Margarete Paulsrud, age 11, of Danbury, Iowa, for her question:

Why must salmon die after spawning?

This is true only of the Pacific salmon. Most of them return after several years of mid ocean voyaging. But a few return from long journeys through fresh water lakes and rivers. Journey's end is always the inland stream where the salmon and their ancestors hatched from eggs. Each generation of parents returns home to spawn eggs and hand on life to the next generation. And after spawning, all the Pacific salmon die. At present, scientists do not know exactly why this must happen, though they are beginning to understand some of the reasons.

The ocean going salmon stop eating when they enter the rivers, perhaps because fresh water food no longer suits them. Their silvery blue color changes to red and the males grow large, hooked jaws. These changes and weeks of fasting must weaken them and the final spawning is exhausting work. We might simply suppose that they die from over exertion. But the fresh water Pacific salmon also die after spawning. Scientists suspect that the secret is in DNA, in the genes that govern the features and behavior of all living things. These salmon inherit a genetic life program that orders them to die after handing on life. The accepted explanation lies in DNA but in this case the hows and whys are still unanswered.