Abundance and Distribution Patterns of Nekton and Micronekton...

515

Journal of Oceanography, Vol. 59, pp. 515 to 535, 2003

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

Copyright © The Oceanographic Society of Japan.

Keywords:⋅ Pelagic nekton,⋅ micronekton,⋅ distribution,⋅ habitat,⋅ coastal,⋅ abundance trends,⋅ Transition Zone,⋅ Northeast PacificOcean.

Abundance and Distribution Patterns of Nekton andMicronekton in the Northern California CurrentTransition Zone

RICHARD D. BRODEUR1*, WILLIAM G. PEARCY2 and STEPHEN RALSTON3

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.3Southwest Fisheries Science Center, NMFS, Santa Cruz, CA 95060, U.S.A.

(Received 19 July 2002; in revised form 14 December 2002; accepted 16 January 2003)

The epipelagic and mesopelagic nekton communities of the northern California Cur-rent have been sampled somewhat continuously over the last four decades with bot-tom and pelagic trawls, small midwater trawls, and purse seines. We review thezoogeography and community and environmental associations of the dominant pelagicmicronekton and nekton species in this region with a view to understanding theirrole in this dynamic marine ecosystem. As is typical of many upwelling eastern bound-ary current regions, the pelagic biomass is dominated by a few species that fluctuatedramatically through time. The abundance trends of pelagic nekton caught in thisregion demonstrated large-scale ecosystem changes about the time of the regime shiftsof 1976/77 and 1989 and possibly another beginning in 1999. The rapidity of thechanges in composition indicates that the response was due to a change in migrationor distribution patterns as opposed to recruitment patterns. The 1989 regime shiftled to a dramatic increase in sardine and a decrease in anchovy populations. Themost pronounced interannual signals were attributed to strong El Niño/SouthernOscillation (ENSO) conditions in 1983 and 1998 that altered the latitudinal rangesand proximity to the coast of many pelagic species. Variations in abundance andcross-shelf distribution patterns were noted for both pelegic nekton andmicronektonic from surveys off California, Oregon, and Washington.

Pelagic and demersal fish populations in the easternNorth Pacific have fluctuated in response to large-scaleocean conditions (Hollowed et al., 1987; Ware andThomson, 1991; Beamish, 1993; Hollowed and Wooster,1995; Mantua et al., 1997; Anderson and Piatt, 1999;McFarlane et al., 2000; Benson and Trites, 2002). Thesestudies clearly suggest that the response to a particularset of conditions may vary among species and even amongdistinct geographic populations of the same species (Hareet al., 1999). On shorter time and space scales, anoma-lous conditions associated with strong El Niño-SouthernOscillation (ENSO) events may alter the abundance anddistribution patterns of many highly mobile nekton(Pearcy et al., 1985; Smith, 1985; Mysak, 1986; Lenarzet al., 1995; Pearcy, 2002). Understanding how oceanicconditions in the past may have affected fish populationsmay provide insight into potential future responses, shouldsimilar conditions recur.

Much of our knowledge of long-term changes inpelagic fish populations and distributions for the coastal

1. IntroductionThere is a general agreement among marine scien-

tists that atmospheric and oceanic processes occur acrossdifferent temporal and spatial scales in the North PacificOcean, making it difficult to predict future oceanic con-ditions and their biological responses. For example, ElNiño/Southern Oscillation (ENSO) events occur on year-to-year time scales, the Pacific Decadal Oscillation (PDO)occurs on decadal time scales, and climatic trends suchas global warming or ice ages occur over centennial ormillennial time scales (Mantua and Hare, 2002). Thepopulations of larger marine nekton (fishes and squids)that may be utilizable by humans are known to have fluc-tuated dramatically over millennia even prior to the rela-tively recent period of exploitation (Baumgartner et al.,1992; Finney et al., 2000, 2002).

516 R. D. Brodeur et al.

upwelling area of the northern California Current off thePacific northwest coast (Washington to northern Califor-nia), has resulted from commercial fisheries catches.These records are often biased due to sampling, report-ing, and regulatory variations that hinder the usefulnessof the data. To effectively examine the abundance anddistribution of pelagic species, it is necessary to conductsystematic surveys using standardized methodologies atsimilar times of the year.

Although there have been numerous scientific sur-veys over the last several decades that have targeted or atleast systematically caught pelagic nekton in this region,there have been few attempts to synthesize these resultsand to compare the different surveys (Emmett andBrodeur, 2000). In the case of the smaller pelagic speciesreferred to as micronekton, information is completelylacking on any interannual trends in adult abundance pat-terns in the eastern North Pacific (Brodeur, 2001). There-fore, in this study we examine interannual andinterdecadal changes in nekton and micronekton abun-dance and distribution patterns. We were particularly in-terested in how cross-shelf and along-shelf distributionpatterns of nekton and micronekton change through time,relative to changes in oceanographic conditions.

2. Physics of the Transition RegionTransitional areas are by definition regions where

water masses converge and mix. In the North Pacific thereare two recognized oceanic transition areas: the Transi-tional Domain located between the Subarctic Frontal Zoneand the Central Subarctic Domain (Favorite et al., 1976),and the Transition Zone between the two major watermasses of the North Pacific, Subarctic Water and Sub-tropical Water. The Transition Zone is an area of mixingbetween the cold, low-salinity, highly productive subarcticand the warmer, more saline and less productive Subtropi-cal Water. The subarctic region is further characterizedby intense winter mixing above a permanent haloclineand large seasonal variations in the mixed layer vs. themore stable subtropical, oligotrophic domain with a per-manent thermocline (Fleming, 1955; Roden, 1991; Pearcy,1991).

