Well steelhead Jacks? Gisle is a more appropriate term. Half-pounder would be more appropriate if it indeed was sea-run and not sexually mature. Wether these were small one salt mature fish or imature fish that followed adult back it is unclear. This determining future run sizes as a representation of that age class of steelhead would be highly unlikely for a river that this occurs in a un regular basis. In Southern Oregon and Northern California it would be easier to suggest future run sizes.

As far as Jacks in Salmon; whole different ball game. For Coho; three years of age is an adult, two is a jack, and yes all male (a female would be a small two salt fish). These fish are physilogically adults too. Although the further North you go, it is more common to see fish that over winter in fresh water for up to three years, in this case you can see true "Jennies," fish that spent one year in salt and two or three in fresh and are typical jack size. As far a a acurate measurement of that age class survival as out-migrants to unceartain ocean conditions, this is a very good indicator of the next years adult return will be.
Chinook jacks are also two years of age, one salt and sexually mature adults, however age's three to six are common, but fish can reach up to nine-ten years of age. however rare. Most stocks have commonly age four or five as normal and some stocks have a higher percentage of age six fish such as Tillamook Bay Rivers. Rivers Inlet and the Kenai River are in there own catagory with fish that commonly reach age six through eight. Jack would represent a good idea of ocean survival also just like Coho, although you can commonly follow this strong age class year-by-year(strong showing of three's the next year and ect.)
As far a Chum and Sockeye one salt fish are very rare, first returning adults are age three. Pink (all mature at age two, one salt fish)
It should be reconized that jacks are typical representation of the total run size on average, although environmental factors such as water temperature adn food availability can sque these averages hence, making it difficult to predict the age classes survival when they mysteriously appear or don't appear.