313

Abstract—We examined the diel vertical distribution, concentration, and

community structure of ichthyoplankton from a single station 69 km off the

central Oregon coast in the northeast

Pacific Ocean. The 74 depth-stratified

samples yielded 1571 fish larvae from

20 taxa, representing 11 families, and

128 fish eggs from 11 taxa within nine

families. Dominant larval taxa were

Sebastes spp. (rockfishes), Stenobrachius leucopsarus (northern lampfish),

Tarletonbeania crenularis (blue lanternfish), and Lyopsetta exilis (slender

sole), and the dominant egg taxa were

Sardinops sagax (Pacific sardine),

Icichthys lockingtoni (medusafish),

and Chauliodus macouni (Pacific

viperfish). Larval concentrations

generally increased from the surface

to 50 m, then decreased with depth.

Larval concentrations were higher at

night than during the day, and there

was evidence of larval diel vertical

migration. Depth stratum was the

most important factor explaining

variability in larval and egg concentrations.

Diel variation in vertical distribution 

of an offshore ichthyoplankton community 

off the Oregon coast

Toby D. Auth (contact author)1

Richard D. Brodeur2

Kathleen M. Fisher3

Email address for T. D. Auth: toby.auth@noaa.gov

1 Cooperative Institute for Marine Resources Studies

Oregon State University, Hatfield Marine Science Center

2030 Marine Science Drive

Newport, Oregon 97365

2 Northwest Fisheries Science Center

NOAA Fisheries, Hatfield Marine Science Center

2030 Marine Science Drive

Newport, Oregon 97365

3 National Ocean Service

NOAA, The Center for Operational Oceanographic Products and Services

1305 East-West Highway

Silver Spring, Maryland 20910

Early life stages of fishes have long

been studied to gain insight into

adult spawning locations and biomass (Hunter et al., 1993), trophic

interactions between fish larvae and

their surrounding zooplankton and

piscivore communities (Hunter and

Kimbrell, 1980), and the relationship

between early life processes and subsequent recruitment success (Bradford, 1992; Houde, 1997). Knowledge

of the diel vertical distribution of fish

eggs and larvae is critical to understanding the structure and ecological

interactions of ichthyoplankton communities, and for the development of

appropriate sampling strategies (Gray,

1998). Furthermore, variation in the

diel vertical distributions of different

larval fish taxa and their associated

predator and prey fields may influence

the occurrence, degree, and timing of

competitive, trophic, and environmental interactions (Neilson and Perry,

1990). In addition, an understanding

of the differences in ichthyoplankton

depth distribution and concentration

is essential for accurate quantitative

estimates of whole water-column abundance when the depth to be sampled

is limited (Ahlstrom, 1959; Comyns

and Lyczkowski-Shultz, 2004). Also,

because in many surveys sampling is

conducted during both night and day,

it is often necessary to separate diel

(e.g., visual net avoidance by larvae

during daylight conditions) and depth

effects when analyzing variable ichthyoplankton distributions (Ahlstrom,

1959; Boehlert et al., 1985).

Larvae of many fish species are

known to perform diel vertical migrations (Ahlstrom, 1959; Neilson

and Perry, 1990). These migrations

can follow several different patterns;

the most common is movement into

the upper water column during night

and into deeper water during the day

(Shoji et al., 1999; Tsukamoto et al.,

2001). However, the reverse pattern

has also been observed (LyczkowskiShultz and Steen, 1991; Brodeur and

Rugen, 1994). In addition, some fish

larvae have been found to disperse

during the night and aggregate during the day (Brewer and Kleppel,

1986; Munk et al., 1989).

In the present study the diel variation in vertical distributions and concentrations of fish eggs and larvae

was examined by repeatedly sampling

fish eggs and larvae from a single offshore station off the central Oregon

coast in the northeast Pacific Ocean.

We used univariate and multivariate

statistical techniques such as ANOManuscript submitted 15 August 2006 

to the Scientific Editor’s Office.

Manuscript approved for publication 

25 October 2006 by the Scientific Editor.

Fish. Bull. 105:313–326 (2007)
314 Fishery Bulletin 105(3)

Table 1

Date, time (Pacific daylight-savings time [PDT]), and number of depth-stratified samples collected during each sampling period. 

All hauls were conducted at the same station (HH-05), located at 44°00ʹN, 125°00ʹW at a station depth of 950 m.

Haul Date Time Day or night Number of depth-stratified samples

HH-05A 3 Aug 2000 1600 Day 8

HH-05B 5 Aug 2000 2356 Night 8

HH-05C 6 Aug 2000 0330 Night 8

HH-05D 6 Aug 2000 0630 Day 8

HH-05E 6 Aug 2000 1623 Day 9

HH-05F 7 Aug 2000 0343 Night 9

HH-05G 7 Aug 2000 0633 Day 7

HH-05B 11 Aug 2002 0350 Night 9

HH-05C 11 Aug 2002 0722 Day 8

VA, diversity and evenness indices, multidimensional

scaling, and cluster analyses to assess the influence of

water depth, diel migration, temperature, and salinity

on the occurrence and concentration of dominant ichthyoplankton taxa and assemblages. This is the first

examination of diel variation in the vertical distributions of ichthyoplankton in the northern California

Current since Boehlert et al. (1985), and the only work

of its kind for an offshore assemblage. In addition, no

similar study has been conducted examining fish egg

diel vertical distributions and assemblages in this region. Results from this study are intended to supplement previous spatial and temporal analyses of Oregon

coast ichthyoplankton (Richardson and Pearcy, 1977;

Auth and Brodeur, 2006)−analyses that were based on

samples collected along the Newport Hydrographic (NH)

line (44°39ʹN) over many years.

Materials and methods

Sampling procedures

Ichthyoplankton samples were collected from a single

station (HH-05) approximately 69 km off of Heceta Head

(44°00ʹN, 125°00ʹW) along the central Oregon coast in

950 m of water. Sampling cruises in August 2000 and

2002 resulted in the collection of 74 depth-stratified

samples from nine diel hauls: five during the day and

four at night (Table 1). Samples collected after the beginning of civil twilight (~0445 Pacific daylight-savings

time [PDT]) were considered day samples, whereas those

collected after the end of civil twilight (~2006 PDT) were

considered night samples. A multiple opening and closing net and environmental sensing system (MOCNESS;

Wiebe et al., 1976) with a 1.2-m2 mouth opening and

333-μm mesh nets was used to collect ichthyoplankton

at 7−9 discrete depths. The MOCNESS was fished as

a continuous oblique tow from a depth of 350 m to the

surface at a retrieval rate of 20−30 m/min and a ship

speed of 1.0−1.5 m/s. Ship and retrieval speeds were

continually adjusted during each tow so as to maintain

the mouth opening at a 45° angle for an effective mouth

opening of 1 m2 at all times. Mean water volume filtered

by each net was 210 m3 (standard error [SE]=18.0), but

was always greater in nets from deeper strata and less

in nets fished in the upper 50 m. Because strata had

unequal depth ranges, towing times were different for

each stratum. Throughout the water column during each

tow, we recorded volume of water filtered, temperature,

salinity, depth of the net, length of wire out, and angle

of the net mouth relative to the geoid.

