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Prey selectivity and diel feeding chronology of juvenilechinook (Oncorhynchus tshawytscha) and coho (O. kisutch)salmon in the Columbia River plume

R. SCHABETSBERGER,1,* C. A. MORGAN,1

R. D. BRODEUR,2 C. L. POTTS,1

W. T. PETERSON2 AND R. L. EMMETT2

1Oregon State University, Cooperative Institute for MarineResources Studies, Hatfield Marine Science Center, Newport,OR 97365, USA2National Marine Fisheries Service, Northwest Fisheries ScienceCenter, Hatfield Marine Science Center, Newport, OR 97365,USA

ABSTRACT

We studied salmon feeding selectivity and diel feedingchronology in the Columbia River plume. Juvenilechinook and coho salmon were caught by trawling at2–3 h intervals throughout a diel period on threeconsecutive days (21–23 June 2000) at stations located14.8 and 37 km offshore from the mouth of theColumbia River. A total of 170 chinook salmon werecaught at the inshore and 79 chinook and 98 cohosalmon were caught at the offshore station. After eachtrawl, potential prey were sampled at different depthswith 2–3 different types of nets (1-m diameter ringnet, bongo net, neuston net). Despite the variability inzooplankton abundance, feeding selectivity was sur-prisingly constant. Both salmon species fed selectivelyon larger and pigmented prey such as hyperiidamphipods, larval and juvenile fish, various crabmegalopae, and euphausiids. Hyperiid amphipods wereabundant in the salmon diets and we hypothesize thataggregations of gelatinous zooplankton may facilitatethe capture of commensal hyperiid amphipods. Smallcopepods and calyptopis and furcilia stages of eup-hausiids dominated the prey field by numbers, but werevirtually absent from salmon diet. Juvenile chinooksalmon, with increasing body size, consumed a larger

proportion of fish. Stomach fullness peaked duringmorning hours and reached a minimum at night,suggesting a predominantly diurnal feeding pattern. Ingeneral, both chinook and coho salmon appear to beselective, diurnal predators, preying mostly on largeand heavily pigmented prey items, in a manner con-sistent with visually oriented, size-selective predation.

Key words: diel feeding, predation, prey availability,salmon, selectivity

INTRODUCTION

Fish seldom ingest prey in proportion to their abun-dance in the environment. A substantial body of lit-erature has accumulated which demonstrates thatmany freshwater and marine fish species selectivelyfeed on relatively large and heavily pigmented prey(Gerking, 1994; Utne-Palm, 1999; Viitasalo et al.,2001). Feeding on larger prey may enhance growthrates and hence decrease mortality (e.g. Keeley andGrant, 2001). As most marine mortality of salmonoccurs shortly after the juveniles enter the ocean(Parker, 1968; Pearcy, 1992), studies on their feedingecology during this phase of early marine life arecrucial. A first step towards a better understanding ofthe factors controlling early marine survival ofjuvenile salmon is to quantify their feeding selectivityand their diel feeding patterns as they adapt to oceanconditions.

The Columbia River is the largest river on the westcoast of North America and has historically harbouredthe largest runs of adult salmon and steelhead (Onc-orhynchus spp.) in the contiguous USA, with estimatesranging from 10 to 16 million fish. During the 1990s,the minimum Columbia River returns averagedapproximately 1 million adults (Oregon Departmentof Fish and Wildlife, 2000). The Columbia River en-ters the ocean with an average volume of approxi-mately 10 000 m3 s)1, forming a shallow (�5–15 m)lens of low-salinity (<31) surface water which his-torically extended up to 400 km from the mouth of theriver during spring freshet flows (Barnes et al., 1972).

*Correspondence. e-mail: [email protected]

Present address: Robert Schabetsberger, Institute of Zoology,

University of Salzburg, Hellbrunnerstrasse 34, A-5020

Salzburg, Austria.

Received 2 January 2002

Revised version accepted 22 July 2002

FISHERIES OCEANOGRAPHY Fish. Oceanogr. 12:6, 523–540, 2003

� 2003 Blackwell Publishing Ltd. 523

However, flow regulation, water withdrawal andchanging precipitation patterns have cut in half thetraditional volumes that now enter the ocean(Simenstad et al., 1990, 1992). Most juvenile chinook(O. tshawytscha) and coho (O. kisutch) salmon enterthe ocean during high flows in spring and early sum-mer following a transition period which ranges fromdays to weeks within the Columbia River estuary andplume, during which time they gradually increase sa-linity preference and tolerance.

How the hydrodynamics of river plumes affect thesurvival of juvenile salmon entering the ocean is underdebate (Beamish et al., 1994; Casillas, 1999).Although the potential benefits of inhabiting theplume have not been quantified for juvenile salmon,the turbid, low-salinity water masses may provideshelter from many marine predators and may createfavourable feeding habitats in areas where plume waterconverges with shelf water to form highly dynamicturbidity fronts. Previous studies have shown thatrelatively high numbers of coho and chinook salmonsmolts are present in the Columbia River plume offOregon and Washington (Pearcy and Fisher, 1990;Fisher and Pearcy, 1995). Studies in other river plumeshave demonstrated that elevated levels of potentialprey items can occur in frontal regions (Grimes andFinucane, 1991; St. John et al., 1992; Fukuwaka andSuzuki, 1998; Grimes, 2001).

Although the food habits and feeding ecology ofjuvenile chinook and coho salmon in their early oceanlife are rather well-studied in this area, previous workhas focused on the diet over a large geographical area(Peterson et al., 1982; Emmett et al., 1986; Brodeur andPearcy, 1990; Brodeur, 1991). Thus, much of thevariability observed in the diets may be attributable togeographical variations in prey availability. To date,the only study to examine diel feeding chronology ofjuvenile coho salmon was based on fish sampled onmultiple days from several different areas (Brodeur andPearcy, 1987). It showed a diurnal feeding pattern withincreased feeding intensity around crepuscular periods.

