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Keywords:

  • diet;
  • Euphausia pacifica;
  • euphausiids;
  • feeding;
  • Lampanyctus jordani;
  • mesopelagic fish;
  • micronekton;
  • myctophids

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

ABSTRACT:  The feeding habits of Lampanyctus jordani, an abundant mesopelagic fish in the subarctic North Pacific, was examined based on the stomach contents of 721 specimens collected over the continental slope off the Tohoku area, Pacific coast of northern Japan during April and October from 1996 to 1998. The prey items comprised mainly crustaceans such as copepods, amphipods, euphausiids and decapods. Euphausiids were the most important items in the diet both during April and October. During April, when the annual maximum of zooplankton biomass occurred and the Oyashio Intrusion Current prevailed, L. jordani fed intensively and consumed a high proportion of Euphausia pacifica. These seasonal variations also influenced the feeding intensity and dietary diversity. Feeding intensity, measured by the stomach contents index, was higher during April than October, reflecting the higher biomass of zooplankton in the Tohoku area during spring. The dietary diversity of L. jordani was lower during April than October, indicating that L. jordani shifted to a wider variety of prey when the availability of E. pacifica was limited.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Mesopelagic micronekton such as small fish, shrimps and small squids inhabit oceanic environments and play an important role in oceanic food webs. Mesopelagic micronekton are also important in near-shore food webs, and these communities, termed as mesopelagic boundary communities, occupy boundary zones between oceanic and island or continental slope areas.1 Myctophids are a dominant micronekton in both oceanic and mesopelagic boundary communities. They generally prey upon crustacean zooplankton and are consumed by various marine upper-level predators.2

Lampanyctus jordani is widely distributed in the subarctic North Pacific Ocean3 and is one of the most dominant myctophids in the western part of the ocean.4Lampanyctus jordani has been reported to be preyed upon by marine mammals such as ribbon seal Phoca fasciata5 and Dall's porpoise Phocoenoides dalli.6 Further, over the continental slope in the Tohoku area, dominant demersal fishes such as Pacific cod Gadus macrocephalus, walleye pollock Theragra chalcogramma and threadfin hakeling Laemonema longipes consume L. jordani.7 Despite the importance of L. jordani in the food webs, the only published report on its feeding habits is a description of the numerical prey composition from the Pacific coast off Hokkaido.8

Oceanography in the Tohoku area is complex and variable with the confluence of the Oyashio Intrusion Current, the Kuroshio Extension and the Tsugaru Warm Current. The Oyashio Intrusion Current extends into the region in spring (March–May) whereas the Tsugaru Warm Current originating from the Sea of Japan flows into the Pacific and prevails in the Tohoku area in autumn (September–November). However, seasonal variability is less obvious in the path of the Kuroshio Extension. The area between the Oyashio and the Kuroshio fronts is regarded as transitional waters.9,10 Such a complex oceanographic structure affects the biomass and the species composition of zooplankton,11–13 and therefore affects the feeding habits of mesopelagic fishes. In this article, we report the prey composition and its seasonal variations in the feeding habits of L. jordani in relation to the varying environment of the Tohoku area.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Specimens were collected during five demersal fish surveys conducted by the Tohoku National Fisheries Research Institute in April 1996–1998 (R/V Tanshu Maru) and in October 1996 and 1997 (R/V Wakataka Maru). The area surveyed ranged 150–1800 m in depth and 36–41°N during October, and 38°30′−41°N during April, and Lampanyctus jordani were collected from 250 to 1800 m (Table 1). A bottom trawl net with a mouth opening of approximately 3 × 20 m was towed for 15–30 min during the daytime at an average ship speed of 3 knots (5.7 km/h). A total of 190 tows were made, and L. jordani were collected during 129 tows. Water temperature was measured from the sea surface to just above the sea bottom using either an expendable bathythermograph (Tsurumi Seiki, Kanagawa, Japan) or a conductivity–temperature–depth (Sea-Bird Electronics, Bellevue, WA, USA) probe at each sampling station.