These transitional regions extend across the PacificOcean, and wind forcing drives sluggish eastward-flow-ing surface currents of the West Wind Drift or SubarcticCurrent. These currents bifurcate off North America toform the Alaska Current that flows to the north and theCalifornia Current that flows to the south, from southernBritish Columbia to Baja California, from approximately50°N to 30°N. Mixing occurs between oceanic waters withthose from the south, and these water masses are subse-quently modified by coastal processes such as runoff andupwelling in this eastern boundary current. Based onanalyses of water mass characteristics (method of Tibby

(1941)), the waters in the upper 200 m off Oregon for theperiod 1961–68 were usually more than 60% subarctic inorigin (Pearcy, unpublished data). Within the CaliforniaCurrent (CC), the focus of this paper, Schwing et al.(1998) delineated three geographical regions based onwind stress which include 1) the northern region (42–48°N) where wind stress varies from poleward toequatorward, 2) the central region (34–42°N) with strong-est wind stress, mainly equatorward, and 3) the southernregion (22–34°N) where wind stress is predominantly tothe south.

This transitional region in the California Current ishighly dynamic and ocean conditions fluctuate widelyover time and space. The region has distinct seasonal aswell as interannual and interdecadal fluctuations. Coastalupwelling along the coast is episodic off Oregon duringthe summer, but more persistent off northern California.During the upwelling season, a coastal jet is formed anddynamic frontal zones sometimes extend far out to sea asjets. In the winter off the Pacific Northwest coast, theCalifornia Current either moves offshore or is replacedby the poleward flow of the Davidson Current. The Cali-fornia Undercurrent is a subsurface poleward current gen-erally confined to the continental slope and most intenseduring the summer (Hickey, 1998). During El Niño events,the equatorward flow of the California Current is anoma-lously weak, and southern water intrusions with their as-sociated fauna are found far to the north (Wooster andFluharty, 1985; Pearcy and Schoener, 1987; Pearcy, 2002).Thus, interannual variations in the northern CaliforniaCurrent are likely to result from both regional and glo-bal-scale processes (Landry et al., 1989; Hickey, 1998).Interdecadal fluctuations with periods of 15–25 years havealso been documented in the North Pacific and in thecoastal transition region (Hare and Mantua, 2000). Thesewere based on variations in sea-surface temperatures andabrupt changes in the survival or species composition ofpelagic animals, such as occurred in 1977 and 1989.

3. ZoogeographyThe zoogeographic patterns of zooplankton of the

oceanic North Pacific have been described and related towater masses by Brinton (1962), Fager and McGowan(1963) and Reid et al. (1978). Several papers describethe zoogeographic distributions and abundances of com-mon epipelagic nekton of the North Pacific Ocean.Brodeur (1988) reviewed the zoogeographic distributionof dominant epipelagic nekton fishes, including fishessuch as Pacific saury (Cololabis saira), albacore tuna(Thunnus alalunga), and pomfret (Brama japonica) thatwere found mostly in the Transition Zone. This study in-cluded the common nekton (fishes, cephalopods, seabirds,marine mammals) in an analysis of species associationsand food web relationships from the oceanic Transition

Abundance and Distribution Patterns of Nekton and Micronekton in the Northern California Current Transition Zone 517

Zone. Brodeur and Ware (1995) examined interdecadalvariability in catch rates and distribution of pelagic nektonfrom the eastern North Pacific Subarctic and Transitionzones. Distinct species associations of epipelagic nektonamong different oceanographic regions of the North Pa-cific including the Transition Zone were reported byPearcy et al. (1996) and Brodeur et al. (1999).

The zoogeographic distributions and species asso-ciations of the coastal nekton of the northern CaliforniaCurrent are incompletely known. Horn and Allen (1978)analyzed the distribution patterns of over 500 coastalmarine fishes of California. They found faunaldiscontinuities at Point Conception (33–34°N) and PugetSound (47–48°N). Unlike previous studies, they did notrecognize a faunal boundary at Cape Blanco (42°N) be-tween the Vancouveran and California zoogeographic re-gions. This may be an artifact of examining only speciesoff California and not including many species normallyfound to north, which may have a southern endpoint atthis latitude. They found a rather low (2.4%) degree ofendemism in the California fish fauna. Following an ear-lier classification system suggested by Briggs (1974),Allen and Smith (1988) grouped the shelf region fromnorthern Washington to Point Conception in central Cali-fornia as the Oregonian Province, within the larger East-ern Boreal Pacific Region, based mainly on demersal fishdistributions. A similar analysis has not been attemptedfor pelagic species, but few pelagic nekton are endemicto this province. Analyses have been conducted of thecoastal distribution patterns and assemblages of pelagicnekton in this region (Brodeur and Pearcy, 1986; Brodeuret al., 2003a, b).

For micronekton, Willis et al. (1988) recognizedthree transition zone patterns of mesopelagic fishes basedon extensive midwater trawl collections: an Eastern Tran-sition Zone that included the California Current region, aWestern Pacific Transition Zone, and an entire Transi-tion Zone. Lampanyctus ritteri was a myctophid speciesthat was closely associated with the California Currentregion. Jefferts (1988) also found that most species ofcephalopods were associated with water masses, and thata number were limited to the California Current and theeastern Transition Zone, including the common squidGonatus californiensis. Krygier and Wasmer (1988) listed13 species of pelagic shrimps that were confined mainlyto North Pacific Transitional Waters. From these studies,it is clear that some species of nekton and micronektonare endemic to Transitional Waters and that some appearto be endemic to the eastern transition region and theCalifornia Current region.

In summary, the pelagic fauna of the northern Cali-fornia Current is composed of a small number of endemiccoastal and offshore species within a larger mixture ofsubarctic, transitional, and subtropical species, some ofwhich are at the limits of their geographic distribution.