Little is known about the feeding selectivity ofjuvenile chinook and coho salmon entering the oceanenvironment. Previous studies suggest that they prefercomparatively large and/or highly pigmented prey(Peterson et al., 1982; Brodeur, 1989, 1991) such asamphipods, euphausiids, insects, late developmentalstages of decapods, and fish as opposed to smaller, lesspigmented zooplankton such as copepods or decapodzoeae. Prey abundance changes with currents and tidesand many zooplankton prey undergo diel verticalmigration, thus impacting their relative availability topredators throughout the diel cycle. Adequately des-

cribing the prey field in a highly dynamic environmentsuch as the Columbia River plume therefore requiresintensive sampling throughout the diel period.

In June 2000, a study was conducted inside theColumbia River plume to examine diel catch rates,food habits, and the prey field of juvenile chinook andcoho salmon inshore and offshore of the plume core.In this paper, we examine the diel availability of preyat both locations and the relative feeding selectivity ofsalmon upon these prey resources. We also describethe diel feeding chronology of juvenile coho andchinook salmon along with diel changes in the preyconsumed by both species to better understand if theirearly marine feeding behaviour in the Columbia Riverplume is consistent with the concept of salmon beingdiurnal, size-selective predators.

MATERIALS AND METHODS

Fish sampling

Following an initial trawl survey of an inshore–off-shore transect just south of the mouth of the ColumbiaRiver by the RV W. E. Ricker, we established stationsat two locations which yielded high juvenile salmoncatches (Fig. 1). The first was at a station named CR8,approximately 14.8 km from shore and a water depthof 60 m. At this station, termed the ‘inshore study’,the Ricker was joined by the FV Sea Eagle for a depth-stratified study on 21–22 June 2000.

Both vessels towed similar nets along parallelcourses perpendicular from shore approximately0.7 km apart, with the Sea Eagle towing at the surfaceand the Ricker towing subsurface. Trawling was con-ducted approximately every 2 h (daytime: 04:45–21:45hours). The Ricker used a 264 Nordic rope trawl builtby Nor’Eastern Trawl Systems, Inc. (Bainbridge Island,WA, USA), which has variable mesh sizes (162.6 cmat mouth to 8.9 cm at cod end) and has a fishingmouth opening of approximately 30 m wide · 18 mdeep. This net was quickly lowered to equilibriumdepth, fished for half an hour, and then quickly re-trieved to minimize surface contamination. Themouth opening and depth of the headrope (around18 m) of this trawl was continuously monitored usinga Simrad FS3300 backwards-looking net sounder(Simrad A/S, Horten, Norway). This net fished thelayer between 18 and 36 m depth, although it mostlikely also fished a short period of time in the surfacelayer, especially during retrieval. The Sea Eagle used apelagic rope trawl of similar dimensions and mesh sizeas the Nordic trawl but rigged with floats to fish at thesurface. Trawling continued at regular intervals

524 R. Schabetsberger et al.

� 2003 Blackwell Publishing Ltd, Fish. Oceanogr., 12:6, 523–540.

throughout the diel period. Fish captured in trawlswere identified, counted, measured to the nearest mmfork length (FL), and frozen for later laboratory ana-lyses. Relatively few (<23%) fish were caught in thedeeper trawls, and these were therefore pooled withfish caught at the surface. The depth-specific catchrates will be addressed in detail in a future publication.

Once this sampling was completed, the two vesselsmoved to a deeper station (133 m; CR20) approxi-mately 37 km offshore at the same latitude on 22–23June 2000. At this station, termed the ‘offshore study’,the Sea Eagle trawled at the surface (0–18 m stratum)conducting one tow approximately every 3 h, whilethe Ricker sampled the planktonic prey resources.

Plankton sampling

During the inshore study, the Ricker sampledzooplankton prey between each of the fish trawldeployments, although the constraints of samplingwithin 2 h blocks of time allowed only two differentplankton gear types to be used. Surface prey weresampled with a 1.0 by 0.3-m neuston net with 335-lmmesh, towed for 5 min at a distance of 60 m and out ofthe wake of the vessel, while underway at 3.7 km h)1.Simultaneously, deeper prey were sampled with a 1-mdiameter, 335-lm mesh ring net (termed meter net).The net was fished obliquely by letting out 60 m ofcable and retrieving it immediately at 30 m min)1.Wire angle was maintained so that the net fished to amaximum depth of 20–30 m. A calibrated GeneralOceanics (General Oceanics Inc., Miami, FL, USA)

or TSK (TSK America Inc., North Bend, WA, USA)flowmeter located inside the mouth of each net wasused to estimate the amount of water filtered. At theoffshore station, a 60-cm diameter, 335-lm meshbongo net was used as an additional gear type to des-cribe zooplankton abundance and was fished in thesame way as the meter net. All prey samples werepreserved in 5% buffered formaldehyde. In the labor-atory, the entire sample was rinsed in freshwater andscanned for large, rare organisms using a light tableand a magnifying light. Two to four subsamples of thesmaller, more abundant organisms were counted usinga dissecting microscope. Subsamples were taken usinga Folsom splitter or a 10-ml Hensen stempel pipetteand counted to achieve approximately 30 individualsof each taxon. Densities for each group counted werecalculated as number of individuals per 1000 m3. Totallengths were measured in approximately 30 individualsof each taxon per sample using an ocular micrometer.

