
salmonaremoreabundantand haveamoreextensive east-
westrangethandoAsiansockeyesalmon(Fig.2).
 Asymmetricaldistributions ofAsian and NorthAmeri-
cansalmonin the open oceanmight reect density-depen-
Winter
Summer
Fig. 1. A general conceptual model of seasonal distribution and movements of Pacic salmon in the open ocean. Salmon are distributed in both
the Bering Sea and North Pacic Ocean in the summer and primarily in the North Pacic Ocean in the winter. Immature salmon generally move
to the south and east in winter (black arrows) and to the north and west in summer (grey arrows). Base map showing oceanographic features
and approximate current speed (km/d) is from Quinn (2005).

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Myers et al.
dentinteractions, as explainedby the theory of “ideal free
distribution” (Fretwell and Lucas 1970).  That is, as com-
petitiveinteractionsincreaseingrowingsalmonpopulations,
the population’s geographic distribution increases until it
reachesanewequilibrium.OguraandIto(1994)suggested
thatlarge-scalereleases ofhatchery chumsalmoninJapan
resultedinanexpansiontotheirknownoceanicrange.How-
ever,detectingstocksatthelimitsoftheirgeographicrange
mightsimplybeeasierwhentheyareabundant.Asymmet-
ricaldistributions of Asian and NorthAmerican salmon in
theopenoceanhavealsobeenattributedtophysicaloceanic
factors,suchascoldwinterseatemperaturesinthewestern
NorthPacic(e.g.,Shepardetal.1968;Neaveetal.1976)or
passive(eastward)transportofimmatureAsianshbywind-
drivenandgeostrophiccurrents(Uenoetal.1999;Azumaya
andIshida2004).
 Inthelate 1970s, compositeconceptualmodels of the
distribution and migration routes of major regional stock
complexes ofAsian and North American salmon were de-
velopedbyINPFCresearchersusinginformationfromhigh-
seasresearch and commercialshingcatchandeffortdata,
biological data, tag recovery data, and stock-identication
results(primarilynaturalparasitetagsandanalysisof scale
patterns,e.g.,Frenchetal.1976;Takagietal.1981).Burgn-
er(1991)updatedtheFrench et al. (1976) sockeye salmon
migrationmodelswiththe results of scale patternanalyses
conducted in the 1980s in the open ocean region south of
46°N(Harris1987;seereviewbyMyersetal.1993).These
conceptual models were a major breakthrough in our un-
derstandingof stock-specic migratory behavior ofsalmon
Fig. 2. Composite map showing overlap in open ocean distributions of Asian and North American salmon as observed in high-seas tagging ex-
periments (1956–2004). Closed (black) diamonds = Asian stocks; closed (grey) triangles = North American stocks; open box = region of overlap.
Data source: High Seas Salmon Research Program, University of Washington, Seattle.

NPAFC Bulletin No. 4
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Stock-specic ocean distribution of salmon
intheopenocean, andarestillfrequentlyusedandcitedin
thescienticliterature.  These modelsneedto be updated,
however,because they are primarily based on datacollect-
edduringthemid-1950stolate1960s,when(1)theNorth
Pacic climate regime was in a different phase than after
the1977 regime shift(Mantua et al.1997), (2) there were
no large-scale releases of hatchery chum and pink salmon
into the North Pacic Ocean (Mahnken et al. 1998), and
(3) large-scale high-seas driftnet sheries were harvesting
largepercentagesofsalmonreturningto RussiaandAlaska
(e.g.,Fredinetal.1977;Harris1987).Inaddition,theseold
conceptualmodelsdonottellusanythingaboutinterannual
variationand the effects of oceanconditions on stock-spe-
cicdistributionandmigrationpatterns.
 Morerecently,researchershavebeenattemptingtode-
velop quantitative models of open ocean distribution and
movements of some numerically dominant salmon species
andstocks(e.g.,HiramatsuandIshida1989;Thomsonetal.
1992,1994; Datetal.1995;Rand etal.1997;Walteretal.
1997;AzumayaandIshida2004).Forthemostpart,howev-
er,thesequantitativemodelshavefailedtosuccessfullycap-
turerelativelyclear differencesin the open oceandistribu-
tionandmigratoryorientationofAsianandNorthAmerican
salmonstocks.Anaddeddifcultyisthattheexistingtime
seriesofempiricaldataareusuallynotsufcienttovalidate
computermodels.
 Weare nowinthemidst ofageneticrevolution thatis
beginningtoprovidereliable mid- to small-scale estimates
ofsalmonstockcompositionneededtodevelopandvalidate
quantitativemodels of interannual variation in open ocean
distribution and migration patterns of salmon (see Fig. 3).
Chum salmon were the focal species for the initial devel-
opmentofacomprehensive PacicRimgenetic(allozyme)
baseline.Thesedatawereusedtoestimatethestockcompo-
sitionofchumsalmoninsamples collectedduringresearch
vessel surveys and to develop new conceptual models of
chumsalmondistributionand migration patterns for major
regionalstocks(e.g.,Figs.3and4,toppanel; Urawa2000,
2004;Urawaetal.2001).Theresultsofanalysesusing20al-
lozymelocifrom356chumsalmonpopulationshaveshown
(1)ahigherdegreeofoverlapintheoceanicdistribution of
Asian and NorthAmerica stocks than that extrapolated by
previousmethods,(2)substantialintra-annualuctuationsin
stockcomposition over short time periods, and (3) greater
useoftheBeringSeabyimmatureandmaturingstocksfrom
throughoutthespecies’rangethanthatindicatedbytagging
studies(Seeb et al. 2004). Seeb et al.(2004) also suggest
that geographically but not genetically similar populations
ofchumsalmonfollowsimilarmigrationroutes.