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4. Data SourcesPelagic nekton and micronekton abundance, species

compositions, and distribution patterns came from sev-eral different investigations (Table 1). In some cases, thepurpose of the investigation was not to directly assessthe nekton or micronekton taxa examined here, but themethodology allowed for quantitative estimates of theirabundance in space and time. For each study, we exam-ined only the dominant species that comprised the bulkof the catch in any given survey.

a) Soviet Union Trawl SurveysTo examine long-term abundance patterns prior to

the beginning of regular National Marine Fisheries Serv-ice (NMFS) surveys along the west coast, we examinedcatches from a series of research cruises conducted bythe Soviet Union during the 1960s and 1970s from Canadato northern California, with more limited collections dur-ing the 1980s mainly in the Columbia region. Trawling

was done using a flare trawl with a mouth opening of27 × 8 m and 20-mm mesh in the codend (Ermakov andStepanenko, 1996). More than 1700 bottom trawls weremade, but we restricted our analysis to those done fromJuly through September and extending from 40.5 to48.5°N to make them comparable to the NMFS surveydata (Fig. 1). We grouped the data into the same geo-graphic areas as the NMFS data (see below).

b) NMFS Groundfish SurveysThe Alaska Fisheries Science Center of NMFS has

conducted Triennial Shelf Trawl Surveys of groundfishresources along the West Coast of North America from1977 through 2001 (Dark and Wilkins, 1994). Althoughthe trawl survey is not designed to target small pelagicspecies, it is conducted with a randomized design andequal sampling effort each year, thus permitting compari-sons of catches between survey periods. All trawling wasdone using a Nor’Eastern trawl constructed with 8.9-cm

Fig. 1. Locations of Soviet Union Trawl Survey stations by year from 1965 to 1978, during July through September, fromlatitudes 40.5°N to 48.5°N only.


nylon mesh in the body and lined with 3.2-cm mesh toretain small fish. Surveys were conducted from July toOctober from northern Washington to Monterey Bay,California. For this study, data were summarized by theU.S. Vancouver, Columbia, and Eureka InternationalNorth Pacific Fisheries Commission (INPFC) GeneralAreas (Fig. 2).

c) OSU Micronekton CollectionsCollections were made along the Oregon coast with

four transects off Astoria (AH), Brookings (BH), CoosBay (CH) and Newport (NH) (Fig. 3). Cruises were madeat monthly intervals from Nov. 1961 to April 1964, al-though not every transect was sampled each cruise. Inaddition, collections were made along the Newport lineintermittently until 1969. Stations were located at 28, 46,84, 120, and >120 km along each transect. Oblique towswere made to ca. 200 m, except at the inshore stationswhere approximately half the water column was sampled.A 1.8-m Isaacs-Kidd Midwater Trawl (IKMT) with a 5-mm liner was used to collect micronekton (Pearcy andLaurs, 1966; Pearcy, 1976) throughout the study period.

d) OSU Purse-Seine SurveysData on pelagic nekton abundance off Oregon and

Washington were collected from purse seine surveys con-

ducted from May to September 1981 to 1985 by OregonState University (OSU) (see Brodeur and Pearcy (1986)and Pearcy and Fisher (1990) for sampling dates and de-tails). Briefly, fine mesh (32-mm) herring purse-seineswere fished along east-west transects from Cape Flatteryoff northern Washington to Cape Blanco off Oregon. Thenet size varied from 457 m to 495 m in length and 20 mto 60 m in depth among the 15 cruises. Most of thesecollections were conducted during daylight. Only the Junecruises were analyzed in this study, and three transects(Grays Harbor, Columbia River, and Newport) which wereconsistently sampled each year were examined for in-shore-offshore trends (Fig. 4).

e) NMFS Columbia River Plume SurveysThis data set consists of the nekton captured during

a NMFS pelagic trawl survey for juvenile salmonids offWashington and Oregon during June and September of1998 (Fig. 4). This survey sampled as far north as La Push,WA and south to Cape Perpetua, OR. Sampling consistedof 15–30 minute tows with a pelagic rope trawl alongtransects perpendicular from shore at a variety of loca-tions. The rope trawl, which contained variable mesh sizes(162.6 cm at mouth to 8.9 cm at cod end), had a fishingmouth opening of approximately 30 m × 20 m. To main-tain catches of small nekton, a 6.1-m long, 0.8-cm knotlessliner was sewn into the cod end. Both chartered commer-cial fishing vessels and research vessels (R/V W.E. Ricker)were used to conduct the sampling. Initial trawls wereconducted in the shallowest depth possible while farthestseaward sampling was conducted approximately 50 kmfrom shore. Most sampling was conducted during day-light and all tows were 30 min in duration. All nekton

Fig. 2. Location of sampling stations for first four years ofNMFS Triennial Groundfish Surveys. Also shown are theINPFC General Areas used in the summary graphs (fromDark and Wilkins, 1994).

Fig. 3. Location of transects summarized for the OSUMicronekton Collections from 1961 to 1969 along with the200 m and 1000 m isobaths.


caught were counted and measured at sea. Again, onlyJune data were summarized and the same three transectswere examined in detail.

f) NMFS Juvenile Rockfish SurveysAnnual trawl surveys designed to estimate the dis-

tribution and abundance of pelagic juvenile rockfishesalong the central California coast have been conductedaboard the R/V David Starr Jordan since 1983 (Wyllie-Echeverria et al., 1990). Cruises have been conductedduring May and June when the pelagic juvenile-stage fishare most susceptible to capture by midwater trawling.These surveys use a modified 26- by 26-m Cobb midwatertrawl, with a codend liner of 1.27-cm stretched mesh.Beginning in 1986, three spatially replicated “sweeps”of a series of standard stations were conducted in a studyarea ranging from 36.0–38.4°N (Fig. 4). At each station a15-minute nighttime trawl sample was taken at standarddepth (30 m where possible, 10 m at shallow stations).

