Blackwell Science, LtdOxford, UKFISFisheries Science0919 92682006 Blackwell Science Asia Pty LtdFebruary 2006721105114Original ArticleFood sources for corbicula in lakes A Kasai et al. FISHERIES SCIENCE 2006; 72: 105–114 Food sources for the bivalve Corbicula japonica in the foremost fishing lakes estimated from stable isotope analysis Akihide KASAI,1* Haruhiko TOYOHARA,1 Akiko NAKATA,1 Tsunehiro MIURA2 AND Nobuyuki AZUMA3 1Graduate School of Agriculture, Kyoto University, Sakyo, Kyoto 606-8502, 2Shimane Prefectural Inland Fisheries Experimental Station, Hirata, Shimane 691-0076 and 3Faculty of Agriculture and Life Science, Hirosaki University, Bunkyo, Hirosaki 036-8561, Japan ABSTRACT: Carbon and nitrogen stable isotope ratios in tissue of the bivalve corbicula Corbicula japonica and particulate organic matter (POM) were measured in Lake Jusan, Lake Ogawara and Lake Shinji, which are the foremost fishing grounds for the corbicula in Japan, to determine their food sources. The bivalves in Lake Ogawara and Lake Shinji showed enriched isotope composition, while those in Lake Jusan were depleted. In addition, the difference in the isotope ratios between the sam- pling sites was remarkable in Lake Jusan. Chlorophyll concentrations were significantly higher in Lake Ogawara and Lake Shinji than those in the inflow rivers, although that in Lake Jusan was equivalent to that in the river. Residence time of river water was estimated at 1 day, 455 days and 88 days in Lake Jusan, Lake Ogawara and Lake Shinji, respectively. These values indicate that the bivalves in Lake Ogawara and Lake Shinji assimilate autochthonous phytoplankton, while those in Lake Jusan assim- ilate terrestrial matter in the upper reaches and marine phytoplankton in the lower reaches because of low production in the lake. KEY WORDS: bivalve, brackish lake, Corbicula japonica, food source, particulate organic matter, stable isotope. INTRODUCTION diet of organisms. However, among the methods, direct observation of feeding behavior of bivalves is The infaunal suspension-feeding bivalve corbicula unfeasible over long periods in the field. Indirect Corbicula japonica inhabits brackish waters. The techniques, such as gut content analysis of bivalves landing of corbicula in 2001 reached 17 000 tons, may be misleading because it cannot distinguish which was 28% of the total landings of inland fish- ingested material that is not assimilated. Algae, for eries, so that the corbicula is one of the most example, can survive passage through the digestive important fishery resources in Japan. Corbicula tract.3 Therefore, neither direct observation nor gut plays an important role in the ecosystem through content analysis are suitable for determining food feeding and nutrient excretion activities, because sources of corbicula. they often dominate the macrobenthic community In contrast to these conventional methods, sta- in brackish waters.1,2 It is therefore essential to ble isotope analysis has received increasing inter- know corbicula nutrition for the elucidation of car- est as it can measure assimilation by potential bon and nitrogen cycling in the ecosystem. Esti- producers. The 13C/12C and 15N/14N ratios of an ani- mates of food sources are also important for the mal directly reflect the composition of food management of the corbicula resources, because it sources assimilated and incorporated over time.4 would affect their growth rates and reproduction. Since the isotope composition in each primary Corbicula obtains particles through filtration by source of organic matter shows different character- holding its inhalant siphon above the sediment istics, this method has been used successfully in surface. There are several ways to understand the many studies of spatial and temporal variations in potential diets of invertebrates in various estuarine 5 *Corresponding author: Tel: 075-753-6314. Fax: 075-753-6468. and saltmarsh food webs. In our previous paper, Email: [email protected] carbon and nitrogen isotope ratios were measured Received 6 May 2005. Accepted 10 August 2005. in the body of C. japonica in the lower reaches of 106 FISHERIES SCIENCE A Kasai et al. the Kushida River.6 The results indicated that the the river mouth were quantitatively estimated by contribution of terrestrial organic matter is signif- Kasai and Nakata,6 but those in brackish lakes are icantly important for the corbicula diet, although still unknown. The differences in the environmen- the contribution gradually changes among sam- tal conditions would cause different contributions pling sites. between rivers and lakes. Therefore, the present Corbicula japonica inhabits lower reaches of study investigates the isotopic composition of rivers and brackish lakes, so that the corbicula C. japonica in brackish lakes. Based on the isotopic fisheries are prospering in many lakes. The