Fish size classes

Wild juvenile chinook salmon in the Columbia Riversystem enter the marine environment after spendingvarying amounts of time in freshwater. Offspring fromadults migrating up the river in fall migrate down assubyearlings after several months in freshwater, whereasoffspring from adults migrating up the river in spring orsummer spend more than a year in freshwater beforemigrating to the ocean (for a detailed description seeNicholas and Hankin, 1988). Size thresholds derivedfrom the analysis of length and age data from previous

Figure 1. Map of the study site indicating the starting points of fish trawls during the inshore and offshore study eight and 20nautical miles (14.8 and 37 km, respectively) off the mouth of the Columbia River.


Feeding of juvenile salmon 525

cruises (1998/99) at this time of year were used to dis-criminate between subyearling and yearling chinooksalmon (June: subyearlings £140 mm FL, yearlings>140 and <280 mm FL; J. Fisher, Oregon State Uni-versity, unpublished data). Juvenile coho salmon re-main in freshwater for more than a year prior tomigration to sea. In addition to these naturally spawnedfish, many of the salmon juveniles caught may haveoriginated from hatcheries along the Columbia Riverand its tributaries. These fish are normally released inspring and summer. We were not able to completelydiscriminate between wild and hatchery fish, becausenot all hatchery fish were marked prior to release.

Stomach analysis

Frozen fish were thawed in the laboratory, measuredand the stomachs were dissected. Stomachs were pre-served individually in 10% buffered formaldehyde.After at least 4 weeks in the fixative, the stomachswere rinsed three times and soaked in water for a totalof 24 h and transferred to 70% ethanol. Stomachcontents were removed and examined under a dis-secting microscope. The relative condition of thewhole stomach contents, as well as those of eachindividual prey category were semi-quantitatively ratedon a scale of 0–4 ranging from totally digested to freshprey. Prey items were identified to the lowest possibletaxonomic level. When possible, total length wasmeasured to the nearest 0.1 or 1 mm using an ocularmicrometer or a stage ruler, respectively. Each preytaxon was blotted on absorbent paper to remove excessmoisture and weighed to the nearest milligram.

Data analysis

We calculated the percent numerical composition(%N) and the percent gravimetric composition (%W)for each prey species and life-history stage by station.The species and developmental stages were pooled toderive total numbers for six major prey categories(‘Copepoda’, ‘Amphipoda’, ‘Decapoda’, ‘Euphausia-cea’, ‘Osteichthyes’, and ‘Other’). The category‘Other’ was composed of polychaetes, cirripede cyprislarvae, mysids, isopods and insects. We tested for dif-ferences in diet between subyearling and yearlingchinook salmon from both stations using contingencytables (Crow, 1982; Cortes, 1997). The same test wasused to compare diets of coho salmon and each age-class of chinook salmon.

Feeding selectivity was estimated using the ‘log ofthe odds ratio’ (LOR; Gabriel, 1978), a measure that issymmetrical around 0 and ranges from 0 to +¥ (pos-itive prey selection), and from 0 to )¥ (negative preyselection):

LOR ¼ lndið100 � eiÞeið100 � diÞ

� �;

where di and ei are the numerical percentages of taxoni in the diet and environment, respectively. Todescribe the prey field, we first calculated averagedensities (numbers per 1000 m3) for day and nightfrom each plankton net and then calculated averagedensities for the combined net types (inshore study:meter and neuston nets; offshore study: meter, bongo,and neuston nets). As salmon ingested primarily largezooplankton prey (95% > 2.5 mm), prey selectivitywas computed for prey items >2.5 mm, unless thetaxon in question was strongly pigmented and hadbeen found previously in the diet of juvenile coho andchinook (e.g. pteropods, cirripede cypris larvae, var-ious crab larvae; Brodeur, 1989; Brodeur and Pearcy,1990; R. Schabetsberger, unpublished data).

Average weights for prey items were calculatedfrom relatively undigested specimens (condition >2).Size frequency distributions of prey items were com-pared between the diet and the environment bysummarizing all available length measurements withineach data set. Linear regressions were fitted to therelationships between selectivity indices and the log ofthe average prey weight and between prey fish lengthand predator FL.

Stomach content wet weight was expressed aspercentage wet body weight (minus the stomachcontents weight) to standardize for differences in bodysize (Brodeur and Pearcy, 1987). Fish of different sizeswere pooled by time period after no significant corre-lation between percentage stomach contents and fishweight was detected. Non-parametric Kruskal–Wallistests and Mann–Whitney U-tests (Bonferroni correc-ted) were used to test for differences in stomachfullness.

Differences in the six major prey categoriesbetween chinook salmon caught during day and night(inshore study: six hauls ‘day’, two hauls ‘night’;offshore study: five hauls ‘day’, two hauls ‘night’) weretested using chi-square contingency tables. Too fewcoho were caught during night-time to test for dieldifferences in their feeding. For the inshore study‘Euphausiacea’ was pooled with ‘Other’ to avoid hav-ing prey numbers less than five per cell.

RESULTS

Salmon catches

A total of 170 chinook (109 subyearlings and 61yearlings) salmon were caught during the inshorestudy and 79 chinook (51 subyearlings, 28 yearlings)



and 98 coho salmon were caught during the offshorestudy. No coho salmon were caught at the inshorestation. All salmon specimens caught were examinedin the diet studies. Juvenile chinook salmon exhibiteda bimodal size distribution reflecting the differentgrowth patterns of subyearlings and yearlings. Incontrast, coho were on average larger and occupied anarrower size range (Fig. 2).

