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Last Post by GrimJack
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The salmon thing is only partially true - there are some species that spawn more than once. The species that spawn only once; begin to die on the way back. Their mouths deform and they are no longer able to eat - every last bit of energy that they have is used to get them back to the spawning grounds. I would have to look into the senescence (okay, I did - I cold not resist):

The antagonistic pleiotropy theory of senescence postulates genes or traits that have opposite effects on early-life and late-life performances. Because selection is generally weaker late in life, genes or traits that improve early-life performance but impair late-life performance should come to predominate. Variation in the strength of age-specific selection should then generate adaptive variation in senescence. We demonstrate this mechanism by comparing early and late breeders within a population of semelparous capital-breeding sockeye salmon (Oncorhynchus nerka). We show that early breeders (but not late breeders) are under strong selection for a long reproductive lifespan (RLS), which facilitates defence of their nests against disturbance by later females. Accordingly, early females invest less energy in egg production while reserving more for nest defence. Variation along this reproductive trade-off causes delayed or slower senescence in early females (average RLS of 26 days) than in late females (reproductive lifespan of 12 days). We use microsatellites to confirm that gene flow is sufficiently limited between early and late breeders to allow adaptive divergence in response to selection. Because reproductive trade-offs should be almost universal and selection acting on them should typically vary in time and space, the mechanism described herein may explain much of the natural variation in senescence.

Uh oh! I can't find any references to support my statement that some salmon spawn more than once (salmon and trout are related, trout being essentially a salmon that stays in fresh water). Apologies! I live in Seattle and thought I knew it all about salmon, sigh! If I find something, I will get back.
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All coho and chinook salmon die after spawning as part of their life cycles. Brown trout, steelhead and brook trout do not automatically die following spawning, although some will die simply from old age, stress from spawning or an infection such as fungus brought on by spawning activity. Depending on their age, some trout will return to Lake Michigan.

A little more:
Oncorhynchus is a genus of in the family Salmonidae; it contains the Pacific salmons and Pacific trouts.
Atlantic salmon, known scientifically as Salmo salar, is a species of fish in the family Salmonidae, which is found in the northern Atlantic Ocean and in rivers that flow into the Atlantic and the Pacific.

But I digress, you were talking about the brain - I will stop now.

This topic has been dead for over six months. Start a new discussion instead.
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