Open-ocean orientation and return migration routes of chum salmon based on temperature data from data storage tags

Temperature data storage tags were applied to maturing chum salmon Oncorhynchus keta in the Bering Sea during summer 1998, 5 of which were recovered in the coastal waters around Japan. These tags recorded water temperatures experienced by the fish for periods of 62 to 118 d at resolutions of 15 to 30 min, We considered fine-scale aspects of the data by analyzing diel temperature patterns in regard to the migration behavior and orientation of the fish. During the night, temperatures experienced by the fish changed little, and were among the highest temperatures experienced whereas daytime temperatures often showed dramatic changes as the fish changed depth. We analyzed a time series of temperature differences between sunrise and sunset as an indication of progress made by the fish against horizontal thermal gradients, which was assumed to represent changes in latitudinal position. Daytime differences were positive when the fish were migrating to warmer waters, whereas night-time differences were zero or slightly negative, This suggests that the fish made more progress migrating during the day than at night. We also considered large-scale aspects by examining oceanographic data in relation to the temperatures recorded by the tags. Analysis of possible migration routes based on large-scale patterns of sea surface temperature distributions suggests that zonal (east-west) routes are preferred. These data support the hypothesis that chum salmon utilize orientation cues associated with the sun during open ocean migration. However, whether salmon also utilize additional orientation or navigation mechanisms remains to be determined.