Zooplankton abundance

At the inshore station, overall zooplankton densityand the number of species were greater and decapodabundance was greater than at the offshore station(Table 1). Larval stages of the bay ghost shrimp(Neotrypaea californiensis), furcilia and calyptopisstages of euphausiids, Cancer oregonensis/productusmegalopae, and polychaetes dominated the zooplank-ton at the inshore station, whereas euphausiid larvaeand Calanus marshallae (CV and adults) dominatedoffshore. Hyperiid amphipods (Vibilia australis andLycea pulex) and Cancer magister megalopae wereeither not caught or only reached densities of <200individuals per 1000 m3 (Table 1). The abundance ofmost prey species tended to increase during darkness atboth study locations.

Diet composition

Salmon from the inshore station ate comparativelymore decapod larvae and ingested a broader spectrumof prey than did fish from the offshore station(Table 2, Fig. 3). In terms of numbers, decapods,hyperiid amphipods, and prey items in the category‘Other’ dominated the diet at the inshore station. Thenumerical proportion of crab larvae increased withincreasing size of chinook salmon. At the offshorestation, hyperiid amphipods numerically dominatedthe diet of both yearling and subyearling chinook andcoho salmon. One species, V. australis, comprisedmore than 95% of the amphipods consumed. Onejuvenile coho stomach contained 938 individuals ofV. australis.

In terms of weight, fish, decapods, amphipods and‘Other’ were the dominant prey categories during theinshore study, whereas fish, euphausiids and amphi-pods dominated the prey at the offshore station. Ingeneral, the proportion of fish biomass in the dietincreased with increasing size in chinook and cohosalmon from both studies. In the offshore study theproportion of euphausiid biomass decreased withincreasing salmon size (Fig. 3).

The diets of subyearling and yearling chinooksalmon were significantly different in both studies(inshore study: v2

5 ¼ 190:7; P < 0:001; offshore study:v2

5 ¼ 40:5; P < 0:001). Subyearling chinook fromthe inshore station ingested relatively more copepods(5.8% versus 0.1% of total prey by number), amphipods(21.4% versus 14.6%), and ‘Other’ (20.1% versus12.1%) and fewer decapods (49.7% versus 68.3%)than yearling chinook. Subyearling chinook fromthe offshore study ingested a higher numericalproportion of euphausiids than yearling chinook

Figure 2. Size frequency distributions of fork lengths ofjuvenile salmon (5-mm intervals) from the inshore andoffshore studies. The dashed line separates subyearling andyearling chinook salmon. Mean fork length is given in eachpanel.



(4.2% versus 1.5%). The diet of coho from the off-shore study was significantly different from that ofsubyearling (v2

5 ¼ 88:6; P < 0:001) and yearling(v2

5 ¼ 197:5; P < 0:001) chinook salmon. Cohopreyed less on decapods (1.6% versus 5.1%) andcopepods (1.1% versus 2.2%) than subyearling chi-nook. They preyed more on euphausiids (3.3%zversus 1.5%) and less on fish (0.5% versus 1.2%) anddecapods (1.6% versus 6.0%) than yearling chinooksalmon.

Feeding selectivity

Both salmon species positively selected for amphipods(Atylus tridens, V. australis, L. pulex, Hyperoche medus-arum), euphausiids (Thysanoessa spinifera, Euphausiapacifica), various crab megalopae (mostly Cancer mag-ister and C. oregonensis/productus) and large copepods(Euchaeta sp.) (Figs 4 and 5). Cnidarians, pteropods,cirripede cypris larvae, calyptopis and furcilia stages ofeuphausiids, and chaetognaths were under-represented

Table 1. Average densities (individualsper 1000 m3) of the dominantzooplankton categories at both samplinglocations during day and night. Thenumbers of tows with each of the dif-ferent plankton nets is presented inbrackets. See Materials and methodssection for a description of the differentnets and how the average densities werecalculated.

Inshore Offshore

Day (3) Night (1) Day (6) Night (2)

Cnidaria 2736 270 10 0Pteropoda 2669 6940 96 272Polychaeta

Tomopteris sp. 7312 0 1 3Copepoda

Calanus marshallae (CV and adult) 2576 30 692 2194 46 167Euchaeta sp. 0 0 0 0Other Copepoda 509 8077 237 2797

Cirripedia (cypris) 1287 787 16 26Mysidacea

Alienancanthomysis macropsis 18 194 0 0Other Mysidacea 226 456 0 0

Isopoda 6 3 0 0Amphipoda

Atylus tridens 73 1893 1 1Other Gammaridae 6 71 20 9Hyperoche medusarum 54 83 0 1Vibilia australis 6 3 20 90Lycea pulex 0 3 68 3Other Hyperiidae 51 224 5 34Caprellidae 2 0 0 0

EuphausiaceaEuphausia pacifica

(juveniles and adults)30 787 1 307

Thysanoessa spinifera(juveniles and adults)

0 0 1 37

Euphausiacea (furciliae) 9527 9065 1014 136Euphausiacea (calyptopis) 9568 26 332 4318 4107

DecapodaCrangon sp. 2255 2281 31 0Neotrypaea californiensis 8836 41 060 30 5Pachycheles rudis (megalopae) 3 0 0 0Cancer magister (megalopae) 7 30 0 159Cancer oregonensis/productus(megalopae)

6928 15 0 9

Other crab megalopae 477 86 1 3Crab zoeae 237 1958 104 190

Other Crustacea 194 4786 32 10Insecta 20 18 7 1Chaetognatha 2005 3149 39 96Osteichthyes 50 45 42 20



Table 2. Percentage by number and weight of prey categories in the diets of chinook and coho salmon during the inshore andoffshore study.

Inshore study Offshore study

Chinook Chinook Chinook Chinook Cohosubyearling yearling subyearling yearling yearling

Prey %N %W %N %W %N %W %N %W %N %W

PolychaetaTomopteris sp. – – – – – – – – 0.27 0.73

CopepodaCalanus marshallae – – – – 1.04 0.12 0.04 – 0.37 0.04Euchaeta sp. – – – – 0.82 0.71 1.54 0.76 0.57 0.37Other and unidentified copepods (Epilabidocera

longipedata, Euchirella rostrata, Gaussia princeps,Neocalanus cristatus, unid.)