Table 1.  Number of specimens of Lampanyctus jordani, the stomachs of which were examined in the present study
Depth (m)Month
AprilOctoberTotal
250–400261541
401–5005361114
501–6007436110
601–7008862150
701–100081183264
1001–1800123042
Total334387721

Fish samples were fixed in a 10% buffered formaldehyde–seawater solution at sea and then transferred to 50% isopropanol in the laboratory. Each fish was measured to the nearest 0.1 mm standard length (SL), weighed to the nearest 0.01 g and then dissected. Prey were identified to the lowest taxon possible, counted and weighed to the nearest 0.01 mg. Fish specimens with everted stomachs or digested prey in their mouth cavities were excluded from the analysis. Fish scales (probably from myctophids) and ophiuroid arms found in the stomachs were excluded from the dietary analysis, since they were considered to be ingested incidentally as fish were trawled in the net. The contribution of each prey in the diet of L. jordani was expressed as the percentage number (Cn), percentage wet mass (W) and percentage frequency of occurrence (F). For the calculation of Cn and W, total number and mass of identifiable prey were used. Based on these indices, the index of relative importance (IRI14) was calculated for each prey category as equation 1:

  • image(1)

where i represents the ith prey category and IRI for each category was then standardized to %IRI in equation 2:15

  • image(2)

where n is the total number of prey categories considered. Dietary diversity in terms of number was calculated using an index proposed by Levins:16

  • image(3)

where B is the dietary diversity and pi is the total number of the nth prey category found from the stomachs. The calculation was made separately for April and October. The index was standardized to fractions (0–1) for comparison:17

  • image(4)

where Bs is the relative dietary diversity. The percent stomach contents index (SCI) was calculated as equation 5:

  • image(5)

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Hydrography

In the Tohoku area, the Kuroshio front is defined by the 14°C isotherm at a depth of 100 m in April and 16°C in October, and the Oyashio front is defined by 5°C isotherm at 100 m depth in April and the 7°C isotherm in October.9,10 The area delimited by the fronts is regarded as transitional waters. The Kuroshio front was found near 37°N during both April and October 1997 (Fig. 1). The Oyashio front was found during April 1997 and 1998. Neither front was observed during April nor October 1996, but a seasonal difference of temperature regime was evident; the greater part of the upper 100-m layer was occupied by ≤ 10°C water during April, whereas the same area was covered with > 10°C water during October, indicating that the temperature regime during April and October 1996 was also influenced by the Oyashio Intrusion Current and Kuroshio Extension, respectively. The upper 100-m layer of the northern part of study area was covered by warm water > 16°C during October, which was the outflow of the Tsugaru Warm Current. A clear seasonal change of temperature regime caused by the alternation of water masses was found. The temperature at 100 m depth (mean ± standard deviation, SD) was lower during April (8.1 ± 3.1°C) than October (12.1 ± 3.0°C), due to the Oyashio Intrusion Current during April and the Kuroshio Extension and Tsugaru Warm Current during October.

image

Figure 1. Chart showing the isotherms at a depth of 100 m and location of stations (●) during (a) April 1996, (b) October 1996, (c) April 1997, (d) October 1997 and (e) April 1998. (f) Depth contours in the Tohoku area.

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Diet composition

The stomach contents of 721 fish ranging from 31.6 to 127.8 mm SL (110.3 ± 12.6 mm, mean ± SD) were examined. Of these, 63 individuals had empty stomachs. Prey items included more than 66 taxa, but crustaceans such as euphausiids, decapods, amphipods and copepods formed the largest component (>99% by all indices) (Table 2). The most important prey group was euphausiids in terms of number (Cn = 52.8%) and frequency of occurrence (F = 54.7%) (Table 2). Among euphausiids, Euphausia pacifica was the only important species and constituted 38.6 and 26.5% of the total number and wet mass of the prey identified, respectively. Decapod crustaceans were the most important by wet mass composition (W = 50.0%), but were low in numerical importance (Cn = 4.9%). Copepods were the most diverse group in the diet: at least 38 species of copepods including Neocalanus cristatus and Metridia pacifica were identified. It was the secondary most important prey group by number (Cn = 24.2%) and frequency of occurrence (F = 43.4%); however, its gravimetric contribution was very limited (W = 2.7%). Ostracods such as Alacia alata and amphipods such as Cyphocaris challengeri occurred in more than 14% of fish, but their contributions were low both in terms of W and Cn (<10%). Lampanyctus jordani ingested five other prey groups such as chaetognaths and fish (myctophids), but they were of limited importance (<3% by all indices).

