Remove eyes and gills from carcass. Cut carcass into about 4" pieces. Cover with cold water. After 30 minutes, drain and rinse under cold water briefly. Cover with fresh cold water. After 30 minutes, drain and rinse under cold water briefly.
Sweat mire poix (no leaves on celery) and bouquet garni. Add fish carcass. Moisten with white wine. Let alcohol burn off a bit. Add cold water Bring up to almost boil, skimming the entire time. Reduce heat to a simmer. Cook 20 minutes. Turn heat off and let sit 10 minutes
Strain through etamine.
Too much cooking time is the usual culprit. Celery leaves can also make it bitter. So can skin, gills, and eyes. Too high a heat as well. I know some chefs that blanch their onions before using them in their mirepoix to eliminate the bitter juices.
@cheflayne is right on the money. I am very fortunate to have a poker buddy that is a fish monger. He has shown me how to make many stocks,soups, broths, etc.
When I make a broth with Salmon or Tuna, I actually use a thermometer. I bring it up pretty quick to 185 F and keep it there for 15-20 min, I also use a pinch of salt contrary to the normal belief.
My preference for a mock poix is the lighter pale vegies and spice. Scallion bottoms, lemongrass, etc. I save the darker leaf for the garnish.
Salmon stock is definitely an assertive stock so I use it for some applications, but not straight across the board for all salmon dishes. I don't find tuna stock to be near as assertive. Both fish have a high fat content so they are not as prone to yield a clear stock which is why some steer away from using them. For me it depends upon the final application of the stock as to the type of fish carcass used.