Like has been said it could have been stress from the handling/hauling/stocking and they just were weakened to the point they got a fungal disease.

FHM typically only live to about 18 months age. So if you do not currently have predators big enough to eat the adults (and make them disappear) as time goes by you will have some natural mortality from old age. After they have spawned several times they just get old and slow down so something eats them. But if there is nothing to eat them they die of old age.

Probably just the stress of stocking.


John

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