Feeding Habits of Albacore Thunnus Alalunga in the Transition Region of the Central North Paciﬁc

Home , Gonatopsis borealis, Phronima sedentaria

Blackwell Science, LtdOxford, UK FISFisheries Science0919-92682004 Blackwell Science Asia Pty Ltd 704August 2004 843 Feeding habits of albacore H Watanabe et al. 10.1111/j.1444-2906.2004.00843.x Original Article573579BEES SGML

FISHERIES SCIENCE 2004; 70: 573–579

Feeding habits of albacore Thunnus alalunga in the transition region of the central North Paciﬁc

Hikaru WATANABE,1* Tsunemi KUBODERA,2 Suguru MASUDA3 AND Shigeyuki KAWAHARA1

1National Research Institute of Far Seas Fisheries, Shimizu-Orido, Shizuoka 424-8633, 2National Science Museum, Shinjyuku, Tokyo 169-0073 and 3Shizuoka Prefectural Fisheries Experiment Station, Yaizu, Shizuoka 425-0033, Japan

ABSTRACT: The feeding habits of albacore Thunnus alalunga (fork length: 48.9Ð76.2 cm, n = 132) were examined from late spring to early autumn in relation to its northward migration in the transition region between the subtropical and subarctic fronts in the central North Paciﬁc. Samples were collected at night using surface gill nets or during daytime pole-and-line surveys in 2001 and 2002. During May and June, albacore fed mainly on Japanese anchovy Engraulis japonicus, which accounted for 27.2%, 67.0%, and 45.5% of the total stomach contents by number (Cn), wet weight (WW), and frequency of occurrence (F), respectively, and secondarily on the subarctic gonatid squid Gonatopsis borealis (Cn, 15.8%; WW, 10.8%; F, 28.8%). From July to September, albacore continued to depend on Japanese anchovy (Cn, 48.2Ð52.8%; WW, 79.9Ð95.2%; F, 27.8Ð85.4%). These results corresponded well with the remarkable rebound of the Japanese anchovy stock since the 1990s. Gonatopsis borealis, the main squid prey from May to June, almost disappeared from the stomachs of albacore from July to September, probably due to the northward migration of this squid to subarctic waters in summer. The feeding impact of albacore on the Japanese anchovy stock in the transition region was conservatively estimated to be from 1400 to 2100 tons per day from late spring to early autumn.

KEY WORDS: albacore Thunnus alalunga, central North Paciﬁc, feeding habits, feeding impact on Japanese anchovy, transition region.

INTRODUCTION pods were important prey in the offshore transition region.1,2,13 Off Oregon, albacore feed largely on Albacore Thunnus alalunga is widely distributed Pacific saury in the coastal water area, whereas between 10∞N and 50∞N in the North Pacific.1 Pre- squid consituted the majority of the stomach con- spawning albacore, 2–5 years old, undertake a tents in the offshore area.3 Comparisons to data of northward feeding migration from the subtropical the prey composition in the stomachs and envi- waters to the transition region, which is located ronmental condition among the different areas between the subtropical and subarctic fronts, suggest that albacore are generally opportunistic during spring and summer.1–7 predators, similar to other scombrid fish spe- Considerable knowledge of the feeding habits of cies.1,8,11,12 In the central and western North Pacific, this species in the transition region of the eastern stomach content analyses of albacore also have North Pacific has been accumulated; they feed been conducted,14–18 but quantitative data on their mainly on small pelagic fish such as Pacific saury diets are extremely limited. This knowledge is Cololabis saira and northern anchovy Engraulis essential for understanding the food web struc- mordax, and crustacean zooplankton such as tures in these regions, because albacore are euphausiids and sergestid shrimps in the Califor- thought to play an important role in the fourth nia Current System.8–12 Pacific saury, gonatid squid trophic level in the oceanic ecosystems.2 Our Berryteuthis anonychus, and sometimes amphi- research attempted to quantify the feeding habits of albacore in the transition region of the central North Pacific. We also attempted to estimate the *Corresponding author: Tel: 81-543-36-6057. impact of feeding by albacore on Japanese anchovy Fax: 81-543-35-9642. Email: [email protected] Engraulis japonicus during late spring and early Received 1 August 2003. Accepted 12 March 2004. autumn. 574 FISHERIES SCIENCE H Watanabe et al.