 Our conceptual model of the seasonal migrations of
BristolBaysockeyesalmon(Fig.4,bottompanel),whichin-
corporatesrecentdatafromgenetic(DNA)analysis(Habicht
etal. 2005), scale patternanalysis(Bugaev 2005), andex-
ploratoryshing(Farleyetal.2005),pointstoamoreexten-
sivedistributionofjuvenileandimmatureNorthAmerican
sockeyesalmonintheBeringSeainsummer and fall than
wasindicatedbyearliermodels(Burgner1991).However,
earliermodels may accurately reect seasonal distributions
ofsalmonintheBeringSeaduring“cool”periods,because
mostofthedatawerecollectedduringrelativelycoolperiods
160W 140W 180 160E 140E
40N
50N
60N
+
+
+
+
+
+
+
+
+
+
+
+
+ +
+ +
+
+
+
+
North Pacific Ocean
Bering Sea
+
500
100
10
0
+
+
+
+
+
0
30
60
90
Central NPO
180º
n=127 fish
0
20
40
60
Central G
A
145-148ºW
n=150 fish
0
20
40
60
Western G
A
163-168ºW
n=49 fish
Fig. 3. Example of geographic variation in the regional stock composition of immature and maturing chum salmon in their 2nd-4th winters at
sea, using a comprehensive baseline for 20 allozyme loci from stocks throughout the Pacic Rim (Urawa and Ueno 1997, 1999; Urawa 2000).
Samples were collected during NPAFC-coordinated cooperative winter surveys of salmon aboard the Japanese research vessel Kaiyo maru in
January 1996 and February 1998. The relative sizes of the solid circles represent catch per unit effort in a research trawl towed at each station.
Crosses indicate zero catches. Bars indicate percentages of each regional stock group, from left to right: Japan (downward diagonal), Russia
(black), northwestern Alaska (upward diagonal), Alaska Peninsula and Kodiak (white), southeastern Alaska and British Columbia (horizontal
brick), on three different survey lines. NPO = North Pacic Ocean, GA = Gulf of Alaska.

NPAFC Bulletin No. 4
164
Myers et al.
Fig. 4. Examples of seasonal stock-specic migration models for regional stocks of Asian and North American salmon. Top panel: Model for
Japanese hatchery chum salmon as estimated by genetic stock identication (Urawa 2000, 2004; Urawa et al. 2001). In their rst summer-fall,
juveniles are distributed in the Okhotsk Sea. In their rst winter, they are distributed in a narrow region of the western North Pacic. By their
second summer-fall, they have migrated into the Bering Sea, and in late fall they migrate south and east and spend their second winter in the
Gulf of Alaska. In subsequent years, they migrate between their summer-fall feeding grounds in the Bering Sea and their winter habitat in the
Gulf of Alaska. In their last summer and fall, maturing sh migrate back to Japan through the western Bering Sea and western North Pacic.
Bottom panel: Migration model for Bristol Bay sockeye salmon as indicated by tag recoveries (Myers et al. 1996), scale pattern analyses (Myers
et al. 1993, Bugaev 2005), parasite tags (Burgner 1991), genetic (DNA) stock identication (Habicht et al. 2005), and exploratory shing (Farley
et al. 2005). In their rst oceanic summer and fall, juveniles are distributed on the eastern Bering Sea shelf, and by the following spring immature
salmon are distributed across a broad region of the central and eastern North Pacic. In their second summer and fall, immature sh migrate
to the west in a band along the south side of the Aleutian chain and northward through the Aleutian passes into the Bering Sea. In subsequent
years, immature sh migrate between their summer/fall feeding grounds in the Aleutians and Bering Sea and their winter habitat in the North
Pacic. In their last spring, maturing sh migrate across a broad, east-west front from their winter/spring feeding grounds in the North Pacic,
northward through the Aleutian passes into the Bering Sea, and eastward to Bristol Bay.
Odd-year cycle of high abundance
of maturing Russian pink salmon
in odd years
Fig. 5. Example of interannual variation in sockeye, chum, and pink salmon catch per unit effort (CPUE; 1 tan = 50 m of gill net) in Japanese
research vessel catches in the Bering Sea. Because of their two-year life cycle (including one winter in the ocean), maturing pink salmon are
genetically different in even and odd years. Maturing pink salmon returning to spawn in rivers in eastern Kamchatka, Russia, are the dominant
regional stock in the Bering Sea in odd-numbered years. Data and gure source: Ishida et al. 2005.
160140180 140
40N
50N
60N
Winter-
Spring
Summe
r
-
Fall
1s
t
Fall
Okhotsk
Sea
Bering
Sea
North Pacific Ocean
1s
t
Winter-
Spring
Japanese
Chum Salmon
160W 140W 180160E140E
Winter-Spring
Summe
r
-
Fall
Okhotsk
Sea
North Pacific Ocean
Gulf of
Alaska
Bristol Bay
Sockeye Salmon
60N
50N
40N
Gulf of
Alaska
1s
t
Fall
Bering
Sea

NPAFC Bulletin No. 4
165
Stock-specic ocean distribution of salmon
inthe1950s–60s,whilemostrecentdatawerecollecteddur-
ingaperiodofwarmingintheBeringSea(KhenandBasyuk
2005). 
