Oceanography and ecology of the Aleutian Archipelago

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Title
Oceanography and ecology of the Aleutian Archipelago: Spatial and temporal variation
Permalink
https://escholarship.org/uc/item/2p50v2wv
Journal
Fisheries Oceanography, 14(SUPPL. 1)
ISSN
1054-6006
Authors
Hunt, GL
Stabeno, PJ
Publication Date
2005
DOI
10.1111/j.1365-2419.2005.00378.x
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FISHERIES OCEANOGRAPHY Fish. Oceanogr. 14(Suppl. 1),292–306,2005
Oceanography and ecology of the Aleutian Archipelago:
spatial and temporal variation
GEORGE L. HUNT, JR1,* AND PHYLLIS J. in populations of Steller sea lions and other species,
STABENO2 andhasimportantimplicationsforthemanagementof
1DepartmentofEcologyandEvolutionaryBiology,Universityof regionalmarineresources.Weconcludethatthemarine
waters of the Aleutian Archipelago are divided into
California, Irvine, CA92697, USA
2National Oceanicand Atmospheric Administration, Pacific atleasttwodifferentecologicalregions,withpotential
MarineEnvironmentalLaboratory,7600SandPointWay,NE, foraconcomitantseparationofsomefisheryresources.
Seattle, WA98115, USA
Key words: Aleutian Islands, biogeographic patterns,
biophysicalcouplingandecosystemstructure,bottom-
ABSTRACT up forcing, island passes, physical forcing of marine
ecosystem, Steller sea lion
This compilation of new information and summaries
of earlier work emphasizes variability within marine
waters of the Aleutian Archipelago. From the Alaska
INTRODUCTION
PeninsulatoNearStrait,netflowthroughthepassesis
northward, with four passes (Amukta, Amchitka, AlthoughtheAleutiansareasiteofimportantfisheries
Buldir,andNearStrait)contributingmostoftheflow. andthecenterofdistributionoftheendangeredwestern
EastofSamalgaPass(169(cid:1)W),watersderivedfromthe stock of Steller sea lions (Eumetopias jubatus), their
Alaska Coastal Current predominate, whereas west of remoteness has limited oceanographic investigation.
Samalga Pass, waters of the Alaskan Stream predom- Nevertheless, it has been known for some time that
inate. Thepatternofstormtrackscreatesaclimatolo- there were spatial discontinuities in the diets and
gical (interannual and long term) transition zone in populationtrajectoriesofStellersealions(Yorket al.,
weather features (e.g. surface air temperature) near 1996; Sinclair and Zeppelin, 2002) that suggested
170(cid:1)W.ThemarineecosystemoftheAleutianArchi- possible underlying spatial heterogeneity in Aleutian
pelagoalsohasastrongdiscontinuityatSamalgaPass, marine ecosystems. Additionally, among marine orni-
where cold-water corals, zooplankton, fish, marine thologists, the shallow, narrow passes of the Aleutian
mammals and foraging seabirds show a step change in Islandsarerenownedfortheirimmenseaggregationsof
species composition. Diets of ground fish, Steller sea foraging seabirds, whereas many of the larger, deeper
lions(Eumetopiasjubatus)andsomeseabirdsalsochange passeswerethoughttosupportfewforagingbirds.Thus,
there.Lowergrowthratesofsomefishspeciesandstable ecologicallyimportantspatialheterogeneitywasknown
isotope data indicate that productivity declines west- toexistatavarietyofscales.Thesepatternsofvariation
ward along the archipelago. The available data dem- gave rise to questions about the causal basis of the
onstrate considerable ecosystem variability over time observedpatterns,andtheirimplicationsforthestruc-
scales of decades to millennia. Abrupt changes in ture and function of the Aleutian marine ecosystem.
composition of fish communities at several of the When funding became available to investigate poten-
majorpassessuggestthatSamalgaPassmaymarkonly tial bottom-up and top-down impacts on Steller sea
one of several ecological divisions of Aleutian waters. lions in the Aleutians in 2001 and 2002, there were a
This spatial and temporal heterogeneity provides an numberofscientificquestionsofbroad,generalinterest
importantcontextwithinwhichtoviewrecentdeclines thathelpeddirecttheresultingfieldstudy(theAleutian
Passes Project). As a result, research cruises were
organized in 2001 and 2002 to investigate spatial pat-
ternsinecosystempropertiesatavarietyofscalesfrom
*Correspondence. e-mail: [email protected].
that of frontal systems within passes to large blocks of
Presentaddress:SchoolofAquaticandFisherySciences,Box
thearchipelago,andatlevelsoforganizationfromocean
355020, UniversityofWashington, WA98195, USA.
physics and chemistry to the distribution, abundance
Received 10March2005
andforagingecologyofmarinebirdsandmammals.
Revised version accepted 25April2005
292 (cid:2)2005Blackwell PublishingLtd.