MATERIALS AND METHODS The maximum diameter of sagittal fish otoliths and rostral length of the lower beak of squid were also Samples were collected in the transition region measured to estimate the body size [standard from 6 to 10 September 2001, 25 May-3 June 2002, length (SL) for fish and dorsal mantle length (DML) and 1–26 July 2002 in the central North Pacific for squid] and wet weight of the digested prey, between 34∞N and 45∞N, and 151∞E and 175∞E applying the relations obtained from intact speci- (Fig. 1). The sampling localities shifted northward mens of the main prey items.19–24 with season due to the summertime feeding migra- The stomach content index (SCI) was calculated tion of albacore (Fig. 1). From 6 to 10 September as follows: 2001 and 25 May-3 June 2002, samples were col- SCI (%) = (wet weight of stomach contents/body lected during daytime with pole and line using wet weight excluding wet weight of stomach sardine and anchovy as bait. In July 2002, some contents) ¥ 100 samples were collected using this method and additional samples were collected using a surface The frequency of occurrence (F) of each food item gill net, which consisted of 50 units (1 unit mea- in the total number of stomachs examined, per- sured 7 m in height and 50 m in width) with mesh centage of each food item to the total number of sizes of 48, 55, 63, 72, 82, 93, 106, 115, 121, 138, and food items (Cn), and percentage wet weight of each 157 mm. The gill net was set at dusk and retrieved food item to the total wet weight of the stomach the following dawn. Samples were frozen at -30∞C contents (WW) were calculated for species i as: for further analysis in the laboratory. A total of 132 Fi (%) = (number of stomachs including food item individuals ranging from 48.9 to 76.2 cm in fork i/total number of stomachs examined) ¥ 100 length (FL) were subjected to stomach content analysis. For the samples collected in the pole-and- Cni (%) = (total number of food item i/total line survey, we excluded all fresh and minimally number of food items in the stomachs) ¥ 100 digested sardine and anchovy prey from the stom- WWi (%) = (total wet weight of food item i/total ach content analysis to avoid contamination due wet weight of the stomach contents) ¥ 100 to the chum. We combined the gill net and pole- and-line samples in the analysis because the prey Using these three indices, an index of relative species compositions were similar in July 2002. importance (IRI)11 for food item i was calculated We measured FL, body wet weight of the alba- using the equation core, and wet weight of the stomach contents. IRIi = (Cni + WWi) ¥ Fi Zooplankton prey were identified to species or genera, depending on their state of digestion, and This index is represented by the size of a rectangle counted for each taxon. All fish and squid prey resulting from plotting the three values on a three- were identified to the possible lowest taxonomic way graph. level based on morphology of sagittal otoliths of fish and lower beaks of squid, respectively, because these prey items were generally highly digested. RESULTS

Diet composition

Kamuchatka Pen. 50°N Sea of Okhotsk We identiﬁed 19 prey species belonging to 20 6–10 September, 2001 1–26 July, 2002 genera in the 132 stomachs examined, of which 25 May–3 June, 2002 eight (6.1%) were empty (Table 1). 17 19 18

16 40 18 17 19 17 May–June Japan 20 19 18 19 18 Fish were the most common prey, with the highest 19