 Research vessel catches of salmon in the open ocean
vary signicantly from year to year (e.g., Fig. 5; Ishida et
al.2002;Ishidaetal.2005).Thesevariationslikelyresult
fromchangesinstockabundanceandcomposition,distribu-
tion,migrationroutes,migrationtiming,andphysicalhabitat
(temperature,salinity,currents,e.g.),as well as prey abun-
danceordistribution.Atpresent,however,timeseriesofge-
neticstockidenticationdataaretoolimitedto providede-
tailedinformationoninterannualvariationinstock-specic
distributionandmigrationroutesintheopenocean.Perhaps
the best available genetic (allozyme) data time series de-
scribeschumsalmoncaughtinJuly(1995–2001)inresearch
gillnetsin thecentralBeringSea (Fig.6).Thestrongodd-
evenyear variation in researchgillnetcatch per unit effort
(CPUE)ofmaturingchumsalmonislikelyduetoadensity-
dependent change in the salmon’sdistribution (though not
survival) in years when maturing eastern Kamchatka pink
salmonwere abundant in the Bering Sea, as theygenerally
areinJulyofoddyears(Fig.6,toppanel;Fig.5;Azumaya
andIshida2000;Ishidaetal.2002).Thereisnodirectrela-
tionbetweenestimatedrelativeabundanceofmaturingJapa-
nesechumsalmoninthecentralBeringSeainJulyandsub-
sequentadultreturnstoJapan(Fig.6,centerpanel).There
isa strong negative relation, however,betweentherelative
abundanceofRussianchumsalmonandseasurfacetemper-
atures(SST)inthecentralBeringSeainJuly(Fig.6,bottom
panel).Thiscorrelationmightreecttheinuenceofocean
temperatureonruntiming,i.e.,inwarmSSTyearsRussian
salmonmay maturefasterandleavethe central BeringSea
sooner,resulting inlowerCPUEs in July.There is no ap-
parentrelationbetweenresearchgillnetCPUEsofmaturing
RussianchumsalmoninthecentralBeringSeainJuly(Fig.
6)andsubsequentadultreturnstoRussia(commercialcatch,
seePICES2004).
 Evenmoreeffectivethanallozymebaselinesinidentify-
ingindividualpopulations,comprehensiveDNAbaselines
forchumsalmonandotherspeciesarebeingdevelopedand
appliedtoquestionsaboutopenoceandistributionandmi-
grationpatterns of salmon (NPAFC2004).  Unfortunately,
this genetic “revolution” is happening at a time when the
numberofsalmonresearchvesselsurveysintheopenocean
isdiminishing,dueinparttodecreasinggovernmentsupport
forsuchsurveys.Nevertheless,throughcooperativeresearch
programscoordinatedbyNPAFC(forexample,BASIS,Ber-
ing Aleutian Salmon International Survey, 2002-present),
wearerapidlyadvancinginourknowledgeofstock-specic
distributionandmigrationpatternsofsalmon(NPAFC2005;
Urawaetal.2005).
Interannual Variation in Salmon Distribution Relative to
Ocean Conditions
 Inthissection,we willreviewacasestudythatsought
relationsbetweeninterannualchangesinsalmondistribution
andoceanconditions.SSTisthemainindexofinterannual
variation in natural open ocean habitats considered in this
section.Wedonotreviewotherwell-knownclimaticindi-
ces,suchasALPI(AleutianLowPressure Index)andPDO
(Pacic Decadal Oscillation), which are believed to char-
acterize long-term climatic trends over the North Pacic.
Ishidaet al. (2002) did notnd any signicantcorrelation
betweenSSTsand theAleutianlow-pressureindex (ALPI)
0
30
60
90
120
150
180
210
CPUE
1995199619971998199920002001
Year
Alaska
Russia
Japan
0
5
10
15
20
25
30
35
40
45
567 8 9
SST (C)
CPUE
JAPAN
RUSSIA
ALASKA
0
20
40
60
80
100
0 50 100 150
CPUE
Adult returns
(millions of fish
)
Odd years
Even years
Fig. 6. Top panel: Interannual changes in mean catch per unit effort
(CPUE) of maturing chum salmon stocks in research gill nets in the
central Bering Sea (180°), July 1995–2001. Center panel: Relation
between chum salmon returns to Hokkaido, Japan, and Bering Sea
CPUEs of maturing Japanese chum salmon. Bottom panel: Relation
between sea surface temperatures (SSTs) and CPUEs of maturing
chum salmon stocks in the Bering Sea in July of even years. Data
source: S. Urawa, National Salmon Resources Center, Japan.

0
10
20
30
40
50
60
70
80
90
100
1231 2 3
Ten-day period of month
The share, %
A
p
ril May
0
10
20
30
40
50
60
70
80
90
100
123123
Ten-day period of month
The share, %
April May
Chum Salmon Sockeye Salmon
1997
1999
NPAFC Bulletin No. 4
166
Myers et al.
orSSTandresearchgillnet CPUEs for each speciesinthe
central Bering Sea in July (1970–2000).  They speculated
thatSST,particularly at higher temperatures,causesashift
insalmondistributionthataffectedtheirCPUEvalues.
 Themajorpremiseofthispartofourreviewisthatin-
terannualvariationinsalmondistributionintheopenocean
duringthe spring-summerseasondependslargelyonocean
conditionsduring the precedingwinter. In particular, spa-
tio-temporalpatternsofsalmon distribution in spring-sum-
mervarydependingonthesynoptictypeofwinter(coldor
warm).
 The migration routes of salmon in the western North
PacicOceaninspringappeartofollowthenorthwestward
progressionofthe2°CSSTisotherm(Birman1985;Erokh-
in1990).  In years that differbywinterclimatictype(cold
or warm), the spring CPUEs of salmon in research gillnet
catchesand corresponding concentrations ofsalmonin the
openoceanvaryduringthesameten-dayperiod.Forexam-
ple, hydro-meteorological conditions in the western North
PacicinApril–Mayof1997and1999werequitedifferent.
The winter of 1996–1997 was warm.  In the rst ten-day
periodofApril1997,theSSTintheNorthPacicwatersoff
Kamchatka reached 3°C, and by the second half ofApril-
early May 1997, SSTs had already reached 3.5–4°C (Fig.
