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Diet of Euphausia Pacifica Hansen in Sanriku Waters Off Northeastern Japan

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Diet of Euphausia Pacifica Hansen in Sanriku Waters Off Northeastern Japan Plankton Biol. Ecol. 48 (1): 68-77, 2001 plankton biology & ecology £.< The Plankton Society of Japan 2001 Diet of Euphausia pacifica Hansen in Sanriku waters off northeastern Japan Yoshizumi Nakagawa1, Yoshinari Endo1 & Kenji Taki2 'Graduate School ofAgriculture, Tohoku University, Sendai 981-8555. Japan 2Tohoku National Fisheries Research Institute, Shiogama 985-0001, Japan Received 15 May 2000; accepted 23 June 2000 Abstract: Stomach contents of Euphausia pacifica in Sanriku waters off northeastern Japan were ex amined bimonthly from April 1997 to February 1998 and compared to ambient microplankton abun dance. Copepods were consumed by E. pacifica in proportion to their abundance in the water column in all sampling periods except April. Few copepods were consumed by £ pacifica in April despite their high abundance in the water column. Instead, in April the stomachs contained large numbers of diatoms. Copepods proved to be the most important food item, in terms of carbon, for most of the year in Sanriku waters. A tentative calculation of the impact of predation by E. pacifica on copepods in Sanriku waters showed that the daily impact accounted for 0.1-2.2% of copepod biomass. Since £ pacifica can suspension-feed on diatoms, dinoflagellates, tintinnids and invertebrate eggs, as well as raptorial-feed on small and slow-moving copepods, it appears to be capable of feeding on a wide va riety of organisms by switching the feeding behavior according to ambient food conditions. key words: Euphausia pacifica, diet, copepods, Sanriku waters (Mauchline & Fisher 1969). Diatoms were abundant in the Introduction diet in spring and autumn, and crustacean remains were fre Euphausia pacifica Hansen is the dominant euphausiid quent in summer and autumn. Switching from phytoplank- species in the northern North Pacific (Mauchline & Fisher ton to zooplankton prey was also reported for the copepod 1969). It also occurs across the southern part of the Bering Calanus pacificus, when phytoplankton levels declined Sea, the Sea of Okhotsk and the Sea of Japan (Brinton (Landry 1981). 1962; Ponomareva 1963). E. pacifica is considered a key Studies on the feeding activities of E. pacifica have great species in Sanriku waters off northeastern Japan because importance for understanding the structure of marine many endemic and migrant predators, including pelagic and ecosystems off Sanriku. However, since little is known demersal fish, marine mammals, seabirds and benthic or about the food habits of E. pacifica in Sanriku waters (e.g. ganisms, depend on this species for food (Nemoto 1962; Endo 1981), or the relationship between diet and growth, Odate 1991; Nicol & Endo 1997; Yamamura et al. 1998). the flow of energy from lower to higher trophic levels in Mauchline (1967) pointed out three food types utilized food webs containing E. pacifica is not clear. In Sanriku by euphausiids: (1) material such as diatoms, dinoflagel waters, this species is known to be distributed both in the lates, and tintinnids filtered by the mouthparts from the cold and productive Oyashio area and the warmer and less water, (2) zooplankton, and (3) detrital material obtained productive Transition area (e.g. Taki et al. 1996). However, from bottom sediments. Many species of euphausiids are it is not known which area is superior from the viewpoint of omnivorous and exhibit multiple feeding behaviors, being food quality and quantity on a yearly basis. able to filter-feed using 'compression filtration' and to feed In this study, the stomach contents of E. pacifica in San raptorially on zooplankton (Hamner 1988). Seasonal shifts riku waters were examined bimonthly throughout the year. in diet composition have been observed for the euphausiids Stomach contents were compared to ambient microplank Meganyctiphanes norvegica and Thysanoessa inermis ton abundance to determine if the species exhibits food se lectivity. Stomach contents analysis underestimates soft Corresponding author: Yoshizumi Nakagawa; e-mail, yosizumi@bios. bodied organisms and maceration of food makes identifica tohoku.ac.jp tion of food items difficult. In order to compensate for the Diet of Euphausia pacifica 69 underestimation, we made several calculations to obtain 125° reasonable daily rations for E. pacifica and discussed which 50'N food items were greatly underestimated. 