DECADAL VARIATION IN THE TRANS-PACIFIC MIGRATION OF NORTHERN BLUEFIN-TUNA (THUNNUS-THYNNUS) COHERENT WITH CLIMATE INDUCED CHANGE IN PREY ABUNDANCE

Northern bluefin tuna, Thunnus thynnus, apparently spawn only in the w estern Pacific and a portion of the juveniles migrate to the eastern P acific. During the past decade, catches of northern bluefin in the eas tern Pacific have declined. One possible cause for this decline, propo sed by bluefin stock assessment studies, is a decline in the proportio n of bluefin that migrate out of the western Pacific. This hypothesis is examined with several indices of the relative abundance of bluefin tuna in the western and eastern Pacific. These indices suggest a decli ne in the proportion of bluefin migrating to the eastern Pacific since 1977. This period of reduced bluefin migration coincides with a perio d when a prey of bluefin, Japanese sardine, Sardinops melanosticta, we re abundant off Japan. It is hypothesized that in years when sardines are abundant off Japan, a higher proportion of bluefin stay in the wes tern Pacific compared with years when sardines are scarce. Currently, the abundance of sardines off Japan is declining. If this decline cont inues, this hypothesis predicts an increase in bluefin migrating north of Hawaii and into the eastern Pacific.