Aleutian oceanography andecology 293
In assembling this volume, it was felt that it was to the Commander Islands off the Kamchatka Penin-
important to include recent work by investigators sula in eastern Asia (Fig. 1). From its northernmost
working in the Aleutian Archipelago in addition to point in Alaska at False Pass to its southernmost
thosewhotookpartincruisesin2001and2002.Thus, islands in the Delarof group at 179(cid:1)10¢W, the Aleu-
the papers in this supplemental volume of Fisheries tian Archipelago spans over four degrees of latitude
Oceanography have been motivated by a variety of (425 km). This long, narrow chain of volcanic
interests, including fisheries management (Heifetz mountaintops separates the waters of the sub-arctic
et al., 2005; Logerwell et al., 2005; McDermott et al., NorthPacificOceanfromtheBeringSea.Recognizing
2005), the biology and ecology of sea lions (Call and the importance of the region for wildlife, the Aleu-
Loughlin, 2005; Fadely et al., 2005; Sinclair et al., tians are a designated UNESCO International Bio-
2005), an interest in the spatial patterns of variability sphere Reserve and most of the islands are within the
in the ocean environment occupied by the sea lions Alaska Maritime National Wildlife Refuge. The
andothertoppredators(Byrdet al.,2005;Coyle,2005; Aleutians form the southern boundary of two Large
Jahnckeet al.,2005;Laddet al.,2005a,b;Mordyet al., Marine Ecosystems, the Eastern and the Western
2005; Rodionov et al., 2005; Stabeno et al., 2005; Bering Sea (e.g. http://www.edc.uri.edu/lme/, May
Vlietstraet al.,2005),understandingchange,overtime 2005).
scales of millennia, in human occupation and ecology The passes between the islands vary from narrow,
in the Aleutians (Causey et al., 2005), and sustaining shallow channels between large islands, to wide, deep
theservicesthatthisecosystemprovides(Schumacher passages bordered by small islands perched on narrow
and Kruse, 2005). In this overview, we provide a syn- surrounding shelves (Table 1) (Favorite, 1974). The
thesis of recent and past studies to update current passes, particularly the deep passes, allow transfer of
knowledgeofthespatialandtemporalvariabilityofthe water between the North Pacific Ocean and the Ber-
marine ecosystems of the Aleutian Archipelago. We ingSea(Stabenoet al.,2005).Thepassesalsoprovide
also identify some ofthe questions that remain. habitat for a vast array of species. The eastern Aleu-
tians, as far west as SamalgaPass,are characterized by
shallow, generally narrow passes, and a mixture of
PHYSICAL SETTING
large and small islands. The shelf (200 m depth con-
The Aleutian Archipelago stretches westward over tour) around these eastern islands is narrow on the
2000 kmfromtheAlaskaPeninsulainNorthAmerica northside(<10 km),buttothesouth,itstartsoffwide
Figure1. Map of the Aleutian Islands
showing200-misobathandplacenames
mentioned in the text. ACC, Alaska
Coastalcurrent;ANSC,AleutiansNorth
Slope Current; BSC, Bering Slope
Current.
(cid:2)2005Blackwell Publishing Ltd,Fish. Oceanogr., 14(Suppl.1), 292–306.
294 G.L.HuntandP.J.Stabeno
Table1. DepthandareaofmajorpassesintheAleutian-Commanderislandarc(afterFavorite,1974;Laddet al.,2005a).Flow
fromArsen’ev (1967)inFavorite(1974), and Stabeno etal. (2004).
Cross-sectional Flow intoBering
Region Pass/strait Depth(m) area (km2) Sea(106m3s)1)
Eastern Aleutians Unimak 52 1.0 0.1
Akutan 30 0.1 0.1
Umnak 7 1.6 ??
Regionaltotal 3.9 (cid:1)0.2
Central Aleutians Samalga 200 3.9 ??
Chuginadak 210 1.0 ??
Herbert 275 4.8 ??
Yunaska 457 6.6 ??
Amukta 430 19.3 4.0
Seguam 165 2.1 0.4
Tanaga* 235 3.6
Amchitka* 1155 45.7
Regionaltotal 87.0 >8.8
Western Aleutians Kiska 110 6.8
Buldir 640 28.0
Semichi 105 1.7
Regionaltotal 36.5 0.7
Commander-Near Straits Near 2000 239
Commander 105 3.5
Regionaltotal 242.5 14.4
Kamchatka Strait(cid:1) 4420 335.3 2.6
*Favoritegives the combinedflow forTanagaPass and Amchitka Passas4.4Sv.
(cid:1)Below 3000m.
