An East-West Comparison of the Transition Zone Coastal ... National Fisheries Research Institute,...

415

Journal of Oceanography, Vol. 59, pp. 415 to 434, 2003

Keywords:⋅ Pelagic nekton,⋅ assemblages,⋅ habitat,⋅ coastal,⋅ Transition Zone,⋅ North PacificOcean.

* Corresponding author. E-mail: [email protected]

Copyright © The Oceanographic Society of Japan.

An East-West Comparison of the Transition Zone CoastalPelagic Nekton of the North Pacific Ocean

RICHARD D. BRODEUR1*, JOSEPH P. FISHER2, YASUHIRO UENO3,KAZUYA NAGASAWA4 and WILLIAM G. PEARCY2

1Northwest Fisheries Science Center, NMFS, Newport, OR 97365, U.S.A.2College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, U.S.A.3Tohoku National Fisheries Research Institute, FRA, Hachinohe 031-0841, Japan4National Research Institute of Aquaculture, FRA, Nikko 321-1661, Japan

(Received 30 May 2002; in revised form 25 October 2002; accepted 12 December 2002)

During the 1980s and 1990s, scientific research cruises were conducted in both theeastern and western boundary regions of the North Pacific Ocean. The main purposeof these cruises was to examine the abundance and distribution patterns of juvenilesalmon in coastal waters. These studies created one of the most extensive databasesever collected on the species composition of coastal Transition Zone epipelagic nektonin the North Pacific Ocean. Catch data from two purse seine and two surface trawlsurveys (one each from off northern Japan and eastern Russia and off the West Coastof the U.S.) were examined using multivariate techniques to analyze the communitystructure of nektonic cephalopods, elasmobranchs, and teleosts in the coastal zoneduring the summer and autumn months. Juvenile salmonids are generally among themost common species caught, but in terms of overall abundance, other potential com-petitors with juvenile salmon, such as small squids and clupeoid fishes predominatedthe catches. Species diversity and dominance varied among areas and gear types.Distinct assemblages were found in each area, but the two regions had closely relatedspecies occupying similar ecological positions in each habitat.

the dominant nekton have been described for the easternand western Transition Regions of the North Pacific(Brodeur and Pearcy, 1986; Ueno et al., 1990; Shuntov etal., 1993, 1995, 1998; Radchenko et al., 1997; Nagasawaet al., 1998; Brodeur et al., 2003), little attempt has beenmade to examine species and environmental associationsof these pelagic species. Surprisingly, more quantitativeanalysis has been undertaken of these relationships foropen ocean nekton (Pearcy et al., 1996; Brodeur et al.,1999) than for their coastal counterparts.

The purpose of our study was to investigate speciesand environmental relationships of the dominantepipelagic nekton in coastal waters off the west coast ofthe U.S. and off northern Japan and eastern Russia. Ourunderlying assumption was that species have adapted tothe physical and biological conditions present in each ofthese systems; therefore, by using a comparative ap-proach, we can infer how variations in the environmentmay affect species composition and abundance patterns.Our specific study objectives were to: (1) describe andcharacterize the types and diversity of the dominantnektonic species (cephalopods, elasmobranchs, andteleosts) found in these two distinct geographic areas at

1. IntroductionSpecies diversity within an ecosystem is influenced

by a wide range of temporal and spatial variability in thephysical environment, the timing and magnitude of bio-logical productivity, and interactions within the commu-nity such as competition and predation. Understandingthe composition and distribution patterns of pelagicnektonic organisms is of great interest to scientists andfisheries managers involved in ecosystem managementand maintenance of biological diversity within pelagicecosystems.

During the 1980s and 1990s, scientific researchcruises were conducted in both the eastern and westernregions of the North Pacific Ocean. The main purpose ofthese cruises was to examine the abundance and distribu-tion patterns of juvenile salmon in coastal waters. Thesestudies created one of the most extensive databases evercollected on the relative species composition of coastalepipelagic nekton in the North Pacific Ocean. Although

416 R. D. Brodeur et al.

Fig. 1. Location of CTD collections (dots) and contours of sea surface temperatures for each cruise examined in this study fromthe Western and Easter Pacific Ocean.

An East-West Comparison of the Transition Zone Coastal Pelagic Nekton of the North Pacific Ocean 417

approximately the same latitude but separated by thou-sands of kilometers of open ocean, (2) use multivariateanalyses to determine species associations and relation-ship of these associations to environmental variables, and(3) compare the species compositions and associationscollected with different gear types within geographic ar-eas.

2. Methods

2.1 Data collectionA number of large-scale coastal pelagic surveys have

been conducted since the early 1980s off Oregon andWashington on the west coast of the United States andoff Hokkaido Island in Japan and the Kurile Islands inRussia. The purpose of these surveys was primarily toassess the distribution of juvenile salmon, but detailedcatch information was obtained on a number of speciesthat were quantitatively retained in the sampling gear. Wewere interested in examining cruises that were not anoma-lous in terms of environmental conditions (e.g., El Niñoyears). Catch data for all nekton were analyzed from fourrepresentative surveys: two (one purse seine and one trawlsurvey) were from the eastern North Pacific Ocean offOregon and Washington and two (also one purse seineand one trawl survey) were from the western North Pa-cific Ocean off the Russian and Japanese coasts (Table 1;Fig. 1).

More information on the sampling methodology andpreliminary catch results can be found in technical re-ports of the School of Oceanography, Oregon State Uni-versity (OSU) (Fisher and Pearcy, 1985), the NationalResearch Institute of Far Seas Fisheries, Japan FisheriesResearch Agency (FRA) (Ueno et al., 1990; Nagasawa etal., 1998), and the Northwest Fisheries Science Centerof National Marine Fisheries Service (NMFS) (Casillasand Bucher, 2000).

