Reverse migration of adult pink salmon (Oncorhynchus gorbuscha) to the sea after their return to fresh water

Few studies have examined the movement of adult Pacific salmon Oncorhynchus spp. between the sea and rivers after they have entered a river. In this study, archival tags that recorded temperature and salinity were attached to pink salmon (Oncorhynchus gorbuscha) that had ascended one of four rivers in northern Japan, to examine their individual movement patterns among fresh, brackish, and salt water. Of 44 pink salmon tagged and released in the rivers, nine were recaptured in the same river where they had been tagged and released, four were recaptured in marine nearshore trap fisheries, and one was recaptured in a neighboring river from where it was tagged and released. Among the 14 recaptured tagged fish, abrupt changes in salinity records were found in seven tag records, indicating that tagged fish had returned to brackish or seawater. Six of the seven individuals were males, and the sex ratio was biased toward males when compared with all tag-released individuals. The results suggest that a substantial proportion of homing adult pink salmon may make short-term movements between fresh and saltwater during their spawning migration. Analysis of the tag-recorded water temperatures showed significantly cooler riverine temperatures compared with inferred temperatures of the coastal water (up to 20.5 degrees C). While many factors, including orienting behavior and male-biased dispersal, likely influenced the observed 'probing' behavior, I speculate that the temporal use of freshwaters rivers may be a thermoregulatory behavior to avoid high, stressful temperatures. Future research relating the short-term movements between fresh water and salt water to local environmental conditions is needed to investigate this hypothesis.

Automated summary

This study investigated the movements of pink salmon between freshwater and saltwater during their spawning migration, finding that a significant proportion of homing adult pink salmon may make short-term movements between fresh and salt water. Analysis of recaptured tagged fish showed abrupt changes in salinity records, with a male bias in sex ratio.