((cid:1)100 km) near the Alaska Peninsula and narrows to Low is strongest in winter, and practically disappears
<25 kmbySamalgaPass.ThecentralAleutians,from insummer(Rodionovet al.,2005).TheAleutianLow
Samalga Pass to Amchitka Pass, are a combination of is the statistical combination of two patterns, one a
small to medium-sized islands perched on narrow region of low pressure centered east of 180(cid:1), and the
shelves (e.g. Amukta, Yunaska Islands) and deep, other a split between a low-pressure centre east of
wide passes (e.g. Amukta, Tanaga, Amchitka) KamchatkaPeninsulaandasecondcenterintheGulf
(Fig. 1). There are also narrow, shallow passes (e.g. ofAlaska.Typically,stormsoriginateeastofJapanand
AmiliaPass)wherewaterrushespastclustersofislands move northeastward along the Aleutian Archipelago
that, in aggregate, create a large landmass (Fig. 1). totheGulfofAlaska;lessfrequently,theyoriginatein
Westward from Amchitka Island, much of the Rat theGulfitself(Rodionovet al.,2005).Intheregionof
Island group (Amchitka, Kiska, and islands in cyclogenesis off Japan,storm tracks show considerable
between)intheAleutianArchipelagoischaracterized consistency in direction; storm tracks originating in
ashavingsmallislandsandwidepasses,someofwhich the Gulf of Alaska are more variable. Sea level pres-
contain deep canyons (e.g. Murray Canyon, west of sure variability reaches its maximum in the gap
Kiska Island) (Fig. 1). In contrast, the Near Islands betweenthesetwoareasofhighstormfrequency(170–
(e.g.AgattuandAttu)aresituatedonacomparatively 150(cid:1)W, south of the Alaska Peninsula) (Rodionov
broad, shallow shelf. Between Attu and the Com- et al., 2005).
manderIslands,NearStraitprovidesadeeppassagefor Although the seasonal signal in surface air tem-
exchange of water between the North Pacific Ocean peratures is similar throughout the Aleutian Islands,
and the Bering Sea. interannual and interdecadal variations in surface air
The Aleutian Islands have a cool, wet and windy temperatures differ substantially in the western and
maritime climate. The region is frequented by storms, easternportionsoftheAleutianArchipelago,withthe
andtheAleutianLowisanindex ofwherestorms,on dividing line between the two regions in the vicinity
average, reach their lowest pressure. The Aleutian of 170(cid:1)W (Rodionov et al., 2005). In the eastern
(cid:2)2005Blackwell Publishing Ltd,Fish. Oceanogr.,14(Suppl. 1),292–306.
Aleutian oceanography andecology 295
Aleutians, there was a shift in 1977 toward a warmer Pass, through which it enters the southeastern Bering
climate, whereas in the western Aleutians there has Sea (Stabeno et al., 2002; Ladd et al., 2005a). How-
been a slow, steady decrease in winter surface air ever,Laddet al.(2005a)showthatwhilethe freshest,
temperatures since the 1950s (Rodionov et al., 2005). inshore portion of the Alaska Coastal Current enters
Varianceinwintersurfacetemperaturesinthewestern Unimak Pass, the saltier, offshore portion flows west-
Aleutians doubled in the period 1986–2002 when ward to Samalga Pass, where it turns northward on
comparedwith1965–85,resultinginadoublingofthe the east side of the pass to join the Aleutian North
seasonal cooling rate since the 1950s. On a compar- Slope Current.Thus,fromFalse Pass toSamalga Pass,
ativenote,insoutheastAlaska,thevarianceofsurface the Alaska Coastal Current is the primary source of
air temperature decreased by a factor of 2 (Rodionov northward flowing water in the passes. West of
et al., 2005). The palaeoenvironmental record indi- Samalga Pass, as far as Near Strait, the Alaskan
cates that during the Medieval Warm Period of Streamanditsextensionarethemajorsourceofwater
Europe(c.AD900–1350),temperaturesonBeringand entering the passes from the North Pacific (Reed and
Shemya Islands were elevated, whereas in the eastern Stabeno, 1993; Stabeno et al., 2002). The trajectories
Aleutian Islands and Gulf of Alaska, temperatures ofdriftersreleasedintheGulfofAlaskasupportthese
weredepressed(Heusseret al.,1985;Savinetskyet al., results (Ladd et al., 2005a).
2004;Causeyet al.,2005).Thus,theremaybealong- The differences in source water along the southern
standing split in climate between the eastern and side of the eastern and central Aleutian Islands result
western extremes of the Aleutian Archipelago. instrikinglydifferentphysicalandchemicalproperties
Three currents (Fig. 1) supply the waters bathing in the passes. The waters of the Alaska Coastal Cur-
the eastern and central Aleutian Islands: the Alaska rentarewarmerandfresherthanthoseoftheAlaskan
Coastal Current (Schumacher and Reed, 1986; Reed, Stream, and this results in a front as the last of the
1987) and the Alaskan Stream (Favorite, 1967; Fav- Alaska Coastal Current turns northward at Samalga
orite et al., 1976; Onishi and Ohtani, 1999) flow Pass (Fig. 2) (Ladd et al., 2005a). A front also occurs
westward along the south side of the archipelago, and to the north of Samalga Pass with much fresher water
the Aleutian North Slope Current flows eastward occurringtotheeastofthepass,suggestingthatfluxes
along the north side from Amchitka Pass (Stabeno throughtheeasternpassesfreshenthesurfacewatersof
and Reed, 1994, 2004; Reed and Stabeno, 1999). The the Aleutian North Slope Current (Fig. 2).