In addition to catch information on nekton species,length frequency and weight data were taken for asubsample of all dominant species caught. In the analy-ses to follow, we distinguished juvenile salmon fromadults, as the migratory patterns and habitat preferencescan be quite distinct in the two life history stages. In mostcases, we were able to assign specific names to all taxabut in the case of several small juvenile specimens, wehad to classify them at a higher taxonomic level (genusor family). Although some larval forms were collected,we analyzed only those taxa that were likely to be truenekton (juvenile and adult stages). Temperature, salinity,and density throughout the water column were measuredwith a CTD at each station. In addition, for some cruises,surface chlorophyll and Secchi depth (a measure of wa-ter transparency) were measured. The distributions of seasurface temperature are shown in Fig. 1 for each cruise.

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2.2 Data analysis1) Species diversity and dominance indices

We used three indices to assess broad differencesbetween areas and gear types. As a diversity measure-ment we used the Shannon-Weiner (H′) Index, whichmeasures both species number (richness) and equitability(evenness). Hutcheson’s (1970) method was used to testfor differences in diversity among the cruises. We usedPielou’s Index (E) to measure evenness. This index rangesfrom 0 to 1.0 with the maximum value representing a situ-ation in which all taxa are equally abundant. Dominancewas measured using Simpson’s (D) Index, which givesthe probability that any two indviduals drawn at randomfrom an infinitely large community belong to differentspecies (Magurran, 1988).2) Cluster analysis

We first eliminated rare species from the data ma-trix. Rare species were defined as those that had a fre-quency of occurrence of less than 5% of the stations foreach cruise. A hierarchical cluster analysis was then per-formed separately on both stations and species, using therelative Bray-Curtis dissimilarity coefficient as the dis-tance measure (Clifford and Stephenson, 1975). This dis-tance measure was chosen because it standardizes by sam-ple unit totals (McCune and Mefford, 1999). The flexiblebeta method (β = –0.25) was chosen for the agglomera-tive clustering algorithm. Both station and speciesdendrograms were examined, and a qualitative decisionwas made as to what distance should be used to deter-mine cluster groupings based on biological interpretation.The data was then reordered according to the dendrogramsand a two-way table was created based on the originalstations-by-species matrix of abundance showing the as-sociations between species groups and station groups.Station groups were then plotted on a map of the areasampled to see if they were geographically distinct. Thesoftware PC-ORD (McCune and Mefford, 1999) was usedfor both clustering and ordination methods.3) TWINSPAN analysis

Species assemblages were identified by classifyingsamples (stations) and attributes (species) using Two-wayIndicator Species Analysis (TWINSPAN) (Hill, 1979b;Gauch and Whittaker, 1981; Gauch, 1982). Hierarchicalclassifications of stations and species are producedthrough a process of repeated ordinations and divisionsof the samples, starting with all the samples in one groupand continuing through several levels of division. Spe-cies are classified based on their affinities for the differ-ent sample groups, and “indicator” species are identified,the abundance patterns of which most clearly distinguishthe different sample groups (Hill, 1979b). Station group-ings were also determined, but the divisions were termi-nated at the second level, yielding 3–4 distinct stationgroupings for each cruise.

4) Detrended correspondence analysisDetrended correspondence analysis (DCA) (Hill,

1979a; Hill and Gauch, 1980; Gauch, 1982), was used toshow association among species and samples by plottingcombinations of the first three DCA ordination scores. InDCA, the rate of change of species turnover along a sam-ple DCA axis is constant. Abundance curves of speciesalong the sample axis is assumed to be bell-shaped, andaxes are scaled so that standard deviations of the abun-dance curves for all species are about the same and equalto 1 unit along the axes. The DCA axis score for eachspecies equals the weighted mean of the DCA scores ofthe samples in which it occurs. Four standard deviations(SD) approximate the distance over which a species risesto a mode and disappears. Therefore, species having or-dination scores that differ by 4 SD should rarely be foundin the same sample unit. The ordination scores for spe-cies were plotted to see if they grouped in the same wayin ordination space as they did in the cluster analysis.Correlation analysis of the ordination axes with stationdata (latitude, longitude, distance offshore, and depth) andenvironmental variables (temperature, salinity, chloro-phyll, and Secchi depth) was conducted for each cruiseto determine which factors could best explain variabilityalong the axes.

3. Results

3.1 Species composition and abundanceThe taxonomic composition of the catch from the

different surveys is listed in Appendix Tables 1–4. In theJune 1985 purse-seine survey off the West Coast of theU.S., salmonids and scorpaenids represented the mostdiverse families (Fig. 1, Appendix Table 1). Salmonidsoccurred most frequently, and were the only taxa to bepresent in more than 50% of the collections. Salmonidsand clupeids dominated in terms of overall catch, althoughthe most frequent species caught, coho salmon, made uponly a little more than 20% of the catch. The next mostabundant species was the market squid, but this speciesoccurred in relatively fewer hauls. Individuals of mosttaxa were relatively small, with the general exception ofthe elasmobranchs and adult salmon. Overall species com-position of the 1999 trawl survey conducted in the samearea was somewhat similar to that of the 1985 survey.Osmerids, salmonids, and clupeids were the most com-monly occurring and abundant families overall, but squidsand scorpaenids were relatively insignificant (Fig. 2).However, there were some distinctive differences betweenthese surveys (Appendix Table 2). The dominant speciesin 1999, Pacific sardine, comprised over two-thirds of thenumerical abundance, although it did not occur at all inthe 1985 sampling.