20 Cn, WW, and F values (59.1%, 70.8%, and 71.2%, respectively), followed by squid (27.0%, 24.9%, and 30 63.6%, respectively; Fig. 2a). The IRI was highest for 140 150 160 170°E fish (9253), followed by squid (3305; Fig. 2a). The Fig. 1 Sampling localities of albacore Thunnus ala- IRI of other taxa were <380, showing that fish and lunga in the transition region of the central North Pacific squid were the primary prey of albacore. Of the fish and distribution of the sea surface temperature during prey, Japanese anchovy E. japonicus was the com- each sampling period. monest with Cn, WW, and F values of 27.2, 67.0, Feeding habits of albacore FISHERIES SCIENCE 575

Table 1 Prey species composition of albacore Thunnus alalunga from May to September in the transition region of the central North Paciﬁc 2002 2002 2001 25 May-3 June 1–26 July 6–10 September n = 66 (2 empty) n = 48 (2 empty) n = 18 (4 empty) 50.5–75.8 (cm FL) 48.9–76.2 (cm FL) 51.7–56.4 (cm FL) Cn WW F Cn WW F Cn WW F Food items (%) (%) (%) IRI (%) (%) (%) IRI (%) (%) (%) IRI Euphausiids 0.1 <0.1 1.5 <1 12.8 0.3 25.0 328 11.1 0.5 16.7 195

Amphipods Phronima sedentaria 0.1 0.1 3.0 1 0.8 0.3 10.4 11 – – – – Phronima spp. 0.2 0.1 4.5 1 0.7 0.1 12.5 11 1.6 0.1 11.1 20 Brachyscelus crusculum 6.2 0.7 13.6 93 0.6 <0.1 2.1 1 – – – – Brachyscelus spp. 0.1 0.1 3.0 1 0.2 <0.1 4.2 1 – – – – Phrosina semilunata 1.7 0.7 6.1 14 0.3 0.1 4.2 2 – – – – Phrosina spp. 2.5 1.0 7.6 27 – – – – – – – – Paraprone crusculum 0.2 0.1 1.5 <1 ––– – –––– Paraprone spp. 0.6 0.1 3.0 2 – – – – – – – – Platyscelus ovoides 0.1 <0.1 1.5 <1 ––– – –––– Leptocotis tenuirostris 0.1 <0.1 1.5 <1 0.1 <0.1 2.1 <1 –––– Unidentiﬁed 1.7 1.5 19.7 – – 0.1 12.5 – 2.4 0.2 16.7 44

Decapods 0.1 <0.1 3.0 <1 ––– – ––––

Pteropods Diacria sp. 0.1 <0.1 1.5 <1 ––– – ––––

Squid Gonatopsis borealis 15.8 10.8 28.8 766 1.7 0.3 12.5 25 8.9 3.2 5.6 68 Onychoteuthis borealijaponica 1.3 1.1 7.6 18 0.3 0.3 4.2 2 2.4 1.6 11.1 45 Onychoteuthis banksii 0.1 1.0 1.5 2 – – – – – – – – Gonatus berryi 0.1 <0.1 1.5 <1 ––– – –––– Gonatus spp. 1.7 0.6 7.6 17 – – – – – – – – Sthenoteuthis oualaniensis 0.2 0.7 4.5 4 – – – – – – – – Ommastrephes bartrami 0.2 6.0 1.5 9 – – – – – – – – Todarodes paciﬁcus 0.2 0.4 1.5 1 – – – – – – – – Abraliopsis felis 0.3 0.2 1.5 1 – – – – – – – – Abraliopsis spp. 0.3 0.2 6.1 3 0.3 0.6 2.1 2 – – – – Octopoteuthis spp. 0.2 0.1 1.5 <1 0.1 0.1 2.1 0 – – – – Ommastrephidae 0.3 3.9 6.1 26 0.1 1.0 2.1 2 – – – – Unidentiﬁed 6.6 – 28.8 – 2.2 – 20.8 – 2.4 – 11.1 –