Alaska Coastal Current begins along the coast of The macronutrient levels also differ between the
southeastern Alaska and flows (cid:1)1000 km along the Alaska Coastal Current and the Alaskan Stream.
coast of Alaska to Samalga Pass. It is forced by winds Surface nitrate levels are higher west of Samalga Pass
and modified by coastal freshwater discharge in the than to the east of it (Ladd et al., 2005a), and Mordy
Gulf of Alaska (Royer, 1979; Royer et al., 1979; Sta- et al.(2005)showthatthisislargelytheresultofdeep
beno et al., 2004). There is a strong seasonal signal in tidal mixing within the passes. In May and June of
transport in response to local wind forcing, and in 2001 and 2002, chlorophyll concentrations were high
salinity in response to freshwater discharges in the in waters east of Samalga Pass and low between
Gulf of Alaska (Schumacher and Reed, 1980, 1986; Samalga Pass and Seguam Pass (Mordy et al., 2005).
Stabenoet al.,2004).Incontrast,theAlaskanStream, Primary production was relatively low between Uni-
a western boundary current of the North Pacific sub- mak and Tanaga Passes, except for the north end of
arctic gyre (Thomson, 1972; Reed, 1984), forms Seguam Pass and the shelf edge in the southeastern
southeast of Kodiak Island and follows the shelf break BeringSeawherenutrients,mixedupwardfromdeeper
as a narrow, high-speed current to Amchitka Pass. At layersbytidalactioninthepasses,weretrappedabove
Amchitka ((cid:1)180(cid:1)W), the archipelago turns north- the pycnocline north of the Aleutians (Mordy et al.,
westward and the stream broadens and separates from 2005).
theslope.Thus,westofAmchitkaPass,meandersand Flow in the passes of the Aleutian Archipelago is
eddies exist in theoretical, model and observational dominated by the tides (Stabeno et al., 2005). Strong
studies (Thomson, 1972; Favorite et al., 1976; Over- tidal currents mix the water column top to bottom
land et al., 1994; Stabeno and Reed, 1994). This over the shallow sills (e.g. to 80 m in Akutan and
portion of the Alaskan Stream remains the predom- 140 m in Seguam Pass). In passes that are wider than
inate source of water bathing the western Aleutians the internal Rossby radius (e.g. Amukta Pass), flow is
(Stabeno and Reed, 1992). bi-directional, resulting in northward flow on the east
Priortothepresentwork,thewesternextentofthe side and southward flow on the west side (Ladd et al.,
Alaska Coastal Current was assumed to be Unimak 2005a; Stabeno et al., 2005). Except in Kamchatka
(cid:2)2005Blackwell Publishing Ltd,Fish. Oceanogr., 14(Suppl.1), 292–306.
296 G.L.HuntandP.J.Stabeno
(a)
Bering Sea
North Pacific
S = 32.7 ± 0.3
S = 31.9 ± 0.1
S = 31.4 ± 0.1
(b)
34.0
33.8
33.6
33.4
33.2
33.0
Islands of the Unimak 32.8
Four Mountains Pass 32.6 Figure 2. Sea surface salinity (psu)
32.4 measured underway on the 2001 cruise
32.2 (from Ladd etal., 2005a). (a) Salinity
Yunaska Isl. 32.0
plotted against latitude south of the
31.8
AleutianIslands(black)andnorthofthe
31.6
Samalga islands (red). (b) Average along-track
31.4
Pass salinity by region, including the region
31.2
east of Unimak Pass, between Unimak
Amukta 31.0
Salinity
Pass 30.8 and Samalga Passes, and between
30.6 Samalga and Amukta Passes.
Strait where flow is southward from the Bering Sea, species),cup(fourspecies)andhydrocorals(17species)
netflowhasbeentothenorthinallpasseswhereithas predominated to the west of Samalga Pass (Heifetz,
been measured to date. The northward transport is a 2002; Heifetz et al., 2005). Although research into
source of nutrients for the Bering Sea. Monthly and these cold-water corals is in a preliminary stage, it is
long-term variability in transport through Amukta alreadyclearthatthewatersofthecentralandwestern
Pass is related to the position and strength of the Aleutian Archipelago support remarkably high con-
Alaskan Stream (Stabeno et al., 2005). centrationsanddiversityofthesepoorlyknowntaxa.In
contrast,thecoralpopulationsfoundinthewatersofthe
eastern Aleutians differ little from those found across
SPATIAL VARIABILITY AND
theshelfandupperslopeoftheGulfofAlaska(Heifetz,
BIOGEOGRAPHIC PATTERNS
2002;Heifetzet al.,2005).
A number of taxa have distribution patterns in the Within the water column, zooplankton species
AleutianArchipelagothatsuggestresponsestohabitat compositioneastofSamalgaPassistypicalofaneritic
variables,includingthechangeinwaterpropertiesand community and includes species such as Calanus
chemistry at Samalga Pass. For example, the species marshallae, Pseudocalanus spp., Acartia spp., and Thys-
diversity of cold-water corals increases sharply west of anoessa inermis, all characteristic inhabitants of shelf
Samalga Pass (Heifetz et al., 2005). East of Samalga ecosystems. In contrast, the waters between Samalga
Pass, 96% of bottom trawls containing corals had but and Tanaga Passes are dominated by oceanic species
oneortwotaxapresent,whereaswestofSamalgaPass, of zooplankton such as Neocalanus spp., Eucalanus
33%ofhaulscontainedmorethantwotaxa.Seawhips bungii and Euphausia pacifica (Hunt et al., 1998;
(35% of species) and sea pens (20.5% of species) pre- Coyle, 2005). Coyle (2005) found that up to 50% of
dominated in the east, whereas gorgonian (nine the observed variance in zooplankton abundance
(cid:2)2005Blackwell Publishing Ltd,Fish. Oceanogr.,14(Suppl. 1),292–306.
