The demise of the sea trout

Can any sea trout boffins help me with my enquiries?
I am new to fly fishing, so excuse my ignorance.
I already understand that a sea trout is a smolted brown trout, and
that mostly females run to sea.
I also understand that male brownies stake-out a territory for food
and mating.
What I don't understand is, if we stock non-native brownies, cockfish
and hens, why do they not compete for food and displace our natural
breeding stock?
Why also would they not consume the natural fry?
Having spent the past year fishing the Coquet at Felton and only
landing 2-3lb "paint-brush" tailed stockies, I could not help
wondering if the lack of sea trout was due to any of the above
factors.
ps I have been assured by the locals that these particular sea trout
are not related to brown trout!.

Alec.