If you are interested in a guided trip on the Upper Pitt River, there are a selected few operators that offer this service.
STS Guiding Service and
Ultimate Sportfishing, both supporters of this website, are available for booking, but it's best to set a date fast as the time availability of this fishery is small. April and May are indeed good months for targeting bull trout, which will be feeding aggressively on juvenile salmon. Most of the fishing is done on the Upper Pitt River, north of the lake. I'm not aware of people who primarily target them in the lake itself, but I could be wrong.
They are not typically targeted for eating, for a couple of reasons. Most areas (all rivers and some lakes) are closed for retention while deep lake fishing is not popularly done since you need a boat and there are also so many other fishing opportunies available. These migratory char on the coastal watersheds were once fished until their populations were endangered. Their growth rate is slow and the population always takes long to rebound after some harvesting, therefore most anglers would recommend that they should not be kept.
Good luck.