Species composition in the western North Pacific


Ocean showed some overlap with that of the eastern side,at least at the family level: the cephalopod catch was morediverse, but the salmonids were far less diverse, and noelasmobranchs were caught, at least in 1989 (Fig. 2, Ap-pendix Table 3). The dominant taxon in terms of abun-dance was sand lance, which did not appear in the purse-seine catches in the eastern Pacific Ocean, although themesh sizes were somewhat different. Again, most of thenekton caught were small (<250 mm), with the exceptionof the adult salmonids and some scombrids. A more var-ied fauna was observed in the 1996 sampling, especiallyamong the salmonids and offshore micronekton, such asthe myctophids and bathylagids (Appendix Table 4).These three groups were well represented in terms ofnumerical composition, as were the gadids,hexagrammids, clupeids, and scombrids (Fig. 2).

3.2 Species richness, diversity and dominanceThere were some geographic and temporal differ-

ences in diversity and other community measures ob-served among the four cruises (Table 2). The averagenumber of unique taxa was similar for both sides of thePacific, although the numerical abundance was muchhigher in the western North Pacific. Diversity was simi-lar between the 1985 U.S. purse-seine cruise and the 1996Japanese trawling cruise, and these were not significantlydifferent from each other (t = 0.006, p = 0.564). All otherpairwise comparisons of diversity were significantly dif-ferent from each other (p < 0.001). Similarly, there werefew dominating species for these two cruises and the even-ness and dominance factors were high and low, respec-tively. Conversely, dominance was very high in the 1999U.S. trawl cruise due to a very large catch of Pacific sar-dine, and was moderate in the 1989 Japanese cruise dueto a large catch of sand lance.

3.3 TWINSPAN analysisWe analyzed TWINSPAN dendrograms based on two

divisions, although in most cases we carried our analysisout for an additional level if the groupings were particu-larly large (Fig. 3). For June 1985, the first division de-lineated all salmon along with inshore species such asLoligo, Pacific herring, and spiny dogfish, while the otherdivision contained the two trout species and other spe-cies found mostly offshore including juvenile Sebastes,hake, juvenile wolfeels, Pacific saury, greenlings, andsculpins (Fig. 3). In 1999, the salmon were clustered to-gether at the first division, but adults and juveniles werefound in different groupings in subsequent divisions. Theother main grouping was fairly small and included pre-dominantly offshore species such as juvenile sablefish,soupfin shark, and jack mackerel (Fig. 3).

Four station groups were selected for the 1985TWINSPAN analysis (Fig. 4). Group A was character-ized by salmon (including juvenile chum salmon), butexcluded market squid and was mainly found to the north(northern Oregon and Washington). Group B was char-acterized by market squid, juvenile and adult coho, andjuvenile chinook, but lacked juvenile chum salmon. Thisgroup showed a wide latitudinal distribution was predomi-nantly inshore. Group C was mainly found off southernOregon and was characterized mainly by juvenile Sebastesbut showed a general lack of salmon, market squid, andspiny dogfish. Group D was fairly small (two stations)and was distinguished by the occurrence of wolfeels andcutthroat trout.

Four station groups were delineated in 1999 (Fig.4). In contrast to groups in 1985, which appeared to re-flect mainly latitudinal gradients, those in 1999 seemedto show both inshore-offshore and latitudinal gradients.Group A was found mainly at the inshore stations (exceptfor the southernmost transects) and was dominated byjuvenile chinook salmon, Pacific herring, and spiny dog-fish. Prominent species of Group B were jack mackereland juvenile coho salmon. The inshore-offshore positions

Fig. 2. Composition of catches for each cruise by major taxo-nomic categories.

Table 2. Summary of diversity, evenness, and dominance meas-ures for each cruise.

Measure U.S. Cruises Japanese Cruises

1985 1999 1989 1996

# of Taxa 39 28 28 34# of Individuals 5615 28561 8933 100590Diversity (H′) 2.329 1.012 1.532 2.328

Variance (var H′) 0.018 0.043 0.037 0.007

Evenness (J) 0.636 0.304 0.446 0.660Dominance (D) 0.138 0.553 0.341 0.120


of Groups B and C overlapped, but Group C was foundentirely off Washington, and was dominated by Pacificsardine, indicating the northerly distribution of sardinesin September of 1999. Group D comprised a few offshorestations containing mainly juvenile sablefish and soupfinsharks.

The TWINSPAN species groupings for samples takenoff Hokkaido Island were quite different from those takenoff the U.S. west coast (Fig. 5). Both cruises containedan outlying group of just a few species with all the re-maining taxa in a single cluster, which was further subdi-vided into smaller groups. For the July–August 1989cruise, the arabesque greenling was the only taxon in theoutlying group and the remaining taxa were fairly evenlydistributed among the remaining groups. For the August–November 1996 cruise, adult pink salmon, Japanese mack-erel, and flying squid were grouped together in the outly-ing group. Four divisions of the remaining large groupwere required to achieve reasonably small clusters (Fig.5).

Only three TWINSPAN station groupings were de-lineated for 1989, with a strong break in the assemblagesoccurring around 148°E (Fig. 6). All but one station tothe west of this line was classified as Group A, domi-nated by Japanese anchovy, Pacific saury, chum salmonjuveniles, and Japanese sardines. The area east of this linewas comprised mostly of Group B and was characterizedby gonatid squids and pink salmon juveniles. Group Coccurred infrequently across the middle shelf and wasdominated by the presence of sand lance.

For the 1996 cruises, the TWINSPAN analysisformed four groups (Fig. 6). A northern cluster (GroupA) found entirely in the Sea of Okhotsk was character-ized by high abundances of juvenile pink and chumsalmon, as well as arabesque greenling, which is com-pletely absent from the remaining groups. Group B, foundclose to the Kurile Islands, contained high abundances ofadult chum and chinook salmon and Pacific pomfret.Group C, found mainly off the eastern side of HokkaidoIsland, contained mainly Pacific mackerel and flyingsquid, whereas Group D (2 stations only) was found off-shore and contained mainly Japanese anchovy.