Ichthyoplankton samples were preserved at sea in

a 10% buffered-formalin seawater solution. Fish eggs

and larvae from each sample were completely sorted,

counted, and identified to the lowest taxonomic level

possible in the laboratory with a dissecting microscope. The lesser of either all larvae or a random

subsample of 50 individuals from each species in each

sample was measured to the nearest 0.1 mm standard

length (SL) (or notochord length for preflexon larvae)

using an ocular micrometer mounted on the sorting

microscope.

It should also be noted that Sebastes spp. were not

identifiable below the generic level based on meristics

and pigmentation patterns (see Richardson and Pearcy

[1977] and Matarese et al. [1989] for a more complete

discussion of this problem); therefore no species-specific

inferences are intended for this taxon in this study. Recent work has allowed the identification of rockfishes to

species level based on mitochondrial markers (Gray et

al., 2006), and this identification should enable future

researchers to discern specific patterns in larval rockfish distribution and abundance.

Data analyses

Fish egg and larval concentrations for each depth-stratified sample (Di

) were expressed as the number of individuals per 1000 m3. Weighted mean water-column

densities for each haul (DHaul

) were calculated according

to the following equation:
Auth et al.: Diel variation in vertical distribution of an offshore ichthyoplankton community 315

DH = D r / r , (1) aul ∑ i i ∑ i

where ri = the depth range (m) of each depth-stratified

sample.

Weighted mean depths (WMDs) of dominant larval taxa

were calculated according to the following equation

(Pearre, 1973):

WMD = ∑n d / n , (2) i i ∑ i

where ni = the number of individuals in each depthstratified sample; and

di = the mean depth (m) of each sample.

To facilitate vertical distribution analyses, the water column was divided into seven depth strata: 0−10,

10−20, 20−50, 50−100, 100−150, 150−200, and 200−350

m. The weighted mean density of eggs and larvae in all

samples collected in each depth stratum per haul were

calculated as the strata densities for each haul. A typeII ANOVA model was used to test the null hypothesis

that egg and larval densities did not differ between day

and night periods or between depth strata, and that

there was no interaction among these factors (Dunn

and Clark, 1974; Lough and Potter, 1993). ANOVA

and a Tukey’s multiple range test were applied to the

loge(n+0.1)-transformed haul and depth-strata densities to test for significant differences between day and

night periods and depth strata. Weighted mean (based

on density) larval lengths of important species were also

calculated for each haul and depth stratum, and were

similarly tested for significant differences between day

and night periods and depth strata.

Taxa diversity and evenness for day and night and

total diversity and evenness for each depth stratum

were analyzed for all identifiable egg (n=11) and larval

(n=20) taxa. The Shannon-Wiener diversity index (Hʹ)

was used to measure egg and larval diversity, where

higher Hʹ values denote greater diversity. Taxa evenness was assessed by using Pielou’s evenness index

(Jʹ), which ranges from zero to one, with the maximum

Jʹ value indicating that all taxa are represented in

the same relative concentrations. Both Hʹ and Jʹ were

calculated according to the formulas of Shannon and

Weaver (1949) and Krebs (1989).

On Field et al.’s (1982) recommendation, we performed

a hierarchical cluster analyses in conjunction with nonmetric multidimensional scaling (MDS) ordinations to

identify assemblages by potential taxa, depth-strata,

and diel egg and larval concentrations. For analyses

of taxa assemblages, only those egg (n=4) and larval

(n=12) taxa found during more than 5% of the sampling events were included, whereas all identifiable

egg (n=10) and larval (n=19) taxa were included in the

other assemblage analyses. Concentrations for each egg

and larval taxon were averaged for each diel tow (eggs:

n=8; larvae: n=9) and each depth stratum from all

tows (eggs: n=22; larvae: n=53), which constituted the

sampling units in the respective multivariate matrices.

Sampling units for which no taxa were found were excluded from the analyses.

Dendrograms of egg and larval taxa, depth strata,

and diel presence were created using hierarchical,

group-averaged clustering from Bray-Curtis ranked

similarities on standardized, fourth root-transformed

egg and larval concentrations (Clarke and Warwick,

2001). Dendrograms were cut to produce ecologically

interpretable clusters when they were apparent. In order to verify our interpretations of the dendrograms, we

performed nonmetric MDS ordinations using similarity

matrices from the cluster analyses, with 20 random

restarts each to minimize stress levels. A two-dimensional ordination approach was adopted because stress

levels were sufficiently low (≤0.08) in all cases and were

not appreciably reduced by the addition of a third dimension, and the results were sufficiently interpretable

ecologically in two-dimensional space (Clarke and Warwick, 2001).

A nonparametric, multivariate procedure (BIO-ENV)

was used to analyze the relationship between select

environmental variables and egg and larval community

structures. The details of the BIO-ENV algorithm and

its suitability for use in analyzing the interactions of

biological and environmental data are described by

Clarke and Gorley (2001) and Clarke and Warwick

(2001). Two separate analyses were performed: one with

a similarity matrix of depth-stratified samples by egg

taxa (24 samples × 9 taxa), and the other with a similarity matrix of depth-stratified samples by larval taxa

(61 samples × 19 taxa). These matrices were analyzed in

association with three environmental variables: mean

depth (m), mean temperature (°C), and mean salinity of

each depth-stratified sample. Both BIO-ENV analyses

were performed by using the Spearman rank correlation

method on the normalized Euclidean distance similarity

matrices of the loge(n+0.1)-transformed, nonstandardized environmental variables by depth-stratified samples (Clarke and Gorley, 2001). All diversity, evenness,

cluster, MDS, and BIO-ENV analyses were performed

by using PRIMER statistical software (PRIMER, vers.

5.2.9, PRIMER-E Ltd, Plymouth, UK).