results, the use of different components of organic environmental conditions in brackish lakes are matter in the bivalves’ diet is discussed in relation usually different from those in river mouths. From to the difference in the environmental conditions the physical point of view, for example, it is among brackish lakes and lower reaches of rivers. expected that surface water temperature is higher in brackish lakes than river mouths in summer. However, active primary production caused by the MATERIALS AND METHODS long residence time of water might increase abun- dance of phytoplankton and lead to improved bio- This study was conducted in Lake Jusan, Lake logical conditions in brackish lakes.7 The relative Ogawara and Lake Shinji, which are the foremost contributions of food sources of the corbicula in fishing grounds for corbicula in Japan (Fig. 1a). (a) 130°E 140°E (c) L. Jusan L. Ogawara N 40°N Sea of Japan 5 O2 OM 10 O1 L. Shinji Japan O4 O3 OK2 PacificOcean O5 O6 30°N R. Doba 20 R. Takase O7 O8 (b) OR OK1 15 O9 J2 N 2km JM J1 J5 O10 J9 R. Sadoro J3 J6 J10 R. Anenuma J8 J4 J7 1 .2 J-K 5 JR R. Toriya 2km R. Iwaki L. Shinji SM (d) L. Nakaumi S13 N S15 Fig. 1 (a) Site locations. Topog- S12 S1 2.5 S2 raphy and sampling stations in S11 S14 5 S3 (b) Lake Jusan, (c) Lake Ogawara S10 S4 and (d) Lake Shinji. K, M and R in the labels indicate stations from R. Hii S6 S9 S5 which benthic microalgae were S8 S7 collected, marine stations and SR 2km riverine stations, respectively. Depth contours in lakes are in meters. Food sources for corbicula in lakes FISHERIES SCIENCE 107 Landing in the three lakes accounted for over 70% and 23.7 ± 2.93 mm (average ± standard deviation) of the total corbicula landing in Japan in 2001. The in Lake Jusan, Lake Ogawara and Lake Shinji, corbicula is the main component of the animal respectively, and their differences between stations biomass in each lake, and thus the most important in each lake were small. Bivalve samples were fisheries resources. stored at −40°C until analysis. The foot muscle for Lake Jusan is a shallow mesohaline lake situated each sample was excised, dried in an oven at 60°C, on the coast of Sea of Japan (Fig. 1b). The lake has and ground to a fine powder with a mortar and an area of 18.1 km2, and an average depth of 2 m pestle. with a maximum of 3 m. The River Iwaki and River To investigate the characteristics of organic Toriya empty into the lake at the south-eastern sources from rivers and lakes, the isotopes for par- and eastern ends, respectively. Adequate water ticulate organic matter (POM) were also measured. exchange between the lake and ocean through the For POM sampling in the lakes, 1 L of bottom water north-western channel leads higher salinity in (0.5–1 m above the bottom) was collected with a the north-west (>5), while salinity is kept lower van Dorn water sampler from stations where the by the freshwater discharge in the southern and bivalves were sampled. Fresh water at the middle eastern area (<5).8 Corbicula mainly inhabits the or lower reaches of River Iwaki, River Takase and northern part of the lake.9 River Hii was also sampled (JR, OR and SR, Fig. 1) Lake Ogawara is a deep oligohaline lake on the to provide data on terrestrial particulate organic coast of Pacific Ocean (Fig. 1c). It has an average matter that is transported to the lakes. In addition, depth of 11 m with a maximum of 25 m in the cen- as representatives of organic matter produced in tral part. The lake is approximately 4 km wide and the lower side of each lake, the water at the down- 14 km long with a surface area of 65.6 km.2 Fresh stream side of the lakes was collected (JM, OM and water is supplied mainly from the River Takase with SM, Fig. 1). However, these sampling sites may not additional supply from the River Sadoro, River offer pure marine POM, but potentially offer vari- Doba and River Anenuma at the southern or south- ous mixtures of riverine, lake and marine POM. western ends, while the lake water flows out to the The POM was defined by the particles collected on Pacific Ocean at the north-eastern end. The salinity a precombusted Whatman GF/F glass microfiber in the lake is significantly low (∼1) because water filter. In addition to terrestrial organic matter and exchange between the lake and ocean is strongly phytoplankton, benthic microalgae have recently limited.9 Corbicula mostly inhabits shallow coastal been recognized as important food sources for area of the whole lake. estuarine secondary producers by their high palat- Lake Shinji is a shallow oligohaline lake on the ability.6,12–14 Therefore, benthic microalgae were coast of the Sea of Japan (Fig. 1d). The surface area extracted from the surface sediments of the mud- of Lake Shinji is 79.2 km2 and its mean depth is 5 m flat in Lake Jusan (JK) and Lake Ogawara (OK1 and with a maximum depth of 6 m at its center.