Aleutian oceanography andecology 297
between Unimak and Tanaga Passes could be Growth rates of some fish species show a decline
accounted for by salinity and temperature. The from east to west. Length-at-age for northern rockfish
abundances of C. marshallae and T. inermis were and Atka mackerel decreased from east to west
positively correlated with temperature and negatively through the Aleutians (Logerwell et al., 2005).
with salinity, whereas the abundances of E. pacifica GrowthratesofPacificoceanperchalsodeclinedfrom
and N. cristatus showed negative correlations with easttowest,butthedifferenceswerenotasstrikingas
temperature and positive correlations with salinity. those in Atka mackerel and northern rockfish. These
Likewise, in spring the biomass of near-surface meso- patternssuggestthatpreybiomassmaybelessplentiful
zooplankton near Kiska Island, western Aleutian Is- in the western Aleutian Islands than in the eastern
lands, was dominated by oceanic species of copepods portion of the archipelago.
(Coyle and Hunt, 2000). At least 26 species of seabirds nest in the Aleutian
Demersalfishalsoshowstronglongitudinalpatterns Archipelago (Byrd et al., 2005). Their nesting and
infaunalaffinities,speciesdistributionandabundance, foragingdistributionsreflecthabitatandpreyneeds,as
diets, and growth rates (Logerwell et al., 2005). Mov- well as the influence of terrestrial predators such as
ing westward from the Alaska Peninsula, species introduced foxes and rats (Byrd et al., 2005). Samalga
richnessdropssharplyatSamalgaPass(38%),whereas Pass does not seem to be a break point in the distri-
there islittle decline betweenSamalgaand Amchitka bution of breeding seabirds based on a species by
Passes(4%).Thereisafurtherdeclineof20%westof species evaluation (Byrd et al., 2005). Nevertheless, if
BuldirIsland.Ofthe36Aleutiandemersalfishspecies species are aggregated by foraging guild, there are
associatedwiththeOregonianzoogeographicprovince apparent large-scale patterns (Table 2). In particular,
(Allen and Smith, 1988), there is a 30% decline in about 68% of the 1.7 million piscivorous seabirds
species richness between UnimakandSamalgaPass,a nesting in the Aleutian Islands occur to the east of
54% drop between Samalga and Amukta Passes and a Samalga Pass. In contrast, 92.5% of the 7.9 million
further drop from nine species to two species between planktivorous seabirds nest to the west of Samalga
Amchitka Pass and Buldir Island (Logerwell et al., (U.S. Fish and Wildlife Service, 2000). These species
2005).AsLogerwellet al.(2005)pointout,thelarvae include auklets (least Aethia pusilla, crested A. crista-
of many of these fish species are pelagic, and their tella, whiskered A. pygmaea and parakeet A. psittacula
distributions are likely to be strongly influenced by auklets) that forage on zooplankton (e.g. large
water mass origins and movements. Additionally, the copepods – Neocalanus plumbcrus, N. cristatus, and
deep passes may be potential barriers to dispersal euphausiids–Thysanoessaraschii,T.inermis,andlarval
(Logerwell et al., 2005). Abundance patterns of fish) and nest predominately on islands between
widespreadspeciesshowlongitudinalshiftsinnumbers TanagaandBuldirPasses(Joneset al.,2001;Byrdet al.,
(Atkamackerel,Pleurogrammusmonoterygius,aremore 2005). These birds forage inand around passes, where
abundant west of Samalga Pass; walleye pollock, tidal currents result in concentrations of prey (Day
Theragra chalcogramma, are less abundant west of andByrd,1989;Huntet al.,1998;Byrdet al.,2005).
BuldirIsland)ordepthpreferences(Pacificcod,Gadus Marine bird species that are inshore foragers, such
macrocephalus; walleye pollock). At smaller spatial as cormorants, glaucous-winged gulls (Larus glauces-
scales, several of the passes (Samalga, Seguam, and cens) and pigeon guillemots (Cepphus columba), nest
Tanaga)appeartosupportunusuallyhighbiomassesof predominatelyonlargeislands,whichtypicallyhavea
fish such as Atka mackerel and Pacific ocean perch larger area of shallow shelf (Byrd et al., 2005). Inter-
(Sebastes alutus). estingly, about 31% of inshore-foraging seabirds nest
Diets of some demersal fish species vary with loca- in the eastern Aleutians and about 31% nest in the
tion along the Aleutian Archipelago. Euphausiids Near Islands, at the far western extreme of the Aleu-
constituted 50–90% of the diets of walleye pollock, tian Archipelago. These two regions hold in common
Atka mackerel and Pacific ocean perch east of an unusually wide extent of shallow shelf waters,
Samalga Pass, and <50% of the diets of these fish to appropriate foraging habitat for inshore-foraging sea-
the west of this pass, where copepods and myctophid birds (Springer, 1991; Springer et al., 1996). In recent
fishes were more important prey (Logerwell et al., years, cormorants in the Near Islands have suffered
2005). Pacific ocean perch also showed a further in- major population declines, possibly as a result of the
crease in their consumption of myctophids west of destructionofkelpforestssubsequenttothedeclinein
BuldirIsland.PacificcoddietsshiftedatSamalgaPass, sea otters (Enhydra leutris) (Doroff et al., 2003; Byrd
with walleye pollock predominating to the east of the et al.,2005).Overall,thereisnoindicationofaspatial
pass and Atka mackerel to the west. pattern in either reproductive success or population
(cid:2)2005Blackwell Publishing Ltd,Fish. Oceanogr., 14(Suppl.1), 292–306.
