My theory...and it is just that...my theory!!

Anyways...I have alwayys been under the impression that the task of jacks has been to be sort of safety net to the true adult population in case there is some sort of natural disaster or other unexpected and negative enviromental condition to hit a population.

Because pinks, chum and sockeye spawn in such great numbers and during a more concentrated period, the need for jacks is minimized because there are enough adult male fish around to act as a safety net.

Coho and chinook do not spawn in the large concentrated numbers that the other aforementioned species do and thus it is neccasary for these populations to have a safety net.

I also believe that the operational sex ratio (males:females) is higher in sockeye, chum and pinks and thus there maybe adult males doing the same job that chinook and coho jacks do.