|Fish display at John Yi, Reading Terminal Market|
But silver trout is whiting, which is really hake, part of the cod family. I haven't the foggiest idea about mountain trout, though it certainly isn't a trout and it certainly doesn't come from a mountain stream or lake. Striped bass and rockfish? They're the same, except when they're not.
You'll find all these items at the fish mongers at the Reading Terminal Market, but it's hard to know what the actual species is. More than any other food, fishes tend to have very local names.
The weakfish, for example, has been called bastard trout, squwteague, sea trout, grey trout, sand trout, shecutts, silver sea trout, squeteague and squit.
In culinary terms, however, nomenclature is secondary. Just pick the broad type of fish you're hungry for (white flesh or oily, large or thin) and buy the freshest you can find. Most recipes for cod work just as well with whiting or haddock. Fluke or flounder? Doesn't matter! Mackerel or Spanish mackerel? There's a bit of a size difference, and while you might be able to distinguish their flavors, it's not that big a difference.
Rockfish and striped bass are the same, delicious, meaty fish, it's just that rockfish is the name in the Chesapeake and striped bass on the Hudson River northwards (New Jersey seems to be the dividing line between the names). With one big caveat. Some striped bass are true striped bass, caught in the wild. Others, under either name, may be a factory or farm raised hybrid of striped bass and white bass. The former is anadronmous, living most of its life at sea but spawning in fresh or brackish rivers; the latter is strictly a fresh water variety. Whether wild or hybrid, both have a good, meaty taste. You can usually tell the difference because the wild striped bass has a blue tag affixed to its jaw; also, the "stripes" on the hybrid tend to be jagged, and the fish are frequently smaller than the striped bass.
If you're as fascinated by culinary fish as I and would like to learn more about the denizens of the sea off Mid-Atlantic waters, I highly recommend Alan Davidson's North Atlantic Seafood. This British diplomat's tome covers fish on both sides of the Atlantic, is chock full of great stories, local recipes from Denmark to South Carolina, and entries for each North Atlantic food fish with discussions of range, best cooking methods, and names in various languages; before he died in 2003 he wrote similar books about the fish of the Mediterranean and South-East Asia, and was the editor of the Oxford Companion to Food.