The unappreciated jack

posted in: About Salmon | 0

By Richard M. Johnson

The jack salmon is looked at as either a fun fish to catch on an ultra light spinning rod, or the failure of a perfectly good fish to reach it’s potential. When I say “jack”, I’m referring to a precocious male salmon that decides to grow up in a hurry. It spends one year or less in salt water before returning to fresh water to spawn like the big boys. You may wonder why they mature so quickly and are they successful spawners? The answer to the first question is that this might be nature’s way of spreading out the genetic contribution of a particular brood year over several years. In the case of chinook salmon, the dominant year class is the age four group. However, some return at age two (jacks), some at age three, and there are even a few five year olds (six year old chinook are rare in the south Puget Sound watersheds). An environmental roadblock such as flooding or poor marine survival might limit the number of spawners in a given year. Fortunately, the jacks would have returned before the problem arose (take El Nino years as an example, when adultsalmon, especially coho, were hard hit due to poor feeding conditions). As to how jack salmon fare in the competitive world of reproduction – they have developed a special tactic that we know as the “sneak attack”. While the big brutes are jousting for the right to sire the eggs of an unsuspecting female, the jack waits in the shadows and races in at just the opportune moment to add some of his own milt to the pot. In spite of its diminutive size (one to four pounds), the jack is governed by the same biological clock as its bigger adversaries. It dies shortly after spawning, and its carcass decomposes, putting important nutrients back into the stream.

Despite the importance of jacks in nature, fisheries managers are constantly trying to figure out ways to prevent jacking in hatchery stocks. More jacks means fewer adults back to the rack for spawning (and fewer adults available for harvest). There appears to be a nutritional link to precocious maturation such that the faster the growth of juvenile fish, the higher the jacking rate. It follows that salmon that go through “extended” rearing in the hatchery, where they receive plentiful, high quality rations, tend to have a higher rate of jacking than fish that rear in the wild the same length of time. High concentrations of forage fish available to young salmon in the marine estuary or nearby vicinity could result in jacking due to the rapid growth experienced by the voracious feeders. Genetics, as well, plays a role in determining precocity. More jacks in the spawning population results in more jacks in the offspring. Generally, about 3 to 10 percent of the naturally produced males mature as jacks, depending on species and location.

In spite of the fact that a jack will never attain the size to make it a prized sport fish or a brawny super-stud on the spawning ground, a good showing of jacks is often used as a predictor of a good adult return the following year. That makes them worth their weight in gold.