A jack is a male salmon that matures and returns one year earlier than other adult salmon to carry out spawning. This is quite common for both chinook and coho salmon. For example, adult chinook salmon typically spend three years in the ocean, a chinook jack only spend two years in the ocean.
Because jacks spend one year less in the ocean than other salmon, they are much smaller. A chinook jack is defined as any fish that is between 30 and 50cm (or 62cm, depending on which river it is from). A coho jack is typically between 30 and 40cm long.
Jacks carry out spawning with other adult salmon in the stream. They are known as "sneaker males", which deposit their sperm on eggs discretely while other large adult male salmon fight over their territory.
The importance of jacks is often overlooked. Because they are spawning with runs from other years, they are responsible for gene exchange between runs. In the long run, this could possibly save vulnerable stocks that are recovering.
When salmon fishing, jacks are often the easiest fish to entice as they will strike bait, lure and fly anytime when the opportunities are given.