NOAA Technical Memorandum NMFS-AFSC-162


Abundance and Spatial Pattern 

of Salps Within the North Pacific 

Subarctic Frontal Zone, August 1991 

by 

S. E. Ignell, B. L. Wing, B. D. Ebberts, and M. M. Masuda 

U.S. DEPARTMENT OF COMMERCE 

National Oceanic and Atmospheric Administration


National Marine Fisheries Service


Alaska Fisheries Science Center


February 2006 
NOAA Technical Memorandum NMFS 

The National Marine Fisheries Service's Alaska Fisheries Science Center 

uses the NOAA Technical Memorandum series to issue informal scientific and 

technical publications when complete formal review and editorial processing 

are not appropriate or feasible. Documents within this series reflect sound 

professional work and may be referenced in the formal scientific and technical 

literature. 

The NMFS-AFSC Technical Memorandum series of the Alaska Fisheries 

Science Center continues the NMFS-F/NWC series established in 1970 by the 

Northwest Fisheries Center. The NMFS-NWFSC series is currently used by 

the Northwest Fisheries Science Center. 

This document should be cited as follows: 

Ignell, S. E., B. L. Wing, B. D. Ebberts, and M. M. Masuda. 2006. 

Abundance and spatial pattern of salps within the North Pacific 

Subarctic Frontal Zone, August 1991. U.S. Dep. Commer., NOAA 

Tech. Memo. NMFS-AFSC-162, 26 p. 

Reference in this document to trade names does not imply endorsement by 

the National Marine Fisheries Service, NOAA. 
NOAA Technical Memorandum NMFS-AFSC-162


Abundance and Spatial Pattern


of Salps Within the North Pacific


Subarctic Frontal Zone, August 1991


by 

S. E. Ignell, B. L. Wing, B. D. Ebberts*, and M. M. Masuda 

Alaska Fisheries Science Center 

7600 Sand Point Way N.E. 

Seattle, WA 98115 

www.afsc.noaa.gov 

*present address:


U.S. Army Corps of Engineers


CENPP-PE-RPF


 333 S.W. First Avenue 

Portland, Oregon 97208 

U.S. DEPARTMENT OF COMMERCE 

Carlos M. Gutierrez, Secretary 

National Oceanic and Atmospheric Administration 

Vice Admiral Conrad C. Lautenbacher, Jr., U.S. Navy (ret.), Under Secretary and Administrator 

National Marine Fisheries Service 

William T. Hogarth, Assistant Administrator for Fisheries 

February 2006 
This document is available to the public through: 

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

www.ntis.gov 
Notice to Users of this Document 

This document is being made available in .PDF format for the convenience of users; however, the 

accuracy and correctness of the document can only be certified as was presented in the original hard 

copy format. 

iii

ABSTRACT 

An intensive oceanographic and zooplankton survey of a small portion of the North Pacific 

Ocean=s Subarctic Frontal Zone (SAFZ) was conducted in August 1991. Over 91% of the total 

displacement volume of the zooplankton samples consisted of salps, followed by copepods (2.1%), 

euphausiids (2.0%), chaetognaths (1.7%), and amphipods (1.0%). Salps also accounted for almost half 

(44.7%) of the total caloric value in the samples. Salp abundance was not correlated with abundances of 

the crustacean zooplankton groups. Salps exhibited a reverse diel vertical migration pattern; other 

species exhibited diel patterns. 

v 

CONTENTS 

ABSTRACT...............................................................................................................................................iii 

INTRODUCTION.......................................................................................................................................1 

MATERIALS AND METHODS.................................................................................................................2 

RESULTS....................................................................................................................................................4 

Oceanographic data.........................................................................................................................4 

Zooplankton species composition...................................................................................................4 

Zooplankton displacement volume and caloric value......................................................................6 

DISCUSSION..............................................................................................................................................8 

ACKNOWLEDGMENTS.........................................................................................................................11 

LITERATURE CITED..............................................................................................................................12 

TABLES....................................................................................................................................................17 

FIGURES...................................................................................................................................................21 

The mid-Pacific Subarctic Frontal Zone (SAFZ) is an important but poorly understood portion of 

the Transition Zone separating the Subarctic Domain from the Subtropical Domain. The SAFZ is 

usually defined in terms of salinity and temperature fronts above the permanent pycnocline: its southern 

limit is defined by the surfacing of the 33.8 practical salinity units (psu) isohaline and disappearance of 

the temperature inversion in the subarctic halocline and its northern limit is defined by the outcropping 

of the 33.0 psu isohaline (Roden 1991). In the central and western North Pacific Ocean, the SAFZ is 

approximately 300 km wide, generally ranging from 40EN to 43EN latitude. 