298 G.L.HuntandP.J.Stabeno
Table2. Distributionandabundance(individuals)ofmarinebirdsbreedingontheAleutianIslands(datasource:U.S.Fishand
WildlifeService,2000).
West Central Eastern
Far West Amchitka to Samalga to Unimak to
Species orspecies group ‘Near Islands’ Buldir Pass Amchitka Pass Samalga Pass
Piscivores
Red-legged kittiwake 0 12680 28 700
Black-legged kittiwake 14518 30726 10073 6221
Murres (both species) 23268 19574 61743 54578
Ancient murrelet 0 12240 16542 25581
Puffins (bothspecies) 53915 101756 164403 1 047768
Totalpiscivores 91701 176976 252789 1 130848
Planktivores
Northern fulmar 0 1360 509100 10
Storm-petrels (both species) 2500 3 003500 1256000 576198
Smallauklets (allspecies) 54 2 549515 778699 13480
Totalplanktivores 2554 5 554375 1765878 589688
Inshoreforagers
Cormorants (allspecies) 21133 5679 3853 9386
Glaucous-winged gull 12537 10893 10709 20418
Pigeon guillemot 336 2084 8349 4034
Totalinshoreforagers 34006 18656 22911 33838
trendsforseabirdsnestingintheAleutianIslandsthat bathymetry cause the aggregation of vertically migra-
wouldsuggestaresponsetolowerprimaryproductivity ting or depth-tending zooplankton (Simard et al.,
west of Samalga Pass (Mordy et al., 2005). In fact, 1986; Vermeer et al., 1987; Genin et al., 1988; Coyle
both murre and kittiwake populations at Buldir Island et al., 1992; Hunt et al., 1998).
are increasing (Byrd et al., 2005). Pinnipedsandcetaceansalsoshowscale-dependent
Limited at-sea surveys of migrant and resident patternsofdistributionandabundanceintheAleutian
marine birds in 2001 and 2002 reinforce the notion Archipelago. A number of researchers using different
that the ecosystems of the eastern Aleutian Islands statistical techniques and data sets have obtained
differ significantly from those of the central region. strikinglysimilargroupingsofStellersealionrookeries
EastofSamalgaPassbirdsseen atsea weredominated in the Aleutian Archipelago and western Gulf of
by piscivorous tufted puffins (Fratercula cirrhata) and Alaska(Yorket al.,1996;SinclairandZeppelin,2002;
planktivorous short-tailed shearwaters (Puffinus tenui- Call and Loughlin, 2005; Sinclair et al., 2005). Call
rostris),amigratoryspeciesthatbreedsinthesouthern and Loughlin (2005), using diets, population traject-
hemisphere (Jahncke et al., 2005). West of Samalga ories,andhabitatwithin10milesoftherookery,were
Pass, small auklets and northern fulmars (Fulmarus able to discern five distinct classes of Steller sea lion
glacialis), which were observed feeding on large ocea- rookeries,includingonegroupfromAttutoAmchitka
nic copepods, dominated the marine avifauna in Pass, a second from Amchitka to Samalga Pass, and
Tanaga and Seguam Passes, respectively (Hunt et al., threeothergroupingsthatweretotheeastofSamalga
1998; Jahncke et al., 2005). At smaller spatial scales, Pass.Sinclairet al.(2005),inanindependentanalysis,
surface-foraging fulmars and near-surface-foraging employed an additional 3 yr of diet and population
shearwaterswereconcentrated near frontalfeaturesin trend data and focused on the 16 Steller sea lion
the passes (Ladd et al., 2005b; Vlietstra et al., 2005). rookeries within the area covered by the 2001 and
Interestingly, in frontal regions in Akutan Pass where 2002 cruises to the Aleutian passes. They found that
euphausiidswerehighlyconcentratednearthesurface, thestrongdivisionofdiettypesandpopulationtrends
shearwaters consumed much smaller individuals than previously identified by Sinclair and Zeppelin (2002)
they did in regions where they had to dive to capture as occurring at Samalga Pass was still present. To the
euphausiids at depth (Vlietstra et al., 2005). These east of Samalga Pass, diets, on average, had a higher
passes are likely prime foraging areas for marine birds diversityofpreyspeciesandpopulationtrendsshowed
because strong tidal currents impinging on the lesser rates of decline than did diets and population
(cid:2)2005Blackwell Publishing Ltd,Fish. Oceanogr.,14(Suppl. 1),292–306.
