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Fig. 5. Schematic drawing of experimental raceway. In order to study the roles of sex steroid hormones in downstream behavior (negative rheotaxis), the fish were transferred into the upper pond (2 × 4 × 0.5 m) of a two-step raceway connected to the lower pond (2 × 8 × 0.5 m) through a fishway (20 cm in diameter by 4 m in length made of a polyvinyl chloride (PVC) half-cut pipe (Munakata et al. 2000b). Spring water was supplied into the upper pond. Flow rate (volume) and velocity of the water in the fishway ranged between 10–20 l/s and 70–85 cm/s, respectively. Water temperature fluctuated between 9–10°C. At the downstream edge of the fishway in the lower pond, a net trap (2 × 0.7 × 0.7 m) was placed to capture fish that moved down from the upper pond. An individual experimental fish was identified as a downstream migrant if it moved from the upper pond into the net trap in the lower pond. In order to investigate the effects of sex steroid hormones on the occurrence of upstream behavior, the experimental fish and net trap were transferred into the lower and upper pond, respectively. The frequency of downstream or upstream migrations is expressed as a percentage of the initial fish numbers. Reprinted with permission from Fish. Sci., 78, Munakata et al., Involvement of sex steroids, luteinizing hormone and thyroid hormones in upstream and downstream migratory behaviors in land-locked sockeye salmon Oncorhynchus nerka, 81–90, Fig. 1, © 2012b, The Japanese Society of Fisheries Science.

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