41*N Materials and Methods Samples were collected bimonthly from April 1997 to February 1998 at 4 stations along a transect at 40°N and at 5 stations along a transect at 38°20'N in Sanriku waters off northeastern Japan (Fig. 1). The number and locations of sampling stations in April were different from those in the other months. The profiles of temperature and salinity were determined using a CTD or STD. Water masses were deter mined for each sampling station based on the Tempera ture-Salinity (T-S) diagram and the classification scheme of Hanawa & Mitsudera (1987) (Table 1). 38' A ring net, with a mouth diameter of 130 cm and mesh aperture of 0.45 mm (Watanabe 1992), was towed obliquely from 15 m above the bottom to the surface at stations shal lower than 300 m, and from 150-m depth to the surface at stations deeper than 300 m. Net sampling was confined to 37* 37' 142° 143° 144°E nighttime when the species occurs in the upper 150 m. The 140° towing duration ranged from 5 to 31 min with a mean of Fig. 1. Locations of sampling stations in this study from April 12.2 min. Collected euphausiid specimens were preserved 1997 to February 1998. Those in April were different from the in 5% formalin. other months and are marked by an apostrophe numbers. Under A 100-ml water sample was collected from each of 6 lined stations were shallower than 300 m. depths (0, 10, 20, 30, 50, 75 m), filtered through a Whatman GF/F glass fiber filter, and analyzed by the method of Table 1. Water masses defined by Hanawa & Mitsudera (1987) Yentsch & Menzel (1963) with a Hitachi 139 spectrofluo- which dominated at each sampling station in this study from April rometer for chlorophyll a. Chlorophyll a was integrated 1997 to February 1998. TW, the Tsugaru Warm Current water sys over the upper 75 m, and then averaged for each cruise. A tem; OW, the Oyashio water system; KW, the Kuroshio water sys tem; SW, the surface-layer water system. The number of sampling 100-ml water sample from 20-m depth was preserved in 5% stations in April was different from the other months. formalin, assuming that Euphausia pacifica feeds in the surface layer as the percentage of full stomachs in E. April June August October December February pacifica is reported to be highest at 0-50-m depth at night for the Sea of Japan (Ponomareva 1963). A 10-100-ml sub- Stn 1/1' OW SW TW TW TW OW sample was taken from this sample, depending on mi- Stn 2/2' TW TW TW SW TW TW croplankton abundance, allowed to settle for 24 h, and then Stn 3/3' TW TW OW OW TW TW the numbers of different groups of ambient microplankton Stn 4/4' OW TW OW OW SW OW were enumerated with a modified Utermdhl method Stn 5/5' TW SW TW SW TW OW (Taniguchi 1977). Stn 6/6' TW TW TW SW TW OW Ten adult E. pacifica were sorted randomly form each Stn 7/7' OW SW TW KW TW OW OW station, and their total length, from the anterior tip of the Stn 8/8' OW OW TW KW TW Stn 9 OW TW KW SW OW rostrum to the distal end of the telson, was measured. A total of 435 stomachs were dissected out and the contents spread on glass slides. Stomachs were examined under a dissecting microscope, and scored into 5 classes based on ity & Langdon (1984). Invertebrate eggs found in the stom their fullness: empty stomachs, class 0; less than 25% full, achs were thought to be copepod eggs and carbon content class 1; 25-50% full, class 2; 50-75% full, class 3; more was estimated from egg size with a conversion factor of than 75% full, class 4. Food organisms were identified, enu 0.14 pgC/mT3 (Kiorboe et al. 1985). Copepods in the merated and dimensions measured to allow calculation of stomachs were identified by their mandibles, and their num both total carbon content of the stomach contents and the ber was determined from the number of pairs of mandible relative contribution of the various prey types. The carbon blades of a given size. Total length of copepods was mea contents of diatoms and dinoflagellates were estimated sured when found intact in the stomach. The carbon content from cell volumes according to Strathmann (1967). Tintinnid of copepods was obtained by estimating the prosome length carbon was estimated from lorica volume according to Ver- from the width of the mandible blades using the equation of 70 Y. Nakagawa, Y. Endo & K. Taki St. 1' 2' 31 4' 2 3 n ' i 50- ? i ^ A \ S150- Q200- X 250- b> 300- Apri 300-1 142.0 142.4 142.8'E 142.0 142.4 142.8*E 141.5 142.0 1415*E 141.5 142.0 142.5*E Fig. 2. Vertical profiles of temperature along the 40°N line from April 1997 to February 1998. Fig. 3. Vertical profiles of temperature along the 38°20'N line from April 1997 to February 1998. Karlson & Bamstedt (1994), then calculating dry mass from the prosome length of the copepod and assuming that 250 -i 46% of the dry mass was carbon (Vidal 1980). The carbon E contents of intact copepods in the stomach were calculated M200- from total length with the equation of Hirota (1986). c A chi-square test of independence (Sokal & Rohlf 1969) j= 150 H was used to check for seasonal differences in stomach full ness.
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