Vol. 396: 111–122, 2009 MARINE ECOLOGY PROGRESS SERIES Published December 9 doi: 10.3354/meps08330 Mar Ecol Prog Ser Feeding ecology of the swordfish Xiphias gladius in the subtropical region and transition zone of the western North Pacific Hikaru Watanabe1,*, Tsunemi Kubodera2, Kotaro Yokawa3 1National Research Institute of Far Seas Fisheries (Yokohama), 2-12-4 Fukuura Kanazawa Yokohama, Kanagawa 236-8648, Japan 2National Science Museum, 3-23-1 Hyakunin-cho Shinjyuku, Tokyo 169-0073, Japan 3National Research Institute of Far Seas Fisheries (Shimizu), 1-15-1 Orido Shimizu, Shizuoka 424-8633, Japan ABSTRACT: This is the first time a quantitative analysis has been carried out on feeding habits of swordfish Xiphias gladius (n = 455), mainly ranging from 1200 to 2100 mm in eye–fork length, in the western North Pacific. Based on these data, we examined the feeding ecology of this species in rela- tion to its seasonal south–north migration between subtropical and transition waters. The main X. gladius prey size was 80 to 500 mm, and the size spectrum of the prey shifted to a smaller range from spring to summer. In winter and spring, X. gladius was distributed in the subtropical region and fed mainly on the winter–spring cohort of neon flying squid Ommastrephes bartramii, followed by Pacific pomfret Brama japonica. In summer and autumn, X. gladius migrated to the transition zone between the Kuroshio front and subarctic boundary and fed mainly on the winter–spring cohort of O. bar- tramii, which also migrates into the transition zone from the subtropical region in summer. Other common prey species in summer and autumn were the gonatid squid Gonatopsis borealis and the myctophid fish Diaphus gigas, which are endemic to transition and/or subarctic regions. B. japonica almost disappeared from the stomachs of X. gladius during these seasons because this species migrates in summer into the subarctic region, north of the range of X. gladius. Therefore, seasonal changes in stomach contents of X. gladius could be explained by the difference in seasonal south–north migration patterns between predator and prey. The seasonal south–north migration of X. gladius seems to be a feeding migration, following the migration of the winter–spring cohort of O. bartramii. KEY WORDS: Xiphias gladius · Feeding habits · Ommastrephes bartramii · Seasonal migration · Subtropical and transition waters · Western North Pacific Resale or republication not permitted without written consent of the publisher INTRODUCTION ing spring and summer, and southward during autumn and winter for reproduction (Polovina et al. 2000, Seki The swordfish Xiphias gladius is among the largest et al. 2002, Takahashi et al. 2003). Previous studies pelagic fish, reaching 2500 to 3000 mm in eye–fork have indicated that X. gladius feeds mainly on large length (EFL). The species occurs worldwide from trop- squid and finned fish, suggesting that this species ical to transition waters (Sakagawa & Bell 1980, Naka- plays an important role as a top predator in high-sea mura 1985). The main habitat of this fish in the western ecosystems (Scott & Tibbo 1968, Stillwell & Kohler North Pacific is the subtropical region, which consists 1985, Pearcy 1991, Hernandez-Garcia 1995). Although of the subtropical frontal zone north of the subtropical considerable knowledge has been accumulated per- front, the subtropical domain to the south, and the taining to the feeding habits of X. gladius in the North transition zone between the Kuroshio front and the Atlantic and the eastern North Pacific (Scott & Tibbo subarctic boundary (Favorite et al. 1976, Pearcy 1991, 1968, Toll & Hess 1981, Stillwell & Kohler 1985, Mor- Roden 1991; Fig. 1). X. gladius migrates northward eira 1990, Markaida & Sosa-Nishizaki 1998), such from the subtropical region to the transition zone dur- information is extremely restricted in the western *Email: [email protected] © Inter-Research 2009 · www.int-res.com 112 Mar Ecol Prog Ser 396: 111–122, 2009 Fig. 1. Sampling localities in each season in the western North Pacific North Pacific, especially in the subtropical and transi- ern North Pacific (Table 1). Sampling localities corre- tion regions (Moteki et al. 2001). These data are essen- sponded well with the distribution centers of Xiphias tial not only for estimating the feeding strategy of gladius in each season (see ‘Discussion’), and gener- swordfish in this region, but also for establishing new ally shifted northward in summer and autumn due to methods for the sustainable exploitation of this impor- summertime migrations of the fish (Fig. 1). All X. glad- tant fisheries resource based on ecosystemic informa- ius were collected by commercial swordfish fishing tion, which is recognized worldwide as an important vessels using long lines in the 0–100 m layer, which management approach for future fisheries (World corresponds with the main habitat