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There is a fair amount of info out on this.. some say it's a disease, I disagree as I see it in many locations pre-spawn through post-spawn, then you may not see it again until the next spring.
In Hatchery setting, we see the males show this trait but not every year, some may be very small patches, sometimes larger around the head area.
Personally, I have nothing scientifically to offer on this, but our SW Hatchery manager says it is not a "sunburn" in his eyes, he believes it has more to do with environmental factors and said he thinks he sees it more with a warming trend which would align with my observations of pre to post-spawn.
From jpsdad's article:
"Melanosis is not an infection but an increase in the melanin in the skin."
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