Aleutian oceanography andecology 299
trendstothewestofSamalgaPass.Atasmallerspatial Duringthe2001and2002cruises,severalspeciesof
scale, Sinclair et al. (2005) used generalized additive cetaceans were seen with sufficient frequency to per-
models to examine the relationships among popula- mit assessment of their habitat preferences. At the
tion trends, diet diversity, and the distance to, and scaleofthecentralandeasternAleutianArchipelago,
characteristics of, the passes investigated in 2001 and humpback whales (Megaptera novaeangliae) were most
2002 (Unimak, Akutan, Umnak, Samalga, Amukta, common in the region between Samalga and Unimak
SeguamandTanaga).Sinclairet al.(2005)foundthat Island, with only three of 259 individuals seen to the
the frequency of occurrence of herring and salmon in west of Samalga. In contrast, only one of 118 fin
sea lion diets was important for predicting the steep- whales (Balaenoptera physalus) that was seen was west
ness in the rate of population decline, and that with of Unimak Pass (Sinclair et al., 2005). All sperm
thesedietconstituentsincluded,thepopulationtrends whales (Physeter macrocephalus) that were seen (56
were positively related to increasing depth of the individuals)wereencounteredtothewestofSamalga,
nearest of these passes. Why sea lion population and killer (Orcinus orca) and minke whales (B. acut-
declines in rookeries near deep passes are reduced orostrata) were seen predominantly west of Unimak
when diets are considered is unclear. Pass. These patterns in cetacean distribution mirrored
AlthoughthereweretoofewsightingsofStellersea thespatialgroupingsofStellersealionrookeriesbased
lions or other pinnipeds during the 2001 and 2002 on similarity in diet composition and population
cruises to assess their at-sea habitat preferences from trends (Sinclair et al., 2005). Killer whales were
shipboard surveys, the recent use of satellite-linked encountered most frequently on the north side of the
time depth recorders has yielded a wealth of infor- Aleutian Islands and in the vicinity of passes. Most
mation on the foraging habits of juvenile Steller sea killerwhalesidentifiedtoecotypewereoftheresident,
lions. Fadely et al. (2005) extended the work of fish-eatingtype,andthefewtransient,mammal-eating
Loughlinet al.(2003)andRaum-Suryanet al.(2004), typesseenwereintheUnimakPassregion,wheregrey
showing that juvenile sea lions consistently conduct whales (Eschrichtius robustus) and their calves were in
mostoftheirforagingdiveswithin18.5 kmofshorein migration into the Bering Sea (Sinclair et al., 2005).
water <100 m deep. Average trip durations for sea
lions <10 months old were shorter than those of lac-
TEMPORAL VARIABILITY
tating females tracked by Merrick and Loughlin
(1997) and observed by Trites and Porter (2002), Abundant recent evidence indicates that decadal-
consistent with maternal dependency. However, after scalefluctuationsinclimatehaveaffectedzooplankton
May of the year following birth, as juveniles aged, andfishpopulationsinthenortheasternPacificOcean
many travelled farther and dove in deeper off-shelf and eastern BeringSea(e.g. Brodeur and Ware, 1992;
waters. Increases in dive ability and trip duration and Piatt and Anderson, 1996; Mantua et al., 1997; Hare
distance occurred coincident with seasonal oceano- and Mantua, 2000; Hollowed et al., 2001; Wilderbuer
graphic changes. These results emphasize the need for et al., 2002). Climate variability has impacted North
greater understanding of the determinants of prey Pacific fish populations for millennia (e.g. Baumgart-
abundance in the near-shore waters of the Aleutian ner et al., 1992; Finney et al., 2002). Evidence is also
Archipelago and its passes, as these small-scale fea- accumulatingthatmarinebirdsandmammalsrespond
tures of the sea lions’ environment are likely of con- to these changes with shifts in diet or changes in
siderable importance. populationdynamics(Deckeret al.,1995;Hunt et al.,
CallandLoughlin(2005)notethatStellersealion 1996; Springer, 1998; Hunt and Byrd, 1999; Trites
rookeries are spaced, on average, at intervals of et al., 1999).
108 km, though many were at distances of 60 km or IntheAleutianArchipelago,Aleutmiddensdating
less from each other. Steller sea lion females typically to 3500 yr before present (yr BP) provide evidence for
forage within 20 km of their rookeries during the complex faunal changes over time (Causey et al.,
breeding season (Merrick and Loughlin, 1997). Call 2005). Examination of the bones of marine birds
and Loughlin (2005) suggest that the spacing of rook- consumedbytheAleutsshowsthatsignificantchanges
eries may reflect past competition between sea lions in the marine avifauna coincided with shifts in cli-
from neighbouring rookeries when populations were matic conditions. Approximately 1800–2100 yr BP,
larger; the preference for foraging over shallow shelf during a period of cooling, piscivorous birds that feed
waterswouldexacerbatethiscompetition,particularly offshore, such as kittiwakes and murres, increased in
in the central and western Aleutians where many of abundance, whereas between 650 and 1100 yr BP,
the smallislands have very restricted areas of shelf. a period of warming and increased precipitation,
(cid:2)2005Blackwell Publishing Ltd,Fish. Oceanogr., 14(Suppl.1), 292–306.