depth of X. gladius Summit on Sustainable Development 2002, Garcia & at night. Long lines were deployed at dusk and Zerbi 2003). Several projects under the North Pacific Marine Science Organization (PICES) and Global Ocean Ecosystem Dynamics (GLOBEC) are ongoing in Table 1. Xiphias gladius. Numbers of swordfish collected in the North Pacific in relation to the development of such each year and month ecosystem approaches to fisheries. We aimed to quantitatively examine the feeding Month Year Total habits of Xiphias gladius in the subtropical and transi- 1996 2000 2001 2002 2003 tion waters of the western North Pacific during each 100005959 season, and to determine their feeding strategies in 200044650 relation to their south–north migrations and diel verti- 30125175286 cal migrations. 40356141570 5 0 17 20 16 0 53 662315044 701482051 MATERIALS AND METHODS 800110011 9008008 Samples were taken from both the subtropical 10 0 0 1 0 0 1 11 0 0 3 0 0 3 region and transition zone during June 1996, March– 12 0 0 7 12 0 19 July 2000, March–December 2001, February–July and Total 6 67 140 70 172 455 December 2002, and January–April 2003 in the west- Watanabe et al.: Feeding ecology of swordfish 113 retrieved before dawn on the following day. During We calculated the proportion of each prey item each sampling period, sea surface temperature (SST) among the total number of food items identified (N), was measured. Aboard the ship, swordfish wet body the wet wt contribution of each food item to the total weights (BW) were measured and stomachs were wet wt of the stomach contents (W ), and the frequency removed. Stomachs were frozen at –30°C for further of the occurrence (F) of each food item in the total analysis in the laboratory ashore. We estimated EFL number of stomachs examined. Using these 3 indices, (cm) of each individual from the BW (kg) values using an index of the relative importance (IRI; Pinkas et al. the following equations obtained in this study area in 1971) of each food item i was calculated using the each season (K. Yokawa unpubl. data): equation: Winter (Dec to Feb): BW = e–12.1389 × EFL3.2231 (1) IRIi = (Ni + Wi) × Fi (6) Spring (Mar to May): BW = e–12.4565 × EFL3.2915 (2) This index is represented by the size of the rectangle resolved by plotting the 3 values on a 3-way graph. To Summer (Jun to Aug): BW = e–11.6042 × EFL3.1003 (3) readily allow comparison among prey items, the IRI Autumn (Sep to Nov): BW = e–10.7864 × EFL2.9558 (4) was standardized to %IRI for each prey item (Cortes 1997). In total, 455 individuals were examined for stomach content analysis. We excluded all fresh and minimally digested Japanese common squid Todarodes pacifi- RESULTS cus, mackerel Scomber spp., and Jack mackerel Tra- churus japonicus from the stomach content analysis to Habitat, SST, and size composition avoid contamination of the samples with long line bait. Zooplankton prey items were counted and iden- Sea surface water temperatures in which Xiphias tified to the lowest taxonomic level possible, and wet gladius were generally captured were 17–20°C in win- weights of each taxon were measured. All fresh and ter, spring, and autumn, and 17–21°C in summer, sug- minimally digested cephalopod and fish prey were gesting that X. gladius were distributed mainly in identified to the lowest taxonomic level possible, and areas with similar SSTs in all seasons (Fig. 2a). EFL their body length (dorsal mantle length, DML, for size-frequency distributions showed a clear mode cephalopods and standard length, SL, for fish) and between 1200 and 2000 mm from winter to summer, wet wt were recorded. Heavily digested cephalopod representing 4 to 7 year old individuals (Sun et al. and fish prey items were identified based on the mor- 2002; Fig. 2b). In autumn, these modes were not found phology of their lower beaks and sagittal otoliths, due to the small sample size, but the size range of cap- respectively. For these items, the rostral lengths of the tured individuals was 1200 to 2100 mm EFL, which was lower beaks of cephalopods and the maximum diame- similar to the main size range of the fish collected in ters of sagittal fish otoliths were also measured to esti- other seasons. Therefore, the age composition of the mate the original body sizes and wet weights of the present X. gladius sample may have been similar prey, using relationships generated from intact speci- throughout the seasons. mens of the main prey species (Kubodera 1982, Clarke 1986, Kubodera & Furuhashi 1987, Kubodera & Shimazaki 1989, Smale et al. 1995, Ohizumi et al. Diet composition 2001, T. Kubodera & H. Watanabe unpubl. data). These relationships could not be obtained for some We identified 70 prey species belonging to 57 genera minor prey species, such as the octopus Japetella in the 455 examined stomachs (Table 2). Of these stom- diaphana and gonostomatid fishes like Gonostoma achs, 21 (4.6%) were empty.