The SAFZ encompasses the southern limit of salmonid (Oncorhynchus spp.) migrations and the 

northern limit of many subtropical and transitional squid and fish species. The complex variations in 

thermohaline structure of the frontal zone are matched by large variations in nekton abundances at 

various space and time scales. For example, catch rates of salmonids differ sharply from north to south 

near the SAFZ (Ignell et al. 1995; Welch et al. 1995; Murphy 1995) and by several orders of magnitude 

between the western and eastern portions (longitudes 170EE and 145EW, respectively) of the SAFZ 

(Ignell, 1991). This same level of variability persists at much smaller scales (Ignell and Murphy 1993), 

especially in the western portion; near 170EE, salmonid catches range from 0 to 2000 fish per driftnet 

operation in the space of a few tens of kilometers. Using satellite imagery, Ignell and Murphy (1993) 

also showed that at least some of this variability was associated with changes in sea surface temperature 

(SST), and that patchiness in salmonid spatial patterns was associated positively with mean SST and 

variance in SST. 

Spatial differences in nekton abundances, however, may also reflect changes in their forage 

(Murphy 1995). Fish growth and survival is a function of habitat richness which is modified by 

competitive interactions and may be constrained by predator avoidance (Sogard 1994). Forage type and 
2 

availability are major factors determining habitat richness, particularly for salmon in the oceanic waters, 

where their increased size reduces predation risk and enables them to select optimal habitat based on 

growth potential (Pearcy 1992). 

To better understand the relative importance of forage and oceanographic conditions in 

determining nekton spatial patterns, an oceanographic and zooplankton survey of a portion of the SAFZ 

(Fig.1) was conducted in 1991 from the R/V Acania. The survey occurred in conjunction with an 

international monitoring program of neon flying squid (Ommastrephes bartrami) driftnet vessels 

operating in that region, resulting in fine-scale data on the physical environment, zooplankton 

abundances, and nekton spatial patterns. This paper reports on the widespread occurrence of salps 

found in the zooplankton samples. Patterns in salp abundance are compared with temperature and 

salinity data collected from CTD (conductivity, temperature, and depth) casts, and with abundance 

patterns of other zooplankton taxa. 

MATERIALS AND METHODS 

The survey location was originally set at 45E04.5NN, 171E55.6NE, but then moved eastward to 

45E00.0NN to 43E40.0NN and 174E00.0NE to 178E00.0NE, because of inclement weather. The final 

survey consisted of 41 stations arranged in a grid and generally spaced every 20 minutes of latitude and 

every 30 minutes of longitude (Fig. 1). CTD casts to 300-500 m were made at each station (Murphy and 

Ebberts 1997). Salinity and temperature data at 10- and 100-m depths (temperature data are hereafter 

denoted as t10 and t100) were extracted from the CTD data. 
3 

Biological sampling at each station consisted of a double oblique bongo net tow to 100 m. The 

bongo nets were 0.6 m in diameter, with 0.505-mm mesh nets. Depth of tow was measured by a 

bathykymograph attached to the bongo net. The volume of water filtered through each net was 

measured with a flow meter attached inside the net opening. 

After the net was retrieved, zooplankton were washed from the codend and preserved 

immediately in buffered 10% formalin. In the laboratory, the samples were rinsed in tap water for 1-3 

days and then examined using a microscope. Squid and fish larvae were removed for later 

identification. Displacement volume (DV) of the total sample was measured using methods described in 

Smith and Richardson (1977). The percent composition of the sample volume was then estimated 

visually for the following seven taxonomic categories: gelatinous zooplankton (primarily salps), 

chaetognaths, copepods, amphipods, euphausiids, fish, and other. 

The total volume of zooplankton per cubic meter was computed by dividing DV by the volume 

of water filtered, and then apportioned into the seven taxonomic categories according to the estimate of 

sample composition, and averaged across each pair of bongo tows. The total zooplankton volume was 

also converted into calories per cubic meter by a species-independent ratio of DV to wet weight (WW) 

computed from data in Nakai and Honjo (1961) and WW to caloric value data in Davis (1993) (Table 1). 

 The concentration of non-gelatinous zooplankton was computed by first subtracting estimated salp DV 

from measured total DV, and then multiplying the result by the DV/WW ratio. Means of total volume, 

caloric value, and concentration are presented with " one standard deviation. 

