The Journal of Experimental Biology, 203, 1825-1833 (2000)
Behavioural thermoregulation of chum salmon during homing migration in coastal waters.
H.Tanaka, Y.Takagi, and Y. Naito
Homing Pacific salmon are suggested to have directional cues in shallow water while migrating in coastal waters. Since salmonids are typical cold water fish, they may have to deal with warm surface water while gathering directional information. We studied behavioural thermoregulation of 31 free ranging chum salmon Oncorhynchus keta using micro data loggers off the Sanriku coast from early October to December. The surface water temperature was ca. 20 ° C in early October and decreased to ca. 12 ° C in December. The seasonal change of water temperature transformed the behaviour of salmon markedly from deep diving to shallow swimming. In October, salmon frequently dived to depths exceeding 100 m. Duration of deep dives tended to be prolonged as the thermal difference between sea surface and bottom water increased. The results indicated that salmon sought the coolest thermal refuge as they could exploit by their vertical movement. Thermal refuge can lead salmon to minimizing metabolic energy cost. However, salmon repeatedly returned to the surface water column. We found a positive correlation between surface swimming of salmon and the presence of cool covering water, which could contain river waters. It suggests that salmon tend to be risk prone under the high concentration of directional cues. Salmon in December spent most of their time in shallow water. These findings suggest that salmon adjusted their behavioural strategy with the hydrographic structure of the sea in order to balance between acquiring directional cues and behavioural thermoregulation.
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