7).Incontrast, the winter of 1998–1999wascold. In the
secondhalfofApril1999,SSTsintheNorthPacicwaters
off Kamchatkahadnotreached2°C.Bythe beginning of
May,thesurface layer had warmed to2.3°C.Only by the
endofMaydidthemaximumSSTreachabout4.0°C,which
waslowerthanthemeanlong-termvaluesforthisperiodby
about1.5°C.  In1999, the temperaturedifference between
the surface and 100 m below the surface did not exceed
1.0–1.5°C,whichischaracteristicofthehydrologicalwinter.
Thelowwatertemperatureinspring1999,comparedtothe
sameperiodin1997,resultedinfewersalmonincatchesin
thewesternNorthPacicOceanoffKamchatka(Fig.7).
 InApril–Mayof1997and1999,threespeciesofsalmon
(sockeye,chum, and chinook)werecaughtbyresearchgill
netsin North Pacic waters off Kamchatka (Klovachetal.
2000;Klovach2003).Sockeyeandchumsalmonwereob-
servedduringthesameperiod,andchinooksalmonappeared
incatchesinmidMay.Theratioofsockeyeandchumsalm-
oninthecatcheswasdifferentduringwarm(1997)andcold
(1999)years.Theproportionofsockeyesalmonwashigher
in1999than in 1997 (Klovachet al. 2000; Klovach2003;
Fig.8).Sockeyesalmonwerealsothepredominantspecies
inNorthPaciccatchesoffKamchatka during subsequent,
cold years (2000 and 2001).  We hypothesize that this in-
creaseintherelativeabundanceofsockeyesalmonwasas-
sociated with cooling of the western North Pacic Ocean
duringthesecondhalfofthe1990s.Atthattime,icecover
inwaters off the western andeasterncoasts of Kamchatka
increased,andretreatedlaterintheyear(Fig.9).Intheoce-
anic region where Russian sockeye salmon overwinter (in
the North Pacic Ocean, south of theAleutian Islands), it
0
1
2
3
4
1 2 312 3
Т°С
1997
1999
A
0
5
10
15
20
123123
April May
CPUE, kg per net
1999
1997
B
Fig. 7. Comparison of (А) sea surface temperatures (T°C) and (B)
salmon catch per unit effort (CPUE, kg of salmon per net) in 1997
and 1999 in the western North Pacic Ocean off Kamchatka in April
and May. 1, 2, 3 = 10-day periods in April and May.
Fig. 8. The share (%) of chum and sockeye salmon in catches made
near East Kamchatka during April and May in 1997 and 1999.

NPAFC Bulletin No. 4
167
Stock-specic ocean distribution of salmon
wasobservedthatthecoldseasonlastedlongerand spring
warmingbeganlaterintheyear(Fig.10).
 Thesecoolerconditionsapparentlycausedadelay(com-
paredtomeanlong-termdates)inthemigrationsofmaturing
salmontotheeast andwestcoasts ofKamchatka,a change
inthemigrationroutesofsockeyeandchumsalmonreturn-
ingto the East Kamchatkacoast,andashiftintheoceanic
feedingpatterns of differentsalmonspeciesandstocks.  In
particular,the low temperature of North Pacicwaters off
Kamchatkainthespringof1999and2000causedthesock-
eyesalmon’smainmigrationroutestoshiftsouthby2–4°,
arealignmentthatcoincidedwithadisplacedzoneofwater
with optimal temperatures for sockeye salmon (Gritsenko
etal. 2000,2002;Fig.11).Instead of migratingacrossthe
southwesternBeringSea,asoccursinwarmyears,sockeye
salmon returning to the Kamchatka River migrated north
alongthecoastofEastKamchatkatothemouthoftheriver.
Asa result, notonly in Maybut also inJune of 1999and
2000,theCPUEsofsockeyesalmoninNorthPacicwaters
offKamchatkawerehigherthaninwarmyears(Gritsenkoet
Fig. 9. Trends in sea ice cover in the western North Pacic Ocean off the western (A) and eastern (B) coasts of Kamchatka at 53°N, 1995–2001.
1-date that the sea ice margin crossed 53°N during spring warming. Dates (months): 1 = January, 2 = February, 3 = March, 4 = April, 5 = May.
2-duration (number of days) of sea ice cover at 53°N.
0
1
2
3
4
5
6
1994 19961998 2000
Years
Months
0
20
40
60
80
100
120
140
160
Duration, number of days
1
2
3
Fig. 10. Trends in the 5°C isotherm in the eastern North Pacic
Ocean at 50°N, 170°W during spring warming and autumn cooling:
1-date that the 5°C isotherm crossed 50°N, 170°W when warming;
2-duration of 5°C-isotherm at 50°N, 170°W (number of days); 3-date
that the 5°C isotherm crossed 50°N, 170°W when cooling. Dates
(Months): 0 = December, 1 = January, 2 = February, 3 = March, 4 =
April, 5 = May.
Fig. 11. Interannual variability in the dates that the 2°C-isotherm con-
sistently crossed the line from Bering Island to 51°N, 160°E during
spring warming in the western North Pacic Ocean. Dates (Months):
3 = March, 4 = April, 5 = May, 6 = June.
1
2
3
4
5
19941996 1998 2000
Years
s h t n o M
0
20
40
60
80
100
120
) s y a d f o r e b m u n ( n i g r a m e c I
1
2
A
1
2
3
4
5
6
1994 1996 1998 2000
Years
s h t n o M
0
20
40
60
80
100
120
) s y a d f o r e b m u n ( n i g r a m e c I
1
2
B
3
4
5
6
1993 1995 1997 1999 2001
Years
s h t n o M

NPAFC Bulletin No. 4
168
Myers et al.
al.2000,2002;Fig.12).