Catches were sorted at sea, subsampled when necessary,and enumerated to the lowest possible taxon. Data foreach of the 10 dominant nekton were averaged over thethree sweeps (i.e., sweeps were treated as replicates). Theresulting data set included year (1986–2001), station (40unique locations), species, and average CPUE (over threesweeps).

5. Interannual Variability in Dominant Pelagic Spe-cies

5.1 Soviet Union Trawl SurveysSampling was highly variable from year to year (Fig.

1), so the time series within the different regions wereincomplete, especially for the Eureka Region. Within asingle year (1969 for example, Fig. 5), several of the domi-nant species occurred over a broad geographic area (Pa-cific hake, spiny dogfish, eulachon, surf smelt) while oth-ers were more patchy and localized in their distributions(jack mackerel, northern anchovy, Pacific herring, and

Fig. 4. Locations of typical stations sampled during a singlesummer (2000) off Washington, Oregon, and California.Also shown are the transect lines examined for inshore-off-shore distribution patterns of pelagic nekton as describedin the text.

Fig. 5. Overall sampling stations and relative biomass distri-bution of predatory nekton (top) and forage nekton (bot-tom) from the Soviet Union Trawl Survey for July throughSeptember of 1969.


American shad). Interannual variability was low with theexception of several years with large catches of predomi-nantly Pacific herring (Fig. 6). The predatory fishes showan inverse relationship in their catches from the northernregion (Vancouver) versus the Columbia and Eureka re-gions (Fig. 7). This was seen as a dramatic decline imme-diately following the 1976/1977 regime shift in Vancou-ver with a corresponding increase, particularly of Pacifichake, off Columbia and Eureka. The suddenness of thischange suggests that the migration pattern may havechanged at about this time, with less movement to thenorth following the regime shift.

5.2 NMFS Groundfish SurveysPacific herring was the dominant pelagic forage spe-

cies collected in the NMFS triennial surveys in all three

regions, but particularly in the Eureka Region (Fig. 8).The highest biomass in the Vancouver Region was esti-mated to have occurred in 1977. It dropped off dramati-cally according to the following two surveys but hasslowly rebounded since. In the Columbia Region, thebiomass was generally low for late 1970s to mid 1980sbut has been generally high since 1989. In the EurekaRegion, a similar pattern was observed, but the increasewas first apparent in the 1992 survey. Pacific sardines,although never a major component of the catch, first ap-peared in the 1992 survey. These increases are coinci-dent and perhaps related to the change in ocean condi-tions off the West Coast that occurred around 1989 (Hareand Mantua, 2000).

The pelagic predatory fish biomass is dominated by

Fig. 6. Biomass of forage nekton caught by year during theSoviet Union Trawl Survey for the three INPFC generalareas. Years with “×” indicate insufficient effort to deter-mine biomass and those with “�” were not found in thatregion.

Fig. 7. Biomass of predatory nekton caught by year during theSoviet Union Trawl Survey for the three INPFC generalareas. Years with “×” indicate insufficient effort to deter-mine biomass and those with “�” were not found in thatregion.


Pacific hake in all regions, although spiny dogfish is alsoimportant, especially in the Vancouver Region (Fig. 9).There has been a general increase during the 1989–1998period in the two northern regions, with a correspondingdecrease in the Eureka Region. This likely indicated anortherly migration pattern for these predators, particu-larly during the warm period of the early 1990s. It alsosuggests that examining only one region likely will givean incomplete picture of the abundance trends (Emmettand Brodeur, 2000). Although Pacific hake contributedmainly to this increase, this period is generally the onlytime when chub and jack mackerel contributed substan-tially to the catch.

5.3 OSU Micronekton CollectionsFour species of myctophid fishes dominated the

midwater trawl collections off Oregon. Stenobrachiusleucopsarus clearly was most abundant, followed by

Diaphus theta , Tarletonbeania crenularis , andLampanyctus ritteri (Fig. 10). Although sampling vari-ability of these fishes is large between individual stationsor times of the year, interannual differences were appar-ent in catches among the eight years. Catches were high-est during the downwelling season of 1965, especially atthe offshore stations, and were lowest during theupwelling season during 1965–1967 (Fig. 10).

5.4 OSU Purse-Seine SurveysThe 10 species examined comprised up to 80% of

the total catch in June cruises during all years, with muchof the remainder belonging to the various salmonid spe-cies (Emmett and Brodeur, 2000). As reflected in the pro-portion of the catch made up of the dominant species, thenekton fauna underwent substantial changes during theearly 1980s (Fig. 11). During the strong El Niño, whichstarted in 1983 and continued on through at least June of

Fig. 8. Biomass of forage nekton caught by year during theNMFS Triennial Groundfish Survey for the three INPFCGeneral Areas.

Fig. 9. Biomass of predatory nekton caught by year during theNMFS Triennial Groundfish Survey for the three NPAFCstatistical regions.


1984, there was a dramatic drop in forage fishes and aninflux of large predatory pelagic species, particularly chubmackerel, jack mackerel, and sablefish (Pearcy et al.,1985; Pearcy and Schoener, 1987; Brodeur and Pearcy,1992). Most of these predatory species inhabit warmerwater and perhaps were able to occupy habitats that werefurther north (chub mackerel) or onshore (sablefish) thantheir usual geographic ranges. During the relative cold,strong upwelling year of 1985, the pelagic fauna was simi-lar to the pre-El Niño composition dominated by marketsquid, herring and northern anchovies (Fig. 11).

