Please forgive me for re-igniting an old post.
I have worked quite a bit with trout, both brown and rainbow as well as a few other variations not to be limited to fresh or salt water.
I have also worked with mackerel equally. Not to say i am an expert on the subject, i certainly am not and the information i provide is only true in my perception.
Rainbow trout is indeed the most readily available of the trout species depending on you location. If you live near costal waters you will likely see saltwater trout species commonly but of the fresh water species rainbow is most common. This is due to trout farming as mentioned above in another post. More on farmed vs. wild later. brown trout tends to not survive farming conditions, this is here-say information i have been told, not based on any experience or reading. I prefer brown trout over rainbow, so much so that i often travel north to washington and canada to fish for it, rainbow trout the only trou you will find in texas, though i have seen one brown trout, that was one time out of countless fishing adventures. Up north brown trout are more abundant but rainbow still win in numbers. (I used to throw rainbow back out of determination to catch brown). Time to answer the question though. When profiling the flavor of fish you must first imagine the habitat; if possible. Habitat has a huge impact on flavor of fish, this is why i dont bother with farmed fish at grocery stores. My rule of thumb (edit: i don't always follow this rule there are exceptions). when selecting a place to fish is based on weather or not i would drink the water the fish lives in. A small murky pond will give the fish a very fishy almost polutted flavor. A clean stream will have a crisp taste in white flaky fish like trout, allowing the rich oily flavors to really shine through. Trout are very oily and rich in omegas, not as much so as mackerel, but not far off. Brown trout to me is best descriped as a light, flaky fish where the meat has woody, and nutty undertones. While rainbow tends to be just as flaky but not as oily or meaty as the ladder. The undertones translates more as a grassy citus flavor. My favorite preparations for trout both rainbow and brown include sauteed hot and fast on a cast iron skillet with lightly browned butter, a pinch of salt, white pepper, and thyme finished with a squeeze of lemon peel (not juice, i want the oils in the peel). Sometimes my fishing trips are too generous and i catch more that i can eat that night and because brown trout is a special treat for me i just cant let it go unless its undersized. When this happens i will cure it, smoke it, salt it, or pickle it. I always use fins, organs, heads and bones to make a fermented fish sauce with shitake or woodear mushrooms. (Edit: trout skin should be left on and eaten with the fish. Its where most of the fat on that fish is and when cooked properly has a more jellyish texture between the skin and meat).
On mackerel, one of my favorites from the see, try pickling in a hot brine using apple cider, not only will this preserve it for a few extra days but the oils and the acidity make a interesting combo, you can add spices and herbs for flavoring.
Wild vs. farm- as said above farm raise is some icky stuff, the fish is swimming with so many other fish, not enough movement, no fast flowing streams to give it that clean flavor and to build strong muscle which creates all those yummy healthy fish oils. Wild fish feed on insects that have been feeding on surrounding vegetationsuchas fruits, nuts, and some cases the sap and bark of trees, imparting a very pleasant flavor that skips right over your head if you don't look for it.
The times i make excemptions to my rule of thumb is when im in a densely wooded national park, and i come across a decent sized undesturbed quite pond with insects swarming over the surface and a stream or river near by with the flood plane. When i see this i know there are trout, tramp and fattening on an abundant food supply. This habitat gives the brown trout an almost greenish hue to it (trouts colors change slightly due to environment because they are stealth predators relying on camoflage). The flavor becomes more intense, fishier, not as crisp, but with more intense underlying flavors of woody nuttiness. The best way i can describe it is wildly rustic. I serve this at dinner parties when i find it. Saute whole(organs removed) with salt and butter until a crispy gold brown crusted has formed, i place the fish in a shallow bowl shaped platter or saucer and then pour a broth of bay leaves and sticks, lichen, reindeer moss, rosemary, thyme, wild mushrooms (morel) and the 2 secret ingredients (white oak leaves and grape leaves) into the bottom of the dish table side to create a aroma that smells like the forest and impart a very umami and foresty flavor to the fish.
Could it be that the taste of trout is our only difference in this area?