Pairwise correlation analyses were also conducted to

assess the relationship between concentrations of several prominent egg (Sardinops sagax [Pacific sardine],

Icichthys lockingtoni [medusafish], and Chauliodus macouni [Pacific viperfish]) and larval (Sebastes spp. [rockfishes], Stenobrachius leucopsarus [northern lampfish],

Tarletonbeania crenularis [blue lanternfish], and Lyopsetta exilis [slender sole]) taxa as well as total eggs and

larvae and the environmental variables salinity and

temperature. Mean egg and larval densities, salinity,

and temperature per depth-stratified sample were used

as variable measures. Before inclusion in the analyses,

egg and larval concentrations were loge(n+0.1)-transformed to normalize the data and homogenize residual

variances. Statistical significance was determined at

α = 0.05. All ANOVA and correlation analyses were

performed using the statistical software JMP (JMP,

vers. 5.1., SAS Inst., Inc., Cary, NC).
316 Fishery Bulletin 105(3)

28 29 30 31 32 33 34 35

0

25

50

75

100

125

150

175

200

225

250

275

300

325

350

A B

0 2 4 6 8 10 12 14 16 18 

0 

25 

50 

75 

100 

125 

150 

175 

200 

225 

250 

275 

300 

325 

350 

HH-05A (2000) HH-05B (2000) HH-05C (2000) 

HH-05D (2000) HH-05E (2000) HH-05F (2000) 

HH-05G (2000) HH-05B (2002) HH-05C (2002) 

Haul 

Temperature (°C) Salinity (psu) 

Depth (m) 

Figure 1

Temperature (A) and salinity (B) water-column profiles from a single offshore

station (HH-05 [44°00ʹN, 125°00ʹW]) sampled repeatedly off the central Oregon

coast during August in 2000 and 2002.

Results

Hydrography

The water column at the sampling station was similarly stratified for each of the nine diel hauls, and a

thermocline was centered approximately at 20 m (Fig.

1, A and B). Mean water temperature was 14.7°C (standard deviation [SD]=1.6) at the surface, and decreased

to 6.2°C (SD=0.2) at 350 m. Salinity increased from

a mean of 31.9 psu (SD=0.7) at the surface to 34.0

psu (SD=0.03) at 350 m. The unusually low salinities

recorded in the upper 35 m of the water column during

the last haul in 2000 (HH-05G) may have resulted from

an infusion of low-salinity Columbia River plume water

from the north.

Egg concentrations and distributions

A surprisingly small sum of 128 fish eggs representing

11 taxa from nine families was collected throughout

the study. Three taxa were dominant according to

total mean concentration and frequency of occurrence

from all depth-stratified samples: S. sagax (3.42/1000

m3; 0.07), I. lockingtoni (2.00/1000 m3; 0.14), and C.

macouni (1.07/1000 m3; 0.12). Together these three

taxa accounted for over 71% of the total mean egg

concentration. Trachurus symmetricus (jack mackerel)

eggs were found at a high concentration (241.29/1000

m3) in a single daytime surface (0−10 m) sample in

2000 but were found in only two other samples and

at much lower densities (6.80 and 17.54/1000 m3).

These two samples were collected from two depths
Auth et al.: Diel variation in vertical distribution of an offshore ichthyoplankton community 317

Table 2

Day, night, and total mean densities (number/1000 m3) of fish eggs collected at all different depth strata from a single offshore 

station off the central Oregon coast in 2000 and 2002 (1 standard error in parentheses). For between-stratum comparisons of 

each taxon within each diel category, different superscripts indicate significant differences (ANOVA, P<0.05). 

Depth (m)

100– 150–

Taxa 0–10 10–20 20–50 50–100 150 200 200–350 Total

Day

Total eggs 159.90 (79.0)a 31.01 (14.1)ab 18.60 (4.1)a 0.70 (0.7)b 0 (0)b 0 (0)b 1.30 (1.3)b 13.43 (5.8)

Sardinops sagax 84.11 (78.9) 6.26 (6.3) 1.36 (1.4) 0 (0) 0 (0) 0 (0) 0 (0) 5.99 (5.6)

Lipolagus ochotensis 2.10 (2.1) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.14 (0.1)

Chauliodus macouni 2.14 (2.1)ab 9.39 (6.3)ab 8.70 (2.5)a 0 (0)b 0 (0)b 0 (0)b 0 (0)b 1.74 (0.8)

Merluccius productus 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Trachipterus altivelus 0 (0) 2.09 (2.1) 4.45 (4.4) 0 (0) 0 (0) 0 (0) 0.43 (0.4) 0.75 (0.4)

Trachurus symmetricus 50.36 (47.8) 3.51 (3.5) 1.36 (1.4) 0 (0) 0 (0) 0 (0) 0 (0) 2.10 (1.9)

Icichthys lockingtoni 14.76 (7.9) 6.26 (6.3) 1.37 (1.4) 0.70 (0.7) 0 (0) 0 (0) 0 (0) 1.83 (0.7)

Tetragonurus cuvieri 0 (0) 0 (0) 1.37 (1.4) 0 (0) 0 (0) 0 (0) 0 (0) 0.14 (0.1)

Lyopsetta exilis 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Microstomus pacificus 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.87 (0.9) 0.28 (0.3)

Undetermined spp. 6.42 (6.4) 3.51 (3.5) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.44 (0.3)

Night

Total eggs 42.42 (40.2) 26.66 (11.2) 1.02 (1.0) 0.95 (1.0) 0 (0) 0.67 (0.7) 0.58 (0.6) 3.65 (2.2)

Sardinops sagax 0 (0) 7.28 (7.3) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.21 (0.2)

Lipolagus ochotensis 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Chauliodus macouni 0 (0) 3.64 (3.6) 1.02 (1.0) 0 (0) 0 (0) 0 (0) 0 (0) 0.24 (0.1)

Merluccius productus 0 (0) 3.88 (3.9) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.14 (0.1)

Trachipterus altivelus 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Trachurus symmetricus 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Icichthys lockingtoni 34.28 (32.0) 11.87 (7.5) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 2.21 (1.9)

Tetragonurus cuvieri 2.71 (2.7) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.14 (0.1)

Lyopsetta exilis 0 (0) 0 (0) 0 (0) 0.95 (1.0) 0 (0) 0 (0) 0 (0) 0.13 (0.1)

Microstomus pacificus 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.67 (0.7) 0.58 (0.6) 0.31 (0.3)

Undetermined spp. 5.43 (5.4) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.28 (0.3)

Total

Total eggs 107.69 (49.3)a 29.08 (8.7)ab 10.78 (3.8)abc 0.81 (0.5)bc 0 (0)c0.33 (0.3)c 0.98 (0.7)bc 9.08 (3.6)

Sardinops sagax 46.73 (44.1) 6.71 (4.4) 0.76 (0.8) 0 (0) 0 (0) 0 (0) 0 (0) 3.42 (3.1)