5.5 NMFS Columbia River Plume SurveysMore recent trawl survey data show a substantially

different pattern than the purse seine data collected in theearly 1980s. Forage nekton dominated the catch in allyears, comprising over 90% of the total catch (Fig. 12).Sardines were an important component of the catch in allyears except 2001, but they occurred in very low num-bers in only one haul during the purse-seine cruises inthe 1980s. This increase in sardines in the Pacific North-west and off Canada has been documented a number of

times (Hargreaves et al., 1994; Brodeur et al., 2000;McFarlane and Beamish, 2001) and probably reflects in-creased recruitment as well as expansion of the range ofthis species into favorable habitats. The results for 2001suggest that the sardines were not present in substantialnumbers off Oregon and Washington and they were re-placed by Pacific herring and market squid. The preda-tory nekton species exhibit a declining trend over the yearssampled, with Pacific hake being present mainly duringthe 1998 ENSO event (Fig. 12).

5.6 NMFS Juvenile Rockfish SurveysThe following observations stand out in the analysis

of interannual variability in the catches of the largernekton off central California (Fig. 13, upper panel). Pa-cific hake appears to have undergone a long-term declinebut has started to rebound in the last two years. This re-cent increase in the abundance of Pacific hake off centralCalifornia in this survey may be due to a southerly shiftin the summer feeding distribution of hake following the1999 La Niña. The catch of market squid has been highlyvariable over the last 16 years. Market squid abundance

Fig. 10. Interannual variation in the catch of the dominant myctophid species collected during the OSU Micronekton Surveysalong the Newport transect by distance offshore. Data are summarized by upwelling (May–October) and downwelling (No-vember–April) periods for all collections combined within each season. Too few fish were collected at the 28-km station.


was absent and low during the 1986 and 1998 warm ENSOevents, respectively, but was quite high during the 1992ENSO event. Northern anchovy has been variable but hasnot noticeably trended up or down in abundance in thisarea of the coast, whereas Pacific sardine has increasedmarkedly through 1998, but has declined since. Amongthe less dominant species, spiny dogfish has declinedmarkedly, and Pacific herring and shortbelly rockfish havedeclined, but to a lesser extent (Fig. 13, bottom panel).Butterfish were relatively more abundant during the early1990s. Whitebait smelt and sanddab abundance data showno consistent patterns.

Catches of micronekton in this survey show a dis-tinct synchrony in the pattern of juvenile young-of-the-year (YOY) abundances of Pacific hake, shortbellyrockfish, and Pacific and speckled sanddabs (Fig. 14,upper panel). Much of this synchronous pattern, however,is due to low abundances during the 1986, 1992, and 1998ENSO events, which apparently had an adverse effect onsurvival, or possibly distribution. It is again noteworthythat, following the 1998 event, the combined abundanceof these early life history stages has increased markedlyeach year, presumably due to the existence of conditionsmore favorable to larval survival. For the mesopelagic

micronekton (Fig. 14, lower panel), in aggregateLeuroglossus stilbius, Tarletonbeania crenularis, Diaphustheta, and Argentina sialis have declined in abundanceover the 1986–2001 period of the survey (r = –0.47, df =62, P < 0.0001). The declines have been most apparent inDiaphus and Argentina (i.e., r = –0.73 and –0.50, respec-tively), although each is now less abundant than it wasduring the 1980s. For example, the average log-catch ofLeuroglossus and Tarletonbeania from 1997–2001 is onlytwo-thirds of that taken during the first five years of thesurvey. Whether these declines have been gradual, or oc-curred more precipitously sometime during the mid-1990s, is difficult to determine. More recently,Leuroglossus has increased in abundance but the otherspecies, especially Diaphus theta, have not rebounded.

6. Spatial Variability in Dominant Pelagic Species

6.1 OSU Micronekton CollectionsAlong the Newport Transect, the most frequently

sampled of the four transects, catches were usually lownear the coast (Fig. 15). The 28 km station on this transectwas the only station among the four transects that wasover the continental shelf (depth of water about 90 m)

Fig. 11. Summary of the catch (as a percent of total) for theforage (top) and predatory (bottom) species collected offOregon and Washington from 1981 to 1985 during the OSUPurse-Seine sampling.

Fig. 12. Summary of the catch (as a percent of total) for theforage (top) and predatory (bottom) species collected offOregon and Washington from 1998 to 2001 during NMFSPelagic Trawl sampling.


and myctophids were not usually caught here. Catchesincreased in deeper water offshore generally peaking atabout 80 km, and then declining farther offshore.

Differences are also apparent in the inshore/offshorecatches during the two oceanographic seasons along theNewport Transect (Fig. 15). Generally catches were largerat the 46 km station during the downwelling than theupwelling season, but significant differences were foundonly for D. theta and T. crenularis (Wilcoxon Rank SumTest, both p < 0.03). Catches were higher at the offshorestations (84 and 120 km) during the upwelling seasons(Fig. 15). These results agree with the findings of Pearcy(1976), which showed same trends in overall fish biomass,with significantly higher biomass inshore (28 and 46 km)during the winter, and offshore (84 and 120 km) duringthe summer. These trends are associated with winters char-acterized by a greater inshore transport (downwelling) andlonger residence time in the Ekman layer, when thesevertical migrators reside longer in the upper layers, andsummers characterized by upwelling associated with off-

shore transport and enhanced prey production. Laurs(1967) found similar inshore-offshore trends off the south-ern Oregon coast near Brookings, Oregon.

The general trend along the Newport (NH) Transectwas apparent along the other transects, with peak catchesof myctophids over the continental slope and decreasedcatches farther offshore over the abyssal plain (Fig. 15).The continental shelf along the Coos Bay (CH) Hydrolinewas narrow, and the 28 km station was over the deepestwater (about 400 m) of any of the 28 km stations. Herewe caught the greatest numbers of S. leucopsarus, bothduring the upwelling and downwelling seasons at the 28km station, while the other myctophids had peak catches46 km offshore. Along the Astoria Hydroline (AH) verylarge catches of S. leucopsarus were acquired at the 28km station during the downwelling season when fish wereprobably advected into the Astoria Canyon and concen-trated there. This included one catch of over 250 indi-viduals during a strong downwelling event. Astoria Can-yon may act to funnel and concentrate mesopelagic ani-mals (Pereyra et al., 1969). Brookings Hydroline (BH)catches usually peaked at 46 or 84 km, and although the28 km station is only slightly shallower than the one at

Fig. 13. Summary of the catch (log abundance) for the domi-nant (top) and secondary (bottom) species collected off cen-tral California from 1986 to 2001 during the NMFS Juve-nile Rockfish Trawl sampling.