Caloric and DV values were normalized by log-transformation for use in statistical tests and 

contour plots. Statistical tests consisted of correlations between taxonomic categories and t-tests of 

night-day differences in zooplankton abundance. Zooplankton abundance data were fitted by a lowess 
4 

function to a generalized additive model (GAM) with a single variable (time of day), which allowed for 

a nonlinearity in the explanatory term (Hastie 1992). Fitted values of the resulting GAM models were 

plotted as a function of time of day, and the residuals plotted as a function of latitude and longitude. 

The latter plots allowed us to identify spatial variation in zooplankton abundances independent of timeof-day effects. 

RESULTS 

Oceanographic Data 

Based on latitude-by-depth sectional plots of temperature along each end of the survey area and 

on contour plots of temperature and salinity, the SAFZ=s northern boundary was located inside the 

survey area, whereas the southern boundary was outside the survey area (Figs. 2 and 3). Along the 

174EE longitude transect, the northern boundary was characterized by a front ranging from 44.4EN to 

44.8EN; along the 178EE longitude transect, the northern boundary-defining front ranged from south of 

the survey area to 44.0EN latitude. Surface salinities north of the northern boundary were less than 32.7 

psu; south of the northern boundary, surface salinities were 33.0 psu or greater, confirming the 

placement of the SAFZ=s northern boundary based on temperature gradients. The northern boundary 

was also highly variable, exhibiting sharp turns and meanders across the survey area (see plot of 

temperature at 100 m, Fig. 2). 

Zooplankton species composition 

Although more than 80 species of zooplankton were identified during the processing of the 

samples, we did not attempt a detailed analysis by species or life-history stages. Nearly all of the 
5 

gelatinous zooplankton consisted of salps; of these, about 80% were Salpa fusiformis, with a few 

individuals of Cyclosalpa bakeri, Thalia democratica, Thetys vagina, and Dolioletta gegenbauri. Other 

gelatinous zooplankton included hydromedusae, siphonophores, scyphomedusae, ctenophores, 

pteropods, and heteropods. 

Chaetognaths were dominated by Sagitta elegans. Sagitta scrippsae occurred frequently but in 

very low numbers, and Eukhronia hamata occurred rarely. 

Copepods were dominated by Neocalanus cristatus (~60%), N. plumchrus or N. flemengeri 

(~20%), and Calanus marshallae and C. pacificus (~20%, both species combined). We did not attempt 

to distinguish N. plumchrus from N. flemengeri. Other calanoid copepods (14 species) were present but 

did not make up a significant portion of the non-gelatinous zooplanktonic volume; often only one or two 

individuals of a species were found in a sample. 

Amphipods were dominated by the free-living Themisto pacifica. Other hyperiid amphipods (10 

genera) known or believed to be associated with gelantinous zooplankton were observed in small 

numbers and did not contribute significantly to the non-gelatinous zooplankton volume. The gammarid 

Cyphocaris challengeri was observed in small numbers at only eight stations. 

Euphausiids were dominated by Thysanoessa inspinata (~60%), Euphausia pacifica (~40%), and 

larval forms. Nematocelis difficilis, Nematobrachion flexipes, and Tessarabrachion oculata ocurred 

rarely in the samples. 

Fish and fish eggs were never numerous in our samples. Larval myctophids occurred most 

frequently but never contributed significantly to the total zooplankton volume in our study, although 

rare adult or large juvenile specimens made up 5-35% of the total volume in occasional samples. 
6 

Squid larvae, although frequent in occurrence (~50% of the samples), were a significant portion 

of the total zooplankton volume at only one station (45.0oN, 175.5oE). Other zooplankton that were 

present but never a significant portion of the total zooplankton volume were pelagic polychaetes 

(Tomopteris septentrionalis, Sagitella kowalewskii, and Travisiopsis lobifera), larval cirripeds (Lepas 

spp. cyprids), ostracods (Conchoecia alata, C. daphnoides, and Conchoecia spp.), mysids 

(Caesaromysis hispida), decapod shrimp (Serjestes similis), and megalopa of a grapsoid crab. 

Zooplankton displacement volume and caloric value 

Total DV of net zooplankton varied between 0.048 and 9.529 ml@m-3, a 198-fold difference 

between stations. Computed caloric value ranged from 23.6 to 1,111.0 cal·m-3, a 47-fold difference 

between stations. Concentrations of non-gelatinous zooplankton ranged from 1.9 to 515.6 g·1,000 m-3, a 

271-fold difference between stations. Mean DV was 1.7 ± 2.2 ml·m-3, mean caloric value was 286.0 ± 

251.1 cal·m-3, and mean concentration of non-gelatinous zooplankton was 110.8 ± 126.8·1,000 m-3. 

