A Shift in Pink Salmon Dominance in the Okhotsk Sea of Hokkaido in Relation to Coastal Environments during Early Sea Life

摘要

Although dominance by the even-year line of pink salmon in the Okhotsk Sea of Hokkaido was maintained from the 1990 to 2000 brood-years, a shift back to odd-numbered year returns occurred. We have monitored the distribution of juvenile pink salmon and the ocean environment in the Abashiri coastal waters of the Okhotsk Sea since 2002 when juveniles from 2001 brood-year pink salmon went to sea. SST measurements were much higher when odd-year juveniles entered coastal waters in 2002 (8.2-9.7°C) and 2004 (5.5-12.0°C) than when even-year juveniles entered coastal waters in 2003 (5.3-6.8°C). Pink salmon juveniles were widely distributed along the coast in May 2002 and 2004, while juveniles in 2003 were densely distributed and restricted to the littoral zone. Mean fork length in 2003 was significantly smaller than those in the other two years. The final number of juveniles captured along the coast from May to July 2003 was 4,700, much lower than the numbers caught in the other two years (19,200 and 21,900, respectively). More interestingly, the abundance of adults in 2003 from pink juveniles (2001 brood year) that experienced the warm temperatures in 2002 was much higher than adults in 2004 that had experienced the cooler temperatures in 2003, resulting in the shift in dominance. These results suggest that shifting between dominance lines might be caused by thermal conditions in coastal waters that result in either long estuary residence times with larger aggregations of fish or rapid dispersal with wider occupation of nursery grounds by pink salmon juveniles after seaward migration.