NAGASAWA*, Syuzo EGUSA* Department of Fisheries, Faculty of Agriculture, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan. HARA* and Isao YAGISAWA*

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1 Kazuya NAGASAWA*, Syuzo EGUSA* Department of Fisheries, Faculty of Agriculture, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan Takeshi HARA* Isao YAGISAWA* Nikko Branch, National Research Institute of Aquaculture, Chugushi, Nikko, Tochigi Prefecture321-16, Japan (Received July 18, 1983) Ecological factors influencing infection levels of salmonids by Acanthocephalus opsari ichthydis in Lake Yunoko, Japan, analysed. The most important factor is feeding habits of definitive host. Brook trout, Salvelinus fontinalis, heavily infected with this parasite as y fed on intermediate host, Asellus hilgendorfi. By contrast, Biwa salmon, Oncorhynchus rhodurus, kokanee, O. nerka, preyed mainly on fish plankton, respectively, hence, y infected to a lesser extent with this parasite. The overlapping habitats of definitive intermediate hosts noted. These are factors accelerating greater contact between both hosts. Most brook trout inhabited littoral zone, to which A. hilgendorfi restricted. How ever, Biwa salmon kokanee widely dispersed in lake. A close distributional relation ship between plant materials A. hilgendorfi was also suggested. Introduction The acanthocephalans of family Echinorhynchidae often exhibit broad speci ficity toward fish definitive hosts. A number of hosts are recorded in literature for some species of this family. Echinorhynchus salmonis has been found in 57 species subspecies (PETRUSHEVSKI SHULMAN, 1958) Acanthocephalus anguillae in 36 species (PETROCHENKO, 1956). CHUBB (1964) reported differences in occurrence of A. clavula (=E. clavula) among * Present addresses: KN, Hokkaido Fisheries Experimental Station, Hama-cho, Kushiro, Hokkaido 085, Japan; SE, Nippon Veteri nary Zootechnical College, Musashino, Tokyo 180, Japan; TH, National Research Institute of Aquaculture, Tamachi-cho, Watarai-gun, Mie Prefecture , Japan; IY, Chitose Branch, Hokkaido Salmon Hatchery, Rankoshi, Chitose, Hokkaido 066, Japan four species of fishes in Llyn Tegid (Bala Lake), Merionethshire. He attributed m to difference in feeding of fishes on Asellus meridianus, intermediate host of parasite, except for pike, which appeared to acquire infections not by feeding on isopod but by secondary process of eating already infected fishes. The feeding habits are primary factors since natural infection occurs when a fish ingests an inter mediate host containing an infective larva. The intermediate hosts for species of Echinorhynchidae are amphipod or isopod crustaceans, hence, carnivorous fishes are more readily infected with echinorhynchid acanthocephalans through ingestion than are hervivorous omnivorous fishes. How ever, re must be additional factors in fluencing infection levels rar than just feeding habits. For example, factors promoting greater contact between definitive intermediate hosts would lead to heavier

2 Kazuya 54 NAGASAWA, Syuzo EGUSA, Takeshi HARA Isao YAGISAWA infections. The present study was intended to elucidate those factors that influence levels of acanthocephalan infections. In initiating study, we justifiably concerned with selection of a suitable site examined several possible locations that might best serve as a study site. As a result, Lake Yunoko was chosen because of (1) heavy infections of fishes with "Acanthocephalus? sp."(matsumoto, 1972),(2) availability of ecologically different fish populations, (3) its small size, (4) its location close to laboratory of Nikko Branch of National Research Institute of Aquaculture with appropriate facilities. MATSUMOTOfirst noted acanthocephalan infections of salmonid fishes in this lake reported that isopod, Asellus hilgendorfi, harvored parasite larva. The acanthocephalan was identified as Acanthocephalus opsariichthydis YAMAGUTI, 1935, its life cycle was confirmed by present authors (unpublished). Materials Lake Yunoko Methods 36 47'N,139 25'E Fig. is 1. Sampling small lake located in Tochigi Prefecture, measuring approximately 900m long, 400m wide, up to 15 m in depth (Fig. 1). This lake has no inlet streams, its water is supplied by springs. At outlet, re is a big waterfall, which prevents upstream fish migration. A belt of submerged plants fringes margin of lake. For detailed descriptions of lake see JIBP-PF RESEARCHGROUP OF LAKE YUNOKO(1975). The following salmonids selected for study: brook trout, Saluelinus fontinalis; Biwa salmon, Oncorhynchus rhodurus; kokanee, O. nerka. Fishes collected with nylon gill nets at monthly intervals during period July 1978 to June A fleet of gill nets composed of four 10-m by 2-, 4-, 6-, 8-m panels of each of five mesh sizes was set along bottom at Station A, a 10-m by 10-m gill net of same mesh sizes was employed at Station B (Fig. 1). During winter, however, fishes could not be sampled at Station B because of icecover of lake. The nets laid for 5-h stations of salmonids (A & B) Asellus hilgendorfi (C) in Lake Yunoko. Depths on contour lines are in meters. a periods seasons, in after brought to removed spring, recorded for also food fied to weight In of lowest or fishes summer positions ir summer, fixed The in capture taken nets to clarify lake. in lifted When ir analyses. binocular or overnight y laboratory. from in order within summer which stomachs 10% in contents possible distributions formalin taxon dissecting microscope. respective stomach identiunder The contents a wet was recorded. In October 1978, Asellus hilgendorfi collected with an EKMANN-BIRGE bottom sampler( m) at depths of 0.2, 1, 2, 3, 4, 6, 8, 10 m at Station C (Fig.1). Throughout text, each season is defined as: winter (January-March), spring (AprilJune), summer (July-September), autumn (October-December).