3.4 Cluster analysisFor the cluster analysis by species, we divided the

groups around the 25% level, which generated four clus-ter groups for all cruises. For the 1985 cruise, juvenilecoho, chinook and chum salmon, adult pink and cohosalmon, and spiny dogfish formed a distinct cluster (Fig.7). This cluster was strongly associated with station Clus-

Fig. 3. Two-way Indicator Species Analysis of the U.S. cruisesin 1985 and 1999.

Fig. 4. Two-way Indicator Species Analysis station groupingsfor the U.S. cruises in 1985 and 1999.


ter B and to a lesser extent Cluster A (Table 3). Adultchinook, chum and sockeye salmon formed another smallcluster most closely associated with station Cluster B,while cutthroat and steelhead trout and wolfeel juvenileswere grouped together and found mainly at station Clus-ters A–C. All other taxa were combined into a fourth het-erogeneous cluster with strong affinities to station Clus-ter A (Table 3).

Somewhat different results were found for the 1999trawl cluster analysis (Fig. 7). Juvenile coho salmon wereclustered with several offshore species, including jack andchub mackerel, Pacific sardines, soupfin shark, andsablefish and showed the highest affinities with stationCluster C (Table 4). Juvenile chinook salmon were clus-tered with several inshore forage species such as Pacificherring, northern anchovy, and several smelt taxa and thiscluster had high mean abundances at Clusters B and D(Table 4). Adult chum and chinook salmon were associ-ated with market squid, most notably in Cluster D,whereas adult coho salmon were most closely associatedwith spiny dogfish and starry flounder of Clusters C andD.

In the species cluster analysis for the 1989 purse-seine cruise, pink salmon were associated with Pacificpomfret, Pacific saury, and flying squid (Fig. 8) and wasassociated mainly with Cluster B and to a lesser extent Aand C (Table 5). Another major cluster included most ofthe numerically dominant species, including sand lance,

Japanese anchovy, and sardine, which were most preva-lent in Clusters A and C (Table 5). The third cluster in-cluded shortarm squid and banded Irish lords and had thehighest affinity with station Clusters A and D. The re-maining group included two less abundant species withno particular station group affinities.

In the 1996 species analysis, the first cluster includedseveral juvenile salmon and walleye pollock (Fig. 8) andwas found mainly in Cluster D (Table 6). The second clus-ter was larger and included adult salmon, sardines, an-chovy, chub mackerel, saury, flying squid, and other lessabundant pelagic species and was associated with stationClusters A and C. A third smaller grouping was comprisedof juvenile chum and pink salmon collected mainly at sta-tion Cluster D (Table 6). The fourth group was dominatedby offshore mesopelagic fishes and squids and was found

Fig. 5. Two-way Indicator Species Analysis of the Japanesecruises in 1989 and 1996.

Fig. 6. Two-way Indicator Species Analysis station groupingsfor the Japanese cruises in 1989 and 1996.


Fig. 7. Cluster species groups for the 1985 and 1999 cruises off Oregon and Washington.

Table 3. Two-way incidence table showing dominant pelagic nekton captured in purse seines during June 1985 and mean abun-dance (no. haul–1) in each cluster station grouping. Dashed lines indicate that species did not occur in the particular stationgroup. Abundances >10 are in boldface type. Juvenile (j) and adult (a) stages are denoted.


Fig. 8. Cluster species groups for the 1989 and 1996 cruises off Japan and Russia.

Table 4. Two-way incidence table showing dominant pelagic nekton captured in purse seines during September 1999 and meanabundance (no. haul–1) in each cluster station grouping. Dashed lines indicate that species did not occur in the particularstation group. Abundances >10 are in boldface type. Juvenile (j) and adult (a) stages are denoted.

Species groups Scientific name Station groups

A B C D

1 Loligo opalescens 0.1 0.1 — 1.1Oncorhynchus keta (a) 0.5 0.2 — 1.1O. tschawytscha (a) — 1.2 3.0 4.3

2 Squalus acanthias 0.1 0.1 0.3 0.1O. kisutch (a) 0.3 — 0.4 0.3Platichthys stellatus 0.1 0.1 0.1 0.4

3 Engraulis mordax — 1.7 1.9 0.2Hypomesus pretiosus 0.3 3.0 2.5 2 5 . 6Osmeridae — 1.1 — —Clupea pallasi 0.2 1 1 9 . 2 7.1 3 5 . 0O. tschawytscha (j) 1.3 1 3 . 4 5.7 1 2 . 5

4 Sardinops sagax 1.6 0.3 2 6 0 3 . 1 0.4Anoplopoma fimbria — 0.3 0.3 —Scomber japonicus — — 0.3 0.8Trachurus symmetricus 6.6 — 1 7 . 8 0.7Galeorhinus zyopterus — 0.2 0.1 —O. kisutch (j) 1 0 . 2 0.5 1.5 0.8


Table 5. Two-way incidence table showing dominant pelagic nekton captured in purse seines during July–August 1989 and meanabundance (no. haul–1) in each cluster station grouping. Dashed lines indicate that species did not occur in the particularstation group. Abundances >10 are in boldface type.

