by Jerry Burton, NMT President
From the September 2014 Newsletter
The size of the trout population in a stream greatly depends upon the success the fish have when they spawn. To be successful they need flowing water that circulates oxygen and is free of sediment. They also need the right size gravel that they can easily move when building redds. Stream gradient is also important because it will determine whether a stream has boulders and cobble or the gravel they prefer. Spawning substrate can be a factor limiting the size of a trout population.
The number of eggs a female trout will deposit depends upon her body weight and will range from 900 to 1000 eggs per pound of body weight. Light and water temperature is what will trigger spawning activity. In trout hatcheries it is possible to obtain eggs just about every month of the year by placing trout inside where both light and water temperature can be controlled. Because of this it is possible to grow trout to catchable size for at least six months out of a year. Most of the spawning is done by two to four year old fish.
Browns are fall spawners that start spawning in early October. Brook trout are late fall to early December spawners, rainbows are spring spawners, and cutthroats spawn from late spring to early summer. One of the reasons brown trout are so successful in establishing populations in New Mexico is the low flows we have in the fall. Because of this they have enhanced egg survival and hatching success, plus the fry have a head start on the some of the other trout species. Brook trout have a similar advantage. Considering all the thousands of rainbow trout that have been stocked into the various streams in New Mexico it would seem that we would be blessed with many self-sustaining populations. We do not, however, because of two factors; trout have a strong homing instinct that makes them attempt to return to the same location where they were hatched, and if they do attempt to spawn in the spring, it is usually before we have the peak high flow from melting snow runoff that destroys the redds. Cutthroats, however, spawn after the peak of the runoff. It is because of this that cuttbow populations have become established in many streams.
When I used to be guiding clients on the upper Brazos, they probably thought that I was a bit out of it when I would get excited over a four or five inch rainbow they had caught. Of course, the reason I was excited was that the rainbow was a stream spawned fish and was proof of natural reproduction.