Lipolagus ochotensis 1.17 (1.2) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.08 (0.08)

Chauliodus macouni 1.19 (1.2)ab 6.83 (3.8)ab 5.28 (1.9)a 0 (0)b 0 (0)b 0 (0)b 0 (0)b 1.07 (0.5)

Merluccius productus 0 (0) 1.72 (1.7) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.06 (0.06)

Trachipterus altivelus 0 (0) 1.16 (1.2) 2.47 (2.5) 0 (0) 0 (0) 0 (0) 0.24 (0.2) 0.42 (0.2)

Trachurus symmetricus 27.98 (26.7) 1.95 (1.9) 0.76 (0.8) 0 (0) 0 (0) 0 (0) 0 (0) 1.17 (1.1)

Icichthys lockingtoni 23.43 (14.1)a 8.75 (4.6)ab 0.76 (0.8)b 0.39 (0.4)b 0 (0)b 0 (0)b 0 (0)b 2.00 (0.8)

Tetragonurus cuvieri 1.21 (1.2) 0 (0) 0.76 (0.8) 0 (0) 0 (0) 0 (0) 0 (0) 0.14 (0.1)

Lyopsetta exilis 0 (0) 0 (0) 0 (0) 0.42 (0.4) 0 (0) 0 (0) 0 (0) 0.06 (0.06)

Microstomus pacificus 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.33 (0.3) 0.74 (0.5) 0.30 (0.2)

Undetermined spp. 5.98 (4.1) 1.95 (1.9) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.37 (0.2)

(10−20 and 20−50 m, respectively) in the same haul

(HH-05A).

Fish egg concentrations varied across depth-stratified and diel scales (Table 2). However, depth stratum

was the only significant factor explaining variation in

C. macouni, I. lockingtoni, and total egg densities in

the type-II ANOVA model (P<0.05). Almost 91% of the

total egg abundance was found in the upper 100 m of

the water column. Egg densities were generally highest

near the surface and decreased with depth. There were

no S. sagax, C. macouni, or I. lockingtoni eggs found at

depths >100 m.

Mean total water-column concentrations were greater

during the day than at night for S. sagax (5.99/1000 m3
318 Fishery Bulletin 105(3)

[day]; 0.21/1000 m3 [night]), C. macouni (1.74/1000 m3

[day]; 1.07/1000 m3 [night]), and total eggs (13.43/1000

m3 [day]; 3.65/1000 m3 [night]), although differences

were not significant (ANOVA, P>0.05). However, I. lockingtoni eggs occurred in slightly higher concentrations

in the water column at night (2.21/1000 m3) than during the day (1.83/1000 m3). Egg concentrations were

generally higher during the day than at night at each

depth stratum for most taxa (Table 2).

Larval concentrations and distributions

A total of 1571 fish larvae representing 20 taxa from

11 families were collected throughout the study. Three

families accounted for 97.4% of the total standardized larval concentration: Scorpaenidae (55.6%), Myctophidae (35.6%), and Pleuronectidae (6.2%). Within

these families, four taxa were dominant based on total

mean concentration and frequency of occurrence from

all depth-stratified samples: Sebastes spp. (57.75/1000

m3; 0.42), S. leucopsarus (25.23/1000 m3; 0.45), T. crenularis (8.81/1000 m3; 0.35), and L. exilis (3.78/1000 m3;

0.15). Several other taxa were documented at relatively

high frequencies but at lower mean concentrations:

Glyptocephalus zachirus (rex sole) (2.10/1000 m3; 0.18),

Protomyctophum thompsoni (northern flashlightfish)

(1.17/1000 m3; 0.16), I. lockingtoni (1.01/1000 m3; 0.11),

Diaphus theta (California headlightfish) (0.91/1000 m3;

0.11), and Liparis fucensis (slipskin snailfish) (0.66/

1000 m3; 0.11).

Larval concentrations varied across depth-stratified

and diel scales (Table 3). For S. leucopsarus larvae, both

factors and their interaction term (depth, diel presence,

and depth×diel presence) in the type-II ANOVA model

were significant (P<0.05). However, depth stratum was

the only significant factor explaining variation in L.

exilis, Sebastes spp., T. crenularis, and total larval densities in the model. More than 96% of the total larval

abundance was distributed in the upper 100 m of the

water column. Mean concentrations of Sebastes spp., S.

leucopsarus, T. crenularis, and total larvae generally

increased from the surface to the 20−50 m depth, then

declined steadily as depth increased. No Sebastes spp.

larvae were collected below 100 m. In addition, L. exilis

larvae were found only in the 20−100 m depth range,

where concentrations were greater than 3× higher in

the 20−50 than in the 50−100 m depth stratum.

Mean larval densities were generally greater at night

than during the day at all but one depth stratum for

Sebastes spp., S. leucopsarus, T. crenularis, and total

larvae (Table 3). These differences were significant for

S. leucopsarus and total larvae at the 0−10 and 10−20

m depth strata (ANOVA, P<0.05). However, higher larval densities were found during the day than at night

for Sebastes spp., S. leucopsarus, and total larvae at the

50−100 m depth stratum. In addition, L. exilis larvae

were found at higher densities during the day than at

night at all depths at which they were collected.

Analysis of the dominant larval taxa depth distributions over a 16-h period revealed evidence of diel vertical migration (Fig. 2). Weighted mean depth (WMD)

of L. exilis and Sebastes spp. larvae increased from

2324 to 1550 PDT, whereas larval S. leucopsarus WMD

decreased slightly between 2324 and 0303 PDT before increasing as the day progressed. In contrast, T.

crenularis larvae appeared to move up in the water

column from 2324 to 0604 PDT but were not found in

any sample from the haul conducted at 1550 PDT.

Weighted mean standard lengths (SL) of L. exilis,

Sebastes spp., S. leucopsarus, and T. crenularis larvae

generally increased with depth (Table 4). Mean SL for

S. leucopsarus larvae collected during night and during

both day and night combined was significantly greater

at the 20−100 m than at the 10−20 m depth stratum

(ANOVA, P<0.05). Lyopsetta exilis, Sebastes spp., and T.

crenularis larvae collected at night were generally the

same size or larger than those collected during the day

at each depth stratum, although these differences were

not significant (ANOVA, P>0.05).

Diversity and evenness

Larval and egg diversity and evenness varied across

depth strata. Larval diversity generally increased with

depth. Egg diversity increased from the surface to 20

m then declined with increasing depth. Egg and larval

evenness generally increased with depth before declining

slightly at the 200−350 m depth stratum. Egg diversity

and evenness could not be assessed at the 100−150 and

150−200 m depth strata because no eggs were found

between 100 and 150 m, and only one egg (Microstomus

pacificus [Dover sole]) was found at the 150−200 m

depth stratum. There was no appreciable difference in

egg and larval diversity and evenness between day and

night samples.