Table 6. Two-way incidence table showing dominant pelagic nekton captured in purse seines during August-November 1996 andmean abundance (no. haul–1) in each cluster station grouping. Dashed lines indicate that species did not occur in the particularstation group. Abundances >10 are in boldface type. Juvenile (j) and adult (a) stages are denoted.


mainly at station group B and D (Fig. 8).Station clusters for the 1985 purse-seine cruise were

generally arranged by distance offshore with Clusters Aand B found generally inshore and Clusters C and D foundmostly offshore (Fig. 9). There was a lesser degree oflatitudinal separation, with C being found more to thesouth than those of D. In 1999, the station clusters groupedalong a latitudinal basis with Clusters A and C found fur-ther north of B and D. There was relatively little inshore/offshore pattern in the 1999 station groupings (Fig. 9).The station clusters for the 1989 purse-seine cruise offJapan showed some spatial structure in that Cluster A wasmainly found in the western part, Cluster C was predomi-nantly found in the center, and Cluster D was found inthe eastern half of the study area (Fig. 10). Cluster Bspanned the entire range of sampling and tended to belocated at the offshore ends of transects. In 1996, ClusterA was mostly in the southern part of the study area, Clus-ter B was mainly around the Kurile Islands, and groups Cand D were found entirely in the Okhotsk Sea (Fig. 10).

3.5 Detrended correspondence analysisIn the 1985 DCA scores, the first axis was negatively

(r = –0.65) associated with latitude; thus species scoringhigh on this axis (Pacific saury, pelagic juvenilerockfishes, Irish lords, greenlings, and wolfeel) had south-ern affinities, whereas species scoring low on this axis(spiny dogfish, Pacific herring, most salmon) had north-ern affinities (Fig. 11). The second axis was correlatedwith longitude (Loligo was the most inshore species andPacific hake and steelhead were the most offshore), whilethe third was negatively correlated with salinity (Pacificsaury was found in the most saline water and steelhead inthe freshest water).

For September 1999, the first DCA axis was associ-ated with temperature (r = 0.58), with the smelts andLoligo scoring low on this axis (cool water species) and

Fig. 9. Cluster station groupings for the 1985 and 1999 cruisesoff Oregon and Washington.

Fig. 10. Cluster station groupings for the Japanese cruises in1989 and 1996.


Fig. 11. Detrended Correspondence Analysis results for the 1985 and 1999 cruises off Oregon and Washington. Shown are thespecies scores in relation to the first three DCA axes and the environmental variable that shows the highest correlation witheach axis.

Fig. 12. Detrended Correspondence Analysis results for the 1989 and 1996 cruises off Japan and Russia. Shown are the speciesscores in relation to the first three DCA axes and the environmental variable that shows the highest correlation with each axis.


soupfin shark scoring high (Fig. 11). The second DCAaxis showed a negative relationship with latitude such thatthe salmon species were found mainly to the north andjack mackerel was found mainly to the south. The thirdaxis was found to be associated with distance offshore(Fig. 11).

In the analysis of the 1989 Japanese purse seine data,the first DCA axis showed high (0.71) correlation withlatitude, with saury and sardine found mostly to the southand Arabesque greenling found to the north (Fig. 12). Thesecond axis was associated with longitude; species suchas wolffish and sardine were found to the eastern end ofthe sampling area and saury to the western end. It shouldbe noted that due to the northeast-southwest orientationof the transect sampling, longitude and latitude werehighly correlated, and both scored high on the first twoaxes. The third axis was most related to Secchi depth(water transparency), with pomfret found in the clearestoceanic waters and Irish lords found in the most turbidones (Fig. 12).

For the 1996 trawl survey, latitude (r = –0.85) washighly correlated with the first DCA axis, with taxa suchas sablefish, sticklebacks, walleye pollock, and varioussalmon species found in high latitudes and Pacific mack-erel, Japanese anchovy, and neon flying squid found tothe south (Fig. 12). Longitude (r = 0.42) and surface tem-perature (r = –0.26) were the most important explanatoryvariables for the second and third axes, respectively.

4. DiscussionAlthough the latitudinal spread of sampling was simi-

lar between the two regions (roughly 42°N to 53°N), theiroceanographic conditions are quite different from eachother. The eastern North Pacific Transition region is domi-nated by a southward-flowing Eastern Boundary Currentin the offshore region and by episodic upwelling. It isalso affected by an extensive riverine plume in the coastalregion, especially during summer (Hickey, 1989, 1998).In contrast, the western North Pacific study area, particu-larly in waters northeast of Hokkaido and the Kurile Is-lands, is characterized by the dominance of the OyashioCurrent, which is southwestward-flowing and cold, andits mixing with surface waters derived from the KuroshioCurrent, which is northward-flowing and warm (Minoda,1989; Yasuda, 2003). The southern Sea of Okhotsk is char-acterized by low-salinity surface water transported by thecold, southward-flowing East Sakhalin Current, but offthe coast of Hokkaido Island, is seasonally (mainly fromearly summer to autumn) affected by the Soya WarmCurrent that passes between Hokkaido and Sakhalin Is-lands (Takizawa, 1982). Thus the temperature range inthe eastern region is much smaller (6–8°C between themaximum and minimum values within the same cruiseperiod) than that within the western region (10–12°C; Fig.

1). Conversely, the surface salinity range was on the or-der of 10 psu (practical salinity units) in a single cruiseoff Washington and Oregon (Brodeur and Pearcy, 1986),whereas it is generally <1 psu off Japan and Russia (Uenoet al., 1990). Due to the narrow shelf region off Japanand Russia, the range of bottom depths was much greater,resulting in high densities of mesopelagic fishes(Myctophidae and Bathylagidae) at many stations.

Given these contrasting environmental and topo-graphic conditions, one would expect substantially dif-ferent taxonomic compositions in the two regions. How-ever, this is true only to a limited extent, as both regionsshare a number of trans-Pacific species such as spinydogfish, blue shark, chum, sockeye, and chinook salmon,Pacific herring, northern lampfish, sablefish, Pacificsaury, and Pacific mackerel. There are also several ex-amples of taxonomically-related species including siblingspecies of anchovy (Japanese and northern), salmon (cohoand masu), wolfeels, and greenlings. There are severalexamples of ecologically or functionally-related speciesincluding the gadid (hake and pollock), lamprey, andsquids on both sides of the Pacific Ocean. Major differ-ences in the nektonic fauna of the two regions are thepreponderance of cephalopod species in the western Pa-cific and more elasmobranch and flatfish species in theeastern Pacific.

