We spent the third week of July 2014 at the Karta Lake cabin.
1. I can't speak to other times but once we figured out location and presentation we reliably caught as many Coho as we wanted.
2. The trout fishing wasn't as good, but since we were on to the Coho we didn't spend much time chasing trout. Although I did catch a nice rainbow in the river almost up to Salmon Lake. We didn't catch a Dolly.
3. I recommend Karta Lake cabin over the Karta River cabin.
Karta River Pros:
Accessible by boat
You can fish the saltwater bay
You can put out crab pots.
More reliable float plane access in bad weather.
Karta River Cons;
No boat at the cabin.
Practical freshwater fishing is only in the river.
It's a 1+mile hike to the lake and when you get there you won't have a boat.
Karta Lake Pros:
Cabin has a boat. Bring your own motor.
You can fish the lake, the river between Karta Lake and sea, or the river between Karta Lake and Salmon Lake. We found the silvers at the Karta Lake inlet.
Karta Lake Cons;
Tougher floatplane access in bad weather
Hope this helps. We also love the forest service cabins!
Finally, we highly recommend Seawind Aviation in Ketchikan.