Over 91% of the total DV of the samples in the survey area consisted of salps, followed by 

copepods (2.1%), euphausiids (2.0%), chaetognaths (1.7%), and amphipods (1.0%). This order among 

taxonomic categories persisted for the caloric value data; even though salps have a low caloric to DV 

ratio compared to crustaceans (Davis 1993), they still accounted for almost one-half (44.7%) of the total 

caloric value in the samples. 

Caloric value (Table 1) and DV of night samples exceeded that of day samples (Table 2). In the 

survey area, night-day differences were greatest for euphausiids (17.8:1) and least for salps (1.3:1). The 

night-day differences were highly significant (P < 0.0005) for euphausiid and copepod DVs and caloric 
7 

values, and significant (P < 0.005) for amphipod and chaetognath DVs. Salp DV and caloric value, and 

total DV showed no significant night-day differences. 

Lowess plots of log-transformed abundance versus time of day showed that zooplankton 

abundances changed gradually between night and day (Fig. 4). Except for salps, peak abundances 

occurred near midnight and low abundances from about 1000 to 1700 hours (Alaska Daylight Time). 

Fitted lowess values between nighttime peaks and daytime lows varied about 9-fold for euphausiids and 

about 2.5- fold for chaetognaths, copepods, and amphipods. Salps exhibited a reverse diel vertical 

migration (RDVM) pattern, with peak abundances near dawn and dusk (Fig. 5, upper left panel). 

The time-of-day adjusted abundance and distribution of zooplankton varied extensively 

throughout the western survey area. Salps were broadly abundant along the northeastern, southwestern, 

and southeastern regions (Fig. 5) and constituted more than 80% of the total zooplankton in about onehalf of the survey area. Non-gelatinous zooplankters were abundant in the northeastern, central southern 

and northern areas (Fig. 5). Euphausiids, copepods, and amphipods were abundant along two northern 

areas and one southern area. Chaetognaths were abundant primarily in the northwestern area (Fig. 6). 

Except for chaetognaths, highest abundances of each zooplankton taxon occurred in areas exhibiting 

gradual changes in temperature at 100 m (compare Figs. 2 and 6). 

Abundances of gelatinous and non-gelatinous taxa (time-of-day adjusted) were not correlated 

(r = &0.52; P < 0.01); otherwise, they were not correlated with any other zooplankton group. Pair-wise 

correlations between chaetognath, copepod, amphipod, and euphausiid abundances were all significant 

(P < 0.05); of these, euphausiids and amphipods were most closely related (r = 0.71). 

Zooplankton caloric values generally followed the distribution pattern of salps. The greatest 

caloric values occurred in the northeastern and southwestern regions, where salps made up over 90% of 
8 

the total zooplankton volume (Fig 7). High caloric values in a small area along the southern portion of 

the survey area, however, were due to high abundance of copepods and euphausiids. 

Zooplankton abundance and caloric values in areas with a strong horizontal temperature gradient 

(t100) were generally lower than those in weak-gradient areas. Large differences were observed for 

chaetognath (P = 0.089) and euphausiid abundance (P = 0.087) and for total zooplankton caloric value 

(P = 0.048). 

DISCUSSION 

The extensive coarse-scale heterogeneity of zooplankton abundances in our survey area (e.g., 

Fig. 4) is characteristic of biological processes within frontal zones and is comparable with spatial 

variations in zooplankton abundances from data collected in October 1989 along a series of five 

meridional transects (85 total stations) bisecting the SAFZ and ranging from 155EW to 171EW 

longitude and from 37EN to 47EN latitude (Murphy et al. 1998). The much lower number of salps 

observed by Murphy et al. (1998) versus our data may be attributed to differences in study area, season, 

and year, which are all significant factors affecting the geographic distribution and abundance of salps 

(e.g., Hubbard and Pearcy 1971). 

Concentrations of non-gelantinous zooplankton in our study area were more than double 

observed at Ocean Station P, or in the Sargasso Sea, Norwegian Sea, and the Central Equatorial Pacific 

Ocean, but less than those observed in the Bering Sea, coastal waters of British Columbia, and the 

northwestern Pacific Ocean (McAllister 1961). Our relatively high concentrations might be expected, 

because our survey area lies within the SAFZ, a prominent area of convergence across the North Pacific 
9 

Ocean (Roden 1991). Zooplankton production is typically high in areas where physical processes 

promote high primary production. Such areas often occur along coasts, in the centers of cyclonic gyres, 

between currents, and in convergence areas (Daly and Smith 1993). 

