Timing of egg hatch of amphidromous freshwater shrimps in a small river (the Nishida River), western Japan

Larvae of amphidromous freshwater shrimps, which include species of fisheries importance, are released by adult females in freshwater areas of rivers and they drift downstream to the sea. Information on the timing of egg hatch is important to understand the ecology of the amphidromous freshwater shrimps. March et al. clarified that in amphidromous shrimps living in tropical streams, larval drift occurred at night and was slight during the day at low and mid-altitude sites, and no diel pattern was discernible at a high-altitude site. However, the shrimp species was not identified in their study. The methods of larval species identification of Japanese freshwater shrimps were previously developed based on the chromatophore distribution patterns of larvae, enabling us to study reproductive biology of these shrimp species. The purpose of the present study was to clarify the timing of egg hatch of shrimps, with the identification of shrimp species, in a small temperate river in western Japan. Drifting zoeae were hourly sampled during a 72-h period between 2 and 5 July 1998 at two study stations (St. A and St. B) of the Nishida River, Shimonoseki City, Japan (Fig. 1). St. B was located at the lower limit of the freshwater area of the river and St. A was in the middle area, and 1070 m upstream from St. B. Adults of amphidromous freshwater shrimps were observed at high density near the two study stations in preliminary investigations before the present study (Fig. 1). A plankton net with a square mouth (opening, 30 cm ¥ 30 cm; length, 90 cm; mesh size, 212 mm) was used for the sampling. The net was fixed in the study stations for 5 min every hour. Water velocities of the four corners and the center of the net opening were measured using a current meter (Pocket Tachometer Model 3631; Yokogawa, Tokyo, Japan; accuracy, 0.02 cm/s). Filtrated water volume was calculated as follows: filtrated water volume (m) = area of net opening (0.09 m) ¥ average of the five current velocities (m/s) measured ¥ sampling time (300 s). The cross-section of the river at St. B was rectangular in shape because of the head construction (a concrete structure as a river bed retaining work). Therefore, the amount of flow was calculated as follows: the amount of flow (m/s) = river width (7 m) ¥ water depth (m) ¥ the speed of flowing water (m/s). Samples collected were fixed in the 5% formaline and were immediately observed, based on the method by Kikkawa et al. Shrimp species and zoeal stages were identified, based on the methods by Kikkawa et al. and Wada et al. Zoeae (408) were collected at St. B, but no zoea was captured at St. A. Shrimp species and their numbers of zoeae collected at St. B were: Caridina leucosticta (167), Macrobrachium japonicum (10) *Corresponding author: Tel: 81-29-879-6775. Fax: 81-29-879-6748. Email: nakata55@pwri.go. jp Present address: River Restoration Research Team, Water Environment Research Group, Public Works Research Institute, 1-6 Minami-Hara, Tsukuba, Ibaraki 305-8516, Japan. Received 3 October 2005. Accepted 20 February 2006. FISHERIES SCIENCE 2007; 73: 961–963