Fig. 14. Summary of the catch (log abundance) for the juve-nile (top) and mesopelagic (bottom) species collected offcentral California from 1986 to 2001 during the NMFS Ju-venile Rockfish Trawl sampling.


Fig. 15. Inshore-offshore distribution of the dominant myctophid species collected during the OSU Micronekton Surveys bytransect. Data are summarized by upwelling (May–October) and downwelling (November–April) for all years combined.Values are mean ± one standard error of the number caught at each station.


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CH, there were never peak catches at this inshore station.Fast (1960) compared catches of S. leucopsarus in

Monterey Bay and offshore waters and found peak catchesin the bay in early winter and offshore in the summerupwelling season, suggesting that fish are advected intonearshore waters during downwelling (Davidson Current)and displaced offshore during upwelling events. Aron(1962) reported similar trends on his Brown Bear cruise202, with peak catches of myctophids at inshore stations.However, on cruise 199 he found large catches of S.leucopsarus far offshore (145–150°W) as well as an or-der-of-magnitude decrease in myctophid catches in off-shore waters between about 47–50°N and 45–32°N. Thusit appears that the inshore/offshore distribution ofmesopelagic fishes in this regions is strongly influencedby onshore transport either through seasonal downwellingor association with canyons.

6.2 OSU Purse-Seine SurveysThe distribution of the dominant nekton taxa during

the June cruises varied interannually but also with lati-tude and distance offshore. During the strong upwellingyear of 1982, Pacific hake and herring were dominant offGrays Harbor, juvenile salmonids and juvenile rockfisheswere important off the Columbia River, and coho salmonjuveniles and market squid were important inshore on theNewport Line (Table 2). During the ENSO year of 1983,there was a noticeable replacement of the dominantcoldwater species, such as salmon, Pacific herring, andspiny dogfish, with southern species such as chub mack-erel and jack mackerel (Table 2). There was also a com-pression of the distributions closer to shore, especiallyoff Newport. Catches dropped off substantially beyond 9km, although sampling took place out to 37 km (Table 2).During June 1984, oceanographic conditions began toreturn to normal but the fish community was still anoma-lous due to the lingering effects of ENSO (Brodeur andPearcy, 1992). The cross-shelf distribution patterns wereintermediate to those of the previous two years, with thenekton densities offshore of 18 km again dominated bychub and jack mackerel (Table 2).

Fig. 16. Inshore/offshore distribution of pelagic nekton fromthe NMFS Juvenile Rockfish Trawl cruises by transect forall years combined.

Fig. 17. Inshore/offshore distribution of micronekton from theNMFS Juvenile Rockfish Trawl cruises by transect for allyears combined.


6.3 NMFS Columbia River Plume SurveysWe examined the cross- and along-shelf distribution

on nekton from three highly contrasting years: the El Niñoof 1998, the La Niña of 1999, and the strong upwellingyear of 2000. Although predatory warm water species suchas jack and chub mackerel occurred during 1998 (Table3), they were not as important along these transects asthey were during the 1983/84 ENSO (Table 2). In con-trast, forage fishes such as anchovy, smelt, and sardineswere important in all three transects. Similar to 1983, therewere few nekton caught offshore beyond 20 km, whichmay be related to the advection of oceanic water close tothe coast during this ENSO period (Peterson et al., 2002).During the La Niña of 1999, the dominant nekton weredistributed farther offshore on all transects, and there wasan increase in the normal inshore species such as cohoand chinook salmon, herring, and market squid (Table 3).Jack and chub mackerel were still present but were foundgenerally at the offshore stations. Juvenile salmon, an-chovy, and sardines were important on the northern two

transects in 2000, and large numbers of sardines in par-ticular were caught offshore of the Columbia River plumein oceanic water (Table 3). Off Newport, however, mar-ket squid and surf smelt were the main taxa collected atthe inshore and midshelf stations. Despite the differencesin oceanographic conditions during the three years, therewas little difference in the overall densities of nektoncaught in these three years.

6.4 NMFS Juvenile Rockfish SurveysThe combined nekton encountered in this survey

showed a trend of decreasing abundance with distanceoffshore (Fig. 16). However, that pattern is due in largepart to the nearshore distribution of strongly shelf-asso-ciated species, especially market squid and northern an-chovy. Species that often occur well off the continentalshelf, i.e., Pacific hake and sardine, show a more uni-form cross-shelf distribution. Along the north (PointReyes) to south (Davenport) gradient represented by thethree lines, there are some differences in species compo-sition. For example, Pacific hake is most dominant alongthe Point Reyes line, but becomes less important furtherto the south, a pattern in exact opposition to that of thenorthern anchovy (Fig. 16).

Unlike the major nektonic taxa, which tend to de-crease in abundance with distance offshore, themicronekton tend to display the opposite pattern, espe-cially along the Davenport and Pescadero lines (Fig. 17).This is apparently because the terminal stations alongthose two survey lines are located off the continental shelf,where collections tend to pick up substantial quantitiesof mesopelagic organisms (e.g., myctophids andbathylagids). The relatively uniform cross-shelf distribu-tion of young-of-the-year (YOY) shortbelly rockfish alongall three lines is notable.

