Short communicationAtmospheric depression-mediated water temperature changes affect the vertical movement of chum salmon Oncorhynchus keta

- Chum salmon behavior after the passage of atmospheric depression is reported.
- During the passage, they shortened the vertical movement duration as before.
- The cooler water below the thermocline was previously due to strong vertical mixing.
- To minimize energy cost, they likely stayed at cooler surface waters of bays.

The Sanriku coastal area, Japan, is one of the southern-most natural spawning regions of chum salmon Oncorhynchus keta. Here, we report their behavioral response to changes in ambient temperature after the passage of an atmospheric depression during the early spawning season. Before the passage, all electrically tagged fish moved vertically for several hours to depths below the shallow thermocline at >100 m. However, during the atmospheric depression, the salmon shortened the duration of their vertical movements and spent most time at the surface. The water column was homogenous at <150 m deep except for the surface. The descending behavior may have been discontinued because the cooler water below the thermocline was no longer in a thermally defined layer, due to strong vertical mixing by high wave action. Instead, they likely spent time within the cooler water temperatures at the surface of bays to minimize metabolic energy cost during migration.