There are pronounced differences between cruiseswithin one geographic region, although it is difficult toascribe these differences solely to interannual, seasonal,or gear-related effects. For example, there have beenmarked changes in the nektonic species composition inthe eastern North Pacific, even when examining a con-sistent time series (Emmett and Brodeur, 2000; Brodeuret al., 2003). The most notable difference is the replace-ment of market squid and northern anchovy in the 1980swith Pacific sardines in the 1990s. These changes appearto have coincided with a regime shift, which occurredaround 1989 in the eastern Pacific (Emmett and Brodeur,2000; McFarlane and Beamish, 2001). Changes in thenektonic species composition, mainly caused by markedfluctuations in stock size of Japanese sardine and anchovy,have also been reported from the western North Pacific(Shuntov et al., 1996).

Pelagic nekton are particularly affected by anoma-lous conditions such as those occurring during strongENSO events. The surface waters are strongly affectedby warming during these events, and strong-swimmingor highly-migratory species may seek out their preferredthermal habitat at higher latitudes than normal. There havebeen numerous nekton distribution or biomass anomaliesreported for the major ENSO events of 1983/1984 (Pearcyet al., 1985; Brodeur and Pearcy, 1992) and 1997 (Groveret al., 2002; Pearcy, 2002). Similar extensions of warm-water species beyond their normal ranges have been ob-


served in the western Pacific during warming periods(Ivankov et al., 2001). For these reasons, we limited ourcoverage only to years that were considered typical ofthe regions we examined, although we recognize that itis difficult to classify any year as “typical”.

Many of the species we collected in near-surfacewaters are typical Transition Zone species in that theymigrate substantial distances between spawning and feed-ing areas on a seasonal basis. In the Western Pacific, somewarm-water species, such as Pacific saury, Japanese sar-dine and anchovy, and squid (Todarodes pacificus), mi-grate from the Japan Sea to the Okhotsk Sea with the SoyaWarm Current (Yamaguchi and Yamagishi, 1990). Alsoduring summer and early autumn, Pacific saury and othersquid migrate from the oceanic waters of the North Pa-cific to the southern Okhotsk Sea using the warm surfacewater covering the Oyashio Current through some passesof the Kurile Islands. In the Eastern Pacific, Pacific whit-ing, chub mackerel, and Pacific mackerel spawn off Cali-fornia and move into our study area in late summer(Brodeur and Pearcy, 1986). Saury also enters these wa-ters but is generally found well offshore of the study re-gion. In contrast with the Western Pacific, many of theanchovy, sardine, and market squid spawn locally offOregon and Washington. Thus some differences betweenthe two regions can be attributed to seasonal migrationpatterns.

Several studies have examined demersal nekton com-munities in both the eastern (Gabriel and Tyler, 1980;Rogers and Pikitch, 1992; Jay, 1996; Lee and Sampson,2000) and western (Yamamura et al., 1993; Fujita et al.,1993, 1995) Pacific Transition Regions. Multivariateanalyses of these data show that these communities arehighly structured and generally are correlated with depth,with much less relation to other environmental factors.Conversely, there have been almost no attempts at exam-ining species assemblages in the coastal pelagic zone ofthe eastern and western North Pacific. Brodeur and Pearcy(1986) conducted a preliminary assemblage analysis ofnekton from purse-seine collections off Oregon and Wash-ington in 1979–1984. However, this study used a method(Recurrent Group Analysis) that takes into account onlythe presence/absence of species, salmonids were not in-cluded, and all years and months were combined together.Despite the difference in approach, there were some simi-larities between the results of this study and those of thepresent study, in that distance offshore and latitude wereimportant variables in determining species assemblages.In another analysis, somewhat similar to the present study,Brodeur et al. (2003) have examined nekton assemblagesfrom two surface-trawl surveys conducted off Oregonduring 2000 in a region mainly to the south of the regionexamined here. Again the same north-south, inshore-off-shore distribution patterns were found in the cluster analy-

sis, although temperature emerged as the primary explana-tory variable for the first axis in the ordination analysis.

The present analysis suggests that there is some struc-ture to the assemblages, mainly determined by latitudeand longitude, but that boundaries between assemblagesare not as distinct as those observed in demersal commu-nities. In this respect they are far more similar to those ofichthyoplankton assemblages (Richardson et al., 1980;Doyle et al., 2002), which may vary greatly betweencruises. This is probably related to the more dynamic na-ture of the pelagic environment, where a particular fixedlocation may be subjected to rapidly changing environ-mental conditions. Another factor affecting seasonal vari-ability is that many nekton species are pelagic only for aportion of their life cycle, and thus even cruises duringthe same year but in different months may find drasti-cally different nekton composition (Brodeur and Pearcy,1986; Brodeur et al., 2003). More research should be di-rected to understanding the factors leading to variabilityin nekton distributions and interactions. A key questionto resolve is whether these pelagic communities are sta-ble through time, as seen in many demersal nekton as-semblages (Overholtz and Tyler, 1985; Mahon and Smith,1989; Lee and Sampson, 2000), or whether they fluctu-ate through time depending upon an unpredictable physi-cal environment.

5. ConclusionsOur results suggest that there is some structure to

the pelagic nekton communities of the coastal regionsalong the eastern and western North Pacific margins.Some general conclusions are:

1. Major differences in dominant taxonomicgroups represented were found between cruises and be-tween the areas.

2. Some overlap was observed in species or relatedspecies between two geographic areas.

3. Cruises off the coast of Japan and Russia tendedto show a wider range of temperature and depth and tohave more offshore and northern species than those offthe U.S. West Coast.

