Feeding habits of Dall's porpoises (Phocoenoides dalli) in the subarctic North Pacific and the Bering Sea basin and the impact of predation on mesopelagic micronekton

Deep-Sea Research I 50 (2003) 593–610

Feeding habits of Dall’s porpoises (Phocoenoides dalli) in the subarctic North Pacific and the Bering Sea basin and the impact of predation on mesopelagic micronekton Hiroshi Ohizumia,*, Toshiaki Kuramochib, Tsunemi Kuboderab, Motoi Yoshiokac, Nobuyuki Miyazakid a Ocean Research Institute, The University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan Department of Zoology, The National Science Museum, 3-23-1 Hyakunincho, Shinjuku, Tokyo 169-0073, Japan c Faculty of Bioresources, Mie University, 1515 Kamihama, Tsu, Mie 514-8507, Japan d Otsuchi Marine Research Center, Ocean Research Institute, The University of Tokyo, 2-106-1 Akahama, Otsuchi, Iwate 028-1102, Japan b

Received 8 April 2002; received in revised form 26 November 2002; accepted 28 January 2003

Abstract We investigated the stomach contents of Dall’s porpoises collected in pelagic waters spanning most of their range in the North Pacific and the Bering Sea. Analysis revealed the porpoises fed mainly on myctophid fishes in the subarctic North Pacific and on gonatid squids as well as myctophid fishes in the Bering Sea. Most of the prey items were mesopelagic micronekton, primarily fishes and squids that migrate vertically to shallower waters at night. Stomach content was greater during twilight hours, suggesting the porpoises foraged actively on myctophids at night in shallower waters. Stomach contents were strongly characterized by local mesopelagic prey fauna, and prey species selectivity was not apparent. The annual consumption by Dall’s porpoises was estimated to be 2.0–2.8 million tons, or 4.7–6.5% of the biomass of mesopelagic fishes in the subarctic North Pacific, and may account for approximately 24–33% of the overall mortality of mesopelagic micronekton, especially myctophids. Myctophids are also common, but less important, prey of other subarctic predators. Dall’s porpoises are likely the primary consumers of myctophids in the subarctic North Pacific. Since myctophids are the major component of the mesotrophic level, the trophic relationship between myctophids and Dall’s porpoises is thought to be an important pathway of mass and energy in the pelagic food web in the subarctic North Pacific. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Phocoenoides dalli; Feeding habits; Myctophids; Gonatids; Prey consumption; Subarctic North Pacific; Bering Sea basin

1. Introduction *Corresponding author. Cetacean Population Biology Section, National Research Institute of Far Seas Fisheries, 5-7-1 Orido, Shimizu, Shizuoka 424-8633, Japan. E-mail address: [email protected] (H. Ohizumi).

Dolphins and porpoises are top trophic-level predators in marine food webs. This is a wellknown concept, but details of prey items and food consumption have not been evaluated for most

0967-0637/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0967-0637(03)00033-5

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H. Ohizumi et al. / Deep-Sea Research I 50 (2003) 593–610

species, especially in pelagic regions. Moreover, the ecological role and importance of these cetaceans are not well-understood. Recently, marine biologists have attempted to estimate prey consumption by marine mammals including dolphins and porpoises. Trites et al. (1997) calculated that marine mammals in the Pacific consume 150 million tons of biomass annually, and Tamura and Ohsumi (1999) estimated that cetaceans worldwide consume 300–500 million tons of prey. Although these estimates are preliminary, a common conclusion is that the consumption by marine mammals could be comparable to, or exceed the yield of fisheries. Mesopelagic micronekton, especially myctophids and gonostomatids, dominate the pelagic fish fauna (Clarke, 1973; Gj^saeter and Kawaguchi, 1980). The biomass of mesopelagic fishes has been estimated to be six times that of epipelagic fishes (Clarke, 1973). Mesopelagic fishes are important components of the marine ecosystem, but details of the interaction between mesopelagic fishes and their predators are still not well-known. Studies of the feeding habits of dolphins and porpoises from pelagic regions of the Pacific Ocean have reported that mesopelagic micronekton are the main prey items (Perrin et al., 1973; Walker and Jones, 1994; Chou et al., 1995; Robertson and Chivers, 1997; Ohizumi et al., 1998). Dall’s porpoise (Phocoenoides dalli) is widely distributed in the subarctic North Pacific and adjacent waters, including the Bering Sea, the Sea of Japan, and the Sea of Okhotsk. This species reaches 2.1–2.4 m in length and weighs up to about 200 kg (Jefferson, 1988; Houck and Jefferson, 1999). The total population in the North Pacific has been estimated to be about 1.4 million (Miyashita, 1991; Buckland et al., 1994). Dall’s porpoise is the most abundant marine mammal in the subarctic region of the North Pacific (Springer et al., 1999). This study covered most of the range of Dall’s porpoise in the pelagic North Pacific and Bering Sea, except for the exclusive economic zones of the USA, Canada, and Russia. We surveyed stomach contents and analyzed the feeding habits of Dall’s porpoises in the subarctic North Pacific and the Bering Sea by investigating prey species composi-

tion, prey selection, geographical prey variation, time of foraging, and quantity of annual consumption.

2. Methods We analyzed a total of 326 stomach samples that had been stored in the National Science Museum, Tokyo. These stomach samples were collected with other biological data during scientific surveys for incidental catch by commercial salmon gill nets. Stomachs of Dall’s porpoises from the North Pacific and the Bering Sea were collected during the summers of 1984–1989. These specimens were either collected by scientific research vessels or were incidental catches in commercial salmon gill nets. Specimens were also collected from local shore-based porpoise fishing boats (Table 1). A part of the sampling activities were reported to the International North Pacific Fisheries Commission (INPFC) and the Table 1 Outline of the sampling cruises Survey period

Area

Number of stomachs analyzed

May/10–Jun./20/1984

Western Pacific Western Central Pacific Western Pacific Western Central Pacific Bering Sea Bering Sea Eastern Central Pacific Eastern Pacific Western Pacific Western Central Pacific Eastern Central Pacific Eastern Pacific Western Pacific Western Pacific

34(37) 134

Aug./6–Sept./12/1985

Jun./30–Jul./11/1986 Aug./10–Oct./5/1986

Aug./4–Sept./26/1987

Jun./16–Jun./17/1988 May/16/1989

5 1 11(12) 48a 5(6) 23 12 17 3 22 5 1

a Incidental catches by salmon gill net. All other samples were taken by hand harpoon. Numbers in parentheses are samples used for stomach content weight analysis.