Assemblages

Several taxa and depth assemblages were identified on

the basis of cluster analyses and MDS, although no diel

assemblages were apparent (Fig. 3, A−C). Larval taxa

separated out into four assemblages (Fig. 3A). In more

than 5% of the samples too few egg taxa (n=4) were present to permit identification of any assemblages. Cluster

analyses and MDS also indicated the presence of two

egg and larval depth assemblages: <100 m and >100 m

(Fig. 3, B and C).

Environmental relationships

BIO-ENV and correlation analyses revealed significant

relationships between several environmental factors and

egg and larval mean concentrations. A depth-stratified

BIO-ENV analysis, which included mean depth (m), mean

temperature (°C), and mean salinity of each depth-stratified sample, showed that depth alone explained 46% of

the variability in mean egg concentrations and 44% in

larval concentrations. No multiple-factor combination

explained more variability in egg or larval concentration

data. Pairwise correlation analyses revealed significant
319

Table 3

Day, night, and total mean densities (n

u

mber/1000 m3) of fish larvae collected at different depth strata from a single offshore station off the central Oregon coast in 

2000 and 2002 (1 SE in parentheses). For between stratum comparisons of each taxon 

within each diel category, different alphabetic supercripts indicate significant 

differences (A

N

O

VA, P<0.05). For diel comparisions of each taxon 

within each depth stratu

m, different n

u

meric superscripts indicate significant differences (A

N

O

VA, 

P<0.05).

Depth (

m)

Taxa 0–10 10–20 20–50 50–100 100–150 150–200 200–350 Total

Day

Total larvae 247.47 (22.6)abc 2130.11 (71.3)ab 324.49 (104.4)a 150.55 (59.3)a 6.27 (2.6)bcd 1.93 (1.3)d 2.97 (0.8)cd 75.05 (20.1)Sardinops sagax 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)Pseudobathylagus milleri 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.43 (0.4) 0.19 (0.2)Chauliodus macouni 0 (0) 0 (0) 0 (0) 0 (0) 1.59 (1.6) 0 (0) 0 (0) 0.11 (0.1)Tactostoma macropus 2.09 (2.1) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.14 (0.1)Lestidiops ringens 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)Myctophidae spp. 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)Protomyctophum thompsoni 0 (0) 0 (0) 0 (0) 1.70 (1.1) 3.92 (1.5) 0 (0) 0.62 (0.4) 0.88 (0.3)Tarletonbeania crenularis 0 (0)c 12.98 (9.6)abc 26.69 (13.8)a 18.63 (13.9)ab 0.76 (0.8)abc 0 (0)bc 0.26 (0.3)bc 8.40 (5.1)Nannobrachium regale 0 (0) 2.46 (2.5) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.14 (0.1)Stenobrachius leucopsarus 20 (0)b 23.13 (3.1)b 55.79 (34.4)a 46.50 (16.4)a 0 (0)b 0 (0)b 0.79 (0.5)b 15.42 (5.3)Diaphus theta 0 (0) 2.46 (2.5) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.14 (0.1)Brosmophycis marginata 0 (0) 3.13 (3.1) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.14 (0.1)Sebastes spp. 33.45 (26.0)ab 85.61 (51.0)ab 190.12 (54.4)a 64.04 (38.6)a 0 (0)b 0 (0)b 0 (0)b 38.73 (8.9)Liparis fucensis 0 (0) 0 (0) 2.69 (1.6) 2.60 (1.7) 0 (0) 0.53 (0.5) 0 (0) 0.81 (0.4)Icichthys lockingtoni 8.47 (5.2) 10.94 (5.1) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 1.28 (0.6)Citharichthys sordi

dus 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)

Glyptocephalus zachirus 0 (0) 9.39 (9.4) 8.43 (5.2) 4.65 (4.7) 0 (0) 0 (0) 0 (0) 2.21 (0.7)Lyopsetta exilis 0 (0)b 0 (0)b 40.77 (25.7)a 11.72 (5.1)a 0 (0)b 0 (0)b 0 (0)b 5.60 (2.7)Microstomus pacificus 0 (0) 0 (0) 0 (0) 0.70 (0.7) 0 (0) 1.40 (1.4) 0.87 (0.5) 0.66 (0.3)U

ndetermined spp. 3.45 (3.5) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.19 (0.2)

Night

Total larvae 1293.20 (131.3)a 1667.70 (153.9)a 630.54 (281.2)a 84.33 (41.4)ab 13.21 (7.8)b 3.66 (1.2)b 2.68 (1.0)b 139.73 (42.4)

Sardinops sagax 4.42 (2.7) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.27 (0.2)Pseudobathylagus milleri 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)Chauliodus macouni 0 (0) 0 (0) 0 (0) 3.26 (1.1) 0.94 (0.9) 0 (0) 0 (0) 0.58 (0.3)Tactostoma macropus 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)Lestidiops ringens 0 (0) 0 (0) 0 (0) 1.45 (1.4) 0 (0) 0 (0) 0 (0) 0.14 (0.1)Myctophidae spp. 0 (0) 0 (0) 3.05 (3.1) 1.90 (1.9) 0 (0) 0 (0) 0 (0) 0.68 (0.4)Protomyctophum thompsoni 0 (0) 0 (0) 0 (0) 3.61 (2.5) 8.30 (4.8) 1.29 (1.3) 0 (0) 1.52 (0.7)Tarletonbeania crenularis 1.34 (1.3)ab 27.65 (19.4)ab 38.98 (27.9)a 19.26 (10.7)a 3.02 (3.0)ab 0.67 (0.7)ab 0 (0)b 9.34 (5.5)Nannobrachium regale 5.43 (5.4) 12.36 (12.4) 2.03 (1.2) 0 (0) 0 (0) 0 (0) 0 (0) 0.94 (0.4)Stenobrachius leucopsarus 15.09 (2.2)abc 1248.31 (170.3)a 175.64 (61.7)a 34.70 (19.4)ab 0 (0)c 1.71 (1.0)bc 1.59 (0.6)bc 37.50 (13.3)

continued

Auth et al.: Diel variation in vertical distribution of an offshore ichthyoplankton community
320

Table 3 (continued)

Depth (m)

Taxa 0–10 10–20 20–50 50–100 100–150 150–200 200–350 TotalNight (continued)