5.83 0.81 0.07 <0.01 0.37 0.07 0.13 0.03 0.29 0.10

Cirripedia (Lepas sp., unid.) 0.44 0.37 – – – – – – 0.12 0.02Amphipoda

GammaridaeAtylus tridens 1.62 1.09 1.43 0.49 1.34 0.71 0.43 0.21 0.37 0.33Other Gammaridae (Cyphocaris challengeri, unid.) 1.65 0.44 0.11 0.03 0.15 0.11 0.09 0.07 0.02 0.03HyperiidaeHyperoche medusarum 3.39 1.31 1.69 0.19 0.67 0.16 0.30 0.06 0.19 0.08Vibilia australis 12.1 2.15 7.49 0.34 83.57 30.27 86.45 14.15 89.15 27.71Lycea pulex 0.74 0.20 1.95 0.19 0.82 0.90 1.71 1.13 2.82 2.60Other hyperiid amphipods (Hyperia medusarum,

Themisto pacifica, Primno sp.,Vibilia sp., unid.)

1.9 0.51 1.72 1.04 0.37 0.15 0.43 1.14 0.21 0.44

Caprellidae (Metacaprella sp., unid.) – – 0.20 0.08 – – <0.01 <0.01 <0.01 <0.01Isopoda (Idotea sp., Synidotea sp., Sphaeromatidae,Jaeropsis sp., unid.)

1.62 0.89 1.24 0.49 – – – – – –

EuphausiaceaEuphausia pacifica (adults) 0.07 0.24 – – 1.53 10.76 0.62 2.47 0.98 7.43Thysanoessa spinifera (adults) 0.74 3.82 0.39 1.09 2.71 24.07 0.91 4.41 2.33 19.43

MysidaceaAlienacanthomysis macropsis 9.52 12.34 0.13 0.03 – – – – 0.01 0.02Other Mysidacea (Neomysis kadiakensis,

Archaeomysis grebnitzkii, unid.)3.32 4.19 0.85 0.27 – – – – – –

DecapodaCrangon sp. 14.54 4.26 0.20 0.02 4.24 3.12 3.67 1.30 0.24 0.16Cancer magister (megalopae) 0.17 0.68 8.95 12.02 0.13 0.05 0.94 2.42 0.44 1.83Cancer oregonensis/productus (megalopae) 10.67 10.84 51.54 16.27 0.39 0.42 0.81 0.37 0.74 0.79Pinnotheridae (megalopae) (Fabia subquadrata,Pinnixa littoralis, Pinnixa sp., unid.)

2.07 0.35 0.46 0.11 0.15 0.05 0.09 0.01 0.04 0.01

Majidae (megalopae) (Oregonia gracilis,Pugettia productus, Pugettia sp., unid.)

0.22 0.06 0.39 0.04 – – 0.04 – 0.04 0.01

Pachycheles rudis 12.92 3.99 2.35 0.20 – – – – – –Other Porcellanidae (megalopae)

(Pachycheles pubescens, unid.)0.74 0.18 0.07 <0.01 0.07 0.11 – – 0.04 0.01

Grapsidae (megalopae) 0.22 0.01 0.07 0.01 – – – – – –Paguridae (megalopae) (Pagurus beringanus,Pagurus granosimanus, unid.)

0.96 0.22 0.13 <0.01 – – – – – –

Neotrypaea californiensis 1.48 0.06 0.39 0.01 – – – – – –Crab zoeae 1.70 0.57 4.10 0.84 0.07 0.01 0.43 0.11 0.09 0.06

Unid. Crustacea 8.71 6.04 9.38 2.51 0.82 2.02 0.13 0.46 0.14 0.31Insecta 0.52 0.24 0.13 0.11 – – – – – –



in the diet relative to their densities in the planktonnets or were completely avoided. Some of the larger preytaxa (e.g. C. magister megalopae and adult euphausiids)were seldom or never caught in our zooplankton tows atthe station farther offshore, resulting in selectivityindices of +¥. As small fish were relatively rare in thediets, and larger fish (>20 mm) were probably notsampled quantitatively with our plankton nets, theselectivity indices for fish must be treated with caution.

Size was not the only basis for prey selection by ju-venile salmon. Although feeding selectivity was posi-tively related to mean weight of the dominantcrustacean prey, the slopes of the linear regressions ofthe LOR against the log of prey weight in the stomachswere not significant (P > 0.1; Fig. 6). In general, pos-itive selection for hyperiid amphipods was greater thanwould be expected from their body size, suggesting thatother factors such as swimming behaviour or pigmen-tation increase their vulnerability to juvenile salmonpredation. In contrast, C. marshallae, N. californiensis,and decapod zoeae were negatively selected comparedwith other prey of similar body size (Fig. 6).

Prey items were between 1 and 68 mm long, with apeak in size frequency of around 5 mm reflecting thehigh numerical proportion of hyperiid amphipods in thediet of juvenile salmon. The size frequency distributionsof V. australis, L. pulex, and H. medusarum in the dietwere similar to those found in the environment (Fig. 7).On the contrary, salmon were size-selective whenfeeding on euphausiids and mysids. Furciliae and ju-venile euphausiids were rare in the diet, but occurred inrelatively high proportions in the plankton (Fig. 7). Bycontrast, V. australis dominated the diet of fish caughtduring the offshore study, yet it was not abundant in the

plankton. However, the size range of euphausiid furcil-iae and juveniles is similar to that of V. australis. Thissuggests that salmon strongly discriminate betweendifferent prey species, not just prey size.