H. Ohizumi et al. / Deep-Sea Research I 50 (2003) 593–610

International Whaling Commission (IWC) (Yoshioka, 1986; Yoshioka et al., 1990). On the sampling vessels, all specimens were necropsied by trained biologists, including the authors and observers from the INPFC, after body length and body weight were recorded. Stomachs were removed, ligatured at the oesophageal and duodenal ends, and frozen for later analyses. Gonads were fixed in 10% formalin solution. Phocoenid porpoises have four ‘‘stomach’’ compartments, and ingested foods are stored and partly digested in the first compartment, the forestomach. We examined the contents of this compartment only; thus, throughout the following text the term ‘‘stomach contents’’ refers only to forestomach contents. In the laboratory, each stomach was thawed and weighed to the nearest gram. Each stomach was then opened and its contents placed into a plastic pan. The inner lining of the stomach was carefully rinsed to collect small otoliths and cephalopod beaks. The empty stomach was weighed again, and the wet weight of the stomach contents was estimated by subtracting the empty stomach weight from the weight of the stomach with contents. Stomach contents were sorted into the following categories: (1) half-digested fishes with muscle still attached to bones or with sagittal otoliths in the skull, (2) half-digested squid with heads holding the buccal masses, (3) isolated buccal masses, (4) isolated sagittal otoliths and beaks, and (5) other digested remains. Except for the digested remains, contents were secondarily sorted into taxonomic groups by referring to the otolith collection of the senior author, Ohizumi et al. (2001), Smale et al. (1995), and Kubodera and Furuhashi (1987), the lower cephalopod beak collection of the National Science Museum, Tokyo, and Clarke (1986). Because few intact fishes and squids were found, identification of almost all prey species was made by otoliths or lower beaks. The minimum number of fish of each species in the stomach contents was estimated as half the number of isolated sagittal otoliths and those taken from skulls. The number of isolated lower beaks, buccal masses, and heads holding buccal masses were summed to estimate the number of each squid species.

595

The body weights (BW) of squids were estimated from the lower rostral length of the lower beak (LRL), or from the estimated dorsal mantle length (DML) and the equations listed in Table 2. The lower rostral length was measured according to the protocol of Ohizumi et al. (2000). Energy densities of prey squids were extracted from the literature (Table 3). These energy densities and estimated weights of squid were the bases for the estimations of caloric and mass contributions. Five stomachs that had incomplete samples of squid prey were excluded from the analyses of prey composition, but the data of stomach content weight were used in the analyses. Male porpoises that had spermatocytes in more than half of the testicular tubules in a histological section were considered to be sexually mature. When histological observations were not available, we considered a testis weight of at least 40 g as a criterion of sexual maturity (Kasuya and Jones, 1984). For females, the presence of at least one corpus luteum or corpus albicans in the ovaries, evidence of lactation, and pregnancy were indicative of maturity. Because the observers on the commercial salmon gill net ships in the Bering Sea did not collect gonads, we estimated the sexual maturity of these specimens from their body lengths, such that lengths of >170.5 and >182.6 cm were representative of mature females and mature males, respectively (Newby, 1982). To examine prey variation in different geographical locations, the North Pacific was divided into four regions, each about 20
longitudinal lines from 140
E to 137
W. These four areas were termed the western Pacific (WP), the western central Pacific (WCP), the eastern central Pacific (ECP), and the eastern Pacific (EP). The Bering Sea (BE) was defined as the fifth area (Fig. 1). The daily energy requirement (E) was estimated from the metabolic model (Perez and McAlister, 1993; modified) E ¼ 4:184  317  W 0:75 ðkJ=dayÞ; where W is body weight (kg). Annual food consumption by Dall’s porpoises in the North

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H. Ohizumi et al. / Deep-Sea Research I 50 (2003) 593–610

Table 2 Relationships between measured variables and body length or body weight of squids Species

Regression

Y

X

r2

n

References

Gonatopsis borealis

Y ¼ 35:779X þ 17:036 ln Y ¼ 2:037ln X þ 2:145 ln Y ¼ 1:396ln X þ 3:66 ln Y ¼ 2:568ln X  8:340 ln Y ¼ 0:899ln X þ 3:575 ln Y ¼ 3:33ln X  0:655 Y ¼ 19:02X þ 12:82 ln Y ¼ 2:13ln X þ 0:086 Y ¼ 42:87X  43:4 ln Y ¼ 3:33ln X  0:655 Y ¼ 61:43X  12:3 ln Y ¼ 2:19ln X þ 0:786 Y ¼ 40:78X þ 12:2 ln Y ¼ 2:34ln X þ 0:728 Y ¼ 37:44X þ 18:53 ln Y ¼ 2:64ln X þ 1:11 Y ¼ 40:55X  2:66 ln Y ¼ 2:49ln X þ 0:847 Y ¼ 61:0X  28:9 ln Y ¼ 3:70ln X þ 0:58 Y ¼ 40:55X  2:66 ln Y ¼ 2:49ln X þ 0:847 Y ¼ 17:33X  0:4 ln Y ¼ 2:31ln X þ 0:166 Y ¼ 14:55X þ 7:69 ln Y ¼ 2:44ln X þ 1:342 Y ¼ 24:46X þ 11:4 ln Y ¼ 2:7ln X  0:241

DML BW DML BW DML BW DML BW DML BW DML BW DML BW DML BW DML BW DML BW DML BW DML BW DML BW DML BW

LRL LRL LRL DML LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL LRL

0.94 0.93 0.61 0.96 0.67 NA 0.72 0.82 NA NA NA NA NA NA 0.97 0.98 0.93 0.92 0.95 0.89 0.93 0.92 NA NA 0.97 0.98 NA 0.93

50 50 33 22 37 20 NA NA 17 20 72 74 39 38 NA NA NA NA NA NA NA NA 22 30 20 20 23 14

Kubodera (1986)

Berryteuthis anonychus Gonatus middendorffi Gonatus onyx Other Gonatidae spp. Taonius pavo Galiteuthis sp. Todarodes pacificus Watasenia scintillans Onychoteuthis sp. Abraliopsis sp. Octopoteuthidae sp. Histioteuthis sp. Chiroteuthis sp.