 Thus,theSSTinearlyspringisasignal,notonlytrig-
geringmigrationsofmaturingsalmontothecoasts,butalso
determiningtheratesofthesemigrations,thescheduleofsh
maturation,and,eventually,theclosingdatesofmigrations,
whensalmonentertheirnatalrivers.In May–June1997,a
watermasswithabnormallyhightemperaturesformedinthe
areaoftheNorthKurilStraits.Thisabnormalformationpro-
ducedearliermigrationsofWestKamchatkasockeyesalmon
stocksintotheSeaofOkhotsk,comparedtomeanlong-term
dates.Incontrast,in1999–2001,warmingofwesternNorth
PacicwatersadjacenttotheNorthKurilStraitsbeganlate
andcontinued until the endof June-mid July.As aresult,
WestKamchatkasockeyesalmonremainedinNorthPacic
waters off East Kamchatka longer than in previous years,
andhigh CPUEs of sockeyesalmon were observedinthat
areainearlyJuly2001(Fig.13).
 Inwarmyears,pinksalmonappearinNorthPacicwa-
tersoffKamchatkainearlyJune,andbymidJune,research
gillnet catches are relatively high.  For example, in 1997
and1998(warmyears),afewindividualpink salmonwere
caughtinearlyJune(June2)inPacicwatersoffKamchat-
ka,whilemorethan4t/dayofanapproximatetotal10twere
caughtinmidJune(June12).In2001(acoldyear),onlya
fewindividualpinksalmonwerecaughtoffEastKamchatka
inmidJune(June15),butonetofatotal12twascaughton
July4.
 Inyears with different thermalconditions,the relative
abundanceof salmon specieslikewisevariesspatially.For
example,inwarmyearspinkandchumsalmonpredominate
numericallyintheopenoceanoffeastandwestKamchatka
inlateJune-earlyJuly,whilemostsockeyesalmonhaveal-
readymigratedfromthearea.Incoldyears,theratioamong
thesethreesalmonspecieschangesduetothedelayedmigra-
tionsof WestKamchatka sockeye salmon in North Pacic
waters and the later appearance of pink salmon. At these
times,sockeyesalmonpredominateinNorthPaciccatch-
esmade off Kamchatkauntil the secondten-day period in
July.
 The conditions observed in 2003 provide a clear il-
lustration of the effect of winter habitat conditions on the
distributionandbiologicalcharacteristicsofsalmonduring
thespring-summerseason.AnanalysisofSSTdistribution
inthesouthwesternBeringSeaduringwinterof2002–2003
showedthatJanuary2003wasarelativelywarmmonth.In
subsequentwintermonths,SSTsnearedmeanlong-termval-
ues,and,asaresult, overalliceconditionschanged.Later,
the processes of ice erosion exceeded the mean long-term
datesby 8–14days.This extended iceerosionwasassoci-
atedwith bothcyclonicactivityandthe advection ofwarm
andwetairmassesfromMarchtoApril.Astandardhydro-
logicalsurveyin the southwestern BeringSea in mid-June
found that the temperature of the upper 10-m water layer
wasthehighestmeasuredovertheprevioussixyears.From
MaytoJune2003,SSTsinthewesternandeasternNorthPa-
cicwere3–5°Chigherthanthemeanlong-termvaluesfor
thetime period.Dueto theearlierwarmingof thesewater
masses,pinksalmonappearedinresearchgillnetcatchesoff
eastKamchatkaasearlyasthebeginningofJune,andpeak
CPUEsofpinksalmon occurredbytheendofJune,asone
wouldexpectinawarmyear.
 Weassumethattheinterannualdifferencesin research
gillnetCPUEsobservedinourcasestudyarenotrelatedto
differencesintheverticaldistributionofsalmon.Ingeneral,
salmon are distributed at the surface of the open ocean at
night (Walker et al., 2007).  The nocturnal distribution of
salmon was conrmed by Klovach and Gruzevich (2004),
whosetresearchgillnets attheocean surfaceatnight,and
retrievedthem 9–12 hours later. Welchetal.(1995,1998)
foundabruptdecreasesintherelativeabundance(CPUE)of
salmoninresearchvesselcatches(usinggillnets,longlines,
0
2
4
6
8
10
12
14
16
18
20
1995 1996 1998 2000
Years
CPUE, number per net
all salmon
red salmon
chum salmon
Fig. 12. Catch per unit effort (CPUE, number of sh per net) of salm-
on in the western North Pacic Ocean off East Kamchatka, June
1995–2000. Red salmon = sockeye salmon.
0
1
2
3
4
5
6
7
8
9
18-20.05
21-25.05
26-31.05
01-05.06
06-10.06
11-15.06
16-20.06
21-25.06
26-30.06
1-5.07
6-10.07
Date
CPUE, individuals per net
1
2
3
Fig. 13. Catch per unit effort (CPUE, individuals per net) in the
western North Pacic Ocean near Kamchatka in 2001. 1 = sockeye
salmon, 2 = chum salmon, 3 = pink salmon. Date = days.month, e.g.,
18–20.05 is May 18–20, 2001.

NPAFC Bulletin No. 4
169
Stock-specic ocean distribution of salmon
andropetrawls)when SSTswereat ornearthe upperther-
mal limit for salmon habitats.  However, we do not know
ifthis decrease inabundanceresultedfroma change inthe
verticalorhorizontaldistributionofsalmon.Ourknowledge
islimitedbecauseSSTsinourcasestudywerenotattheup-
perthermallimitofsalmondistributionintheNorthPacic
Ocean.