Dense patches of salps, such as those observed in our study, have been reported for a wide 

variety of pelagic and neritic waters. At Ocean Station I in the North Atlantic Ocean, salps make up 10­

40% of the total dry weight of zooplankton from April to October (Parsons and Lalli 1988). Salps were 

the dominant zooplankton in open waters of the Weddell Sea (Siegel et al., 1992), and off Tasmania in 

windy periods (Harris et al.1992). Wind stress also affected Mediterranean salp populations, promoting 

the generation of dense blooms through renewed pelagic production (Menard et al. 1994). Salps have 

been found in concentrations ranging from 5 to 275 individuals per cubic meter along coastal regions of 

Ivory Coast and Florida (Paffenhofer and Lee 1987), and swarms of Thalia democratica off California 

have reached densities of 500 salps per cubic meter (Alldredge and Madin 1982). 

These high densities arise because of the extraordinary life histories and foraging characteristics 

of salps, which allow them to react with maximum flexibility to variations in ocean conditions 

(Alldredge and Madin 1982). In optimal environments, salps grow quickly, forming large swarms 

through asexual reproduction. They are also non-selective filter feeders, eating everything that they trap 

in their feeding net, enabling them to filter much larger volumes of water and capture a larger range of 

food sizes than other pelagic herbivores (Alldredge and Madin 1982). As a result, growth rates are 

extremely high: body length of young Thalia democratica increases by 10% per hour, and growth of a 

solitary Salpa fusiformis can exceed 40% of its length each day (Madin and Purcell 1992). The 

combined effect of high feeding rates, asexual budding, and high growth rates enables salps to increase 
10 

population sizes rapidly under favorable conditions, even to the point of replacing other zooplankton in 

a region (Madin and Purcell 1992; Paffenhofer and Lee 1987). 

Gelatinous zooplankton occupy an important, but often overlooked, role in pelagic and neritic 

ecosystems. They are often sampled inadequately by nets or are excluded from processing of plankton 

samples (Parsons and Lalli 1988). For example, McAllister (1961) excluded medusae from measures of 

zooplankton concentration because they were not Aedible@ species. Also, many medusae are damaged 

easily and therefore difficult to enumerate. As a result, analyses of zooplankton populations in the 

subarctic North Pacific Ocean are often restricted to non-gelatinous species, giving results such as those 

summarized in Parsons and Lalli (1988), where copepods are reported to make up from 75-95% of the 

planktonic biomass in subarctic waters. 

Nevertheless, gelatinous zooplankton have a substantial impact on the tropic structure of marine 

communities and in the transport of carbon from the sea surface to the deep sea. Biggs (1977) estimated 

that excreted ammonia from salps in the North Pacific Gyre may provide about one-half of the nitrogen 

requirements for phytoplankton in that region, and may limit abundance of diatoms in the late summer 

(Madin et al. 1997). Gelatinous zooplankton produce large quantities of rapidly sinking fecal pellets, 

which can contribute significantly to benthic communities (Alldredge and Madin 1982; Anderson and 

Nival 1988). Phytoplankton remain largely intact in these pellets because digestion is incomplete. As a 

result, upper-layer carbon that is fixed by phytoplankton is transported quickly to the deep sea in a salpdominant ecosystem. With high filtration rates, a salp swarm can remove a large portion of 

phytoplankton from an area, making them major competitors of other herbivorous zooplankton for 

phytoplankton food resources (Loeb et al. 1997) and of fish for zooplankton resources (Purcell and 

Sturdevant 2001). Salps are also a major component of the diet of some fish species such as the chum 
11 

salmon (Oncorhynchus keta), ocean sunfish (Mola mola), louvar (Luvarus imperiallis), and pelagic 

armorhead (Pentaceros pectoralis) that inhabit our survey area and feed on salps (Eschmeyer et al. 

1983; Kashkina 1986; Salo, 1991). 

ACKNOWLEDGMENTS 

We thank Susan McGregor and John Karinen for assistance in processing the samples, Wendy 

Carlson for assistance in preparing figures, and Molly Sturdevant, Richard Brodeur, and Jack Helle for 

their reviews of the manuscript. 
12 

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17 

Table 1.--Data used to compute calories/ml of net plankton collected aboard the R/V Acania cruise to 

the Subarctic Frontal Zone of the North Pacific Ocean, August 1991. Caloric value (per gram) 

data are from Davis (1993) and the weight/volume (0.674) ratio is an average of data given in 

Nakai and Honjo (1961). Percent composition of taxonomic category in group was based on 

visual estimates by one of the authors (BLW). 