Table 2.  Diet of Lampanyctus jordani
Prey taxaCnWFPrey taxaCnWF
  • Cn, percentage of identifiable prey of the total number; W, percentage of identifiable prey of the total wet mass; F, percentage frequency of occurrence of identifiable prey; unident., unidentifiable; +, <0.1%.

  • Neocalanus plumchrus and N. flemingeri, and Themisto japonica and T. pacifica were grouped as a single taxon each (N. plumchrus/flemingeri and T. japonica/pacifica, respectively) because of difficulty in distinguishing these species in partly digested materials.

COPEPODA (Total)24.22.743.4OSTRACODA (Total)4.00.214.8
Aetideopsis multiserrata++0.2Alacia alata0.5+2.4
Euchirella messinensis indica0.1+0.5Conchoecilla daphnoides++0.2
E. rostrata++0.2Orthoconchoecia bispinosa0.1+0.5
E. truncata++0.3Paraconchoecia oblonga++0.2
Euchirella sp.++0.2OSTRACODA (unident.)3.30.212.8
Gaidius tenuispinus++0.2    
G. variabilis0.2+1.3MYSIDACEA (Total)0.50.42.8
Gaidius spp.0.2+0.9Siriella sp.0.10.10.5
Gaetanus armiger0.1+0.3MYSIDACEA (unident.)0.40.32.4
G. simplex++0.2    
Pseudochirella polyspina++0.3AMPHIPODA (Total)9.66.524.4
Pseudochirella sp.++0.2Cyphocaris challengeri6.15.814.0
Aetideidae (unident.)0.2+0.8Hyperia galba0.1+0.5
Calanus pacificus0.1+0.8Hyperia spp.0.1+0.5
Calanus sp.++0.2Themisto japonica2.20.56.6
Neocalanus cristatus2.80.68.4T. pacifica0.1+0.3
N. flemingeri0.90.12.1T. japonica/pacifica0.3+0.9
N. plumchrus1.70.24.1Hyperiidae (unident.)0.3+1.1
N. plumchrus/flemingeri1.70.13.6Phronimidae (unident.)++0.2
N. gracilis0.1+0.2Primno abyssalis0.30.21.6
Neocalanus spp.1.00.13.5Scina sp.++0.2
Calanidae (unident.)0.3+1.3AMPHIPODA (unident.)0.80.33.8
Candacia bipinnata0.2+1.1    
C. columbiae0.50.13.0EUPHAUSIACEA (Total)52.833.254.7
Candacia sp.++0.2Euphausia pacifica38.626.533.3
Candaciidae (unident.)++0.3E. pacifica (furcilia)++0.3
Eucalanus attenuatus++0.3E. similis0.10.20.3
E. bungii1.60.28.2Nematobrachion boopis+0.10.3
Eucalanus spp.++0.2Nematobrachion spp.0.30.30.9
Rhincalanus nasutus++0.2Nematoscelis sp.++0.2
Eucalanidae (unident.)++0.2Tessarabrachion oculatum0.20.41.4
Euchaeta rimana0.1+0.8Thysanoessa inspinata0.10.10.6
Euchaeta spp.0.2+0.9Thysanoessa spp.2.01.04.7
Paraeuchaeta concinna0.1+0.3Thysanopoda orientalis+0.20.2
P. elongata0.50.12.4Thysanopoda sp.++0.2
P. tuberculata++0.2EUPHAUSIACEA (unident.)11.54.524.0
Paraeuchaeta spp.0.30.11.7    
Euchaetidae (unident.)0.50.12.7DECAPODA (Total)4.950.024.9
Heterorhabdus abyssalis0.3+0.8Sergestes similis3.841.315.5
H. pacificus0.2+0.3Sergestes spp.0.10.30.3
H. tanneri++0.9Sergia japonica0.55.41.9
Heterostylites major++0.3S. talismani+0.30.2
Heterorhabdidae (unident.)0.1+0.5Sergia spp.0.42.81.7
Metridia okhotensis0.4+1.7Sergestidae (unident.)0.72.23.5
M. pacifica1.30.15.2Gennadus incertus+0.20.2
Metridia spp.0.2+0.9G. parvus+0.40.2
Pleuromamma abdominalis++0.3Bentheogennema sp.+0.10.2
P. gracilis0.1+0.5DECAPODA (unident.)0.92.84.3
P. indica++0.2    
P. scutullata++0.3Crustacean eggs0.1+0.2
P. xiphias0.1+0.5CRUSTACEA (unident.)0.80.55.0
Pleuromamma spp.0.4+2.5    
Paracalanidae (unident.)++0.2CHAETOGNATHA (Total)0.30.12.2
Onchocalanus cristatus++0.3Sagitta elegans0.1+0.5
Phaennidae (unident.)++0.2CHAETOGNATHA (unident.)0.30.11.7
Phyllopus sp.++0.2    
Scolecithricella sp.++0.2GELATINOUS PREY (unident.)0.1+0.5
Calanoida (unident.)6.70.619.4    
    CEPHALOPODA (unident.)0.10.10.5
        