7. Temporal Changes in CommunitiesTo more objectively synthesize and depict temporal

patterns in abundance of the dominant taxa, the longertime series (Russian, NMFS groundfish and NMFSrockfish cruises) were evaluated using principal compo-nents analysis (PCA). All data were log-transformed(X+1) before analysis. For examination of temporaltrends, the first principal component (PC1), which gen-erally accounted for >60% of the variance, was plottedagainst year of sampling (Fig. 18). Results for the Sovietsurveys for the Columbia Region only indicate thatthroughout the 1960s, the PC1 fluctuated about zero butremained positive in the early 1970s (Fig. 18A). The val-ues following the 1976 regime shift became strongly nega-tive. Patterns for the NMFS triennial trawl data were simi-lar for the Vancouver and Columbia regions (Fig. 18B),showing mainly negative values of PC1 until 1989 (or1992 for Vancouver) and then strongly positive values in

Fig. 18. Variation in first principal component by year forA) Soviet Union sampling, B) NMFS Triennial Survey byregion, and C) NMFS Juvenile Rockfish Surveys.


the 1990s. The most recent cruise appeared to be trendingdown toward lower values. The Eureka region, however,appeared to be somewhat out of phase with the other tworegions, especially during the strong ENSO years of 1983and 1992 (Fig. 18B). The analysis for the JuvenileRockfish Surveys showed that the assemblage of speciesencountered early in the time series apparently changedabout the time of the 1992 ENSO event (Fig. 18C). Dur-ing the first few years of the survey, the catch was char-acterized by relatively strong representation of Pacifichake, market squid, whitebait smelt, shortbelly rockfish,and spiny dogfish, with scores on the first principal com-ponent axis usually being positive. In contrast, during thelatter years of the survey, when scores were always nega-tive (1993–2001), the catch was much more strongly in-fluenced by the increased availability of sardine and north-ern anchovy. Following the 1999 La Niña event, the com-position of the nekton assemblage in this survey returnedto a state more similar to that seen in the early 1990s, asindicated by increased catches of market squid and Pa-

cific hake and decreased catches of sardine and northernanchovy.

The species loadings of the first two Principal Com-ponents showed which species exhibit trends in similardirections (Table 4). In the Soviet data in the Columbiaregion, positive values of PC1 indicate high dogfish abun-dance while negative values are associated with higherabundances of hake and jack mackerel during years (e.g.,1967–68 and 1976–78) when those species were relativelyabundant. Similarly, smelts had a very low value for PC2,possibly due to the high catches in the early 1970s whenmost smelts species were at low abundance. For the NMFStriennial data from the Columbia region only (Table 4),most species had positive PC1 loadings with the excep-tion of eulachon and anchovy, neither of which wereamong the dominant species. Finally for the NMFS Juve-nile Rockfish survey (Table 4), sardines stood out the mostfor PC1, which may have been caused by their occurrencethrough only part of the sampling period.

Table 4. Summary of species loadings of the first (PC1) and second (PC2) Principal Components by sampling program.

Soviet Union Trawl Surveys (Columbia Region only) PC1 PC2

Pacific hake –0.501 0.324Jack mackerel –0.487 0.145Smelts 0.008 –0.519Herring 0.054 0.344Eulachon 0.067 0.629Anchovy 0.303 0.223Shad 0.321 0.182Spiny dogfish 0.520 0.090

NMFS Triennial Surveys (Columbia Region only)Anchovy –0.235 –0.580Eulachon –0.208 0.665Sardine 0.327 –0.048Herring 0.273 0.230Shad 0.367 –0.166Spiny dogfish 0.360 –0.287Jack mackerel 0.350 0.206Chub mackerel 0.401 0.122Pacific hake 0.416 –0.027

NMFS Juvenile Rockfish SurveySardine –0.410 –0.104Anchovy –0.216 0.460Butterfish 0.005 0.501Sanddabs 0.147 0.454Herring 0.178 0.506Market squid 0.250 0.089Spiny dogfish 0.341 –0.036Pacific hake 0.410 –0.211Shortbelly rockfish 0.430 –0.097Smelts 0.437 0.038


8. DiscussionIn this overview paper we have assembled and

reanalyzed the results of several diverse sampling pro-grams in the northern California Current spanning sev-eral decades. The motivation behind the sampling wasoften dissimilar and, as mentioned previously, the pelagicspecies were often not the main target of the sampling(i.e., demersal trawls). Despite these shortcomings, wefeel that the data do provide a glimpse of some of themajor environmental conditions and ecological associa-tions that have existed over the last few decades in thenorthern California Current. It is beyond the scope of thisstudy to examine detailed species/environmental relation-ships for all the data sets. Indeed, this has been attemptedalready for a subset of the data for a number of the sam-pling programs described herein (Jay, 1996; Emmett andBrodeur, 2000; Brodeur et al., 2003a, b). Many of thesepapers also describe species and community associationsin more detail than was possible here. For example, Jay(1996) examined fish assemblages from the NMFS Tri-ennial Surveys up to 1992 and found that chub and jackmackerel and Pacific hake all grouped together as an as-semblage, and that they all showed dramatic increases inthe 1989 and 1992 surveys although there were some dif-ferences between the northern and southern parts of theirrange, similar to what we found here.

Although some interannual differences in abundancewere likely due to recruitment differences, most of theseshort-term and even some of the longer-term variationsare probably due to changes in migration patterns. Manyof the pelagic species are highly mobile and can migrategreat distances if conditions in their normal habitat aresuboptimal. Many species, including Pacific hake, jackand chub mackerel, and Pacific sardines are known toundertake substantial seasonal migrations from theirspawning habitats off southern California to feeding ar-eas in the northern California Current. Thus our samplingmay completely miss this migration, depending on themovement patterns, or may perhaps overestimate thepopulation if the timing coincides with the passage ofmajor schools. For the data sets we examined forinterannual comparisons, we attempted to be consistenton the timing of the cruises, although we certainly ac-knowledge the potential for substantial seasonal variabil-ity, as seen in the micronekton data.

