Time of Annulus Formation on Scales of Chum Salmon in the North Pacific Ocean in 1998 and 1999

Scales (n = 552) of chum salmon (Oncorhynchus keta) collected in the North Pacific Ocean in May 1998 and 1999 and April 1999 were used to compare time of annulus formation between years, regions, and ages. May samples were collected from approximately the same location and time, during two years of very different ocean conditions, 1998 a strong EI Nino year and 1999 a strong La Nina year. Sea surfaces were generally warmer during 1998 than in 1999 in the eastern North Pacific Ocean. Fish were generally longer in 1998. In 1999, fish length increased from west to east in the North Pacific Ocean. Annulus formation occurred earlier in 1999 than in the warmer EI Niiio year (1998); possible causes are discussed. Annulus formation generally occurred earlier in the eastern North Pacific Ocean than in the central and western North Pacific Ocean. Northern fish formed the annulus before southern fish in both years. Younger fish generally completed the annulus before older fish. Younger fish also had a greater number of circuli beyond the annulus.