Fig. 3

Schematic design of the present study. A In the drifter deployments, the drifter was deployed either from beaches or river mouths or from vessels at offshore locations. When the drifter encountered a tagged fish, the receiver recorded its signals. GPS positions were logged by the drifter and uploaded to the Argos system whenever the drifter communicated with satellites. The drifter was retrieved after becoming stranded on the shore. B Setting of the range tests in shallow nearshore area. First, the two test transmitters were placed at a depth of 60–70 cm, approximately 10 cm above the sandy bottom. After the transmitters were fixed, the drifter was placed near the transmitters (< 5 m) for approximately 10 min. The drifter was then moved toward the center of the river mouth and released into the river flow. The transmitters were left in the water at the same location for the next 4–24 h to ensure that the drifter reached a sufficiently large distance from the transmitters such that the transmitters were out of the drifter’s detection range. The drifter was stranded on the lake shore and retrieved 2–3 days after deployment. C The range test in deep offshore area was conducted using a fishing boat. After arriving at a suitable location where the water depth exceeded 50 m, the boat engine was turned off to reduce noise. The test transmitters were placed in the water at a depth of approximately 20 m before the drifter was placed on the water surface from the boat and released. The boat drifted with its engine turned off for approximately 3.5 h after drifter deployment