Diaphus theta 3.05 (1.8) 11.52 (3.9) 9.20 (6.7) 0 (0) 0 (0) 0 (0) 0 (0) 1.88 (1.0)Brosmophycis marginata 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0)Sebastes spp. 263.65 (124.1)a 356.70 (176.9)a 379.92 (209.6)a 14.96 (12.0)ab 0 (0)b 0 (0)b 0 (0)b 81.52 (25.7)Liparis fucensis 0 (0) 0 (0) 0 (0) 3.23 (1.9) 0 (0) 0 (0) 0 (0) 0.47 (0.3)Icichthys lockingtoni 6.45 (4.8) 0 (0) 1.02 (1.0) 0 (0) 0 (0) 0 (0) 0 (0) 0.67 (0.5)Citharichthys sordidus 1.70 (1.7) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.13 (0.1)Glyptocephalus zachirus 2.07 (2.1) 11.17 (6.9) 7.15 (4.5) 0 (0) 0.94 (0.9) 0 (0) 0.81 (0.8) 1.96 (1.0)Lyopsetta exilis 0 (0)b 0 (0)b 9.23 (3.5)a 1.95 (2.0)ab 0 (0)b 0 (0)b 0 (0)b 1.50 (0.5)Microstomus pacificus 0 (0) 0 (0) 2.28 (1.3) 0 (0) 0 (0) 0 (0) 0.29 (0.3) 0.34 (0.2)Undetermined spp. 0 (0) 0 (0) 2.04 (2.0) 0 (0) 0 (0) 0 (0) 0 (0) 0.28 (0.3)

Total mean

Total larvae 156.68 (69.8)a 369.04 (119.5)a 460.51 (138.2)a 121.11 (37.4)a 9.74 (4.0)b 2.80 (0.9)b 2.84 (0.6)b 103.80 (23.3)Sardinops sagax 1.96 (1.3) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.12 (0.1)Pseudobathylagus milleri 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.24 (0.2) 0.11 (0.1)Chauliodus macouni 0 (0) 0 (0) 0 (0) 1.45 (0.7) 1.26 (0.9) 0 (0) 0 (0) 0.32 (0.1)Tactostoma macropus 1.16 (1.2) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.08 (0.08)Lestidiops ringens 0 (0) 0 (0) 0 (0) 0.64 (0.6) 0 (0) 0 (0) 0 (0) 0.06 (0.06)Myctophidae spp. 0 (0) 0 (0) 1.36 (1.4) 0.85 (0.8) 0 (0) 0 (0) 0 (0) 0.30 (0.2)Protomyctophum thompsoni 0 (0) 0 (0) 0 (0) 2.55 (1.2) 6.11 (2.5) 0.64 (0.6) 0.34 (0.2) 1.17 (0.3)Tarletonbeania crenularis 0.60 (0.6)c 19.50 (9.7)abc 32.15 (13.7)a 18.91 (8.6)ab 1.89 (1.5)bc 0.33 (0.3)c 0.15 (0.1)c 8.81 (3.5)Nannobrachium regale 2.41 (2.4) 6.86 (5.5) 0.90 (0.6) 0 (0) 0 (0) 0 (0) 0 (0) 0.50 (0.2)Stenobrachius leucopsarus 2.26 (1.3)bc 112.1 (81.8)ab 109.06 (37.5)a 41.25 (11.9)a 0 (0)c 0.85 (0.6)bc 1.14 (0.4)bc 25.23 (7.2)Diaphus theta 1.35 (0.9) 6.49 (2.6) 4.09 (3.2) 0 (0) 0 (0) 0 (0) 0 (0) 0.91 (0.5)Brosmophycis marginata 0 (0) 1.74 (1.7) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.08 (0.08)Sebastes spp. 135.76 (66.3)ab 206.09 (90.6)ab 274.47 (96.2)a 42.22 (22.6)b 0 (0)c 0 (0)c 0 (0)c 57.75 (13.7)Liparis fucensis 0 (0) 0 (0) 1.50 (1.0) 2.88 (1.2) 0 (0) 0.27 (0.3) 0 (0) 0.66 (0.2)Icichthys lockingtoni 7.57 (3.4) 6.08 (3.3) 0.45 (0.5) 0 (0) 0 (0) 0 (0) 0 (0) 1.01 (0.4)Citharichthys sordidus 0.76 (0.8) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0.06 (0.06)Glyptocephalus zachirus 0.92 (0.9) 10.18 (5.7) 7.86 (3.3) 2.58 (2.6) 0.47 (0.5) 0 (0) 0.36 (0.4) 2.10 (0.5)Lyopsetta exilis 0 (0)b 0 (0)b 26.75 (14.7)a 7.38 (3.3)a 0 (0)b 0 (0)b 0 (0)b 3.78 (1.6)Microstomus pacificus 0 (0) 0 (0) 1.02 (0.7) 0.39 (0.4) 0 (0) 0.70 (0.7) 0.61 (0.3) 0.52 (0.2)Undetermined spp. 1.92 (1.9) 0 (0) 0.91 (0.9) 0 (0) 0 (0) 0 (0) 0 (0) 0.23 (0.2)

FisheryBulletin105(3)
Auth et al.: Diel variation in vertical distribution of an offshore ichthyoplankton community 321

Figure 2

Diel differences in the proportion of total abundances for (A) Lyopsetta exilis,

(B) Sebastes spp., (C) Stenobrachius leucopsarus, and (D) Tarletonbeania

crenularis larvae collected at different depth strata during a 16-h period at

a single offshore station off the central Oregon coast on 5–6 August 2000.

No T. crenularis larvae were collected in the 1623 PDT haul. The times at

which the first MOCNESS net of each haul was opened are given above

the panels. Numbers in the upper right corner of each panel represent

the weighted mean depth (m) of larvae collected in each haul. During the

sampling period, sunset occurred at 2033 and sunrise at 0610 PDT.

0 0.2 0.4 0.6 0.8 1

0-10

10-20

20-50

50-100

100-150

150-200

200-350

0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

0 0.2 0.4 0.6 0.8 1

0-10

10-20

20-50

50-100

100-150

150-200

200-350

0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

0 0.2 0.4 0.6 0.8 1

0-10

10-20

20-50

50-100

100-150

150-200

200-350

0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

0 0.2 0.4 0.6 0.8 1

0-10

10-20

20-50

50-100

100-150

150-200

200-350

0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1

35 35 43 74

11 24 30 69

45 30 46 74

80 35 34

A

B

C

D

Depth (m) 

Proportion of total abundance 

2356 0330 0630 1623

correlations between temperature and mean concentrations of Sebastes spp. and total larvae, C. macouni, I.

lockingtoni, and total eggs. Significant negative correlations were seen between these same taxa and salinity,

and temperature and salinity were negatively correlated

with each other (P<0.05) (Table 5). Mean concentrations

of L. exilis, S. leucopsarus, and T. crenularis larvae, and

S. sagax eggs were also positively correlated with temperature and negatively correlated with salinity, although

the correlations were not significant (P>0.05).