The size distributions of fish ingested by salmondiffered from those captured in the plankton nets forboth studies (Fig. 7). Fish found in stomachs werealmost entirely juveniles >20 mm TL, whereas fishcaught in the various plankton nets were mostly larvae<20 mm TL. Total length of fish prey increased withincreasing fork length of juvenile salmon; however,the relationship was weak because of the approximatemeasurements of digested prey fish (chinook inshorestudy: R2 ¼ 0.15, P ¼ 0.21; chinook offshore study:R2 ¼ 0.28, P ¼ 0.025; coho offshore study:R2 ¼ 0.068, P ¼ 0.157).

Female hyperiids were more abundant in the diet ofjuvenile salmon; however the sex ratios of hyperiids inthe diet were not significantly different from sex ratiosfound in the environment (chi-square test; P > 0.3 forall hyperiid species). Numerically, female V. australis,L. pulex and H. medusarum comprised 100/100, 91/87and 63/73% of all individuals in the diet and theenvironment, respectively.

Diel feeding chronology

Chinook salmon from both studies and coho salmonfrom the offshore study exhibited statistically signifi-cant differences in stomach fullness throughout theday (Kruskal–Wallis test: P < 0.001). Chinook salmoncaught during the inshore study had less food in theirstomachs compared with fish caught farther offshore(Fig. 8). Stomach fullness during the inshore studyincreased during the morning and stayed relatively

Table 2. (Contd.)

Inshore study Offshore study

Chinook Chinook Chinook Chinook Cohosubyearling yearling subyearling yearling yearling

Prey %N %W %N %W %N %W %N %W %N %W

OsteichthyesClupeidae (Clupea pallasi, Sardinops sagax) – – 0.39 2.03 – – – – – –Engraulidae (Engraulis mordax) 0.07 0.68 0.46 4.17 – – – – – –Osmeridae (Allosmerus elongatus, unid.) – – 0.46 24.63 – – – – – –Myctophidae (Stenobrachius leucopsarus, unid.) – – – – – – 0.17 7.95 0.04 5.30Cottidae (Hemilepidotus spinosus),Scorpaenichthys marmoratus

– – – – 0.07 4.43 0.09 3.88 0.04 5.45

Scorpaenidae (Sebastes sp.) 0.07 8.42 – – – – 0.38 33.46 0.09 18.20Bathymasteridae (Ronquilus jordani) – – – – – – – – 0.10 1.52Pleuronectiformes (Psettichthys melanostictus, unid.) 0.15 0.24 0.20 0.51 – – – – – –Unid. fish remains 1.85 34.80 3.06 32.28 0.67 21.76 0.60 25.61 0.26 7.02



constant throughout the rest of the day. The stomachcondition factor reflected the same pattern (Fig. 8).

For chinook salmon from the offshore study, theweight of stomach contents was significantly greater inthe morning hours than in the afternoon and evening

(Fig. 8). Stomach fullness peaked at 11:00 hours, thendecreased from early afternoon to a minimum around02:00 hours. The highest proportion of fresh stomachcontents preceded the peak in stomach fullness forchinook salmon in the offshore study.

Figure 3. Percent by number andweight of the six major prey categories inthe diet of different size-classes of ju-venile chinook and coho salmon. Samplesize for each size-class is given at the topof the columns in the Percent weightpanel.



No coho salmon were caught during the night, butstomach contents had reached a maximum by 08:00hours, indicating a clear peak of feeding in themorning. Stomach contents were lower later in theday and no second feeding peak in the evening wasobserved. The condition factor first increased andthen significantly decreased during the morninghours.

Significant differences in diet of chinook wereobserved between day and night for both studies(inshore study: v2

4 ¼ 36:3; P < 0:001; offshore study,v2

5 ¼ 1094:4; P < 0:001). At the inshore station,chinook ingested more decapods (73.3% versus 63.4%of total prey) and fewer ‘Other’ (8.2% versus 17.2%)during darkness. During the offshore study, decapodshrimp larvae (Crangon sp.) were found almostexclusively in fish caught just before midnight. How-ever, the fish caught <3 h later had few Crangon intheir stomachs (Fig. 9).

DISCUSSION

Juvenile salmon in the Columbia River plumewere feeding selectively on highly pigmented andcomparatively large prey items. Although some of the

hyperiids were in the same size-range as the furcilia orjuvenile stages of euphausiids, the euphausiids werecompletely absent from the diet, whereas hyperiidswere selected over other prey. Conspicuous pigmen-tation was probably a strong factor relating to thepositive selection of different crustaceans. All hyperiidspecies found in the stomachs exhibited some sort ofopaque pigmentation along their body, whereas earlystages of euphausiids are largely transparent. Mosthyperiids, Cancer spp. megalopae and adult euphausi-ids have large, dark compound eyes. Additionally, themore predictable, constant swimming pattern ofhyperiids may have facilitated prey capture (Petersonet al., 1982). Although hyperiids were less dominant atother locations along the Oregon and Washingtoncoast, they are regularly found in the diet of juvenilesalmon (R. Schabetsberger, unpublished data).Hyperiid amphipods were also eaten by Atlantic sal-mon in much higher proportions than were found inthe zooplankton (Jacobsen and Hansen, 2001).

