Hmmm... so brook trout are doing fine but browns are not? Are you sure the brown trout swimming around in shallow water in distress makes you more aware of them vs. the brooks that could be sinking to the bottom? But then again you did mention the cage situation...
How old, how large, and what sex are each species? Is it possible the browns that are dying are egg bound females or just older fish? It's not usual for trout to only have a lifespan of 3 to 4 years in ponds.
If this truly is only effecting browns then it shouldn't be a water quality issue as if anything brooks are fussier than browns. It may be a pathogen that the brooks have been exposed to previously but the browns have not.
What kind of pathogens are your fish tested for at the hatchery? Are your brook trout IPN free? It's hard to find a source of brook trout that are, but they develop an immunity. Perhaps your brown trout have never been exposed to it until you put them together?
Water temp is not your problem. Browns although a bit more hardy get their hardiness sometimes exaggerated in literature. They still need cool water temps and lots of oxygen.
If you find another one gasping at the surface can you take a look at the gills with a magnifiying glass and see if there are any parasites on the them?
Last edited by Cecil Baird1; 05/22/10 09:38 AM.
