It is hard to explain, but I'll try. Like with stockie vs. wild, there are a number of indicators, and none of them are foolproof. And it is less reliable than the wild/stockie indicators. I can never say with any certainty, but based on a pic I'm right 80% of the time or so.
Also, as a disclaimer, much of this is highly stream specific. So differences I'm associating with Allegheny vs. Susquehanna/Delaware could be stream X vs. stream Y, and it's just coincidence that the ones I fish in each respective watershed have it. Also, I'm not claiming them to all be genetic. I suspect there's a mix of genetics and environment/diet which go into this. Also, I suspect there's differences between Susquehanna fish and Delaware fish as well, but don't have enough experience to identify them. Anyway, here goes.
1. Red spot pattern/placement. Allegheny fish very commonly have 2 distinct lines. Sometimes it's 3, sometimes it's a hodge podge (random placement). But never 1 line. And nomatter what the pattern, none of the spots seem to ever be below the lateral line on the fish.
Susquehanna fish commonly have 1 line. They sometimes also have 2 or 3, or a hodge podge. But when it's more than 1 line, there usually are a few red spots below the lateral line.
2. Dorsal fin - Allegheny fish have spots on the dorsal fin. They can be in a row, but there are spaces between them. Susquehanna/D fish it's usually bars, rather than distinct spots. I need to pay closer attention to this one, it's the one I'm least sure of, but would be the most concrete "tell" if it holds consistent. Unfortunately in most of my pics I smoosh down the dorsal, which makes them all look like bars, so my sample size is low. Mental note to self, get in habit of flaring the dorsal for pics.
3. Tail. On Allegheny fish, there are usually prominant dark markings along the upper and lower edges of the tail. On Susquehanna/D fish, the tail is either absent of markings, or if there are markings, it's more of an edge to edge arc. That ark is visible on Allegheny fish as well, but noticably fainter than the edge lines.
4. Yellow spots. On Allegheny fish, they tend to be most prominant on the back, and fade as you reach the lateral line. On Susquehanna/D fish, they stay prominant on the sides and sometimes encroach on the belly!
5. Shape. Generally, Allegheny fish are longer and more slender. This is very likely environmental, not genetic.
6. Color. Also likely environmental, not genetic. But Allegheny fish run dark, with that greenish black color. In Susquehanna fish, it's often a more brilliant green. The exception is tanic streams, where they often do run real dark.
Also, note, regarding #1. The hallmark of stockies is generally a whole lot of spots in a hodge podge, no particular pattern to it. I've always found it possible that the hodge podge in some wild fish is due to past stockie genetic influence. I don't have much evidence of it, and have no way to prove it. But it seems logical.
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