What commercial fishermen do, without trolling valves, they trail a brake, usually behind their floppers. Also flopper stoppers can be set nose down and act as a brake. Brakes are usually homemade, but maybe be available from a marine store that caters to fishermen. They can be anything from a small sea anchor to a piece of plywood. Depending on the type of brake, some can run alongside the hull, out of the way of fishing.
Another way to counter speed if you fish with with your line from outriggers or poles, is fishtail. The outer pole line is faster and the inner one is slower. Half the time each line is about the right speed. And the change in action often draws salmon attention.
When transmissions were all mechanical, some people changed their props for trolling. And some others used a 2 speed gear box behind the regular transmission.