 Climaticconditionsin2003(warmwinter,earlyspring)
alsoaffectedthebiologicalcharacteristicsofsalmon,partic-
ularlythedegreeofgonadmaturity.Becauseofhighwater
temperatures,salmonmaturedfasterin2003thanin2001or
2002.Maturingsalmonmigratedtotheirspawninggrounds
sooner and immature sh occupied open-ocean feeding
grounds(previouslyoccupiedbymaturingsh)earlierthan
usual.Asaresult,in2003alargenumberofimmaturesalm-
on were caught in North Pacic waters off Kamchatka as
earlyasJune,and in July,about40%ofthe catch in these
regions was immature male sockeye salmon (Fig. 14).  In
coldyears,thesamesituationoccursatleasttwoweekslater
(KlovachandGruzevich2004).Thus,takingintoconsider-
ationthecorrelationbetweensynoptictypeofwinter(coldor
warm)and migrationpatterns,itseemspossible to forecast
thedistributionofsalmonintheopenoceanduringtheirpre-
spawningmigrations,aswellasthedatesoftheirmigrations
tospawninggrounds.
 Weconcludethatmanymeasuresof interannual varia-
tion in salmon populations in the open ocean (e.g., the
number of sockeye and chum salmon in catches, the date
when immature individuals appear on pre-spawning feed-
inggrounds,thedegreeofgonadmaturity,theratiobetween
maleandfemaleindividuals,andthedatewhenpinksalmon
appearincatchesmadeinPacicwatersoffKamchatka)are
determinedtoalargeextentbyclimaticconditions.
 Hodgson et al. (2006) recently evaluated a similar
modeldevelopedbyBlackbourn(1987)forNorthAmerican
sockeyesalmon. Blackbournhypothesizedthatinterannual
variation in the timing of adult sockeye salmon returns to
riversinNorthAmericais related to winter-springSSTs in
theGulf ofAlaska.Accordingto this hypothesis,maturing
NorthAmerican sockeye salmonin the Gulf of Alaska are
distributedfarthertothenorthandwestinwarmwintersthan
incoolwinters.Ifswimmingspeedsandstartdatesofreturn
fromtheopenoceanareconstant,thensalmonrunsinsouth-
easternrivers(e.g.,FraserRiver,BC)willbelate,andrunsin
riversfarthertothenorthandwest(e.g.,BristolBay,Alaska)
willbeearly.Ingeneral,theresultsofHodgsonetal.(2006)
indicatedthat(1)correlationsbetweenmigratorytimingand
SSTarereversedfornorthernandsouthernpopulations,(2)
interannualvariationinsalmondistributionandseatempera-
turearerelated,and (3) the start-of-return dateisapopula-
tion-specictraitthatisnotaffectedbylocationatsea.
 Therearelittle or no empirical dataonsalmon migra-
tionsintheGulfofAlaskatovalidateHodgson’sresults.Al-
mostallhighseastaggingresearchintheGulfofAlaskawas
carriedout duringthe 1960s, but oceanographic conditions
inthatdecadewereirregular—particularlyinthewinterand
0
10
20
30
40
50
60
70
80
90
100
16-
20.06
26-
30.06
16-
20.07
26-
31.07
6-
10.08
Date
Share, of immature individuals%
males
females
Fig. 14. Share of immature sockeye salmon in the western North
Pacic Ocean near Kamchatka, 2003. Date = days.month, e.g., 16-
20.06 is June 16-20, 2003.
Fig. 15. Example of annual variation in stock-specic distribution of
maturing sockeye salmon in the Gulf of Alaska in the spring (April
1965 and 1966), as shown by historical tagging experiments (n = 193
sh). The symbols indicate the high seas release locations of tagged
sh later recovered in western Alaska. Western Alaska (Bristol Bay)
= closed (grey) triangles and southern British Columbia (Fraser R.) =
closed (black) diamonds. Note that while both stocks are distributed
across broad fronts, and distributions of the two stocks overlap signif-
icantly; Bristol Bay sockeye salmon are distributed farther to the west
and north than southern British Columbia sh. Using the Southern
Oscillation Index criteria, it can be determined that 1964–65 was a
moderate La Niña (cold) winter and 1965–66 was an El Niño (warm)
winter. AK = Alaska, YT = Yukon Territory, BC = British Columbia.
Data source: High Seas Salmon Research Program, University of
Washington, Seattle.

NPAFC Bulletin No. 4
170
Myers et al.
spring of 1966, when northward transport was about 50%
ofthe10-year average, summer wind stressconditionsex-
istedin February,and allwestward ow of warmwater in
theAlaskaStreamwasapparentlyre-circulatedintheGulfof
Alaska(Favoriteetal.1967).Thus, dataretrievedin those
yearsdonot conform to expectations. Usingthe Southern
OscillationIndexcriteria,itcanbedeterminedthat1964–65
hadaLaNiña(cold)winterand1965–66anElNiño(warm)
winter(http://www.wrcc.dri.edu/enso/reanlnen.html).Inthe
springof1966,salmonweredistributedfartheroffshoreto
thesouthandwestthantheywereinthespringof1965(IN-
PFC1967a,b).Limitedtagdatasuggestthatbothnorthern
(Bristol Bay) and southern (Fraser River) sockeye salmon
stocksmaybe displaced to the southandwest,rather than
northandwest,inthespringfollowingawarmwinter(Fig.
15).Additionalresearchisneededto develop and validate
models to predict the effects of climatic forcing on stock-
specicopenoceandistributionandruntimingofsalmon.
Interannual Variation in Ocean Conditions in the Gulf of
Alaska and the Bering Sea
 In summer, interannual variation in the relative abun-
danceofsalmon in the BeringSeaappears to be inversely
relatedtothatofsalmonintheGulfofAlaska(Ishidaetal.
