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Sandman & Jake,
The genes absolutely do change. Not all fish (or any animal) have the genetic makeup to grow to max size. Whether they are susceptible to disease or too slow to avoid predators or whatever. Some may even live a long time just not grow large. Maybe they only get 30lbs not 50.
As Sandman stated, the large fish spawn more eggs. When you cull the large fish out of the population, there good genes are no longer dominating the population.
This isn't too say that a fish has to 50lbs to have good genes. That 50lber has been spreading its good genes from the first time it spawns when it was 10lbs. But over time, if there are fewer large fish, there genes become less and less dominant. And as stated, in their prime years they produce more eggs... more than they did earlier in life.
So, say one out of 20 fish is genetically superior. That fish is not producing any more eggs than the other 19 when they are all 10lbs. If the fish never reaches the bigger size where it will become a major egg producer, it has no advantage over the rest of the population.
The point I made that was shown from the cod stocks takes it a step further.
Nature is resilient and will adapt. When the cod stocks where blasted, there were very few breeding adults left. The species adapted by having sexually immature fish spawn out of necessity.
Hope that makes sense... the big fish' advantage comes when she is producing many more eggs than the other 19 fish... when she is in her prime spawning years.
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