Discussion

The species composition, assemblages, and dominant

taxa identified in this study were similar to those found
322 Fishery Bulletin 105(3)

in other studies conducted during the summer in offshore waters off the central Oregon coast (Richardson,

1973; Richardson and Pearcy, 1977; Brodeur et al., 1985;

Auth and Brodeur, 2006). This similarity may indicate

that the composition of dominant larval fishes in this

area is resilient to dramatic environmental fluctuations

such as those observed during the last 35 years in the

Figure 3

Dendrograms of (A) larval taxa, (B) larval depth, and (C) egg depth. Cluster and multidimensional

scaling analyses were performed on fish larvae and eggs collected from a single offshore station off

the central Oregon coast in 2000 and 2002. Taxon (A) and depth (B and C) cluster identifications

are indicated on each figure. Numbers in figure 3A refer to the four identifiable larval taxa

assemblages. Depth strata 1 = 0−10 m, 2 = 10−20 m, 3 = 20−50 m, 4 = 50−100 m, 5 = 100−150 m,

6 = 150−200 m, and 7 = 200−350 m.

Liparis fucensis

Chauliodus macouni

Protomyctophum thompsoni

Microstomus pacificus

Glyptocephalus zachirus

Lyopsetta exilis

Sebastes spp.

Tarletonbeania crenularis

Stenobrachius leucopsarus

Icichthys lockingtoni

Nannobrachium regale

� iaphus theta

50 40 30 20 10 0 

Ranked similarities

Taxa A

1 

2 

3 

4 

1500 1000 500 0 

Ranked similarities

> 100 m

< 100 m

B

Depth

northern California Current (Schwing and Moore, 2000;

Grantham et al., 2004). It also provides evidence in

support of the hypothesis of Auth and Brodeur (2006)

that past sampling of ichthyoplankton along the NH

line during the summer is indeed representative of

summer ichthyoplankton assemblages elsewhere along

the Oregon coast.
Auth et al.: Diel variation in vertical distribution of an offshore ichthyoplankton community 323

Figure 3 (continued)

1 

2 

2 

1 

1 

1 

2 

4 

1 

2 

2 

1 

3 

2 

3 

3 

3 

3 

3 

7 

6 

7 

200 150 100 50 0 

Ranked similarities

Depth strata

< 100 m

> 100 m

C

Table 4

Day, night, and total mean standard lengths (mm) of fish larvae collected at different depth strata from a single offshore station 

off the central Oregon coast in 2000 and 2002 (1 SE in parentheses). For between stratum comparisons of each taxon within each 

diel category, different superscripts indicate significant differences (ANOVA, P<0.05).

Depth (m)

Taxa 0–10 10–20 20–50 50–100 100–150 150–200 200–350 Total

Day

Lyopsetta exilis — — 8.7 (0.6) 10.3 (0.5) — — — 9.1 (0.4)

Sebastes spp. 4.1 (1.4) 5.1 (0.2) 6.6 (0.7) 7.4 (1.4) — — — 6.2 (0.5)

Stenobrachius leucopsarus — 13.6 9.3 (0.9) 9.5 (0.6) — — 14.7 (0.3) 9.5 (0.5)

Tarletonbeania crenularis — 9.2 (2.2) 8.9 (1.1) 10.7 (1.4) 14.3 — 16.3 9.6 (0.7)

Night

Lyopsetta exilis — — 9.6 (0.4) 8.6 — — — 9.4 (0.4)

Sebastes spp. 6.4 (0.2) 6.2 (0.5) 7.9 (0.5) 10.0 (2.3) — — — 6.9 (0.3)

Stenobrachius leucopsarus 5.6 (0.7)ab 7.6 (0.2)b 9.7 (0.8)a 11.4 (0.3)a — 14.8 (0.0)ab 15.4 (0.9)ab 8.8 (0.4)

Tarletonbeania crenularis 8.7 7.7 (0.3) 9.4 (0.5) 12.1 (0.9) 13.4 16.0 — 9.6 (0.6)

Total mean

Lyopsetta exilis — — 8.8 (0.4) 10.1 (0.5) — — — 9.1 (0.3)

Sebastes spp. 6.1 (0.5) 6.0 (0.4) 7.4 (0.4) 7.5 (1.1) — — — 6.7 (0.3)

Stenobrachius leucopsarus 5.6 (0.7)ab 7.7 (0.4)b 9.6 (0.5)a 10.2 (0.5)a — 14.8 (0.0)ab 15.1 (0.6)ab 8.9 (0.3)

Tarletonbeania crenularis 8.7 8.3 (0.9) 9.2 (0.6) 11.3 (0.8) 13.5 (0.4) 16.0 16.3 9.6 (0.4)

However, in our study we collected no Engraulis warmer, less saline, offshore surface waters from the

mordax (northern anchovy) eggs or larvae, which usu- Columbia River (Richardson, 1973; Auth and Brodeur,

ally comprise a significant portion of the offshore ich- 2006). This warmer surface water did not appear to have

thyoplankton in August (Richardson and Pearcy, 1977). occurred as far south as our sampling station, except perEngraulis mordax larvae have been associated with haps during the last haul in 2000 (HH-05G) (Fig. 2).
324 Fishery Bulletin 105(3)

Table 5

Correlation coefficients for the depth-stratified sample 

means (n=74) of eleven variables sampled during the 

day and night from a single station (HH-05) off the 

Oregon coast in August 2000 and 2002: water temperature (°C), salinity (kg/m3), and log -transformed densi- e

ties (number/1000 m3) of Lyopsetta exilis, Sebastes spp., 

Stenobrachius leucpsarus, Tarletonbeania crenularis, and 

total fish larvae, and Chauliodus macouni, Icichthys lockingtoni, Sardinops sagax, and total fish eggs. *=P<0.05, 

*=P<0.001, ***=P<0.0001.