Kubodera and Shimazaki (1989) Kubodera (1986) Kubodera and Shimazaki (1989) Clarke (1986) obtained from Gonatus spp. Wolff (1984) Clarke (1986) obtained from Gonatus spp. Clarke (1986) obtained from Taoniinae spp. Clarke (1986) Wolff (1984) Wolff (1984) obtained from A. felis Wolff (1984) obtained from O.banksi Wolff (1984) obtained from A. felis Clarke (1986) obtained from Octopoteuthis sp. Wolff (1984) obtained from H. dofleini Clarke (1986) Clarke (1980)

BW=Body weight (g); DML=Dorsal mantle length (mm); LRL=Lower rostral length (mm); NA=Not available

Pacific (Q) was calculated from the equation X E  Ni  P i  106 ðtÞ; Q¼ e where Ni is the number of porpoises in population i; Pi is the migration period of population i in the North Pacific, and e is energy density of prey (kJ/ g). The value of each parameter is shown in Table 4. Generally, a mature Dall’s porpoise can weigh up to 200 kg (Houck and Jefferson, 1999), but we used 100 kg as a working value. This working value was based on the observed average body weight (104.6723.2 kg, n ¼ 106) obtained by two trans-Pacific survey cruises conducted in 1986 and 1987. The summer population of Dall’s porpoises in the North Pacific Ocean has been estimated at 1,186,000 individuals (or 991,000–1,420,000 animals with 95% confidence limits; Buckland et al.,

1994). From fall to early summer, truei-type Dall’s porpoises migrate to the northwestern North Pacific from the Sea of Okhotsk. This color-type population was estimated at 217,000 with a minimum CV of 0.23 (Miyashita, 1991). We therefore used the 95% confidence limit range of the North Pacific population and 217,000 for the truei-type to estimate consumption by Dall’s porpoises.

3. Results 3.1. Porpoises examined Males were more numerous than females in all areas except for BE. Body lengths tended to be

H. Ohizumi et al. / Deep-Sea Research I 50 (2003) 593–610

smaller in the eastern North Pacific, particularly among males (Table 5). 3.2. Prey variation by region We found more fishes than squids in the stomach contents from all four areas of the North Pacific; in contrast, squids were more numerous Table 3 Calorific densities of prey squids Species

Calorific References value (kJ/g)

Berryteuthis spp. Gonatopsis borealis Other Gonatidae spp. Cranchiidae spp. Todarodes pacificus Enoploteuthidae spp.

5.52 4.69 3.78

Perez (1994) Perez (1994) Clarke et al. (1985)a

1.69 4.01 4.60 5.40

Clarke et al. (1985)b Clarke et al. (1985)c Resources Council (1982)d Perez (1994)

3.08 2.65

Clarke et al. (1985) Clarke et al. (1985)

Onychoteuthis borealijaponica Octopoteuthis sp. Histioteuthis sp. a

than fishes in the BE area. In total, we found 39 types of fish otoliths, including 24 identified species (Table 6). Bathylagus ochotensis, an unidentified bathylagid fish, Scopelosaurus harryi, Protomyctophum thompsoni, Diaphus theta, Stenobrachius leucopsarus, Stenobrachius nannochir, and Lampanyctus jordani were collected in samples from all areas. Myctophid fishes were the major group of prey in all areas, accounting for an average of 87% of the total number of fishes, and 78% of all prey items among the North Pacific areas. Although the numerical contribution varied by location, D. theta was a fundamental component in all areas except BE. Notoscopelus japonicus was Table 4 Parameters for estimation of prey consumption Parameters

Value

W N (dalli -type populations)

100 kg 991000–1420000 Buckland et al. (1994) 217000 Miyashita (1991) 365 d 180 d 8.4 kJ Childress and Nygaard (1973), for D. theta

N (truei -type population)

Gonatus sp. in Clarke et al. (1985). Teuthowenia sp. in Clarke et al. (1985). c Todarodes sagittatus in Clarke et al. (1985). d Watasenia scintillans in Resouces Council (1982). b

597

P (dalli -type populations) P (truei -type population) e

Reference

Fig. 1. Sampling locations of Dall’s porpoises during 1984–1989 in the northern North Pacific and Bering Sea. Enclosures show areas for analyses. Hatched area is the distributional range of Dall’s porpoises.

H. Ohizumi et al. / Deep-Sea Research I 50 (2003) 593–610 196 2.46 8 162 2.73 14 174 4.03 22 183 1.61 21 160 2.23 16 173 2.31 37 176 1.44 12 172 1.84 17 174 1.27 29 176 2.92 8 164 2.81 8 170 2.44 16 182 0.88 3 169 2.19 3 176 2.95 6 171 — 1 170 3.50 2 170 2.08 3 One mature male was excluded because its body length was not recorded. One individual was excluded because it has no biological data. b

183 1.98 14 172 1.47 24 176 1.48 38

191 1.04 75 178 1.21 38 187 1.00 113 a

Male

200 1.64 36 185 2.78 16 195 1.67 52

Female Male Male Female Male Female Female Female

Male

Western Central Pacifica Western Pacific

Average body length (cm) 182 Standard error 4.40 No. of animals 4 Sexually immature animals Average body length (cm) 168 Standard error 3.75 No. of animals 4 Total Average body length (cm) 175 Standard error 3.73 No. of animals 8 Sexually mature animals

Table 5 Body lengths of Dall’s porpoises from five study areas

Eastern Central Pacific

Eastern Pacific

Bering Seab

598

the main prey in WP. B. ochotensis, Icthyococcus sp., and Ceratoscopelus warmingi were the main prey in WCP. S. leucopsarus was one of the main prey items in EP, and it was the most dominant fish found in BE samples. P. thompsoni was a main prey fish in WP, ECP, and EP (Table 6). We found a total of 24 types of squids, including 13 identified species (Table 6). Gonatopsis borealis, Berryteuthis anonychus, Gonatus onyx, Gonatus berryi, Gonatidae spp. (mainly juvenile Gonatus), Galiteuthis sp., and Taonius pavo were found in all areas. Gonatid squids comprised 82% of the total number of squids in average of all areas. Several squid genera and families were found in the North Pacific areas, while only gonatid and cranchiid squids were found in BE. Numerically, G. onyx was important, but it contributed relatively little in biomass because of its smaller size (Table 7). In contrast, G. borealis was quantitatively the main prey squid in all of the areas. Onychoteuthis borealijaponica was also one of the main squids in the eastern part of the North Pacific. Latitudinal variations in prey species composition and numerical contribution were observed in WCP (Fig. 2A and B). In fishes, B. ochotensis and subtropical myctophids such as Electrona rissoi and C. warmingi contributed numerically more between 36
N and 38
N, with their contribution reducing gradually at higher latitudes. In contrast, the contributions of subarctic myctophids, such as D. theta, L. jordani, P. thompsoni, S. leucopsarus, and Symbolophorus californiensis, increased (Fig. 2A). In squids, the numerical contribution of G. borealis was large between 36
N and 40
N, measuring up to about 50%, but it decreased to about 15% between 40
N and 42
N (Fig. 2B). North of 42
N, a relatively larger contribution of G. borealis was again observed. Abraliopsis sp., which is found throughout the transitional zone, and other enoploteuthid squids contributed relatively more between 38
N and 42
N. G. onyx and B. anonychus, which are subarctic species, tended to increase at higher latitudes, especially above 40
N. 3.3. Prey variation by sex and maturity In BE, numerical prey contributions by sex and maturity were significantly different (Chi-square