    OSTEICHTHYES Diogenichtys sp.+0.10.2
        
    No. of identifiable prey items4012  
    No. of identifiable material37  
    No. of stomachs containing identifiable prey items634  
    No. of empty stomachs63  
    No. of stomachs examined721  

Seasonal variation of diet

For the comparison of fish diet by season, only large fish (≥100 mm SL) were included because of the insufficient number (n = 59) of small fish (<100 mm SL). Euphausiids were the most important prey item during both April and October (Table 3). However, the importance (%IRI) of euphausiids decreased from 71.8% during April to 39.5% during October. The decrease in euphausiids was compensated by copepods (%IRI = 29.9%) and amphipods (%IRI = 11.8%) during October. Decapods such as Sergestes similis were also important during April (%IRI = 23.4%) and October (%IRI =  17.5%), but showed no clear seasonal pattern.

Table 3.  Prey groups in the diet of Lampanyctus jordani during April and October
Prey groupAprilOctober
CnW%IRICnW%IRI
  1. Cn, percentage of the total number; W, percentage of the total wet mass; %IRI, index of percentage relative importance of prey groups in the diet during April and October; +; <0.1%. Percentages of number and wet mass of dominant prey species are shown in parentheses.

COPEPODA10.40.93.334.25.329.9
OSTRACODA2.80.10.34.80.41.2
MYSIDACEA0.30.1+0.60.80.1
AMPHIPODA5.02.61.214.513.611.8
EUPHAUSIACEA69.637.871.840.024.239.5
(Euphausia pacifica)(62.6)(34.8)(21.2)(12.6)
DECAPODA10.657.823.44.454.717.5
(Sergestes similis)(6.7)(43.5)(2.4)(40.4)
CHAETOGNATHA0.1++0.60.2+
Gelatinous prey0.1++0.1++
CEPHALOPODA0.10.2++++
OSTEICHTHYES+0.3+
No. of stomachs examined308  353  
No. of identifiable prey items1466  2267  

Feeding intensity measured as SCI averaged 1.2% showing a higher average during April (1.6 ± 2.1%) than October (0.9 ± 1.5%) (Student's t-test, t = 5.100, P < 0.001). The dietary diversity (Bs) of L. jordani was lower during April (0.014) than October (0.090).

DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Like other myctophids,18–20Lampanyctus jordani fed mainly on crustaceans. Euphausiids, decapods and copepods were the predominant prey items in the diet. Off the Pacific coast of Hokkaido, it has also been reported that euphausiids (mainly Thysanoessa spp. and Euphausia pacifica) are numerically important in the diet of L. jordani.8 Shifts in prey species and size with ontogeny were reported in many myctophids.18 However, dietary shift with ontogeny was not examined, since most (721 of 662) specimens examined in the present study were ≥ 100 mm SL. This remains for future study.

Euphausiids were the predominant prey item throughout the study. Among euphausiids, Euphausia pacifica was predominant in the diet, and their contribution was higher during April than October. Two factors are responsible for the seasonal contribution of E. pacifica to the diet. In the Tohoku area, a high abundance of Euphausia pacifica occurs in the area with 5–10°C temperatures at 100-m depth.21 In this study, the greater part of the study area at 100 m depth was ≤ 10°C during April and > 10°C during October (Fig. 1). The abundance of E. pacifica in this area fluctuates seasonally, showing maxima during March and April.22 The high abundance in the coastal area is associated with reproductive behavior of E. pacifica, which mate and spawn in the shallow waters of coastal areas.23 Therefore, variation in the contribution of E. pacifica reflects the seasonal movements of the biomass of E. pacifica and alternation of water masses, namely the Oyashio Intrusion Current during April and the Kuroshio Extension and the Tsugaru Warm Current from April through October.