Temperature Salinity

Salinity –0.82***

Lyopsetta exilis 0.03 –0.09

Sebastes spp. 0.48*** –0.51***

Stenobrachius leucopsarus 0.16 –0.18

Tarletonbeania crenularis 0.15 –0.14

Total larvae 0.57*** –0.53***

Chauliodus macouni 0.24* –0.40**

Icichthys lockingtoni 0.63*** –0.47***

Sardinops sagax 0.20 –0.23

Total eggs 0.55*** –0.51***

In addition, we found evidence that S. sagax spawn in

this region, as documented since the 1990s (Emmett et

al., 2005; Auth and Brodeur, 2006). Similarly, we collected T. symmetricus eggs, which are normally found

off southern California but which have been reported off

southern Oregon by Kendall and Clark1, off Washington

by Ahlstrom (1956), and most recently in the Columbia

River plume by Parnel et al.2

Our finding that the vast majority (96%) of fish larvae

were present in the upper 100 m of the water column

has been supported by several other studies (Brodeur

and Rugen, 1994; Suntsov, 2002; Sabatés, 2004; Auth

and Brodeur, 2006). This finding has significant implications for strategies to sample ichthyoplankton off

Oregon and potentially throughout the California Current. Sebastes spp. primarily occurred in the 10–50 m

depth range, as previously reported (Ahlstrom, 1961;

Boehlert et al., 1985; Sakuma et al., 1999; Auth and

Brodeur, 2006), whereas L. exilis larvae were found

between 20 and 100 m as observed by Auth and Brodeur

(2006) and during daytime collections made by Boehlert

et al. (1985). Also, myctophid larvae within the subfamily Myctophinae (T. crenularis) were found at greater

1 Kendall, A. W., Jr., and J. Clark. 1982. Ichthyoplankton

off Washington, Oregon, and northern California April−May

1980. AFSC Proc. Rep. 82-11, 44 p. National Oceanic

and Atmospheric Administration, Alaska Fisheries Science

Center, 7600 Sand Point Way NE, Seattle, WA 98115.

2 Parnel, M. M., R. L. Emmett, and R. D. Brodeur. 2006. Interannual and seasonal variation in ichthyoplankton collected

off the Columbia River. Unpubl. manuscr., 20 p. Oregon

State Univ., Hatfield Marine Science Center, 2030 SE Marine

Sci. Dr., Newport, OR 97365.

depths than those within the subfamily Lampanyctinae

(S. leucopsarus) as reported by Sassa et al. (2002), although the vast majority was still found above 100 m.

One implication from these findings is that sampling

in the upper 100 m of the water column should be sufficient to characterize pelagic summer ichthyoplankton

abundances and distributions of the majority of fish

taxa along the northeastern Pacific coast (Brodeur and

Rugen, 1994; Auth and Brodeur, 2006). A further implication is that sampling strategies without discrete

vertical strata, which expend additional effort collecting ichthyoplankton below 100 m, may underestimate

larval fish abundances by using samples from largely

uninhabited portions of the water column.

Differences in the abundance and size of fish larvae

collected in diel samples have often led researchers to

suspect bias due to net avoidance, resulting in underestimation of larvae (especially larger larvae) collected

during the day (Richardson and Pearcy, 1977; Boehlert

et al., 1985). These differences can vary depending on

such factors as gear type, size, and tow speed. The present study supports this contention, particularly for the

upper 20 m of the water column, where light penetration is of greatest concern. Sakuma et al. (1999) found

that total adjusted catches of Sebastes spp. larvae off

the central California coast were significantly greater

at night than during the day. Richardson and Pearcy

(1977) observed that large Sebastes spp. larvae (9−11

mm) were collected exclusively during the night, while

smaller larvae (3−4 mm) were collected during both day

and night at the 0−50 m depth off the central Oregon

coast. However, they did not find evidence of daytime

net avoidance for S. leucopsarus (5−11 mm) or Isopsetta

isolepis (butter sole) (14−23 mm) larvae collected during

the same study. In addition, Laroche and Richardson

(1979) found no evidence of increased net avoidance by

larger Parophrys vetulus (English sole) larvae collected

during the day versus night off the Oregon coast. However, considering results from the present and similar

studies on differences in diel ichthyoplankton abundance (Ahlstrom, 1959), we believe that ichthyoplankton

sampling should be conducted primarily at night if at

all possible to eliminate any potential bias due to net

avoidance. Net avoidance should at least be factored

into any model estimating abundances and depth distributions of larvae collected during both day and night.

As noted earlier, diel vertical migration (DVM) has

been well documented for larvae of many marine fish

species. A variety of theories have been put forth to

explain DVM in larval fish, including predator avoidance (Hunter and Sanchez, 1976; Yamashita et al.,

1985), the pursuit of zooplankton prey (Fortier and

Leggett, 1983; Munk et al., 1989), facilitated larval

transport in varying tidal currents (Norcross and Shaw,

1984; Hare and Govoni, 2005), optimization of the energetic advantage gained by larvae at certain depths

in thermally stratified water (Wurtsbaugh and Neverman, 1988; Neilson and Perry, 1990), and the pursuit

of optimum light conditions for larval survival (Heath

et al., 1988). Although this study was not designed to
Auth et al.: Diel variation in vertical distribution of an offshore ichthyoplankton community 325

determine the underlying causes of the DVM of larval

fish, it does provide some evidence for type-I DVM (up

at night, down during the day) for S. leucopsarus and

Sebastes spp. larvae (and to a lesser extent for L. exilis

larvae), and type-II DVM (down at night, up during the

day) for T. crenularis larvae (Fig. 3). Evidence for either

type of DVM for larval Sebastes spp. has not been found

in previous studies (Sakuma et al., 1999; Bjorkstedt

et al., 2002). This new type-I DVM could help explain

the retention of Sebastes spp. larvae close to settlement

areas along and inshore from the shelf as observed by

Auth and Brodeur (2006)—a retention that is due to

the ability of larvae to regulate their position in the

water column and to take advantage of selective Ekman

transport (Sakuma and Larson, 1995).

Acknowledgments

We thank the captains and crews of all research vessels

participating in this study for their cooperation and

assistance in the sampling. Collection of all MOCNESS

tows discussed in this paper was made possible through

two grants to W. Peterson from the U. S. GLOBEC

program (NA 67RJ10151 and NA 86OP0589). We are

indebted to the Peterson Zooplankton Group for their

efforts in collecting data at sea and for archiving the

samples. In this regard, special thanks to J. Keister,

M. Vance, L. Feinberg, T. Shaw, J. Lamb, A. Røestad,

and D. Swenson. We also thank M. Busby, D. Blood,

and A. Matarese of the Alaska Fisheries Science Center

Recruitment Studies Program for their help in identifying eggs and larvae. We thank G. Boehlert, B. Mundy,

A. Moles, and three anonymous reviewers for critical

reviews of the manuscript. Funding was provided by

NOAA’s Stock Assessment Improvement Program, Fisheries and the Environment Initiative, and the Northeast

Pacific GLOBEC Program. This is contribution number

512 of the U.S. GLOBEC Program.

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