H. Ohizumi et al. / Deep-Sea Research I 50 (2003) 593–610

599

Table 6 Prey compositions in the stomach contents of Dall’s porpoises Prey species

Fishes Engraulidae Clupeidae Scomberesocidae Scombridae Myctophidae

Engraulis japonica Sardinops melanostictus Cololabis saira Scomber sp. Protomyctophum thompsoni Electrona rissoi Symbolophorus californiensis Diaphus theta Diaphus gigas Diaphus sp. Diaphus? (Type 7) Stenobrachius leucopsarus Stenobrachius nannochir Stenobrachius sp. Lampanyctus regalis Lampanyctus jordani Lampanyctus? (Type 2) Ceratoscopelus warmingi Notoscopelus japonicus Notoscopelus resplendens Tarletonbeania taylori Tarletonbeania taylori small Lampadena urophaos Lampadena luminosa Lampadena sp. Myctophidae? (Type 17) Myctophidae? (Type 18)

Bathylagidae

Phosichthydae Notosudidae Argentinidae Paralepididae

Sternoptychidae Gadidae Unidentified Squids Gonatidae

Leuroglossus schmidti Bathylagus ochotensis Bathylagidae sp. Icthyococcus sp. Scopelosaurus harryi Argentinidae sp. Paralepis atlantica Lestidiops sphyraenopsis? Lestidiops sp. (Type 10) Maurolicus muelleri Theragra chalcogramma

Gonatopsis beorealis Berryteuthis magister Berryteuthis anonychus Gonatus onyx Gonatus middendorffi Gonatus madokai

Percent of prey number

Percent of occurrence

WP

WCP

ECP

EP

BE

91.9 o0.1 0.5 — 0.4 84.4 12.4 — 3.4 19.8 0.4 — 0.4 3.4 0.1 — — 12.9 0.2 3.3 27.9 — 0.1 — — o0.1 0.1 — 0.1 1.6 0.7 0.9 0.1 0.3 0.1 0.1 0.3 0.1 — 0.2 o0.1 4.0 0.3 8.1 6.6 1.0 — 0.6 2.9 0.3 —

94.7 o0.1 2.6 o0.1 o0.1 62.2 2.8 7.6 1.4 22.9 2.5 — 1.3 1.7 0.3 0.1 0.4 5.4 0.4 10.2 4.5 o0.1 0.1 — 0.1 o0.1 0.3 o0.1 o0.1 14.9 o0.1 14.8 o0.1 14.1 0.1 o0.1 0.1 o0.1 o0.1 o0.1 o0.1 — 0.7 5.3 4.4 1.5 o0.1 1.3 0.7 o0.1 —

85.0 — — 0.8 — 79.1 42.6 — 0.7 28.3 — — — 2.8 0.6 — — 1.8 1.7 — 0.1 — 0.2 — 0.1 0.1 0.1 — — 1.9 — 0.1 1.9 0.3 0.1 o0.1 0.2 — 0.1 0.1 — — 2.5 15.0 11.1 7.1 — 2.7 0.7 — —

91.6 — — 0.1 — 87.5 11.5 — 2.3 46.7 — o0.1 — 18.8 0.3 — 0.2 0.3 1.2 0.1 o0.1 — 4.4 1.6 0.1 o0.1 o0.1 o0.1 — 1.3 0.1 0.1 1.1 0.1 0.1 0.7 0.2 — 0.1 0.1 — — 1.7 8.4 6.4 3.3 o0.1 1.1 1.1 o0.1 o0.1

30.9 — — — — 27.6 2.1 — — o0.1 — — — 22.7 2.8 — o0.1 o0.1 — — — — — — — — — — — 1.9 1.4 0.1 0.4 — o0.1 — o0.1 — — — — 1.3 0.1 69.1 65.4 20.2 0.2 17.3 18.5 1.3 0.2

WP

WCP ECP

EP

BE

1.8 5.3 — 5.3

2.0 26.3 1.3 1.3

— — 62.5 —

— — 4.4 —

— — — —

63.2 — 40.4 57.9 17.5 — 12.3 29.3 3.5 — — 33.3 5.3 40.4 75.4 — 5.3 — — 1.8 3.5 — 1.8

54.6 33.6 32.2 52.0 38.8 — 26.3 35.5 12.5 3.3 12.5 42.1 17.1 53.3 38.8 3.3 7.9 — 3.9 2.6 15.1 4.6 2.0

75.0 — 37.5 100.0 — — — 62.5 37.5 — — 37.5 12.5 — 12.5 — 25.0 — 12.5 12.5 12.5 — —

62.2 — 15.6 51.1 — 2.2 — 51.1 17.8 — 6.7 13.3 4.4 6.7 6.7 — 20.0 8.9 11.1 2.2 2.2 2.2 —

44.1 — — 1.7 — — — 74.6 27.1 — 1.7 3.4 — — — — — — — — — — —

3.5 12.3 1.8 14.0 5.3 3.5

0.7 33.6 3.3 66.4 13.8 0.7

— 12.5 37.5 37.5 12.5 12.5

4.4 6.7 13.3 6.7 6.7 15.6

39.0 6.8 10.2 — 6.8 —

5.3 — 8.8 1.8 3.5 17.5

2.0 1.3 5.9 3.3 — 34.9

— 12.5 12.5 — — 25.0

— 8.9 15.6 — — 11.1

— — — — 16.9 6.8

36.8 — 3.5 50.9 3.5 —

52.0 0.7 15.1 23.7 2.0 —

87.5 — 50.0 37.5 — —

68.9 2.2 15.6 62.2 2.2 2.2

84.7 5.1 66.1 74.6 37.3 10.2

H. Ohizumi et al. / Deep-Sea Research I 50 (2003) 593–610

600 Table 6 (continued) Prey species

Percent of prey number WP

Gonatus berryi Gonatus pyros Eogonatus tinro? Gonatidae spp.

