Assessing the northern diversion of sockeye salmon returning to the FraserRiver, BC

We examine the oft-quoted relationship between the migration of Fraser Rive r sockeye salmon around the northern end of Vancouver Island and sea surfac e temperatures. We examine the methods used to estimate the northern divers ion and conclude that the estimates have a sufficiently low expected error to form a useful representation of sockeye salmon behaviour. The well-known relationship with Kains Island sea surface temperature is explored and pro blems are pointed out. In particular, we explore why Kains Island temperatu res are good predictors of salmon behaviour in May when the sockeye can be over 1000 km away, but the coastal temperatures are poor predictors in July to September when the salmon are actually close by. We show that a more ro bust predictor can be developed using open ocean temperature fields and we show why Kains Island fails as a predictor during the summer months. Finall y, we show by cross-validation that the northern diversion is predictable w ith an r.m.s. error of about 0.1.