 Composition of 

Group/Taxonomic category taxa in group

 (%) g·ml

-1 cal·g-1 cal·ml-1 

Salp - Gelatinous 

Salpidae 100 9.674 96 64.7 

Chaetognaths 

Sagitta elegans 100 0.674 488 328.9 

Copepods 

Neocalanus cristatus 60 0.674 748 504.2 

Neocalanus plumchrus/flemengeri 20 0.674 995 670.6 

Calanus pacificus 20 0.674 872 587.4 

Weighted average 554.1 
_________________________________________________________________________________________ 

18 

Table 1. –Continued. 

Euphausiids 

Thysanoessa spp. 60 0.674 1197 806.8 

Euphausia pacifica 40 0.674 1138 767.0 

Weighted average 790.9 

Amphipods 

Themisto pacifica 100 0.674 725 488.7 

Wet weight/displacement volume data (Nakai and Honjo, 1961) 

Calanus spp. 0.729 

0.662 

0.529 

0.679 

0.716 

Euphausia recurva 0.679 

Salpa fusiformis 0.724 

Average 0.674 
19 

Table 2.--Caloric and displacement volume (DV; in milliliters) statistics: ratio of night to day averages, overall 

average, and percent of total DV by taxonomic category for stations in the survey area. 

Night/day Fraction of DV 

Ratio Mean (%) 

Calories 2.143 292.460 

DV: Total 1.460 2.033 

Salps 1.349 1.859 91.5 

Chaetognaths 2.150 0.035 1.7 

Copepods 3.535 0.043 2.1 

Amphipods 4.342 0.020 1.0 

Euphausiids 17.811 0.040 2.0 

Stations 18/22 40 
Figure 1.-- Survey area and sampling locations of the R/V Acania North Pacific squid driftnet research cr

 N = night station; D = night station.
Figure 2.-- Contour plot of temperature at 100 m, temperature at 10 m, salinity at 100 m, and salinity at 1
Figure 3.-- Latitude by depth sectional plots of temperature along each end of the survey area.
Figure 4.-- Lowess plot of the log-transformed density of copepods, chaetognaths, amphipods, and eupha
Figure 5.-- Lowess plot of the log-transformed density of salps versus time of day (upper left), image plot 

 of salps in the survey area (upper right), contour plot of the ratio of salp displacement volume 

 volume (lower left), and image plot of the density (ml/m3) of non-gelatinous plankton in the 
174 175 176 177 178

Longitude

0. 54 7. 44 3. 44 0. 44 6. 34

edutit aL

174 175 176 

Longitude

0. 54 7. 44 3. 44 0. 44 6. 34

174 175 176 177 178

Longitude

0. 54 7. 44 3. 44 0. 44 6. 34

edutit aL

174 175 176 

Longitude

0. 54 7. 44 3. 44 0. 44 6. 34

0.8

0.4

-0.4

-0.4

0 0.400

0.4

0.4

-0.8

-0.4

1.2

0.8

0.8

0.40-0.4

-1.2-0.8

0

-0.4-0.4

-0.8-1.2

0.6

1

1 

0.0.6

1

-0.6 -0.2

0.2

0.2-0.2-0.2

-1.8

-1.4

-1

-0.6

0.2 0.61 0.2 0.6 0.2-0.2 -0.6 -1

0.6 0.6

0.2 0.2 0.6

-11

-0.6

0.2

-0.2

-0.6

-1

-0.4 -0.8

-1.2

0.8

0.40-1.2-0.8

0.8 0.4

0

-0.8

Copepods

Amphipods

Chaetogna

Euphausiid

Figure 6.-- Image plots of the densities (ml/m3) of copepods, chaetognaths, amphipods, and euphausiids i
174 175 176 177 

Longitude

0. 5

4

7. 4

4

3. 4

4

0. 4

4

6. 3

4

e

d

utit a

L

0.4

0.2

0

-0.2

-0.2

0.6

-0.6

0.4

0.2 0

-0.4

-0.2 0

0.2

0.6 0.4

0.8

-0.8

-1

-0.6

-0.4

Figure 7.-- Image plot of time-of-day adjusted (residuals from GAM model) caloric value in the survey ar
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