Salmon had a high positive selection for thehyperiid amphipod, V. australis, during the offshorestudy. Densities of V. australis were relatively lowcompared with other prey items in our zooplanktonsamples (Table 1). So far, this species has been

Figure 4. Prey selection (LOR) for the major prey categories found in the diet of subyearling and yearling chinook salmoncaught during the inshore study. Average prey densities from samples taken with two different plankton nets (meter andneuston) were calculated for day- and night-time zooplankton samples. The signs > and < correspond to values of positive andnegative infinity. No point or sign is shown when the item was absent in both zooplankton and stomach samples.



observed infrequently in our surveys off the Oregonand Washington coast. According to Vinogradov et al.(1996), V. australis occurs worldwide in the tropical

zone and its range in the Pacific extends from 40� N tothe southern subtropical convergence. Informationabout its distribution further north is scarce, partly

Figure 5. Prey selection (LOR) for the major prey categories found in the diet of subyearling and yearling chinook and yearlingcoho salmon caught during the offshore study. Average prey densities from samples taken with three different plankton nets(meter, bongo, and neuston) were calculated for day- and night-time zooplankton samples. The signs > and < correspond tovalues of positive and negative infinity. No point or sign is shown when the item was absent in both zooplankton and stomachsamples.



because of taxonomic uncertainties associated withthis species (Behning, 1939; Lorz and Pearcy, 1975;Brusca, 1981).

During the initial survey along the Columbia Rivertransect preceding our two studies, the highest den-sities of salps and V. australis coincided at CR20(C.A. Morgan, unpublished data), suggesting that V.australis may have been associated with the salps.Adult Vibilia have been described as living commen-sally in salps, pyrosomes or siphonophores (Lavanie-gos and Ohman, 1998). Vibilia feed on the food-strandat the opening of the oesophagus of the salps (Madinand Harbison, 1977). As many as 12 adult Vibilia havebeen found in one salp, and they are reported fromaquaria studies to rarely leave the host. In contrast, L.pulex frequently moves from inside the salp to theoutside, among salps in a chain, and also betweenunconnected salps (Madin and Harbison, 1977).Hyperoche medusarum has also been found associatedwith various ctenophores and medusae (Harbisonet al., 1977; Lavaniegos and Ohman, 1998). Madinand Harbison (1977) concluded that most hyperiidamphipods are at some stage of their life associatedwith gelatinous zooplankton.

Juvenile salmon preyed on both sexes of hyperiidsaccording to their availability in the environment;however we do not know if the distribution of gela-tinous zooplankton affects their behaviour whenfeeding on hyperiids. Males of Vibilia and Lycea may belargely free-swimming compared with females that aremore attached to their hosts (Madin and Harbison,1977; Laval, 1980). Juvenile salmon may have evenpicked individual female hyperiids from the salps or atleast fed on female hyperiids aggregated near gelatin-ous zooplankton.

All individuals of V. australis in the salmonstomachs seemed to be females, although sexual dif-ferences are sometimes not clear in this genus

(Brusca, 1981). The skewed sex ratios we observedcould have been the result of a patchy distribution,which is common in hyperiid amphipods (Laval,1980). Alternatively, juvenile salmon may beattracted to aggregations of gelatinous zooplankton.More information about the sex-specific behaviour,population dynamics and small-scale distribution ofhyperiids and their hosts is needed before final con-clusions can be drawn. Given the great importanceof these amphipods in the juvenile salmon diet,future studies should assess the interaction ofhyperiids and gelatinous zooplankton by directobservation in laboratory experiments and in thenatural habitat (e.g. Harbison et al., 1977; Madin andHarbison, 1977).

Small copepods such as Pseudocalanus and Oithona<2.5 mm TL) were 2–3 orders of magnitude moreabundant than C. marshallae at both locations(W. Peterson, unpublished data), yet only the latterwere present (although rarely) in the diet. Juvenilesalmon are likely capable of seeing the small copepods,because visual acuity increases during development(Douglas and Hawryshyn, 1990) and such small itemshave also been found in the diets of other juvenilesalmonids (e.g. Seki and Shimizu, 1998; Auburn andIgnell, 2000). Assuming similar spacing of gill rakers inPacific and Atlantic salmon (Salmo salar), copepoditesof C. marshallae and probably also Pseudocalanus adultscould be retained in the buccal cavity, at least in thesmaller subyearling chinook salmon (Wankowski,1979). Thus, we believe that the absence of smallcopepods in the diet was the result of active avoidancerather than any limitation of prey detection or capturecapabilities. Controlled laboratory experiments shouldbe conducted to determine if juvenile salmon feed onthem when larger and/or more pigmented food itemsare absent.

Several possible sources of error may have affectedour estimates of feeding selectivity. Among these are(i) temporal and spatial variability of zooplankton, (ii)gear selectivity, and (iii) the confounding effect ofpredator size. A majority of the crustacean prey taxaare vertical migrators, and their contribution to thenear surface zooplankton varies throughout the dielcycle. Despite their low abundance in the daytimezooplankton samples, adult euphausiids were fre-quently found in the stomachs and were probablyconsumed at dawn or dusk during their downward orupward migration.

Our calculations of selectivity indices were insen-sitive to time of fish capture (Figs 4 and 5). Given therelatively small sample size of predators and the highvariability in zooplankton abundance, the approach of

Figure 6. Linear regressions of selectivity index (LOR) forcrustacean prey items plotted against their average wetweight in the stomachs of juvenile salmon (log scale). Valuesabove the horizontal dotted line indicate positive selection,those below line negative selection. Linear regressions(continuous lines) and their 95% confidence limits (dashedlines) are given. Cal mar: Calanus marshallae; Euch: Euchaeta

sp.; Ali mac: Alienacanthomysis macropsis; Isop: Isopoda; Atytri: Atylus tridens; Vib aus: Vibilia australis; Hyp med:Hyperoche medusarum; Lyc pul: Lycea pulex; Eup pac: Eup-

hausia pacifica; Thy spi: Thysanoessa spinifera; Cran: Crangonsp.; Neo cal: Neotrypaea californiensis; Pac rud: Pachychelesrudis; Can o/p: Cancer oregonensis/productus; Can mag: Cancer

magister; Zoea: various crab zoeae.

b



Figure 7. Comparison of size frequency distributions of important prey categories in the salmon diet and the environment.Sample sizes are shown in each panel. For some taxa, only a few undigested individuals could be found in the stomachs.



using one and two size-classes for coho and chinooksalmon, respectively, and combining the zooplanktonabundances for day and night samples, seemed to yieldrobust estimates of selectivity.