Fish Engraulis japonicus 27.2 67.0 45.5 4286 52.8 95.2 85.4 12637 48.2 79.9 27.8 3558 Trachurus japonicus ––––––– – 0.8 0.3 5.6 6 Maurolicus imperatorius ––––0.2<0.1 4.2 1 – – – – Diaphus theta ––––––– – 9.8 13.9 5.6 131 Ceratoscopelus warmingi ––––0.10.2 2.1 1 0.8 0.2 5.6 6 Paralepididae – – – – 1.0 0.8 4.2 8 – – – – Carangidae 0.1 1.9 1.5 3 – – – – – – – – Gempylidae 0.1 1.9 1.5 3 0.3 0.6 2.1 2 – – – – Unidentiﬁed 31.8 – 42.4 – 25.2 – 89.6 – 11.4 – 33.3 –

n, no. stomachs examined; FL, fork length; Cn, no. food items (%); WW, wet weight (%); F, frequency of occurrence (%), IRI, index of relative importance. 576 FISHERIES SCIENCE H Watanabe et al.

Fig. 2 Percent composition of prey items of albacore Thunnus alalunga in the transition region of the central North Paciﬁc by number (Cn), wet weight (WW), and frequency of occurrence (F). The index of relative importance (IRI) is represented by the size of the respective rectangles. n = no. stomachs examined, FL, fork length. and 45.5%, respectively (Table 1). The IRI of the growth. The second mode was also recognized in anchovy was 4286, while the IRI of other ﬁsh spe- the 12–13-cm range in September, suggesting that cies were <5. For squid prey, Gonatopsis borealis albacore fed on both juvenile and adult anchovy in was the commonest with Cn, WW, F, and IRI values early autumn. From May to June, the size fre- of 15.8%, 10.8%, 28.8%, and 766, respectively quency distribution of the most common squid (Table 1). The IRI for other squid species were <30. prey, G. borealis, had a clear mode between 4 and 5 cm DML (Fig. 4), suggesting that albacore fed mainly on the juveniles. July

The most common prey was fish by Cn (79.7%), Day–night difference in average SCI WW (96.8%), and F (89.6%; Fig. 2b). The IRI of fish was 15 810, whereas the IRI of other taxa were <330 For the samples collected in July, the average SCI (Fig. 2b). Most of the fish were Japanese anchovy were significantly higher during the daytime with very high Cn (52.8%), WW (95.2%), and F (n = 34) than at night (n = 14; 1.13 ± 0.85% vs (85.4%) values (Table 1). Their IRI was also very 0.42 ± 0.21%, mean ± SD; P < 0.05, Mann–Whitney high (12 637) compared with other fish species U-test), suggesting that albacore feed more actively (<10). The main squid prey in May and June, during the daytime than at night. The SCI values G. borealis, almost disappeared in July (Table 1). were not significantly different among daytime samples collected in May–June (1.08 ± 0.72%), July (1.13 ± 0.85%), and September (0.99 ± 0.68%; September P > 0.05, Kruskal–Wallis test).

Fish were the commonest prey, having Cn, WW, and F values of 71.0%, 94.3%, and 50.0%, respectively DISCUSSION (Fig. 2c). The IRI of fish was also the highest (8262), far exceeding those of other prey taxa, which were Feeding habits <210 (Fig. 2c). Japanese anchovy was the most common fish prey, with very high Cn (48.2%), WW In the transition region of the central North Pacific, (79.9%), F (27.8%), and IRI (3558) values (Table 1). albacore fed mainly on Japanese sardine Sardinops The IRI of other fish species were <135. melanostictus in summer in the mid-1980s when the sardine population exploded.25 In the present study, however, no sardine was found in the stom- Size of the major prey species achs collected in 2001 and 2002, reflecting the decline of the sardine population after 1988.26,27 The size distribution of Japanese anchovy from the The Japanese anchovy E. japonicus has rebounded stomachs had a clear mode of between 3 and 6 cm remarkably and its distribution has expanded SL in May–June, 5 and 8 cm SL in July, and 6 and eastward from the coastal waters off Japan to the 10 cm SL in September (Fig. 3), reflecting their transition region around 175∞E during spring Feeding habits of albacore FISHERIES SCIENCE 577