4. Similar numbers of species were found for bothpurse-seine and trawl catches, despite the higher numberof specimens in the trawl catches.

5. The species groups from the TWINSPAN andcluster analyses were generally similar in both regions.

6. Latitude (or temperature) accounted for mostvariability in station groups and DCA analyses, althoughlongitude or distance offshore was sometimes important.

AcknowledgementsWe thank all the scientists and crew involved with

collecting and processing the environmental and biologi-cal data that we analyzed in these surveys, in particular


R. Emmett, and C. Morgan for the U.S. surveys and Y.Ishida for the Japanese surveys. Two anonymous review-ers provided useful comments on an earlier version ofthe manuscript. We acknowledge support from the Na-tional Marine Fisheries Service and Bonneville PowerAdministration for conducting the U.S. surveys. This iscontribution number 354 of the U.S. GLOBEC program,jointly funded by the National Science Foundation andthe National Oceanic and Atmospheric Administration.

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Appendix Table 1. Phylogenetic listing of pelagic nekton captured in purse seines during June 1985 including frequency ofoccurrence (F.O.) in 80 hauls and total number and length range (mm) caught. All measurements are fork lengths unlessmarked ML (mantle length) or TL (total length).

Common name Scientific name F.O. No. Length range (mm)

Market squid Loligo opalescens 19 1136 44–145 MLPacific lamprey Lampetra tridentata 1 2 570–630 TLSpiny dogfish Squalus acanthius 18 455 365–1574 TLSoupfin shark Galeorhinus zyopterus 2 2 1498–1727 TLBlue shark Prionace glauca 3 3 1422–1803 TLBig skate Raja binoculata 1 1 1828 TLAmerican shad Alosa sapidissima 1 3 304–314Pacific herring Clupea pallasi 10 971 94–218Northern anchovy Engraulis mordax 3 153 141–148Whitebait smelt Allosmerus elongatus 1 180 104–126Surf smelt Hypomesus pretiosus 3 11 153–166Eulachon Thaleichthys pacificus 1 285 104–124Coho salmon (j) Oncorhynchus kisutch 50 996 112–337Coho salmon (a) O. kisutch 43 256 365–612Chinook salmon (j) O. tschawytscha 43 282 100–395Chinook salmon (a) O. tschawytscha 16 38 401–800Chum salmon (j) O. keta 13 106 95–162Chum salmon (a) O. keta 5 6 480–775Pink salmon (a) O. gorbuscha 19 116 342–563Sockeye salmon (j) O. nerka 10 11 114–149Sockeye salmon (a) O. nerka 1 3 480–580Cutthroat trout (j) O. clarki clarki 15 22 175–408Steelhead (j) O. mykiss 5 10 200–313Pacific tomcod Microgadus proximus 4 15 44–53Walleye pollock Theragra chalcogramma 1 2 417–459Pacific hake Merluccius productus 4 254 412–545Pacific saury Cololabis saira 6 5 286–367Widow rockfish Sebastes entomelas 12 151 56–78Yellowtail rockfish S. flavidus 9 27 46–54Shortbelly rockfish S. jordani 5 14 68–76Black rockfish S. melanops 3 21 425–538Blue rockfish S. mystinus 4 18 50–61Canary rockfish S. pinniger 1 1 39Stripetail rockfish S. saxicola 1 1 43Sablefish Anoplopoma fimbria 1 2 172–253Kelp greenling Hexagrammos decagrammus 14 34 57–76Lingcod Ophiodon elongatus 1 1 61Brown Irish lord Hemilepidotus spinosus 4 6 31–35Cabezon Scorpaenichthys marmoratus 1 2 42–44Jack mackerel Trachurus symmetricus 1 1 349Wolf-eel Anarrhichthys ocellatus 5 7 400–570Pacific sanddab Citharichthys sordidus 2 4 213–244Starry flounder Platichthys stellatus 1 1 615


Appendix Table 2. Phylogenetic listing of pelagic nekton captured in surface trawls during September 1999 including frequencyof occurrence (F.O.) in 58 hauls and total number and length range (mm) caught. All measurements are fork lengths unlessmarked ML (mantle length) or TL (total length).


Market squid Loligo opalescens 5 21 71–155 MLThresher shark Alopius vulpinus 1 1 3450 TLSpiny dogfish Squalus acanthius 7 7 434–909 TLSoupfin shark Galeorhinus galeus 3 4 1000–1950 TLBlue shark Prionace glauca 1 2 1060–1370 TLPacific sardine Sardinops sagax 16 20810 155–282American shad Alosa sapidissima 2 3 192–264Pacific herring Clupea pallasi 21 3014 81–227Northern anchovy Engraulis mordax 6 51 57–162Whitebait smelt Allosmerus elongatus 5 502 105–116Surf smelt Hypomesus pretiosus 10 2856 109–180Night smelt Spirinchus starksi 1 4 97–105Eulachon Thaleichthys pacificus 1 24 62–105Northern lampfish Stenobrachius leucopsaurus 1 71 49–75Blue lanternfish Tarletonbeania crenularis 1 115 58–76Coho salmon (j) Oncorhynchus kisutch 19 174 222–371Coho salmon (a) O. kisutch 6 7 398–1104Chinook salmon (j) O. tshawytscha 28 565 115–342Chinook salmon (a) O. tshawytscha 6 11 445–672Chum salmon (j) O. keta 6 13 134–248Black rockfish Sebastes melanops 1 1 472Sablefish Anoplopoma fimbria 4 9 194–221Lingcod Ophiodon elongatus 1 1 675Pacific staghorn sculpin Leptocottus armatus 1 1 202Spearnose poacher Agonopsis vulsa 1 2 60–62Pacific mackerel Scomber japonicus 4 156 198–376Jack mackerel Trachurus symmetricus 8 104 325–567Barracudina Lestidium ringens 1 15 91–175 TLStarry flounder Platichthys stellatus 6 14 152–380Ocean sunfish Mola mola 2 3 380–480 TL

Total 28561


Appendix Table 3. Phylogenetic listing of pelagic nekton captured in purse seines during July–August 1989 including frequencyof occurrence (F.O.) in 59 hauls and total number and length range (mm) caught. All measurements are fork lengths unlessmarked ML (mantle length), SCL (scale length), FL (fork length) or SL (standard length).