Currents, tides and river outflow create a constantlychanging prey field in terms of abundance and com-position. Sharp convergent fronts may appear andsubsequently disappear within a tidal cycle at the edgeof the Columbia River plume (Schabetsberger, pers.obs.), forming transient high food-density areas. Forexample, the dominance of Crangon sp. in stomachsfrom one night-time haul suggests that these fish hadfed opportunistically on a swarm of these decapods,possibly aggregated in a frontal region.

Although we used three different plankton netsthroughout a diel cycle during the offshore study,larger prey items were most likely underestimated.Large euphausiids and juvenile fish may have activelyavoided our sampling gear, thereby skewing selectivityestimates upward for these items. However, our datastill show that many small, potential prey items wereabsent or rare in the salmon diet. The magnitude ofsome of the selectivity values for larger prey items may

have been overestimated, but it is unlikely that thesign would change.

Pronounced ontogenetic changes have been des-cribed in the type and size of prey consumed by ju-venile chinook and coho salmon (Peterson et al.,1982; Brodeur, 1991). In particular, the increasingproportion of fish in the diet of juvenile salmonidswith increasing predator size has been found in manystudies (e.g. Keeley and Grant, 2001). Our data areconsistent with a previous finding that the thresholdsize for onset of piscivory of salmonids in the ocean isaround 80 mm FL (Keeley and Grant, 2001). We be-lieve that this strong effect of predator size on preychoice, the spatial and temporal variations in preyavailability, and the differential gastric evacuationtimes for different prey (Bromley, 1994) masked themore subtle temporal changes in diet choice.

A diurnal feeding pattern with peaks around cre-puscular periods has been found previously for juvenilechinook and coho (Brodeur and Pearcy, 1987; Sagarand Glova, 1988), pink (O. gorbuscha) (Godin, 1981)and sockeye (O. nerka) (Doble and Eggers, 1978), andfor brook charr (Salvelinus fontinalis) (Walsh et al.,

Figure 8. Relationship between time ofday and percent wet bodyweight ofstomach contents (left) and digestivecondition factor (right). Data are arith-metic mean ± 95% confidence limits.Values for stomach fullness (percentbody weight) were ln(x + 1) transformedand then back-calculated. Points with anasterisk above them were found to besignificantly different (Mann–WhitneyU-test, P < 0.01) from those imme-diately preceding them. Sample sizes areshown in the left panels. Bars on topindicate times of twilight (grey) anddarkness (black).



1988). In contrast to what Brodeur and Pearcy (1987)found for juvenile coho salmon, we found the mostdistinct increase in feeding intensity to be during themorning hours for both species. During the night,stomach fullness declined to a minimum, but we donot know if juveniles ceased feeding completely.Several studies from freshwater and marine habitatsconfirm that salmonid parr (Amundsen et al., 1999),juveniles (Bradford and Higgins, 2001), and adults(Davis et al., 2000) may be actively feeding duringdarkness. Salmonids seem to be able to feed at lightlevels below 0.01 lux (Godin, 1981; Valdimarsson andMetcalfe, 1999), although the foraging success of At-lantic salmon dropped to 35% of their daytime effi-ciency at conditions of full moon and clear sky (Fraserand Metcalfe, 1997). Salmon are known to mostlyforage at angles above their horizontal plane (Dun-brack and Dill, 1984), which may allow sufficient

contrast during moonlit nights. During our study, themoon was only half-full and scattered clouds may havelimited the ability of juvenile salmon to feed at night.

In conclusion, juvenile salmon in the ColumbiaRiver plume preyed mostly on relatively large and/orpigmented prey consistent with visually oriented, size-selective predation (for a review see Lazzaro, 1987).We found that juvenile salmon were selective, diurnalpredators, and that hyperiid amphipods were a majorcomponent of their diet within the plume. Futurestudies should focus on the distribution and abundanceof hyperiid amphipods, their association with gelatin-ous zooplankton, and the mechanism by which salmonselect them over other available prey. Additionally,daily food consumption and prey choice of salmonshould be compared between the plume and adjacentareas to evaluate the relative profitability of thishabitat.

Figure 9. Percent by number (left) andweight (right) of the major prey categ-ories in the diet of juvenile salmon col-lected throughout both studies. Bars ontop indicate times of twilight (grey) anddarkness (black).



ACKNOWLEDGEMENTS

We thank the captains and crews of the R/V Ricker andF/V Sea Eagle for their excellent cooperation and di-ligence in carrying out the sampling. Brian Beckman,Paul Bentley, Cindy Bucher, Todd Miller and LaurieWeitkamp assisted with the at-sea collection ofjuvenile salmon. Joe Fisher provided unpublishedestimates of growth rates in salmon. We would also liketo thank Susan Hinton for organizing the groundsupport of our operations. Molly Sturdevant, MarcTrudel and Ian Perry provided critical reviews. Earlierversions of the manuscript were improved by com-ments provided by Ed Casillas, David Damkaer, AlexDe Robertis, Cliff Ryer and Jen Zamon. We thankAlex De Robertis for interesting discussions. This studywas funded by the Bonneville Power Administrationthrough a grant to the National Marine FisheriesService and from NMFS to Oregon State University.

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