300 G.L.HuntandP.J.Stabeno
near-shore foragers such as cormorants and parakeet ecosystems along the Aleutian Archipelago (Fig. 3).
aukletsincreasedinabundance.Overthreemillennia, These differences include the sources of water in the
marine bird populations were negatively correlated passes (Ladd et al., 2005a), the nutrient and chloro-
with temperature and positively correlated with pre- phyllcontentofthosewaters,(Mordyet al.,2005),the
cipitation (Causey et al., 2005). Since the prehistoric speciescompositionofthezooplankton(Coyle,2005),
period, there have been considerable changes in the distribution patterns and growth rates of fish spe-
marine bird populations in the Aleutian Islands, with cies (Logerwell et al., 2005), the species composition
some species increasing locally whereas others have and foraging ecology of seabird communities (Byrd
decreased (Causey et al., 2005). et al., 2005; Jahncke et al., 2005), the food habits and
Dramatic changes in the marine fauna of the distribution patterns of pinniped and cetacean species
Aleutian Archipelago were also recorded during his- (Call and Loughlin, 2005; Sinclair et al., 2005) and
toric times. Pacific cod and Atka mackerel were the distribution of cold-water corals (Heifetz et al.,
apparently rarely encountered at Attu Island prior to 2005). Based on the earlier work of York et al. (1996)
1873, but were abundant there in 1878–81 (Turner, and Sinclair and Zeppelin (2002), scientists involved
1886). At Attu Island, capelin were said to be very with planning the cruises in 2001 and 2002 to the
abundant every third year, as may have been the case Aleutian passes had predicted that there would be a
at Atka Island. At Atka Island, capelin were also difference in the availability of Steller sea lion prey
abundant when Turner visited (1878–81), and ‘dead species east and west of the Samalga Pass region. The
fish [capelin, post-spawning] were so thick on the magnitudeofthechangesandthenumberofaspectsof
beach that it was impossible to walk without stepping the marine ecosystem that differed across Samalga
on hundreds of them’ (Turner, 1886, p. 102). Tem- Pass,however,wereunexpected.Alsounexpectedwas
poral variability in capelin abundance has also been evidence for a transition in climate that roughly
inferred in the Bering Sea through examination of aligned withthe ecologicalboundaryatSamalgaPass.
predator diets (Kajimura, 1984; Sinclair et al., 1994; Informulatingthe fieldresearch in2001and2002,
Decker et al., 1995; Hunt et al., 1996). Capelin were there was an expectation that the configuration of
oncecommonatUnalaskaIsland(JordanandGilbert, passesintheeastern(shallowandnarrow)andcentral
1899). In recent decades, capelin have been largely (wide and deep) Aleutian Islands might be an
absentfromAleutianIslandbeaches,thoughinrecent important factor in differentiating foraging opportun-
yearstherearesomereportsfromtheeasternAleutian ities for higher trophic level predators. However, the
Islands of their returning in low numbers (G.V. Byrd, fieldwork failed to test this hypothesis adequately, as
personal communication). the shallow narrow passes investigated were all to the
During historic times, there have been striking eastofSamalgaPassandthewider,deeperpasseswere
changesinthe populationsofmarine mammalsinthe in the central Aleutians. The confounding variable of
Aleutian Archipelago. Steller sea lion populations aprofoundecosystemchangeatSamalgaPasshadnot
have gone through a series of declines. In the 1870s, been anticipated.
Turner (1886, p. 98) noted that: ‘The natives also At least three hypotheses may explain the abrupt
assert that the coming of these fish [Atka mackerel in shift in ecosystem properties at Samalga Pass: the
the 1870s] was coincident with the disappearance of differencesinwidthanddepthofpasses,theoriginsof
the sea lion.’ In recent years, sea lions have declined water with differing physical and chemical properties
precipitously,withconsiderablecontroversyremaining in the passes east and west of Samalga Pass, and the
as to the cause or causes of the population reduction shiftfromlargeislandsonarelativelywidecontinental
(Trites et al., 1999, 2005;NationalResearch Council, shelf in the east to small islands on relatively con-
2003;Springeret al.,2003;Sinclairet al.,2005).After strainedshelvestothewest.Thesehypothesesarenot
recovering from near extinction from harvesting by mutually exclusive, and, at present, we lack the data
Europeans, sea otters also have recently declined necessary to rank them in importance as contributors
(Doroffet al.,2003;Esteset al.,2005),mostlikelydue to ecological differentiation between the eastern and
to predation by killer whales (Estes et al., 1998). central Aleutian Archipelago. Both the source of
water and shelf area would be expected to influence
water chemistry(Mordy et al.,2005)and zooplankton
DISCUSSION
composition (Coyle, 2005), and the configuration of
The findings reported in this volume support the the passes affects the presence of fronts, eddies, etc.
hypothesis that there is significant spatial variation in that influence the aggregation of prey and its avail-
the physical and biological properties of marine ability to marine birds and mammals (Fadely et al.,
(cid:2)2005Blackwell Publishing Ltd,Fish. Oceanogr.,14(Suppl. 1),292–306.