These seasonal variations also influenced the dietary diversity and feeding intensity. The dietary diversity of L. jordani was lower during April than October, suggesting that L. jordani shifted to a wider variety of prey when the availability of E. pacifica was limited. The SCI increased from 0.9% during October to 1.6% during April. In the Tohoku area, the annual maximum of zooplankton biomass occurs in spring to early summer (March–June), and then the biomass reaches the minimum in autumn to winter (September–January).11,12 Thus, SCI may increase from October to April not only because of the importance of E. pacifica in the diet, but also due to the seasonal change of zooplankton biomass in the Tohoku area.

Sergestes similis, the most important prey by wet mass composition of the overall diet, showed no clear seasonal pattern. Yamamura and Inada7 also reported that the contribution of S. similis in the diets of demersal fishes showed no clear seasonal tendency in the Tohoku area, and suggested that its sporadic occurrence in the stomachs of demersal fishes was a result of the wide temperature tolerance range of S. similis.

Euphausia pacifica undertake diel vertical migration and are distributed in the 100–600 m layer during the daytime and in the epipelagic layer at night.13,24 Dense aggregations of E. pacifica have been observed not only in the surface layer at night,25,26 but in the near-bottom layer during the daytime.27 Aggregations of euphausiids in the near-bottom layers result from the interruption of their downward migration by the shallow sea floor.28 Occurrence of myctophid fishes in the near-bottom layer have been reported in specific bottom topographies such as islands, seamounts or continental slope regions.29,30 Gartner et al.18 suggested that myctophids approach the bottom not only because of the interruption of their downward migration, but because of the high prey concentrations in near-bottom layer over the continental slopes. In the Tohoku area, Uchikawa et al.20 found that the myctophid Notoscopelus japonicus fed heavily on E. pacifica, which would be ingested not only in the surface layer at night but in the near-bottom layer during the daytime. Lampanyctus jordani generally occur at 400–700 m depths during the day and their upper limit of distribution extends to the 100–200 m layers at night.4Lampanyctus jordani included in the present study were caught by bottom trawl nets during the day. Thus, it is assumed that they would have been distributed close to the sea bottom, and that they would have ingested E. pacifica in the bottom layer during the daytime altough some specimens in the samples would have been caught in the mesopelagic layer as the net ascended and descended. In the present study, the state of prey digestion was not analyzed. However, 92% of fresh E. pacifica, which were consumed shortly before the fish were caught, occurred in specimens of ≤ 600-m stations, while 8% of those occurred at > 600-m stations (Uchikawa K., unpubl. data, 1998). This suggests that L. jordani preyed upon E. pacifica in the bottom layer of the ≤ 600-m stations.

In this study we have shown seasonal variation in prey composition and feeding intensity of L. jordani. Seasonal changes of the prey composition of mesopelagic fishes including myctophids,31,32 microstomatids33 and gonostomatids34 have been demonstrated in subarctic to temperate regions. These other studies suggested that the seasonal shifts are attributable to the opportunistic nature of feeding in the mesopelagic realm. As shown by previous authors, the ingestion of E. pacifica also reflects its high availability for L. jordani during April. In the Tohoku area, dominant demersal fishes such as walleye pollock Theragra chalcogramma and threadfin hakeling Laemonema longipes consume L. jordani as well as E. pacifica, and the contribution of L. jordani as prey of demersal fishes is higher during May than November.7,35 These findings imply that the food web over the continental slope in the Tohoku area is supported directly and indirectly by the high abundance of E. pacifica. The heavy predation on E. pacifica by L. jordani suggests that a food chain from E. pacifica through L. jordani to demersal fishes is an important energy pathway in the food web of the Tohoku area during spring.

ACKNOWLEDGMENTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

We thank H Imamura, G Shinohara, T Nobetsu and the officers and crews of R/V Tanshu Maru and R/V Wakataka Maru for help with sampling at sea. We also thank N Shiga, T Komai, Y Yamada and H Kaeriyama for prey identification, and H Ogi, H Ohizumi, M Moku, H Saito and H Sugisaki for helpful discussions.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES
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