0.6 0.7 o0.1 0.5 Onychoteuthidae 0.1 Onychoteuthis borealijaponica 0.1 Onychoteuthidae? (Un. 9) — Enoploteuthidae 0.7 Enoploteutis chuni 0.1 Watasenia scintillans 0.3 Abraliopsis felis? 0.3 Enoploteuthis sp. 0.1 Cranchiidae 0.4 Galiteuthis sp. 0.2 Taonius pavo 0.2 Cranchiidae sp. — Ommastrephidae Todarodes pacificus o0.1 Octopoteuthidae Octopoteuthis sp. — Histioteuthidae Histioteuthis sp. o0.1 Chiroteuthidae Chiroteuthis sp. 0.1 Unidentified 0.1 Absolute number in total

Percent of occurrence

WCP

ECP

EP

BE

WP

WCP ECP

EP

BE

0.2 0.2 o0.1 0.3 0.1 0.1 — 0.6 o0.1 — 0.4 0.2 0.1 o0.1 0.1 o0.1 — o0.1 o0.1 o0.1 o0.1

0.1 — — 0.6 1.4 1.3 0.1 1.2 — — 1.2 — 1.1 0.2 0.9 — — 0.2 — — —

0.1 o0.1 — 0.7 1.1 1.0 o0.1 0.1 — — 0.1 o0.1 0.8 0.1 0.7 o0.1 — 0.1 — — —

1.8 1.4 1.3 3.2 o0.1 — — — — — — — 1.8 1.1 0.7 o0.1 — — — — 1.9

14.0 8.8 1.8 14.0

15.1 6.6 0.7 9.9

12.5 — — 37.5

17.8 2.2 — 33.3

32.2 32.2 42.4 61.0

7.0 —

11.2 —

62.5 12.5

42.2 — 6.7 —

1.8 12.3 10.5 5.3

3.3 — 15.1 5.3

— — 37.5 —

— — 4.4 2.2

— — — —

12.3 8.8 — 1.8 — 1.8 3.5 1.8

6.6 6.6 2.0 — 2.0 2.0 4.6 4.6

12.5 25.0 — — 25.0 — — —

22.2 40.0 2.2 — 6.7 — — —

33.9 20.3 1.7 — — — — 11.9

8

45

59

3,942.5 2,7738.5 1,670.5 9,933 5,796 57 152 prey porpoises

Numbers in italics are subtotals of specific taxa.

test, po0:0001; df ¼ 15). S. leucopsarus and other fishes contributed to about half of the prey items consumed by mature porpoises, while in immature porpoises, squids were dominant (Fig. 3; Chisquare test; po0:0001; df ¼ 5). Of the mature porpoises, females fed on fewer S. leucopsarus than males, while immature females fed on fewer B. anonychus than immature males (Fig. 4). In contrast to the striking difference in prey contribution with sexual maturity, the differences in prey contributions between sexes were less distinct, but remained statistically significant (Chi-square test, po0:0001; df ¼ 5 for both mature and immature porpoises).

stomachs. The maximum relative weight of stomach contents was measured 0–1 h after sunrise, while the minimum was recorded 7–8 h after sunrise (Fig. 5). Many half-digested epipelagic fishes such as Sardinops melanostictus were found 9–10 h after sunrise, while many half-digested mesopelagic myctophids were found 2 h after sunrise (Fig. 6). Most of the half-digested squids were also found at dawn, except for one porpoise that ate 125 B. anonychus around 12 h after sunrise.

3.4. Time of foraging

The daily energy requirement (E) was estimated to be about 41,840 kJ for a 100-kg porpoise. This is equivalent to about 5 kg or 5% of its body weight, assuming that the prey energy density is 8.4 kJ/g. The annual consumption by Dall’s porpoises in

Many of the porpoises taken at dawn and dusk had full or partly full stomachs, while many of those taken in daylight hours had almost empty

3.5. Annual consumption by Dall’s porpoises in the North Pacific

Gonatopsis borealis Berryteuthis magister Berryteuthis anonychus Gonatus onyx Gonatus middendorffi Gonatus madokai Gonatus berryi Gonaus pyros Eogonatus tinro? Gonatidae spp.

NA: Not available. Numbers in italics are subtotals of specific taxa.

Absolute in total

74.1 47.4 — 17.9 4.4 — — 0.4 — — 4.0 9.2 8.8 0.4 8.0 — — 8.0 — 7.6 1.6 6.0 — — 1.2 — — —

318 1,462 251 squids

82.6 27.6 0.1 25.5 13.9 0.5 — 4.2 4.4 0.7 5.8 2.7 2.7 — 11.1 0.4 — 7.3 3.4 2.1 0.7 1.2 0.2 — 0.2 0.2 0.6 0.5

94.6 29.2 0.2 25.0 26.8 1.9 0.2 2.6 2.0 1.9 4.7 — — — — — — — — 2.6 1.6 0.9 o0.1 — — — — 2.8

89.6 32.7 — 10.6 12.4 6.0 — 12.5 14.3 0.2 0.8 6.0 6.0 — 0.6 NA 0.3 0.3 NA 3.0 1.4 1.7 — 0.6 — 0.3 NA NA

838 4,005 8,486 g

75.5 38.9 0.1 13.6 12.8 0.1 0.1 1.2 0.1 — 8.6 12.8 12.4 0.4 0.7 — — 0.6 0.1 9.8 1.7 8.0 0.1 — 1.2 — — — 82,671

89.1 73.5 o0.1 3.7 3.4 0.2 — 3.1 3.7 0.6 0.9 8.9 8.9 — 0.6 NA — 0.4 0.2 1.0 0.3 0.6 0.1 — 0.1 0.1 0.2 NA

WCP

7,301

73.5 53.6 — 18.8 0.9 — — 0.1 — — 0.1 21.8 21.8 NA 0.5 — — 0.5 — 2.4 0.2 2.2 — — 1.8 — — —

ECP

48,887

68.7 52.6 0.3 8.8 4.3 o0.1 o0.1 1.4 0.1 — 1.2 28.1 28.1 NA o0.1 — — o0.1 NA 1.5 0.3 1.3 NA — 1.6 — — —

