I suspect that counting jacks does play into the following year runs but that the gill nets are sized for the adults.
But then again, I am one of suspicious nature. For my paranoia to be true, somebody would have to be gill netting them before they get to Bonneville dam.
"The biggest questions now surround the show of jacks at Bonneville. Jack salmon are males that return a year or two earlier than their age class and with enough regularity (except for the past three years) to predict the subsequent adult run.
By Monday, the jack count at Bonneville was 62,748. That's more than three times the previous record and six times the 10-year average. The count almost certainly means a better run in 2009, although high jack counts last year didn't pan out in an accurate run prediction this year."