2002).Thisrelationmightbedrivenatleasttosomeextent
by SSTsor other oceanographic conditions in the Gulf of
Alaska,e.g.,chumandsockeyesalmonaremoreabundantin
theBeringSeathanintheGulfofAlaskainyearswhensum-
merSSTsarehighintheGulfofAlaska(Ishidaetal.2002).
Inthis section we reviewinformationoninterannualvaria-
tioninoceanenvironmentswheresalmonstocksaredistrib-
uted.  The basic premise of this part of our review is that
thecirculation and climate inthe North PacicOceanand
BeringSea areinterconnected.Anotherpremise isthatthe
oceanographicconditionsintheseregionshavebeenchang-
ingsignicantlyoverthepastseveraldecades,evenpriorto
theregimeshiftof1977(Mantuaetal.1997).
 The1977regimeshiftwasalarge-scaleclimaticevent,
andcanbeseeninmuchofthe environmental data for the
region,asillustratedinMantuaetal.(1997).Thatstudyfo-
cusedonSST,becausethatmeasurementwastheubiquitous
data set, though not necessarily the “best” data to explain
salmonvariability.Unfortunately,otheroceanographicdata
aresparse.Long-termseriesofsmall-tomid-scaledataare
absent.  We must, therefore, deal with the few large-scale
andlong-term dataseriesthatareavailable. Forthenorth-
easternPacic OceanandGulfofAlaska, data setsinclude
hydrographicmeasurementstakenatOceanStationP(OSP;
50°N,145°W)since1958(Freelandetal.1997);coastalob-
servationsofsealevel;measurementsofsurfacetemperature
andsalinitytakenatCanadianlighthousesalongtheBritish
Columbia coast, some from early in the last century
(Freeland,http://wlapwww.gov.bc.ca/air/climate/indicat/pdf/
seasurftemp-tdoc1.pdf);hydrographicobservationsmadeat
GulfofAlaskaStation1(GAK1, 60°N,149°W)from1970
to the present (Royer 2005); and observations of currents,
temperature and salinity per depth, taken since 1995 from
Fig. 16. Gulf of Alaska (GAK1) temperature anomalies at 150 m (°C, upper panel) with SOI (lower panel) since 1970 with responses to ENSO
events noted by vertical lines between panels. There is a 99% correlation between data sets with a C.I. linear trend of 0.03°C increase/year at
150 m and throughout the water column (250 m). From Fig. 14 in Royer (2005): “One standard deviation is indicated with dashed line.” ENSO =
El Niño-Southern Oscillation, SOI = Southern Oscillation Index.

NPAFC Bulletin No. 4
171
Stock-specic ocean distribution of salmon
Fig. 17. Monthly coastal freshwater discharge for the Gulf of Alaska from the Alaska-B.C. border to Cook Inlet. From Fig. 5 in Royer (2005):
“Heavy line is 5-year ltered (Butterworth) discharge.” Note that this volume exceeds that of the Mississippi River’s 14,400 cubic meters per
second.
mooringsatSiteM2onthe70-misobathinthesoutheastern
BeringSea(Stabenoetal.1995,2001,2002a).
 Kingetal.(2005)summarizedoceanographicchanges
in the region since the 1998 regime shift.  These changes
includeincreasedstormactivity and increased mixed layer
depthintheGulfofAlaskaastheregionreturnedtocooler,
stormier conditions.  The Bering Sea andAleutian Islands
apparentlyremainedunaffectedby these cooler conditions,
insteadwarmingandlosingtheiricecover.Recentclimate
variability over the North Pacic Ocean and Bering Sea
supporttheideathattheseregionsmightbelinkedtogether
(Kingetal.2005).
Gulf of Alaska
 Inwinter,thedepthofthemixedlayeratOSPhasbeen
decreasing,whichsuggeststhatthe supplyofnutrientsinto
theeuphoticzonewillalso diminish(Freelandetal. 1997).
This shoaling of the mixed layer is the result of a general
warmingand freshening of theocean’supperlayer,as ob-
servedatOSPandalongthe British Columbia coast(H.K.
Freeland, http://wlapwww.gov.bc.ca/air/climate/indicat/pdf/
seasurftemp-tdoc1.pdf).IncreasedwindstressovertheGulf
ofAlaskaisexpectedtodeepenthewintertimemixedlayer,
butapparentlydiminisheddensityintheupperlayercounter-
actsthetendencytowardincreasedwindmixing.Long-term
hydrographicmeasurementsatGAK1,farthernorth,support
thendingsofFreeland’scoastalmeasurements. Asigni-
canttemperatureincreaseof0.03°Cperyearhasbeenfound
throughout the entire water column (250 m) near Seward,
Alaska (Royer 2005; Fig. 16).  The salinity of the upper
layer(0–100m)isalsodiminishinginresponsetoincreased
coastal precipitation and freshwater discharge since 1970
(Fig.17).UnlikeOSP,thiscoastalsitehasnotdisplayedany
signicant trends in wintertime, mixed-layer depths since
1970(Sarkaretal.2005)
 Therefore,themajorchangesthathavebeentakingplace
inthephysicaloceanographyoftheGulfofAlaskainclude:
arelativelysteadyincreaseinthecoastalwatertemperature
ofthe upper layers, adecreasein the mixed-layer depthat
OSP(Freelandetal.1997),anincreaseinstorminessinthe
Gulf ofAlaska, and a decrease in the upper-layer salinity,
resultingfromincreasedprecipitationandcoastalfreshwater
discharge.Thisstraticationisalsoenhancedbyrapidgla-
cialmeltingincoastalAlaska(Arendtetal.2002).Increased
straticationwillinhibittheux ofnutrientsintothe upper
euphoticzone, trap organismsin thatupper layer, increase
theamplitudeof upper-layer seasonal temperature changes
andadvancethetimingofthespringbloom.Oceanicstrati-
cationistakingplaceinconcertwithincreasedwindstress,
aforcethatcould counteract the increased stabilityinnew
oceanlayers.TheincreasedcirculationintheGulfofAlaska
asaresult of increased straticationandwind stress could
alsoproducemoreeddiesalongtheshelfbreak.Wedonot
haveenoughlong-termdataoneddydynamicstodetermine
whetherthislastconjectureistrue.