3 Infection Levels of Salmonids by A. opsariichthydis 55 Results Occurrence of Acanthocephalus opsariichthydis in salmonids Of three species of salmonids examined, brook trout most frequently heavily infected with A. opsariichthydis. Biwa samlon infected to a lesser extent with this parasite than brook trout, kokanee least infected of se three species. This tendency appeared to be stable through out year (Table 1) between host size classes (Table 2), although sample of Biwa salmon was too small for statistical compari son of infection levels. Distributions of definitive hosts Figure 2 shows fish distribution patterns in summer spring. No fishes taken at depths greater than 4 m at Station B. This is thought to be due to forma tion of un-oxygenated layer caused by eutrophication of lake. Brook trout mainly confined to Table 1. Seasonal occurrenc e of Acanthocephalus opsariichthydis in three species of salmonids from Lake Yunoko

4 56 Kazuya NAGASAWA, Syuzo EGUSA, Takeshi HARA Isao YAGISAWA Fig. 2. Distributions of three species of salmonids in Lake Yunoko. Left: summer spring., right: littoral zone of lake. Only two fishes collected at Station B. By contrast, Biwa salmon kokanee widely dispersed from surface to bottom, did not show as a restricted habitat preference as brook trout. Although exact observations not made in autumn, a similar distributional pattern was recognized, although in October fishes abundant inshore probably for spawning. SHIRAISHI TAKAGI (1955) reported that brook trout more abundant inshore than offshore in this lake contrary to kokanee. TANAKA SHIRAISHI (1970) also found that, although fish distributions varied with seasons, brook trout both Biwa salmon kokanee mainly captured inshore offshore, respectively. Therefore, it can be said that

5 Infection Levels of Salmonids by A. opsariichthydis 57 brook trout is a coastal dweller, while Biwa salmon kokanee are open water inhabitants. Distribution of intermediate host Asellus hilgendorfi restricted to vegetated coastal bottom. They most abundant at a depth of 0.2 m, decreased in number with increasing depth, dis appeared below a depth of 4 m (Fig. 3). Similar tendencies have been noted at different seasons in this lake in or papers by SHIRAISHI (1964), SHIRAISHI et al. (1970), SEKINE TSUCHIYA (1972) KITAGAWA (1974). Of se, SEKINE TSUCHIYA sug gested seasonal vertical migration of this isopod; neverless, its migration range was so confined (within 3 m) that dis tributional range of isopod appears to be stable throughout year. In addi tion, although we collected A. hilgengorfi at Station C where is far from Stations A B, fact that se authors obtained similar results at different sampling sites indicates no or little difference in its distributional pattern within lake. Percentages of total wet weight of food items in diets of three species of salmonids from Lake Feeding habits of definitive hosts Stomach content analysis data, expressed as Fig. 3. Vertical distribution of Asellus hilgendorfi on bottom of Lake Yunoko. percentage of each food item of total wet weight of food eaten, are presented in Table 3. Brook trout preyed on widest variety of food organisms of three species of salmonids examined. Asellus hilgendorfi oc curred only in diet of brook trout. The most important food component was chiro nomid larvae, primarily Chironomus plurnosus, which made up to 65.8% of total weight of food consumed. Benthos such as gastropods, sphaeriid clam Pisidium sp., water mites also ingested. Cladocerans, mainly Daphnia longispina, terrestrial hy menopterans of little importance. Al though SHIRAISHI TAKAGI (1955) found that brook trout of this lake fed heavily on A. hilgendorfi trichopteran larvae, abundance of se animals was small or null in stomachs examined by us.

