Allegedly a brown trout male and possibly rainbows as small as 4 inches have an anal fin that is convex as in like this ) on the outside edge. And allegedly a female brown has an anal fin that is concave, and more or less like this on the outside edge of the anal fin ( (See photo below).
https://www.ginkandgasoline.com/fly-fishing/2-ways-to-determine-the-sex-of-a-trout/Prefer to see fish that are obvious males and females to verify the anal fin differences.
So far all pictures I've seen seem to verify the above.
And yes I know a mature brown trout or rainbow trout is easy to tell from a female due to the kype and coloration during spawning, but immature and small fish may not be so easy.
Here's one source that alleges this and I have a article form a biologist in McClane's Fishing Encyclopedia that says the same under the heading of Brown trout.
As a fish culturist, if this is really true, it would help me out a lot as I can sort the males from the females early on to save on feed costs and increase the number of males in the pond. My taxidermy and replica market is primarily male fish.
