Title Horizontal Distribution and Population Dynamics of The

Horizontal distribution and population dynamics of the Title dominant mysid Hyperacanthomysis longirostris along a temperate macrotidal estuary (Chikugo River estuary, Japan) Author(s) Suzuki, Keita W.; Nakayama, Kouji; Tanaka, Masaru Citation Estuarine, Coastal and Shelf Science (2009), 83(4): 516-528 Issue Date 2009-08 URL http://hdl.handle.net/2433/80150 c 2009 Elsevier Ltd. All rights reserved.; この論文は出版社版でありません。引用の際には出版社版をご確認ご利用 Right ください。; This is not the published version. Please cite only the published version. Type Journal Article Textversion author Kyoto University Horizontal distribution and population dynamics of the dominant mysid Hyperacanthomysis longirostris along a temperate macrotidal estuary (Chikugo River estuary, Japan) Keita W. Suzukia,*, Kouji Nakayamab, Masaru Tanakab aDivision of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan bField Science Education and Research Center, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan *Corresponding author. Tel: 81-75-753-6222. Fax: 81-75-753-6229 E-mail address: [email protected] Postal address: Laboratory of Marine Stock-Enhancement Biology, Division of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan Abstract The estuarine turbidity maximum (ETM) that develops in the lower salinity areas of macrotidal estuaries has been considered as an important nursery for many fish species. Mysids are one of the dominant organisms in the ETM, serving as a key food source for juvenile fish. To investigate the horizontal distribution and population dynamics of dominant mysids in relation to the fluctuation of physical conditions (temperature, salinity, turbidity, and freshwater discharge), we conducted monthly sampling (hauls of a ring net in the surface water) along the macrotidal Chikugo River estuary in Japan from May 2005 to December 2006. Hyperacanthomysis longirostris was the dominant mysid in the estuary, usually showing peaks of density and biomass in or close to the ETM (salinity 1-10). In addition, intra-specific differences (life-cycle stage, sex, and size) in horizontal distribution were found along the estuary. Larger males and females, particularly gravid females, were distributed upstream from the center of distribution where juveniles were overwhelmingly dominant. Juveniles increased in size toward the sea in marked contrast with males and females. The findings suggest a possible system of population maintenance within the estuary; gravid females release juveniles in the upper estuary, juveniles grow during downstream transport, young males and females mature during the upstream migration. Density and biomass were primarily controlled by seasonal changes of temperature, being high at intermediate temperatures (ca. 15-25 ºC in late spring and fall) and being low at the extreme temperatures (ca. 10 ºC in midwinter and 30 ºC in midsummer). High density (up to 666 ind. m-3) and biomass (up to 168 mg dry weight m-3) of H. longirostris were considered to be comparable with those of copepods in the estuary. Keywords: mysid; Hyperacanthomysis longirostris; distribution; population dynamics; 1 salinity; estuarine turbidity maximum; Japan; Ariake Sea; Chikugo River estuary 2 1. Introduction Mysids are especially abundant in coastal and estuarine shallow waters, where they occupy an important position between lower and higher trophic levels (Mauchline, 1980). It is well established that juveniles of various fish species utilize mysid production in coastal and estuarine nursery grounds (Hostens and Mees, 1999; Feyrer et al., 2003; Nemerson and Able, 2004). The distribution and population dynamics of mysids are, therefore, significant in consideration of coastal fish production. In this context, several studies on the ecology of mysids have been conducted in estuaries, emphasizing spatial distribution (e.g. Hulburt, 1957; Baldό et al., 2001) and seasonal production (e.g. Wooldridge, 1986; Mees et al., 1994). However, the mechanism that mysid species use to maintain a population at a certain location within an estuary, where physical conditions drastically change in various spatial and temporal scales, remains unclear. More specifically, extensive research on the different horizontal distributions between sexes and/or life-cycle stages within an estuary has been carried out (Hulburt, 1957; Mauchline, 1970; Orsi, 1986; Hough and Naylor, 1992; Moffat and Jones, 1993; Baldό et al., 2001); however, a comprehensive explanation in terms of population maintenance remains obscure, although mechanisms of position maintenance for respective sexes and life-cycle stages were sometimes discussed in the previous studies. The Chikugo River estuary, which is located