Schoolmaster gonate squid Berryteuthis magister 2 3 9–15 MLCommon clubhook squid Onychoteuthis banksii 1 1 284 MLMadokai gonate squid Gonatus madokai 6 69 10–80 MLShortarm gonate squid Gonatus middendorffi 17 354 8–68 MLJapanese flying squid Todarodes pacificus 8 53 88–190 MLIncirrate octopod Japetella diaphana 1 1 60 MLJapanese sardine Sardinops melanostictus 15 956 42–241 SCLJapanese anchovy Engraulis japonicus 16 1794 23–164 SLChum salmon (j) Oncorhynchus keta 2 14 80–139Chum salmon (a) O. keta 1 1 630Pink salmon (a) O. gorbuscha 10 37 392–536Pacific saury Cololabis saira 16 251 9–348Threespine stickleback Gasterosteus aculeatus 1 1 70 SLPomfret Brama japonica 5 9 418–485Chub mackerel Scomber japonicus 2 26 32–395Jack mackerel Trachurus japonicus 2 2 47–71Sand lance Ammodytes spp. 7 5161 27–55Bering wolffish Anarhichas oreintalis 3 3 56–62 SLSablefish Anoplopoma fimbria 1 1 29 SLWhitespotted greenling Hexagrammos stelleri 1 31 43–55 SLRock greenling H. lagocephalus 4 7 78–87 SLMasked greenling H. octogrammus 1 13 39–52 SLArabesque greenling Pleurogrammus azonus 4 37 68–166 SLBanded Irish lord Hemilepidotus gilberti 6 17 24–34 SLCrested sculpin Blepsias bilobus 1 1 30 SLSilverspotted sculpin B. cirrhosus draciscus 4 43 27–47 SLSea raven Hemitripterus villosus 2 2 38–41 SLFrog sculpin Myoxocephalus stelleri 1 1 16 SLSmooth lumpsucker Aptocyclus ventricosus 1 1 224 SLKamchatka flounder Atheresthes evermanni 8 13 30–40 SLLongsnout prickleback Lumpenella nigricans 1 1 47 SLNutcracker prickleback Bryozoichthys lysimys 2 30 28–37 SL

Total 8933


Appendix Table 4. Phylogenetic listing of pelagic nekton captured in surface trawls during August–November 1996 includingfrequency of occurrence (F.O.) in 80 hauls and total number and length range (mm) caught. All measurements are fork lengthsunless marked ML (mantle length), SCL (scale length), FL (fork length), or TL (total length).


Sparkling squid Watasenia scintillans 3 78 18–39 MLNeon flying squid Ommastrephes bartrami 1 126 272–346 MLBoreopacific gonate squid Gonatopsis borealis 10 882 40–190 MLMako gonate squid Gonatopsis makko 6 12 80–194 MLShortarm gonate squid Gonatus middendorffi 3 9 104–330 MLJapanese flying squid Todarodes pacificus 12 425 120–268 MLArctic lamprey Lethenteron japonicum 1 1 475 TLSpiny dogfish Squalus acanthius 1 1 740 SLBlue shark Prionace glauca 1 1 1950 SLJapanese sardine Sardinops melanostictus 2 2440 116–152 SCLPacific herring Clupea pallasi 1 1 238 SCLJapanese anchovy Engraulis japonicus 6 7090 64–148 SCLNorthern smoothtongue Leuroglossus schmidti 6 4274 42–155 SLOkhotsk deepsea smelt Lipolagus ochotensis 6 383 53–107 SLCoho salmon (j) Oncorhynchus kisutch 9 95 281–370Chinook salmon (j) O. tschawytscha 1 1 259Chinook salmon (a) O. tschawytscha 19 66 414–760Chum salmon (j) O. keta 36 7740 118–310Chum salmon (a) O. keta 55 348 394–785Pink salmon (j) O. gorbuscha 36 18361 118–318Pink salmon (a) O. gorbuscha 14 31 406–584Sockeye salmon (j) O. nerka 3 454 231–300Sockeye salmon (a) O. nerka 6 25 380–594Masu salmon (j) O. masou masou 56 80 233–486Daggertooth Anotopterus nikiparini 4 8 624–937 SLNorth Pacific lanternfish Tarletonbeania taylori 3 4504 53–72 SLNorthern lampfish Stenobrachius leucopsarus 7 9351 41–98 SLCalifornia headlightfish Diaphus theta 4 308 42–84 SLWalleye pollock Theragra chalcogramma 8 16262 64–603 SLPacific saury Cololabis saira 10 1427 162–310Threespine stickleback Gasterosteus aculeatus 4 50 41–77 SLSablefish Anoplopoma fimbria 4 4 180–204 SLArabesque greenling Pleurogrammus azonus 17 8762 154–229 SLAtka mackerel P. monopterygius 1 3 196–201 SLCrested sculpin Blepsias bilobus 9 11 103–175 SLPacific pomfret Brama japonica 9 104 406–646Prowfish Zaprora silenus 6 8 125–230 SLChub mackerel Scomber japonicus 7 16864 170–273

Total 100590

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