Bering Sea
 IncontrastwiththeGulfofAlaska,theBeringSeahas
verylittleprecipitation,andwindstresshasdiminishedsince
the1997–98regimeshift(WirtsandJohnson2005).Asdis-

NPAFC Bulletin No. 4
172
Myers et al.
cussedintheprevioussection,theinowofrelativelywarm
waterfromtheGulfofAlaskawillleadtoincreasedstrati-
cationandenhancedsurfaceandupper-layerwatertempera-
tures. Increased stratication willproduce wider variation
inseasonal temperatures dueto solar heating. Interannual
changesinthemixedlayerinthesoutheastBeringSeafrom
2001to2004(Fig.18)revealanincreaseinthetemperature
ofthemixedlayer,accompaniedbyadecreaseinwatersa-
linityanddensity (Wirtsand Johnson 2005), ashiftthat is
consistentwiththechangingupstreamconditionsintheGulf
ofAlaska.
 Changes in seasonal signals such as temperature, sea
ice,and winds, willaffectsalmon production in theBering
Sea(Huntetal.2002;Fig.19).Thetimingandqualityofthe
springbloomishighlydependent onthepresenceof winds
andseaiceinearlyspring.Whenseaiceispresentinorafter
late March, a strong bloom takes place as the ice retreats.
Ifthere is no ice or the iceretreats before late March, the
bloomtakesplaceinMayorJune.Inadditiontoseaice,ed-
diesapparentlyplayanimportantroleinmigrationpatterns,
thougheddydynamicsandformationareimperfectlyknown
(Stabenoetal.2002a,b).
 Insummary,itappears thatupper-layerwatertempera-
tures,stratication,andwindstressareincreasing,whilesa-
linitiesare decreasing (GulfofAlaskaonly).Althoughthe
inuenceofthesemid-scalefeaturesisyettobedetermined,
eddiesmayplayamajorroleinsalmonproductivity.Con-
tinuedsatellitealtimetrywillprovide enhancededdystatis-
ticsinthefuture,perhapssheddinglightonthisquestion.
CONCLUSIONS
 Atpresentourdataareinsufcienttoanswerthe ques-
tion, “What does annual variation in open-ocean salmon
stockcompositiontellusaboutenvironmentalconditionson
small-tomid-sizescales?”Weconcludefromourbriefre-
view,however,thatclimate-drivenchangeinoceanographic
conditionsinopen-oceanfeedingareasandalongmigratory
routes of Asian and NorthAmerican salmon can result in
predictabledifferencesinthedistributionandmigrationpat-
ternsofsalmon.
 Clearly, advancement in our knowledge of stock-spe-
cic ocean distribution and migration patterns is vital to
understandingthestatusofPacicsalmoninmarineecosys-
tems. Updated models of oceandistributionandmigration
areneeded for most of themajor regional stockgroups of
salmonoriginatingfromriversintheNorthPacicRim.
 Pacic salmon species have evolved over millions of
yearstotakeadvantageofdifferentecologicalnichesinthe
openocean.Thediversityofthesenaturaladaptationsbynu-
merousindividualpopulationshasprovidedsalmonspecies
asawholewitharesilientbuffertotheeffectsof environ-
Fig. 18. Mixed layer changes in the Bering Sea from June 2001 to September 2004. From Fig. 2 in Wirts and Johnson (2005): “Mixed-layer
potential temperature (top left), salinity (top right), potential density anomaly (bottom left), and pressure at the base of the mixed layer (bottom
right) plotted versus time using the oat CTD data (plusses) in the southeast Aleutian Basin with seasonal cycles (solid lines) estimated from
annual and semiannual harmonics t to these data.”

NPAFC Bulletin No. 4
173
Stock-specic ocean distribution of salmon
Fig. 19. Water column temperatures from the M2 mooring in the
middle domain of the Bering Sea, 1995-2000. From Fig. 6 in Hunt
et al. (2002): “Areas of black indicate cold water resulting from the
presence of melting sea ice. The yellow line near the bottom of each
panel indicates uorescence at 11–13 m. For each year, uorometer
traces have been scaled to the highest value in that year. Gaps in the
uorometer record are the result of fouling of the instrument. When
ice is present in or after late March, a strong uorescence peak oc-
curs as the ice retreats (1995, 1997). When there is no ice (1996)
or the ice retreats before late March (1998, 2000), an open-water
bloom occurs in May or June. In 1999, the spring was stormy and ice
recurred in May. There was a bloom in late March, and another weak
and prolonged period of production in late May and June.”
mentalchangeontheirmarinegrowthandsurvival.Chang-
esin climate,oceanicconditions,and migrationpatternsof
salmonintheopenocean are inextricably intertwined, and
improvementsinourabilitytomakepredictionsaboutsalm-
onmay very well improve our ability to make predictions
abouttheenvironment.Globalwarmingisresultinginmore
frequentandunpredictableenvironmentalchangesinopen-
oceanhabitatsthroughwhichsalmonmigrate.Weconclude
that changes in the distribution and migration of indicator
stocksofadultsalmonreturningfromtheopenoceanmight
provide an “advance warning” of interannual changes in
NorthPacicmarineecosystems.
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