EP

95,296

97.7 48.7 0.9 23.2 13.8 1.8 0.4 2.8 3.1 2.5 0.7 — — — — — — — — 2.3 1.8 0.4 o0.1 — — — — NA

BE

WP

BE

WP

WCP ECP EP

Percent of reconstitued squid weight

Percent of squid number

82.4 12.9 — 7.5 36.2 4.1 — 7.5 8.2 0.3 5.7 Onychoteuthidae 1.6 Onychoteuthis borealijaponica 1.6 Onychoteuthidae? (Un.9) — Enoploteuthidae 8.8 Enoploteuthis chuni 0.9 Watasenia scintillans 3.1 Abraliopsis felis? (Un.9) 3.5 Enoploteuthis sp. 1.3 Cranchiidae 5.0 Galiteuthis sp. 2.5 Taonius pavo 2.5 Cranchiidae sp. — Ommastrephidae Todarodes pacificus 0.3 Octopoteuthidae Octopoteuthis sp. — Histioteuthidae Histioteuthis sp. 0.3 Chiroteuthidae Chiroteuthis sp. 0.6 Unidentified Unknown spp. 0.9

Gonatide

Prey squid spcies

Table 7 Number, weight and calory contributions of squids

88.5 74.4 o0.1 4.4 2.7 0.1 — 2.5 3.0 0.5 0.7 10.4 10.4 — 0.6 NA — 0.4 0.2 0.4 0.1 0.2 o0.1 — 0.1 o0.1 NA NA

WCP 73.5 51.4 — 21.2 0.7 — — o0.1 — — 0.1 24.1 24.1 NA 0.5 — — 0.5 — 0.8 0.1 0.8 — — 1.2 — NA —

ECP 67.0 51.1 0.3 10.0 3.4 o0.1 o0.1 1.1 0.1 — 0.9 31.5 31.5 NA o0.1 — — o0.1 NA 0.5 0.1 0.4 NA — 1.0 — NA —

EP

99.2 49.7 1.1 27.9 11.3 1.5 0.3 2.3 2.5 2.0 0.5 — — — — — — — — 0.8 0.7 0.2 o0.1 — — — NA NA

BE

36,480 kJ 382,911 35,689 236,066 437,849

90.0 35.7 — 13.6 10.9 5.3 — 11.0 12.6 0.2 0.7 7.5 7.5 — 0.6 NA 0.3 0.3 NA 1.2 0.5 0.7 — 0.6 — 0.2 NA NA

WP

Percent of estimated squid calory

H. Ohizumi et al. / Deep-Sea Research I 50 (2003) 593–610 601

602

H. Ohizumi et al. / Deep-Sea Research I 50 (2003) 593–610

Fig. 2. Latitudinal variation of prey items of Dall’s porpoise in the Western Central Pacific area (160
E-180
). n=number of fishes; N=number of porpoises.

Fig. 3. Variation of numerical prey contribution between mature and immature Dall’s porpoises in the Bering Sea.

H. Ohizumi et al. / Deep-Sea Research I 50 (2003) 593–610

603

Stomach content weight / Body weight (%)

Fig. 4. Variation of numerical prey contribution among sex and maturity in the Bering Sea.

1.8 1.6 1.4

Bering Sea

1.2

Eastern Central Pacific

1.0

Eastern Pacific

0.8

Western Central Pacific

0.6

Western Pacific

0.4 0.2 0 0

2

4

6

8

10

12

14

Hours after sunrise

Fig. 5. Relative stomach content weight at time of porpoise catch.

the North Pacific (Q) is estimated to be about 2.0– 2.8 million tons.

4. Discussion 4.1. Geographical variation in prey and selection of prey Previous studies have revealed that Dall’s porpoises in the Pacific coastal areas of North America feed mainly on epipelagic and benthopelagic fishes such as jack mackerel, rock fishes, anchovies and herring, as well as several species of squids (Cowan, 1944; Scheffer, 1953; Brown and

Norris, 1956; Norris and Prescott, 1961; Fiscus and Niggol, 1965; Loeb, 1972; Morejohn, 1979; Stroud et al., 1980; Jones, 1981). In the southern Sea of Okhotsk, S. melanostictus is a main prey item for Dall’s porpoises (Walker, 1996; Ohizumi et al., 2000). When S. melanostictus is not available, benthic prey items such as Berryteuthis magister and Theragra chalcogramma are important in the Sea of Okhotsk and in the Sea of Japan, respectively (Ohizumi et al., 2000). All of these observations were made in areas on continental shelves. In pelagic waters, Dall’s porpoises feed mainly on gonatid squids and myctophid fishes in the western North Pacific around the Aleutian Islands

H. Ohizumi et al. / Deep-Sea Research I 50 (2003) 593–610

604

125

Squids

50 45 40 35 30 25 20 15 Number of prey

10 5 0 50 Epipelagic fishes Myctophid fishes

45 40 35 30 25 20 15 10 5 0 0

2

4

6

8

10

12

14

16

Hours after sunrise Fig. 6. Numbers of fresh prey at time of catch.

and the Bering Sea (Mizue and Yoshida, 1965; Mizue et al., 1966; Crawford, 1981; Kuramochi et al., 1993; Fiscus and Jones, 1999). Dall’s porpoises in the western North Pacific off Japan also feed mainly on myctophid fishes and squids (Wilke et al., 1953; Wilke and Nicholson, 1958). Walker (1996) noted that prey items of Dall’s porpoises off the American coast and the southern Sea of Okhotsk reflected the fauna on the continental shelf, while those in the western North Pacific reflected the oceanic fauna. Our results