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7 Infection Levels of Salmonids by A. opsariichthydis 59 ecosystem, including or plants animals, would be essential. Our work performed in Lake Yunoko is still insufficient in this sense, furr detailed study is needed. Acknowledgments We are grateful to Dr. M. TANAKA Mr. T. SHIMADA, Nikko Branch, National Re search Institute of Aquaculture, for ir kind assistance. Thanks are also due to Dr. M. HORIKDSHI, Ocean Research Institute, Uni versity of Tokyo, for his comments on manuscript. References AMIN, O. M. J. M. BURROWS (1977) : Host seasonal associations of Echinorhynchus salmonis (Acanthocephala : Echinorhynchidae) in Lake Michigan fishes. J. Fish. Res. Board Can., 34, CHUBB, J. C. (1964) : Occurrence of Echino rhynchus clavula (DUJARDIN, 1845) NEC HAMANN, 1892 (Acanthocephala) in fish of Llyn Tegid (Bala Lake), Merionethshire. J. Parasitol., 50, HIGURASHI, T., M. KAWAJIRI H. HATA (1931): On fishing experiment of l-locked red salmon (Oncorhynchus nerka) in Lake Aoki. Suisan Kenkiushi, 26, (In Japanese). HNATH, J. G. (1969) : Transfer of an adult acantho cephalan from one fish host to anor. Trans. Amer. Fish. Soc., 98, 332. HOLMES, J. C., R. P. HOBBS T. S. LEONG (1977) : Populations in perspective: community or ganization regulation of parasite popula tions. In Regulation of parasite populations, G. W. ESH (ed.). Academic Press, New York, pp JIBP-PF RESEARCH GROUP OF LAKE YUNOKO (1975) : Productivity of community in Lake Yunoko. In JIBP synsis, Vol. 10. Pro ductivity of communities in Japanese inl waters, S. MORI G. YAMAMOTO (eds.). University of Tokyo Press, Tokyo, pp KADOTA, S. Y. SHIRAISHI M. IMOTO (1973) : On distributions, growth sting crop of submerged plants in Lake Yunoko. Prog. Rep. IBP Studies Prod. Communities in Lake Yunoko, (4), (In Japanese). KATO, F. (1978) : Morphological ecological studies on two forms of Oncorhynchus rhodurus found in Lake Biwa adjoining inlets. Japan. J. Ichthyol., 25, (In Japanese with English summary). KITAGAWA, N. (1974) : Studies on bottom fauna of Lakes Tuzenji-ko, Yuno-ko, Suge numa Maru-numa. Jap. J. Limnol., 35, (In Japanese with English summary). KUBOTA, H. (1973). Sting crop of leaves fallen into Lake Yunoko, Nikko, III. Prog. Rep. IBP Studies Prod. Communities in Lake Yunoko, (4), (In Japanese). MATSUMOTO, K. (1972) : A preliminary note on asellid in Lake Yunoko. ibid., (3), (In Japanese). MAYAMA, H. (1978) : Feeding habit of adult kokanee salmon (Oncorhynchus nerka) in Lake Shikotsu. Sci. Rep. Hokkaido Salmon Hatchery, (32), (In Japanese with English summary). NAKAMURA, M. (1975) : Keys to freshwater fishes of Japan fully illustrated in colors. Fifth edition. Hokuryukan Publishing Co., Ltd., Tokyo, 261 pp. (In Japanese). OSANAI, M. TANAKA (1971) : Studies on transplanted kokanee salmon (Oncorhynchus nerka var. adonis) to Lake Mashu, Hokkai do. Sci. Rep. Hokkaido Fish Hatchery, (26), 33-45, (In Japanese). PETROCHENKO, V. I. (1956) : Acanthocephala of wild domestic animals. Vol. 1. Akad. Nauk SSR, Moscow, viii +465 pp. (Transl. from Russian by Israel Program Sci. Transl., Jerusalem, 1971). PETRUSHEVSKI, G. K. S. S. SHULMAN (1958) : The parasitic diseases of fishes in natural waters of USSR. In Parasitology of fishes, V. A. DOGIEL, G. K. PETRUSHEVSKI YU. I. POLYANSKI (eds.). Leningrad University Press, pp (Transl. from Russian by Z. KABATA, Oliver Boyd Ltd., burgh London). Edin SAWA, J. (1931) : On food of kokanee Sakemasu Iho, 4, (In Japanese). salmon. SEKINE, K. A. TSUCHIYA (1972) : Feeding activity of Asellus hilgendorfi with special reference to its distribution in Lake Yunoko, Nikko. Prog. Rep. IBP Studies Prod. Com munities in Lake Yunoko, (3), (In Japa nese). SHIGA PREFECTURAL FISHERIES EXPERIMENTAL STATION (1942) : The natural food of main fishes in Lake Biwa. 80 pp. (In Japanese). SHIRAISHI, Y. (1964) : Fundamental study of Lake Yunoko for its fisheries development. IV. The vertical distribution of zoobenthic fauna

8 60 Kazuya NAGASAWA, Syuzo EGUSA, Takeshi HARA Isao YAGISAWA SHIRAISHI, Y., A. TUCHIYA K. SEKINE (1970): On benthic fauna of Lake Yunoko, Nikko. Prog. Rep. IBP Studies Prod. Communities in Lake Yunoko, (1), (In Japanese). TANAKA, M. Y. SHIRAISHI (1970) : Estimation of sting crop of fish in Lake Yunoko. ibid., (1), (In Japanese).