40 60 May-June May-June n = 276 30 n = 476 40 20

20 10 Size frequency (%)

0 0 4 6 8 10 12 2 4 6 8 10 Doral mantle length (cm) 40 July Fig. 4 Dorsal mantle length distribution of Gonatopsis borealis in the stomachs of albacore Thunnus alalunga 30 n = 583 in May–June 2002 in the transition region of the central North Paciﬁc. 20

phyll front: they migrate from approximately 10 Size frequency (%) 35∞N in spring to 40∞N in summer in the North Pacific,7,31 and seasonal migration patterns of 0 both species are spatiotemporally similar. This 4 6 8 10 12 would explain how the predator–prey relation- 40 ship between albacore and anchovy is main- September tained from May to September. In contrast to the n = 58 anchovy, the subarctic gonatid squid G. borealis 30 migrates further north from the transition region to subarctic waters during spring and summer.5,22,32–34 We found that albacore fed on this 20 squid species frequently in late spring and early summer, but rarely in mid-summer and early 10 autumn. This observation reflects the overlapping distributions of albacore and squid in the earlier seasons and separation of their habitats from 0 4 6 8 10 12 mid-summer to autumn. Standard length (cm) Predation impact of albacore on anchovy Fig. 3 Standard length distribution of Japanese anchovy Engraulis japonicus in the stomachs of albacore Thunnus alalunga from May to September in the transi- In the present study, the size spectrum of juvenile tion region of the central North Pacific. anchovy eaten by albacore gradually became larger with season. This would reflect growth of the anchovy in albacore habitat, because the growth and summer since the early 1990s.28–30 Our results rate of juvenile anchovy is 0.1–0.3 mm per day reflect this increased abundance of Japanese at sizes between 3 and 10 cm SL.35 Small-sized anchovy in recent years, replacing Japanese sar- anchovy of 3–5 cm SL, which were preyed on by dine. Further, Japanese anchovy is the most com- albacore in May–June, were estimated to have mon small pelagic fish in this study area during late hatched mainly during spring. That is, the preda- spring and summer in the last 5 years (Watanabe tion impact of albacore on the newly recruited H, unpubl. data 2000, 2001, 2003).31 These data anchovy is high during late spring and early probably indicate that albacore are opportunistic autumn. feeders in the central North Pacific, as they are in To estimate the impact of predation on Japanese the eastern North Pacific.1,3,8,13 anchovy, we adopted 1.08% of body weight as the Both albacore and anchovy migrate northward daily ration of albacore, which is the average SCI following a shift of the transition zone chloro- value over the entire survey during the daytime. 578 FISHERIES SCIENCE H Watanabe et al.

For a congener of albacore, southern bluefin tuna 5. Mishima S. On the passing over of subtropic fishes to T. maccoyii, of 40–140 cm FL, daily ration was esti- the subarctic region in the summer season. Res. Inst. mated to be 1.01% of body wet weight at 14–18∞C.36 North Pacific Fish. Hokkaido Univ. Spec. Vol. 1981; 61– Their habitat temperature and feeding habits are 71. similar to those of albacore in this study. The bio- 6. Pearcy WG, Fisher JP, Anma G, Meguro T. Species associa- tions of epipelagic nekton of the North Pacific Ocean, 1978– mass of 2–5-year-old albacore, which correspond 37 93. Fish. Oceanogr. 1996; 5: 1–20. to the albacore collected in the present study, is 7. Polovina JJ, Howell E, Kobayashi DR, Seki MP. The transi- estimated to be at least 200 000 tons in the transition zone chlorophyll front, a dynamic global feature defin- tion region of the central and western North Pacific ing migration and forage habitat for marine resources. 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