suggest that mesopelagic prey is common, not only in the western North Pacific but also in all pelagic areas of Dall’s porpoises distribution. Latitudinal prey variations in the WCP area corresponded to variations in the fauna with environmental changes from cold to warm water. The boundary of warm- to cold-water prey species was around 40
N, near the subarctic boundary. G. borealis was an exception. Although G. borealis is a cold-water species, a larger contribution of this squid was observed between 36
and 40
N. However, this includes an obvious bias from fewer squids being ingested in the southern areas. On average, porpoises ate about six G. borealis in areas between 42
N and 50
N, but only two between 36
N and 38
N. In the northern North Pacific and the Bering Sea, D. theta, S. leucopsarus, and P. thompsoni were the most abundant mesopelagic fishes collected by Isaacs-Kidd midwater trawl nets from depths shallower than 500 m (Willis et al., 1988). These species were also abundant in the stomach contents examined during this study. N. japonicus is distributed mainly in the transition zone of the western North Pacific, and this myctophid was the principal prey in WP, suggesting that Dall’s porpoises fed primarily on the myctophids that are abundant in the area. This conclusion is supported by the geographical variation observed in prey species. However, this does not necessarily suggest that Dall’s porpoises do not actively select prey. Ohizumi et al., (2000) noted that Dall’s porpoises in coastal areas off Hokkaido, Japan, preferred prey items from the epipelagic layer, such as S. melanostictus, because S. melanostictus has a high energy density, and foraging on S. melanostictus in the surface layer is energetically efficient. Benthic and low-energy prey, such as T. chalcogramma, are taken when epipelagic prey are unavailable (Ohizumi et al., 2000). Our study suggests that Dall’s porpoises prefer myctophids, and myctophids are similar to S. melanostictus in having a high energy density (8.9 kJ/g for S. melanostictus (Resources Council, 1982); 8.4 kJ/g for D. theta (Childress and Nygaard, 1973). Furthermore, Dall’s porpoises feed on myctophids at night when these fish migrate into the surface waters. In WCP and ECP, Dall’s porpoises may

H. Ohizumi et al. / Deep-Sea Research I 50 (2003) 593–610

also feed more on epipelagic fishes such as S. melanostictus and Cololabis saira, but contrary to our expectations, their contributions were low. Opportunities to feed on these species are not necessarily rare, because a higher percent of occurrences suggests that many porpoises feed on S. melanostictus and C. saira in WCP and ECP, respectively (Table 6). This suggests that while Dall’s porpoises primarily select for myctophids, they feed opportunistically on S. melanostictus and C. saira. Both S. melanostictus and C. saira are shoaling fishes and their distributions are patchy, whereas myctophids are more evenly distributed in pelagic waters. It is likely that selecting myctophids is advantageous in terms of efficient foraging and high energy gains. 4.2. Time and depth of foraging Most mesopelagic micronekton, including the prey of Dall’s porpoises, engage in diel vertical migration. They inhabit deep layers in the daylight hours and ascend to shallow layers at night (Roper and Young, 1975; Pearcy et al., 1977; Watanabe et al., 1999). Our analysis of stomach content weight and fresh prey relative to the time of catch suggest that Dall’s porpoises feed at dawn and dusk, and probably throughout the night. This feeding period corresponds to the time when mesopelagic fishes and squids migrate into the surface layer and implies that the main foraging depth of Dall’s porpoises is quite shallow. This agrees with conclusions derived from an investigation of the stomach contents of Dall’s porpoises in the coastal waters around Hokkaido, Japan (Ohizumi et al., 2000). However, P. thompsoni, which was one of the numerically important prey species, inhabits depths of 300–400 m throughout the day (Watanabe et al., 1999). Therefore, Dall’s porpoises may dive this deep to forage. S. nannochir, which is the most abundant mesopelagic fish inhabiting layers deeper than 500 m (Willis et al., 1988; Watanabe et al., 1999), and Lampanyctus regalis, which also inhabits deeper layers (Willis et al., 1988; Watanabe et al., 1999), were minor species in the stomach contents of Dall’s porpoises. This suggests that Dall’s porpoises do not usually feed at depths below 500 m.

605

Walker (1996) reported daytime feeding on epipelagic S. melanostictus by Dall’s porpoises in the southern Sea of Okhotsk. Amano et al. (1998) also observed active daytime feeding on S. melanostictus in the southern Sea of Okhotsk. However, Amano et al. (1998) observed active feeding of Dall’s porpoises during morning hours in the North Pacific. The observational data presented by Amano et al. (1998) in the North Pacific were obtained from the same research cruise from which some of our stomach samples were collected. The principal prey items in these samples were myctophids. The observations of feeding behavior reported by Amano et al. (1998), stomach content analyses by Walker (1996), and our data suggest that Dall’s porpoises change their time of feeding based on preferred prey availability. 4.3. Variations in prey by sex and maturity In the Bering Sea, prey composition significantly differed by sex and maturity, although the difference between sexes was not remarkable. Sexually mature males and females fed mostly on S. leucopsarus, while the immatures ones fed mainly on gonatid squids. Although this difference was statistically significant, there is little information available to explain it. S. leucopsarus shows bimodal vertical distribution in 20–200 m, and 400–700 m at night (Watanabe et al., 1999), and immature Dall’s porpoises may lack sufficient diving abilities to forage this deep; it may simply be easier to feed on G. borealis, which is found less than 5 m below the surface at night (Naito et al., 1977a, b; Kubodera et al., 1983). In contrast, to mature porpoises the high energy density of S. leucopsarus (7.4 kJ/g; Childress and Nygaard, 1973) may be more attractive than squids, which have a lower energy value (Table 3). However, these discussions are speculative, because the distribution depths of prey species in the Bering Sea are still not fully known, and the correlation between the diving ability and maturity of Dall’s porpoises is also unknown. In contrast to our study, Crawford (1981) found no significant differences in stomach content volume and prey type among sex, maturity, and

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reproductive status in the Bering Sea. Crawford (1981) pointed out that this was due to small sample sizes, conclusion. Variation in prey based on sex and maturity was not examined in areas other than the Bering Sea, because of the low variation in reproductive status in any confined geographical area. 4.4. Seasonality of predation Many species of fish and squid migrate seasonally to the transition zone in the North Pacific. Therefore, available prey fauna may vary by season; our data support only summer feeding habits. Winter feeding habits of Dall’s porpoises are almost unknown, as is their winter distribution in the pelagic North Pacific. One exception is the truei-type Dall’s porpoise in the western North Pacific. This type feeds mainly on Notoscopelus sp. as well as other myctophid fishes and squids in early spring (Wilke and Nicholson, 1958). One of the authors (HO) observed N. japonicus and D. theta in the stomach contents of truei-type Dall’s porpoises collected in midwinter (Ohizumi, unpublished data). These observations and our study, in which we found N. japonicus in stomach contents collected in May, suggest that prey items of truei-type Dall’s porpoises in the western North Pacific do not vary much from winter to early summer. 4.5. Food consumption by Dall’s porpoises We estimated the daily food requirement of a porpoise to be about 5% of its body weight. The feeding rate, defined as the ratio of daily food intake to body weight, is estimated to be about 5% for Dall’s porpoises in the Sea of Japan (Ohizumi and Miyazaki, 1998). These authors calculated the digestion time of stomach contents (from full to empty) to be about 8 h, and in our study, we also observed a complete digestion time of approximately 8 h. These independent studies resulted in almost identical conclusions regarding the digestion time and the feeding rate. Gj^saeter and Kawaguchi (1980) estimated that 43 million tons of mesopelagic fishes, comprising mainly myctophids, inhabit the transition to

