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

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Feeding Habits of Albacore Thunnus Alalunga in the Transition Region of the Central North Pacific 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: 48.9–76.2 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, 48.2–52.8%; WW, 79.9–95.2%; F, 27.8–85.4%). 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 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.
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