in the innermost part of the Ariake Sea, southwestern Japan (Fig. 1), is a macrotidal estuary with a well-developed estuarine turbidity maximum (ETM). This is one of the most productive fishery areas in Japan, holding a large number of semi-endemic species (i.e. species found only in the Ariake Sea within Japan; reviewed in Sato and Takita, 2000) from several taxonomic groups, such as fish (e.g. Neosalanx reganius, Takita, 1996; Trachidermus fasciatus, 3 Cao et al., in press) and crustaceans (e.g. Sinocalanus sinensis, Hiromi and Ueda, 1987; Tortanus derjugini, Ohtsuka et al., 1995; Acartia ohtsukai, Ueda and Bucklin, 2006). The mysid Hyperacanthomysis longirostris (Fukuoka and Murano, 2000) is also considered as a semi-endemic species (Chihara and Murano, 1997), found particularly in lower salinities in rivers flowing into the innermost part of the Ariake Sea (Ikematsu, 1963; Suzuki et al., 2008b). Semi-endemic species of the Ariake Sea are considered as continental relicts that presently survive in restricted areas in Japan, being derived from the isolation of the Japanese archipelago from the Eurasian continent by marine transgressions during geological history (Sato and Takita, 2000). So far copepods have been well studied as an important food for larval and early juvenile fish in the ETM of the Ariake Sea (e.g. Hibino et al., 1999; Shoji et al., 2006; Islam et al., 2006a, 2006b). In addition, several species of mysids recorded from the Ariake Sea (Ikematsu, 1963) have often been found in stomachs of larger juvenile fish of semi-endemic (Kuno and Takita, 1997; Mizutani and Matsui, 2006) and non-semi-endemic species (Takita and Intong, 1991; Suzuki et al., 2008a). For instance, Japanese temperate bass Lateolabrax japonicus juveniles exclusively feed on mysids in the ETM, after they ontogenetically change prey categories from copepods (Suzuki et al., 2008a). Although ecological information on mysids is important to understand the ecosystem supporting fish production, particularly semi-endemic fish populations in the Ariake Sea, available data from this sea area is restricted to general descriptions of several mysid species (Ikematsu, 1963; Suzuki et al., 2008b). The objective of the present study is to investigate the horizontal distribution and population dynamics of dominant mysid species along the Chikugo River estuary, focusing on relationships with physical conditions (temperature, salinity, turbidity, and 4 freshwater discharge). To characterize seasonal and annual changes, we conducted monthly sampling over the entire tidal area of the estuary for 20 months. We identified biological properties (life-cycle stage, sex, and size) of the dominant mysid Hyperacanthomysis longirostris at each station and hypothesized a possible system that would contribute to the population maintenance by comparison of the properties between stations, months, and years. 2. Methods 2.1. Study area Sampling was conducted along the lower reaches and the tidal channel of the Chikugo River (Fig. 1), the largest river flowing into the Ariake Sea. The Chikugo River estuary is characterized by large tidal ranges, vast tidal flats, and extremely turbid water. Since the water column is almost completely mixed during spring tides (tidal range > 4 m), turbidity exceeds 200 NTU even in the surface water at lower salinities (Shoji et al., 2006; Suzuki et al., 2007). The ETM is transported back and forth over a 20 km range along the estuary with tidal movements between low and high tides (Shirota and Tanaka, 1981), although it is usually located 10 to 20 km upstream from the river mouth at spring high tide (Shoji et al., 2006; Suzuki et al., 2007). As for copepods true estuarine species (i.e. species that complete their entire life cycle within the low salinity areas, Sinocalanus sinensis and Pseudodiaptomus inopinus) are overwhelmingly dominant in or close to the ETM, whereas common coastal species (e.g. Acartia omorii, Oithona davisae, and Paracalanus parvus) are distributed in the lower estuary (Hibino et al., 1999; Islam et al., 2006a). Among mysids, Hyperacanthomysis longirostris is abundant in the upper estuary (Suzuki et al., 2008b), whereas other 5 species (e.g. Neomysis japonica and Neomysis awatschensis) are also recorded in the lower estuary (Ikematsu, 1963). Seven stations (R1-R7; Fig. 1) were set up along the lower reaches of the river, ranging from the river mouth to the upper limit of the tidal area, where the Chikugo Weir is located (23 km upstream). Three stations (E1-E3; Fig. 1) were set up along the tidal channel of the river in such a way that E1 was near the mouth of the river and E3 was at the edge of the tidal flat (9 km offshore).

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