subarctic regions of the North Pacific. We estimated that Dall’s porpoises in the North Pacific consume about 2.0–2.79 million tons of food annually. Accordingly, this suggests that Dall’s porpoises consume about 4.7–6.5% of the mesopelagic micronekton biomass, especially myctophids. Except for our study, no data are available on how many mesopelagic fishes are consumed by a specific predator in the subarctic North Pacific. It is difficult to evaluate whether a biomass consumption of 4.7–6.5% is large or small. Sutton and Hopkins (1996) estimated that 89% of the myctophid standing stock is removed annually by mesopelagic predatory fishes (Stomiidae) in the eastern Gulf of Mexico. This is very different from our results. However, these authors surveyed a tropical area, and the life span of myctophids in tropical areas is usually only 1 yr, or no more than 2 yr (Prut’ko, 1988; Gartner, 1991; Giragosov and Ovcharov, 1992). This implies a high mortality within 1 yr. In contrast, most of the subarctic myctophids have a longer life span than tropical species (Halliday, 1970; Gj^saeter, 1973; Kawaguchi and Mauchline, 1982; Linkovski, 1985; Greenly et al., 1999). For example, D. theta can live for up to 6 yr (Ivanov and Lapko, 1994), and S. leucopsarus can live for up to 8 yr (Smoker and Pearcy, 1970; Nishimura et al., 1999); both of these are important prey of Dall’s porpoises. Assuming that the average life span of subarctic species is 5 yr (Gj^saeter and Kawaguchi, 1980), 20% of the standing stock turns over annually, if the biomass is stable. On this basis, we estimate that Dall’s porpoises are responsible for about 24–33% of the overall mortality. 4.6. Competition for myctophids with other predators Although there is relatively limited information about the diet of higher trophic level predators in the subarctic North Pacific, myctophid fishes are common prey items for predators other than Dall’s porpoises (Springer et al., 1999), although they are not always important prey in the subarctic North Pacific. Northern fur seals (Callorhinus ursinus) feed mainly on gonatid

H. Ohizumi et al. / Deep-Sea Research I 50 (2003) 593–610

squids, onychoteuthid squids, Engraulis mordax, and Merluccius productus in offshore regions from the Gulf of Alaska to southern California (Perez and Bigg, 1986). Myctophids were one of their most important prey items only off the coast of northern California, where they constituted about 25% by volume of stomach contents, second only to onychoteuthid squids, at about 40% (Perez and Bigg, 1986). Northern fur seals in the western North Pacific feed on myctophid fishes as one of their main prey items, but squids and Laemonema longipes are also important (Wada, 1971). Walker and Jones (1994) found many myctophid otoliths in the stomach contents of northern fur seals incidentally collected by squid gill nets in the eastern transition zone of the North Pacific. However, they pointed out that some of these otoliths may have been introduced secondarily from the stomachs of squids that were caught by the gill nets and eaten by the seals. In the pelagic eastern Bering Sea, Leuroglossus schmidti and gonatid squids are the main prey of northern fur seals (Perez and Bigg, 1986; Sinclair et al., 1994). Minke whales (Balenoptera acutorostrata) in the western North Pacific forage on epipelagic shoaling fishes such as Engraulis japonica and C. saira, but they take almost no mesopelagic prey (Tamura et al., 1998). In the western subarctic North Pacific, short-tailed shearwaters (Puffinus tenuirostris) catch myctophids as well as Pleurogrammus spp. However, other marine birds, including albatrosses, fulmars, shearwaters, storm petrels, and murres, eat primarily epipelagic fishes, zooplankton, and squids, in the transition to the subarctic North Pacific and in the Bering Sea (Springer et al., 1999). Crawford (1981) found that salmonid fishes in the western North Pacific and Bering Sea feed mainly on zooplankton, and concluded that salmonids do not significantly compete with Dall’s porpoises for food. Salmon shark (Lamna ditropis) is a major predator, but myctophids are minor prey for this species (Sano, 1960; Sano, 1962). Blue shark (Prionace glauca) o65 cm in precaudal length feed mainly on myctophids and gonatids, but larger individuals prefer large ommastrephid squids (Seki, 1994). Pomfret (Brama japonica) consume epipelagic fishes, crustaceans, and squids, while mesopelagic

607

fishes are only minor prey items (Kubodera and Shimazaki, 1989; Wada and Honda, 1992; Seki and Bigelow, 1994). In the Bering Sea, walleye pollock (T. chalcogramma) inhabits not only the sea floor but also pelagic waters. Important foods for this species are copepods and euphausiids, while myctophids are regionally and seasonally important (Yoshida, 1994). Neon flying squid (Ommastrephes bartrami) is an important consumer of myctophids and gonatids in the transition zone during the summer (Naito et al., 1977b; Seki, 1994). However, they originate from tropical zones, and the overlap in distribution with that of Dall’s porpoise is rather limited. Other larger squids in the transition zone and the subarctic North Pacific, such as the boreal clubhook squid (O. borealijaponica) and the boreopacific gonate squid (G. borealis), feed on smaller fishes (Naito et al., 1977b). They can potentially eat myctophids, but studies of their diets are very limited. Except for G. borealis, these squids migrate in transition and subarctic waters only in summer (Naito et al., 1977a). This review of the diet of some of the common subarctic predators suggests that while myctophids are commonly occurring prey items, they are not of primary importance. Dall’s porpoises are on the other hand, primary consumers of myctophids in these regions. Mesopelagic micronekton, especially myctophids, are the major component in the middle trophic level. The trophic relationship between myctophids and Dall’s porpoise appears to be a major mass and energy pathway from the mesopelagic layer to the surface in the pelagic regions of the subarctic North Pacific.

Acknowledgements We would like to thank the observers, researchers, and vessel crews who helped us in the field study. We also thank Prof. K. Kawaguchi, Dr. H. Watanabe, Dr. K. Uchikawa, and Dr. M. Moku for providing information on the prey species. Dr. T. Jefferson gave us comments to improve the manuscript. This study was partly supported by the Fisheries Agency of Japan.

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