Doesn't that sound like a dumb move.
Not at all - it is a good survival strategy for the species. Jacks are often quite successful at spawning because the larger males are too busy sparring with each other to notice a little jack sneaking in and delivering a quick spurt of milt on the redd. The strategy helps diversify the gene pool of each cohort - a jack spawning with an adult female could not possibly have come from the same parents since they were hatched in different years. Interestingly, studies have shown that offspring of jacks aren't more likely to become jacks themselves - it seems to be an environmental factor that causes some years to have more jacks than others.
Also, large fish are not always more successful at spawning, because they are more likely to be eaten by predators when they are nearing spawning. A study on Lake Washington sockeye found that, even though it is believed that all the fish in the lake came from the same introduction about 100 years ago, the sockeye that return to the smaller tributaries have smaller humps and are smaller than average than those that spawn in the lake - it is believed this is because the larger fish are too visible in small streams.