While the direct links to nekton are unclear, there iscogent evidence that the last century has witnessed long-term climatic shifts affecting marine ecosystems of theNortheast Pacific Ocean (Francis et al., 1998; McGowanet al., 1998) with coincident changes in lower trophic levelproductivity (Polovina et al., 1994; Brodeur et al., 1996;McGowan et al., 1998; Mackas et al., 2001) as well aschanges in higher trophic levels (Beamish, 1993; Brodeurand Ware, 1995; Anderson and Piatt, 1999; McFarlane et

al., 2000). Physical and biological time series show con-gruent shifts around 1976 and 1989, although the lattertransition appears to be less intense (Hare and Mantua,2000). Our aggregate time series show changes occur-ring in the pelagic fish community around these sameperiods, although in some cases there are noticeable lagsin the community response.

Recent data suggest that another new regime mayhave begun in late 1998, which coincided with a strongLa Niña event (Schwing et al., 2000, 2002). The responsein zooplankton populations occurred rapidly, with highbiomass levels not seen since before the 1976 regime shiftoccurring all along the West Coast (Schwing et al., 2000,2002; Mackas et al., 2001). The production response atthe higher trophic levels is likely to lag several years be-hind the zooplankton changes although some short-livedstocks such as salmonids and market squid have alreadybegun to resurge (Schwing et al., 2002). However, weshould be careful not to attribute all changes we observeto climate-driven effects as predator-prey interactions maybe important determinants of within-regime abundancelevels (Meuter and Norcross, 2000; Hunt et al., 2002).Indeed, as Ware and McFarlane (1995) have shown, therewas a substantial increase in consumption of Pacific her-ring by hake in the warm period following the regimeshift of 1977 off Vancouver Island that may have de-pressed the recruitment of herring.

In addition to the long-term changes in the ecosys-tem, the California Current has been affected by somedramatic oceanographic and ecological changes duringthe study period. Included among these were two of thestrongest El Niño events ever recorded in 1983 and 1998(Wooster and Fluharty, 1985; Chavez et al., 2002) and avery strong La Niña event in 1999 (Schwing et al., 2002).The effects of these anomalous conditions are readilyapparent in our data, although all taxa do not universallyshow these effects. For example, ENSO effects are verypronounced in juvenile pelagic fishes off central Califor-nia but are not apparent in mesopelagic fishes caught inthe same sampling. This may be due to the more offshoreor deeper spawning regions of these mesopelagic fishesthat may not be as influenced by these warming events.Market squid is among the most notably affected speciesoff Oregon and Washington occurring in relatively lownumbers during ENSO years. In this region, the main ef-fect of El Niño is probably a collapse of the distributionsof many of the locally-adapted species (salmon, herring,smelts) much closer to shore than in normal years, leav-ing the less productive offshore regions to many south-ern migratory species such as chub and jack mackerel.

Distinct inshore-offshore patterns were found formesopelagic micronektonic fishes that were correlatedwith cross-shelf transport during upwelling anddownwelling seasons. Pearcy (1977) showed similar


cross-shelf and seasonal patterns in the dominant taxa forsampling with larger nets from 1969 to 1973, but alsoshowed that the catches of many of these taxa were ex-tremely patchy, even in repeated tows. In addition,interannual variations in catches of the dominant speciesof myctophids were evident both off Oregon and Califor-nia and the present study represents the first time suchinterannual trends in micronekton have been documented.

North-south differences are also apparent in the com-position and relative distribution of the dominant nekton.Off the northwest coast, there appears to be change infauna occurring around the Columbia River as catchesoff Washington often differ substantially from those offOregon. A similar zoogeographic boundary probably oc-curs around Cape Blanco in Southern Oregon (Brodeuret al., 2003b) or Cape Mendocino off northern Californiabased on the NMFS trawl results, but the resolution ofour sampling was not fine enough to determine the exactboundary.

9. ConclusionsWe have examined the abundance trends,

zoogeographic and inshore-offshore distribution patternsof the dominant pelagic and mesopelagic nekton speciesin the Transition Zone region of the California Currentbased on several disparate data sources. This system ap-pears to be temporally and spatially dynamic and thenekton respond to the variabili ty. Interannualperturbations such as ENSO and longer-term variationsrelated to PDO have profound effects on distribution andrecruitment of marine species at all levels of the food webfrom primary producers up to the top trophic levels(Brodeur and Pearcy, 1992). In this region, strong El Niñoevents have a predominantly negative effect on the lo-cally adapted nekton, but may benefit southern speciesthat are capable of migrating north into this system. Natu-ral variability will continue to effect this dynamic eco-system as it always has, but future analyses of this vari-ability may be confounded by anthropogenic changes dueto harvesting, pollution, and species enhancement andintroductions. We recommend continuation of samplingand observations in this region as a way of evaluating thehuman effects in the context of climate and ecosystemchange.

AcknowledgementsWe thank the multitudes of scientists and crew for

their assistance in conducting the research operationsunder often adverse weather conditions during the manycruises included in this study. We are grateful to BobEmmett of the NWFSC and Mark Wilkins and Jim Ianelliof the AFSC for providing data used in the analysis. SuzanPool and Greg Krutzikowsky of the Cooperative Insti-tute for Marine Resource Studies greatly assisted with

database management and analysis of data. Earlier ver-sions of the manuscript were improved through the help-ful comments of Skip McKinnell, Salvador Lluch-Cota,and an anonymous reviewer. We acknowledge the fund-ing of numerous agencies that have made a commitmentto long-term sampling in coastal waters. Included amongthese are the U.S. Office of Naval Research, theBonneville Power Administration, and the three WestCoast science centers of the National Marine FisheriesService.

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Abundance and Distribution Patterns of Nekton and Micronekton...

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